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Continental Tectonics (1980)

Chapter: III. Intraplate Tectonics

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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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Suggested Citation:"III. Intraplate Tectonics." National Research Council. 1980. Continental Tectonics. Washington, DC: The National Academies Press. doi: 10.17226/203.
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III # INTRAPLATE TECTONICS

Tectonics of Noncollisional Regimes The Modern Ancles and the Mesozoic Cordilleran Orogen of the Western United States 6 INTRODUCTION B. CLARIS BURCHFIEL Massachusetts Institute of Technology An early hypothesis of plate tectonics was that plates moved as rigid pieces of lithosphere and that the relative motion between plates was talcen up at narrow zones along their boundaries. Later studies suggested that inter- action at plate boundaries could produce deformation, magmatic activity, and metamorphism for a considerable distance from those boundaries. Simply stated, the prob- lems to be addressed now are "what intraplate features and events are the result of plate-boundary interactions, what are the interrelations between these features and events, and what are the processes that cause them." All three types of plate boundaries, transform, subduction, and spreading, show widely distributed intraplate effects. In some cases the boundary can be defined along a narrow zone, but in others it is a broad diffuse zone of plate interaction. This latter type of diffuse or soR boundary is common in which one or both of the plates is composed of continental lithosphere. The interrelations between all the geological features and events at these diffuse bound- aries are unclear. The study of intraplate or dif~se-plate 65 boundary features and events within continental crust should be an important focus for any program in crustal dynamics. It is in this setting that most continental crust is formed and modified. The subduction of oceanic lithosphere is considered a stable process because it is denser than the astheno- sphere; thus, subduction could extend over long time periods until it is terminated by changes in relative plate motion or by collisional processes. It is evident from studies of modem plate boundaries that subduction of oceanic lithosphere can cause a variety of structural ex- pressions in an ovemding plate. Extension in He over- riding plate is expressed by the formation of back-arc basins and marginal seas. Oblique subduction can pro- duce strike-slip faulting. Horizontal compression in the overriding plate can lead to the development of complex erogenic belts. All three of these types oftectonic activity in an overriding plate can be coupled in venous ways to produce a wide range of complex Reformational styles. Even though the geological and geophysical evidence in- dicates that these types of structural effects exist in over-

66 riding plates, the condition that led to different tectonic settings is effectively unknown. This chapter examines the nature and extent of plate-boundary-related effects within continental lithosphere. The focus will be princi- pally on the setting where oceanic lithosphere is being subducted beneath continental lithosphere and the leading edge of the continental lithosphere is under compression. To demonstrate the presence of compression in the overriding plate, two lines of evidence can be examined: (1) modem intraplate seismicity and (2) geological inter- pretation of erogenic belts. From the seismic evidence it is reasonably clear that an overriding plate can be under compression. The interpretation of the geological evi- dence is much more complex and must be examined in detail. Interrelations between all the elements of an oro- genic belt and their relation to the plate boundary must be established before a conclusion concerning compression within Me continental lithosphere can be established. SEISMIC EVIDENCE FOR INTRAPLATE COMPRESSION o Parts of We Andes are a modern example of a noncollision erogenic belt. Stauder (197S), Dom a study of seismicity in the Peruvian Andes, has conclucled that the Nazca plate is thrusting under the South American plate beneath central and noncom Pem along a surface of shallow (1~15°) dip. Of particular importance to an understanding of Andean orogenesis is Stauder's additional conclusion that the leading (western) edge of the South American plate is uncler east-west horizontal compression for a distance of approximately 700 km east of the Pent Trench (Figure 6.1~. Numerous hypocenters are present in that plate at depths of 1 90 km and for distances up to 700 km east of the trench. Most lie at distances from the trench of 2~0 km, depending on latitude. Focal mechanisms for nine of ten of these intraplate earthquakes yield hori- zontal compressive stress axes oriented approximately east-west. The tenth shows normal faulting and is near the trench. Of the nine solutions that show horizontal compressive stress axes, three solutions indicate predomi- nantly strike-slip faulting and six indicate reverse Slip faulting. The Andes, a chain characterized by voluminous mag- matic activity, are bounded on the west by the Peru~hile trench, a zone of ongoing plate convergence, and on the east by an active fold and thrust belt, which separates orogen from craton. Rates of subduction are on the order of 7-12 cm per year and can reasonably be extrapolated back at least into the Late Miocene on the basis of marine magnetic anomaly studies. Since Middle Miocene time the central Andes, including Peru, have experienced in- tense igneous activity and accompanying orogenesis. Folds, reverse faults, and thrust faults win a relative east- ward sense of motion have developed along the eastern margin of the Andes in the sub-Andean zone (Audeband B. CLARK BURCHFIEL et al., 1973~. The Andes were uplifted to their present height during this late Cenozoic episode of orogenesis. Thus one interpretation is that the Andean orogenic belt is the result of defonnation related to horizontal compres- sion in a noncollisional subduction system. S~uder's (1975) data are preliminary, and studies ofthis type need to be considerably expanded to establish the state of stress within the leading edge of the plate and its relation to areas of modem magmatism and deformation. Even though horizontal compression can be established, how Me stress field is generated is unknown. Studies directed.toward understanding the origin of the stress fields within the continental crust at convergent bound- aries should be encouraged. GEOLOGICAL EVIDENCE FOR INTRAPLATE DEFORMATION RELATED TO NONCOLLISIONAL PLATE CONVERGENCE lithe geological evidence for compression with an over- riding continental plate is complex and less clear than the seismic evidence. The Mesozoic Cordilleran orogen~c belt of the western United States is an older and more deeply eroded Andean-type mountain belt (Hamilton, 1969; Burchfiel and Davis, 1972, 1975~. South of the latitude of central Oregon, the geological history of the Cordilleran erogenic belt suggests that the eastward sub- cluction of an oceanic plate beneath the North American plate occurrecl contemporaneously with defonnation and magmatic activity that at times extender! more than 1000 km into the North American plate. Evidence suggests that major collisional events were rare or nonexistent along this part of the plate boundary during Mesozoic time. The Mesozoic Cordilleran erogenic belt can be divided into four terranes: (~> ~ western terrane of accreted oceanic rocks, (2) a central pragmatic arc or superposed arcs, (3) an eastern terrane of east~irected thrust faults and related structures, and (4) a locally developed terrane lying east of the thrust belt of Trusted Precambrian base- ment rocks in the Colorad - Wyoming area (Figure 6.2~. The first three terranes shiR spatially with time, but events in each terrane have a crumple contemporaneity (Figure 6.2~. The fourth terrane is restricted in time, latest Cretaceous to earliest Tertiary, to a period when the Corclilleran erogenic belt underwent significant but poorly understood changes. Even Cough Me geological features and events in these four belts are broadly con- temporaneous, their interrelations and their relation to plate-boundary interaction are not well established' par- ticularly for the terranes farther removed from the plate boundary. In earliest Mesozoic time, the western boundary of the North American plate lay along a line from central Oregon through the central Klamath Mountains, central Sierra Nevada and west ofthe San Gabriel Mountains into north- eastern Baja California (Figure 6.2~. The plate boundary is at present very sinuous, which is a result of post-

Tectonics of Noncollisional Regimes . - _> o Hi:\ J , 10° \` 'I ·--= BOL/ V/~ \\ ~ 500 KM \ 7so B ~ too 300 soo 7~M B o - . . ,, at, .. .;.; . . -, ; j o . ..... . r i . ~ ~ At. 200 1~ . , , , ~ 200 FIGURE 6.1 NIajor structural elements of the Peruvian Andes. The belts of Miocene to recent magmatic activity and deforma- tion in Me sub-Andean zone are within Me South Amencan plate and lie east ofthe Peru trench, which marks the site of subduction of the Nazca plate. Section B-B' incorporates selected hypo- centers within the outlined region (from Barazangi and Isacks, 1976). T shows the location of the Bench. Hypocenters define a shallow dipping subduction zone and distribution of intraplate seismicity within the South American plate. Mesozoic deformation. Removing later defonnation would straighten the boundary considerably. Nearly all the rocks west of this line are Mesozoic in age and repre- sent rocks of oceanic origin that were accreted by subduc- tion processes and partially or wholly reworked to form either transitional or continental crust. WESTERN TERRANE Geological data from the western terrane indicate that accretion of oceanic rocks to the North American plate began in Triassic time (Davis et al., 1978~. Triassic cherts locally associated with late Paleozoic and possible Triassic ophiolites were tectonically disrupted and em- placed by subduction processes probably during Middle Triassic to Early Jurassic time (Figure 6.31. Locally these rocks are associated with blueschist metamorphic mineral assemblages, which were formed ~0~210 million years (m.y.) ago. In parts of these accreted sequences, exotic 67 Permian "Tethyan" fusulinid faunas are present in some limestones that are mixed with the Triassic and older rocks suggesting that far-traveled oceanic rocks were in- corporated into the accretionary wedge and reworked by later deforrnational events. Jurassic ophiolites, associated sedimentary rocks, and volcanic rocks lie above and to the west of the rocks accreted in earlier Mesozoic time and represent new additions from oceanic lithosphere to the North American plate. Mast of the Triassic and Jurassic rocks can be interpreted as accreted by eastward subduc- tion of oceanic lithosphere beneath the North American plate. Some workers have argued that one or more arc collisions may have occurred during Jurassic, particularly in Late Jurassic time (Schweickert and Cowen, 1975~. The evidence is equivocal that arc-continent collisions took place at this time, and considerably more work is needed to clarify this question. Even if an arc collision did occur in the Late Jurassic, eastward under~rusting of oceanic lithosphere dominated Triassic and Jurassic plate- boundary activity. Cretaceous and early Tertiary rocks fonn the western part of the western terrane and were accreted to the North American plate during east-directed subduction of Cretaceous and early Tertiary time (figure 6.4~. Like all accreted terranes, it is uncertain whether subduction was continuous or episodic. Blueschists of Cretaceous age are common within these rocks. Geologi- cal data demonstrate that rocks of the western terrane became youngertowarcl the west, indicating retrogression of the subduction boundary during Mesozoic time Davis et al., 19781. CENTRAL OR MAGMATIC-ARC TERRANE The central terrane consists of a magrnatic arc or super- posed arcs of Mesozoic age. Arc plutons intrude Pre- cambrian crystalline rocks in the south and Paleozoic arc rocks belonging to an arc accreted in latest Paleozoic to earliest Triassic time in the norm. All host rocks for the plutons were part of the North American plate; thus the arc was built on the western edge of the Norm American plate, and its structural sewing was similar to the modern Andes. The Mesozoic volcanic-plutonic arc began to develop in Early Triassic time, and the oldest platonic rocks date to approximately 23~240 m.y. ago (Figure 6.3~. Igneous activity occurred throughout most of.\Iesozoic time, but the degree of continuity of activity is controver- sial. Several workers have discussed this problem (Lanphere and Reed, 1973; Armstrong and Suppe, 1973), and the data suggest at least three intrusive epochs: (1) 7~106 m.y. ago; (2) 132-158 m.y. ago; and (3) an unde- fined epoch older than 160 m.y. ago, with the oldest dates at 230 240 m.y. ago. Because a few concordant age pairs fall in the intervals between those epochs, the alternative hypothesis of continuous magrnatism cannot be elimi- nated. Younger igneous rocks are also present in the west- ern United States, but an important change in magmatic and structural events occurred at about 75 m.y. ago as discussed below. Magmatic activity in the arc terrane is J

68 FIGURE 6.2 The four Mesozoic to early Tertiary terraces of the Cordille- ran erogenic belt of the western United States. The western terrane consists of rocks acereted to the North American plate dunog Mesozoic time, the central tenant is foxed by rocks of one or more magmauc arcs, and the eastern terrane consists of an east~irected fold and thrust belt. The Colorado Wyoming Rocky Mountain terrane developed in latest Cretaceous~arly Tertiary time during important rear- rangement of Cordilleran tectonic ele- ments. The line meriting the eastern edge of the western terrane is the alp proximate western edge of the North Amencan plate at the beginning of the Mesozoic. K, Klamad, Mountains; SN, Sierra Nevada; SO, San Gabriel Moun- tains. No attempt has been made to re- move intraplate defonnation such as in the Basin and Range province except for reversing movement on the San An- dreas and some associated faults. broadly over the same time span as subduction activity in the western terrane, which has led many workers to couple the two terranes into an Andean-type arc trench system (Hamilton, 1969; Burchf~el and Davis, 1972~. EASTERN TERRANE The eastern terrane lies east ofthe magmatic arc and con- sists of relatively east-directed thrust faults and associated folds. In early Mesozoic time in western Nevada thrust faults developed within Paleozoic "eugeosynclinal" rocks and early Mesozoic back arc sedimentary and volcanic rocks (Figure 6.3~. The age of these thrust faults is Early and Middle Jurassic, and post-Middle Jurassic and pre- Middle Cretaceous. Recent work has demonstrated the presence of early Mesozoic thrusting and folding in nor~- eastern Nevada, which could range in age from Middle Triassic to Earlylurassic. In the miogeosyncline on either side of the Califom~a-south~entral Nevada state line are several large thrust faults and associated folds that involve rocks as young as Early and Middle Triassic and are cut by plutons 185 m.y. old. These thrusts belong to an early Mesozoic period of thrusting that may be earlier than or synchronous with deformation in western and north- eastern Nevada. Farther southeast in southeastern Cali- B. CLARK BU RCHFIEL ! ~ i -.-.2 -.' \) ~ ~ ~ ,. . ,, 1~ ~ ~ of . , - - , - ~ .,~7 - - - t~=,1---- - - ' - - ' - - ' ' ~ 2 - .':',' ~~ /: LIZ,... `': ~ O .. -' \- ,W'-- 'at; , , _ , ~ ' l~ J - - . . ~ . ,= - . . . , - - ., . ~ ;2.,: ,-,;, 2-' /; ~ t.~: ~ ~~ _ ~ r .. . . . . . · ~ 1~ _ ~ _ · · · — ~ - - . ,q, - - - - - - · · ~ `W - - ~ - . . A=> S G ,~ - ~ ,; .. In- - - . ~ ~ ~ - , , _ o 3^^~ at. - ~ . . ~ . . .^ ;v '2 t-~7 J fornia' thrusts are cut by plutons 200 m.y. old and a newly discovered thrust that is unconformably overlapped by the Upper Triassic (?,~Lower Jurassic Aztec Sandstone. Thrust faults in southeastern Califomia involve Paleozoic miogeosynclinal and cratonal facie s and their Precam- brian crystalline basement. Recent data from southern California have demonstrated that some Reformational events are older than 23~240 m.y. Whether these various events belong to one or more episodes of deformation is not yet known. At present we group them in an undiffer- entiated period of early Mesozoic deformation, which ranges from twiddle Triassic to Middle Jurassic. During late Mesozoic time, arc magmatism encroached eastward into the region of early Mesozoic deformation, with the thrust and fold belt involving rocks even further east (Figure 6.4), except in southeastern Califomia where early and late Mesozoic deformation is superposed. The thrust faults developed from west to east until rock units transitional between miogeosyncline and craton were in- volved in thrusting. In southeastern Califomia, thrust faults strike south and southeast, leaving the Paleozoic geosynclinal ter`ane and cutting through the craton. Nearly all late Mesozoic thrust faults in this region in- volve Precambrian crystalline rocks and strike parallel to and along the eastern edge of the late Mesozoic magrnatic

Tectonics of Noncollis~onal Regimes Aft:: _'— , ~ . . ~_~ . =_.... . . . . . ~.~ . ~ . . . , ~ ... `~ . . . . c'- , . . ... . . . ..... ~ `. ~ ^. 'I _\ \. ~ ' ~ if. I, . ~ . . . . I:-: ~ . An.- , . 2 >,-, -__ ~ ~ ~ ~,:': ll -rat ~ ~ ~ . . . . . . . . . \ . . . O IN FIGURE 6.3 Spatial distribution of the western, central, and eastern terranes from Early Triassic to Late Jurassic time. The western and eastern terranes were probably continuous on both sides of the central terrane, but only their present outcrop distri- bution is shown. arc. This major change in structural trend and style occurs near the Califomia-Nevada state line and presumably continues into Mexico, although data to the southeast are scanty. Late Mesozoic plutonism occulted farther eastward in Nevada and to the north than in early Mesozoic time, but in southeastern California and to the south early and late Mesozoic igneous activity is superposed. Associated with the Mesozoic plutonic and volcanic rocks, but character- istically lying east of their major areas of development, thus transitional between the central and eastern terraces, are numerous isolated areas of metamorphic rocks that probably represent exposed culminations of ~ metamor- phic belt that may be continuous at depth. Metamorphism of amphibolite grade affects miogeosynclinal and cratonal rocks of Precambrian and Paleozoic age as well as Meso- zoic and early Tertiary (?) granitic, volcanic, and sedimen- tary rocks. Meager age data suggest Mat metamorphism began at least 180 m.y. ago and continued into Me Ter- tiary, but it is not known if this represents one long period of metamorphism or several spatially and temporally dis- 69 tinct periods. In some areas of southeastern California and adjacent Arizona, metamorphism is mid-Cenozoic in age and is associated with extensional tectonics, not thrust faulting (see Chapters 8 and 9~. It is likely that metamor- phism is synchronous with magmatism, spans a long period of time, and will be tied ultimately to magmatic epochs. The eastern or frontal thrust belt ofthe Cordilleran oro- gen lies to the east of Me Mesozoic metamorphic terranes mentioned above. In this belt, east-directed thrust plates of Late Jurassic to Late Cretaceous age define a zone of crustal dislocation that extends from British Columbia and Alberta, Canada, to southeastern California and prob- ably into Sonora, Mexico. Dunng the Late Cretaceous an important change took place in the location of deforma- tion by Trust faulting. Formation of folds and thrusts in the eastern thrust belt ceased before the end of Me Creta- ceous in a sector from central Utah to southeastern Cali- fornia (Armstrong, 1968~. North and south of this sector of the thrust belt, deformation continued through the Late Cretaceous into Me early Tertiary and ceased in Middle 1 _— . - ~ ! =` _ . ~~ : ~ '. ~.^ .- -A ~ )7l C" c, ~- Q: - ~ ~ ~! . · . . ~ ~ · · · · ~ ... ~ . ,_, . + I, ~ . . . - . . · · · · · ,, - ~ — 4 · . _ · _ . _ _ _ _ j" ~ _ - . _ _ - — — — — — - / t — - - - - · _ _ , - ~] , . ~ . — , — — — — ~~ r _ . . ~ _ . _ _ . ~ . . _ _ . ,, _ ~ _ — — ~ / , — , _ . _ , · - — ~ _ , , .— _ . . . . _ ~ r . ~ . . _ _ _ ~ I ,,,, ~ ,~ Id_ '': ' ::,:,. ~ — . _ , , , ~ ~ ~ — · _N ,: - ' _ · ~ ~ — - — - _ _ _ ~x `_- · - - - - - - , Car 'W-:-:-:-:-:-:-:.-:- ~g _ _ at- . ... ~ , . ~ --; . . . - 3 0 0 ~ M . _ _ . . . _ , , . . _ . _ _ . _ FIGURE 6.4 Spatial distribution of the western, central, and eastern terranes from Late Jurassic to latest Cretaceous time. The `westem and eastern terranes were probably continuous on both sides of the central terrane, but only their present outcrop distri- bution is shown.

70 or Late Eocene time. The period from latest Cretaceous to Eocene (75~;0 m.y. ago) is Me time of He classic Cyanide orogeny. Low-angle thrust faults developed during Lara- mide deformation have a structural style similar to Hose developed in He earlier Cretaceous events but generally lie somewhat farther east. COLORADO—WYOMING ROCKY MOUNTAIN TE RBANE Lee fourd1 te'Tane is only locally developed both spatially ant] temporally ant! consists of large uplifts of Precarn- bnan crystalline rocks that extent] Tom southern Montana into New Mexico. These uplifts, commonly referred to as He Colorad - Wyoming Rocky Mountains, developed only dunng the Laramide Orogeny of latest Cretaceous and early Tertiary time. The structural geometry of He Faults bounding the uplifts has been controversial, one group postulating Trust faults Blat steepen with depth, a second group postulating that the boundary faults are moderate to low angle (for a review see Sales, 1968; Steams, 1971~. Recent seismic reflection lines across one of the uplifts, the wind River Range, by the Consortium on Continental Reflection Profiling (COCORP), has demon- s~atec! a gently dipping thrust fault bounds the uplift (see FIGURE 6.5 Spatial distribution of die western, central, eastern, and Colorad - Wyoming Rocky Mountain terranes Tom latest Cretaceous to early Tertiary time. The western terrane was probably more extensive than its pres- ent outcrop distribution and may have been continuous along the entire west coast, but it is now largely in the off- shore region. B. Cal ARK BURCHFIEL Chapter 10~. Some of the uplifts can be interpreted to be bounded by thrust faults, but several are still equivocal. The geometry of these uplifts is critical because their Origin has been ascribed to horizontal compression or ver- tical displacement without horizontal crustal shortening. The COCORP data suggest Hat the former interpretation may be correct, in which case it may be possible to relate He structural origin of this terrane to plate-boundary in- teraction even though He boundary is more than 1000 km to the west. Igneous activity In the western part of He Now Ameri- can plate also underwent a significant change during the Late Cretaceous (Figure 6.5~. Dunng most of Mesozoic time, a plutoni~volcanic Andean-type arc was active as discussed above. Although He problem ofthe continuous or episodic character of magmatism has not yet been re- solved satisfactorily, it appears that during most of Meso- zoic time magmatism was characteristic of much of He western part of the Norm American plate in the United States and southern Canada. About 75 m.y. ago a change occurred in the patting of igneous activity. Igneous rocks intruded during Amide time (75 50 m.y. ago) are pres- ent in Canada and Idaho and in southeastern California and Arizona (and presumably western Mexico), but Hey are very rare in central and northern California, Nevada, :~-~l -~# ~~\' 5~. ~ - rl1 ~ ) AN ~ - ~ \ % l \< \ o - D D .. \ \ l I I. ~~ Zip'' .. an of, ~ : cn , . of ~ ~ x, :.,'rNc.~4 Z ' ~ · 'A- ' ~ ~ . ~ . ~ . ~ . ~ ~ ~ · . . . i- , · 4, . . ~ . _ ~ ~ ~ w ; , - ~ — ~ ~ !~ ~ rn , '. D i Z rn '. .i 1\ <; v C, ~ ~ rr1 rn

Tectonics of Noncollisional Regimes and western Utah (Armstrong, 1974; Burchflel and Davis, 1975~. Igneous rocks of Laramide age are also present fiercer east in southwestern Montana (Adel `Mountain, Ell~om Mountain, and Livingston volcanic rocks), central and southwestern Colorado Colorado Mineral Belt), northeast and southern Arizona, and southwestern New Mexico (Figure 6.5~. These latter areas of igneous activity lie east of areas of earlier Cretaceous magmatism in a region that had lacked igneous activity since the Pre- cambnan. The Laramide igneous rocks of Colorado and parts of Arizona lie east of the area where Laramide mag- matic activity of the main batholith trend of the Cordillera ceased (Figure 6.5~. Of considerable importance is the general spatial coincidence of this Laramide igneous ac- tivity with the region of basement uplifts. In summarizing the areal distribution of deformation and magmatism during Late Cretaceous-Early Tertiary time, the following data are important: (1) low-angle thrust faults developed from Canada to Mexico along a continuous belt of Late Jurassic to Late Cretaceous age; (2) the belt of low-angle thrust faults lies along the eastern margin of a terrane or zone of batholiths that were em- placed synchronously with thrusting (23~75 m.y. ago); (3) low-angle thrusting ended in Late Cretaceous time in a sector from central Utah to southeastern California at approximately the same time as magmatism ceased in cor- responding latitudes in the batholith belt to the west; (4) to the norm and south, ~TTIide low-angle thrusting con- tinued in regions where ma~natism persisted in the main platonic belt from 75 m.y. to approximately 50 m.y. ago; and (5) in the central Cordillera, largely adjacent to the gap in ~raTnide magsnatism of the main batholith belt, Laramide igneous activity shif;red eastward and coincides generally with the region of Rocky Mountains basement uplifts of the same age. These data suggest that the devel- opment of the Amide structures in the Colorado Wyoming Rocky Mountains is related to events Mat took place throughout the erogenic belt and are ultimately related to plate-boundary interaction. PROBLE M S The geological data from the four terranes within that part ofthe North American plate considered here demonstrate an approximate contemporaneity for Mesozoic events. Studies of modern plate boundaries clearly indicate that plate motions and plate-boundary geometries can change rapidly, on the order of a few million years. Our dating accuracy of events in the Mesozoic are at best within several million years. Stratigraphic units and deforrna- tional, magrnatic, and metamorphic events require pre- cise dating in each Tulane before their effects can be correlated across the erogenic belt, so that a complete picture of all synchronous events at and related to the convergent plate boundary can be developed. Each of the four temnes presents different geological problems that require solution before a complete picture of the terrane 71 evolves and before adequate correlation between terranes and the convergent boundary can be established. The western terrace is characterized by the presence of ophiolites, which offers geologists an exposure of oceanic crust and its deep marine sediments. Study of ophiolites has shown that they have great variability in their internal structure and composition. Continued study of these rocks is necessary to understand their genesis, tectonic setting, and mode of incorporation into a subduction com- plex. Accurate dating of ophiolite sequences is necessary to establish their time of formation. Recent breakthroughs In the preparation of and study of radiolanans have made great advances possible in the dating of not only ophiolite connation but the sedimentary record of deep~ocean sedi- mentary sequences. Subduction is not an instantaneous process but may continue for long periods of geological time; however, our techniques for unraveling sequential events in an accretionary wedge need special attention. Tectonic styles and processes are still poorly understood and must be examined in light of modern subduction systems. Blueschist metamorphism is associated with subduction processes, but many problems still remain in understanding blueschist connation and preservation in subduction terranes. Island-arc-type volcanic rocks are sometimes associated within accretionary wedges and in- corporatecl into them by later plate-boundary activity. Geological studies aimed at determining the ancient and modern paleogeographic settings of island-arc sequences and how they become incorporated into accreted bounda- ries still require a great deal more effort. All of these studies should be correlated with comparison of gem physical and geological studies of modern subduction systems. Study of the central or magmatic arc terrane still must have as its primary objectives the origin of the magmas and their temporal and spatial relations. There remains the pnocipal problem of how magma generation is linked to subduction, and what are the influences of the upper- mantle and continental crust on magma composition. It is within the realm ofthe magmatic arc that preexisting rock sequences are reworked to form continental crust. Dunug this activity, continental crust, as it is known seismically, begins to develop. Through geological, geochemical, and geophysical study of ancient arc systems such as the Mesozoic arc of the western United States, correlated with similar studies of active arc terraces, a more compre- hensive understanding of these terranes will evolve. It is of great importance, because vast quantities of our mineral wealth are related to magrnatic arc development. A proper geological framewodc of magmatic arcs will greatly aid our exploration of their mineral reserves. The thrusted and folded terrane east of the magmatic arc locally has been mapped in considerable detail, but numerous problems remain concerning the geometry and origin of its structure. East-directed thrust faults dominate the structures of this terrane, and the geometry of the thrust faults is reasonably well understood locally, princi- pally because of seismic profiling by oil companies. The

72 origin of these thrust faults remains a great problem. It is still unclear what the relation of thrust faults that involve only sedimentary rocks is to metamorphic core areas that lie to the west of them. Geologists have been preoccupied with Me eastern or frontal parts of thrust faults and have neglected the western or rear parts of these structures. Until we know the relation between metamorphic core and basement rocks to the thrust Cults at their rear in the source terranes, adequate mechanical models for thrust faulting cannot be made. Evidence suggests that thrust faults of the eastern terrane may be related to crustal shortening within the leading edge of an ovemding conti- nental plate, but more precise geological and time relations are necessary to establish the correct three- dimensional models. The attack must be an integrated study of the metamorphic, structural, and geochrono- logical history of the entire thrusted and folded tenant. More geophysical studies, particularly reflection and re- firaction studies, in complex areas of the rearward parts of the thrustecl terranes are necessary to give a better three- dimens~onal picture of crustal and structural relations. The fourth temne of thrusted Precambrian basement rocks of the Colorad~Wyoming area is of considerable economic interest because of its relation to accumulations of oil and gas and mineral deposits of Precambnan, Mesh zoic, and Tertiary age. The Colorado-Wyoming structural province has been regarded by some workers as unique; however, many similar structural provinces are known throughout the world, such as the Amadeus Basin of central Ausmalia and the Caucasus Mountains of southern Russia. Four major problems can be defined in this ter- rane: (1) Why is it localized where it is? (2) What is its structural configuration with depth? (3) What is the rela- tion and origin of the limited contemporaneous magmatic activity to this province? (4) Is it related to the convergent margin nearly 1000 km farther west? Within the four terranes of the Mesozoic Cor~lilleran orogen, numerous problems of timing of events and struc- tural relations exist; thus timing and structural relations between terranes and within the orogen as a whole are not well unclerstood. Only broad correlations between events can be made at present. Greater detail in establishing contemporaneity of events needs to be compared, and relations between subduction, rnagmatic, metamorphic, and back-arc structures need to be established. Present data suggest that events in all terranes are contempora- neous and all are related to activity at a convergent non- collisional plate boundary: the effects of convergence ex- tend 1000 km from the plate boundary. Greater under- standing of the mte'Telations across the erogenic belt is necessary before accurate models can be developed. At present, a consistent model can be constructed that in- volves compression in an ovem~ing continental plate in B. CLARK BURCHFIEL response to subduction of oceanic lithosphere. The nature of lower crust and mantle involvement is unknown, and only through the study of modem belts such as the Andes can we hope to understand the deeper workings of such a plate boundary. The plate boundary and its effects within the overriding plate should be regarded as a single dy- namic system. The processes involved in the develop ment of the Mesozoic Cordilleran orogen are pro. that lead to the formation and evolution of continental crust. REFERENCES Armstrong, R. L. ( 1968). Sevier erogenic belt in Nevada and tTt~, Ceol. Soc. Am. Bull. 79, 429~8. Annstrong, R. L. (1974). Magmatism, erogenic timing and oro- - genie diachronism in the Cordillera from Mexico to Canada, Nature 247, 348~351. Armstrong, R. L., and l. E. Suppe (1973). Potassium-argon geo- chronology of Mesozoic igneous rocks in Nevada, Utah and Southem California, Geol. Soc. Am. Bull. 84, 137~1392. Audebaud, E. H., R. Capdevila, B. Dalmayrac, J. Debelmas, G. Laubacer, C. Lefevre, R. Marocco, C. Martinez, ~I. Mattauer, F. Megard, J. Paredes, and P. Tomasi (1973). Les traits geologique essentials des Andes central (Perou-Bolivie), Re';. Geogr. Phys. Ceol. Dynam. XV, 7~114. Ba~angi, M., and B. L. Isacks (1976). Spatial distribution of earthquakes and subduction of the N=ca plate beneath South Amenca, Geology 4, ~692. Burchfiel, B. C., and G. A. Davis (1972). Structural framework and evolution of the southern t of the Cordilleran orogen, western United States, Am. J. Sci. 272, 97-118. Burchfiel, B. C., and G. A. Davis (1975). Nature and controls of Cordilleran orogenesis, western United States: extensions of an earlier synthesis, Am. 1. Sci. 275-A, 363 396. Davis, G. A., I. W. H. Monger, Id B. C. Burchfiel (1978). .\Ieso- zoic construction of the Cordilleran "collage," central British Columbia to central California, in Pacific Coast Paleogeog- raphy Symposium 2, Pacific Section, Society of Economic Paleontologists and Mineralogists, pp. 1~2. Hamilton, W. (1969). Mesozoic Califomia and the underflow of Pacific mantle, Geol. Soc. Am. Bull. 80, 2~4~2430. Lanphere, M. A., and B. L. Reed (1973). Timing of Mesozoic and Cenozoic platonic events in circum-Pacif~c North America. Geol. Soc. Am. Bull. 84, 3773 3782. Sales, l. K. (1968). Cordilleran foreland deformation, Am. .~.~.s<~. Petrol. Ceol. Bull. 52, 201~2044. Schweickert, R. A., and D. S. Cowan (1975). Early Mesozoic tectonic evolution of the western Sierra Nevada, Califomia, Geol. Soc. Am. Bull. 86, 132~1336. Stauder, W. (1975). Subduction of the Nazca plate under Peru as evidenced by focal mechanisms and by seismicity,l. Ceophys. Res. 8{), 1053~1064. Stearns, D. W. (1971). Mechanisms of drape folding in the Wyo- ming province, Twenty-Third Annual Field Conference Guidebook, Wyoming Geological Association, pp. 12~143.

7 I NTRODUCTI ON Models for Midcontinent Tectonism WILLIAM J. HINZE and LAWRENCE W. BRAISE Purdue University G. RANDY KELLER University of Texas, E! Paso EDWARD G. LIDIAK University of Pittsburgh The midcontinent region of the United States has long been regarded as part of the stable craton. Geological evidence has led to He assumption that this area has un- dergone only minor tectonism during the past several hundred million years and that this tectonism has largely taken the fonn of broad, slow, vertical movements. However' during the past decade there has been accumu- lating geological evidence and increasing awareness that the midcontinent region has been and is at present tec- tonically active. This change in geological thought has come about because of studies of earthquake activity and enhanced discrimination of lateral crustal variations by geophysical techniques. Earthquake activity has focused attention on the central midcontinent, in the vicinity of He New `Madrid seismic zone at the head of the .\lississippi Embayment, and has encouraged studies of contemporary tectonics as a means of predicting seismicity and the areal limits of He poten- tial seismic activity. This chapter reviews the major pub- lished tectonic hypotheses for the contemporary geody- namics of the midcontinent region. However? to set the firameworic for these hypotheses, the geological history is 73 summarized with emphasis on the structural develop- ment and related tectonic events. This summary is impor- tant because the tectonic events that have acted upon the midcontinent in He past are only interpretable based on the structural, sedimentary, and thermal events reflected in the geological history of He area and nearby plate mar- gins. If geological events involve orderly processes, then we can anticipate that the past in a general way is a clue to the present. Although noncyclic processes are impor- tant in early history and crystal conditions have changed to some degree, previous tectonism provides guidelines for subsequent dynamic processes (Allen, 1975~. It is im- portant to use this structural knowledge to decipher con- temporary tectonic processes. GEOLOGICAL HISTORY The geological history of the midcontinent region has been He subject of many discussions (e.g., Bristol and Buschbach, 1971), and the major tectonic events are shown schematically as a function of time in Figure 7.1. The early history of the area is poorly known because

74 FIGURE 7.1 Schematic diagrams of major tectonic events that have af- fected the midcontinent region of the United States through geological time. :~'\ i? ? ~ ,,,,, , ,, ~ . . · Wi WILLIAM J. HlNZE et al. ~ f . . . . E GRENVAM8=lAN D. LATE PaLEOZOIt . . Hi. ..~ ~- / ,',,, ,~ ~.~ // , _ B. EOCAMBRIAN /! it of/ i/ r~ ' '; - ~ ~ \. 6~6 EN ~ I ";~_D By'" E. MESOZOIC 1 it' _<' it'' C. EARLY ~ MIDDLE PA L E OZ O I C K E Y C CONTINENTAL CRUST a OCEANIC CRUST i' FAULTS a, RlfTS SUBDUCTION ZONE ~ CONTINENT AND/OR ISLAND ARC COLLISION REACTIVATION, LOCALLY INCLUDING FAULTING, IGNEOUS ACTIVITY, AND COMPRESSION C BASINS ANOROGENIC IGNEOUS ROCKS ~ CENOZOIC

Models for~Midcontinent Tectonism Precambnan rocks are generally buried beneath Phanero- zoic sedimentary rocks. The Precambnan events have most recently been summarized by Denison et al. (in press). The Precambnan rocks of the continental interior are characterized by linear erogenic belts prior to about 1.6 billion years (b.y.) ago. Continental stabilization occurred subsequently and is indicated by shelf-type sedimentation, anorogenic igneous activity, continental riding, and epeirogenic deformation. Orogenic activity continued along the eastern and southern continental margin in late Precambnan and Phanerozoic time. Figure 7.1A shows the Grenville basement rock prov- ince, which is bordered on Me west by an anorogenic igneous tenant and a series of basaltic nf;c zones. The anorogenic terrane consists mainly of intrusive and extru- sive rocks of felsic composition dated at about 1.~1.5 b.y. Old. The basaltic nit zones extend from Kansas norm to Lake Supenor and south into Michigan and perhaps Indiana, Ohio, Kentucky, and Tennessee. Basaltic igneous activity in these rifts has been dated in the Lake Supenor region at 1.~1.2 b.y. ago (Goldich, 1968~. These nits may reflect a thermotectonic event that attempted to disrupt central North America—an event that is at least partly contemporaneous with tectonic activity occurring within the Grenville province. The third Precambrian ter- rane is Me subsurface extension of Me Grenville province of Canada. This province is underlain by medium- to high- grade metamorphic rocks, granites, and anor~osites. The last major peno<1 of metamorphism occurred 1.0 b.y. to 1.1 b.y. ago, an event that has been related to continental collision during closing of a Precambnan proto-Atlantic Ocean (Beer, 1976~. These events were followed by uplift and stabilization of the Grenville Front, whose extension into the southeastern United States has not been defined. By latest Precambrian to Eocambnan time, the ancient continental land mass of eastern North America began to split (Figure 7.1B). Each of the continental margins formed was probably of the Atlantic type. The grabens associated with the initial rifting were filled with elastic sedimentary and volcanic rocks. The volcanic rocks of Me Catoctin Formation (820 m.y. old) probably represent this rifting event (Brown, 1970~. Development of aulacogens also occurred on the craton during this penod. The proto-Atlantic or Iapetus Ocean (Ranhn, 1976) continued to grow during early Paleozoic time and by Ordovician time had reached substantial width (Figure 7.1C). The continental margin was transformed into an Andean-type margin along the entire eastern continental margin and probably extended along the southern margin into the Ouachitas. This subduction is indicated by tec- tonic and thennal events associated with the Taconic or- ogeny, which affected an area from Newfoundland to Ala- bama. Widespread cratonic sedimentation occurred in the midcontinent during early and middle Paleozoic time (Figure 7.1C), and the development of numerous basins and arches (Figure 7.2) was initiated. Locally, thick de- posits accumulated in the major basins, perhaps as a result of contemporaneous faulting and regional epeirogenic 75 movements. The arch-basin relationship continued to develop throughout the Paleozoic. The axes of the arches and the centers of the basins shifted through time, but the arches were rarely above sea level. Deposition in basins generally kept pace with subsidence, resulting in pre- dominantly shallow-water marine deposits. By Mid-Devonian to Early Mississippian time (Figure 7.1D) plate consumption was terminated in the northern Appalachians by a collision of continental plates in the Acadian orogeny. Dunng this time, the central and sou~- ern Appalachians were involved in continued subduction, and thermal activity and significant tectonic activity was under way in the Ouachitas. Activity in these areas reached a peak during Pennsylvanian time and is re- flected in the Alleghenian-Ouachita orogeny, which also involved continental collision. Mesozoic time (Figure 7.1E) is marked by the rupturing of Pangaea and He opening of He present Atlantic Ocean. Graben development and subsidence occurred along He eastern and southern margins of North America. The Reelfoot Rift of the Mississippi Embayment was reacti- vated during this time, and this reactivation continued into the Cenozoic era (Figure 7.1F). CONTEMPORARY GEODYNAMICS The above discussion of geological history sets the frame- work for consideration of the contemporary geodynamics of the midcontinent region. These geodynamic processes are inadequately known and poorly understood at pres- ent. However, the reality of tectonic activity is dramati- cally illustrated by the recent faulting ant! earthquake activity in the New Madrid seismic zone. Evidence for this activity is largely indirect, as surface indications are rare because of an overburden of recent sedimentary deposits. Also, evidence is limited by the short (-200 year) duration of the historical record. However, obse~va- tions of current patterns of seismicity, stress distribution, Cenozoic faulting, and vertical movements are relevant and are therefore summarized below. SEISMICITY A review of seismicity studies of the midcontinent (e.g., NuKli, 1974) suggests the following major conclusions: ( 1) The pattern of seismicity is rawer diffuse (Figure 7.3), but there are zones in which activity is concentrated (the New Madrid seismic zone, sou~en~ Appalachians, St. Law- rence Valley, and the Boston~Ottawa trend). (2) lithe most intense historical earthquake activity has been centered in the New Madrid seismic zone in southeast Missouri and adjacent areas (Figure 7.4~. (3) The historical ear~- quake record of the New Madrid seismic zone is domi- nated by the 1811-1812 earthquake sequence and asso- ciated aftershocks, which represent a major earthquake occurrence on any scale of companson. Nuttli (1974) has shown that the Tree major shocks of the 1811-1812 se-

76 WILLIAM 3. HIDE et al. 4~ 44o 42° 4oo 38° 368 34o r At Y. at- ~ ~ ~: M ,4~_~--_-- ' - ~ ~ C~, ~ / ~ ~ , , _ ..... _ .._..... . - . ~ ~..: WISC Slur ~ ~ 4: ARCH ~ ~ \ - t~ 1 ) ~ Ni /~1. 1; ^.. I l. ~ 1, .i : 94° g2° 900 438° 8'6O 84° ~ , 82° 80° FIGURE 7.2 Major structural elements of the central midcontinent, United States. Modified from King (1969). quence had approximate magnitudes (m',) of 7.2, 7.1, and 7.4. (4) lithe seismicity of the eastern United States is con- siderably less than the west, but for a given earthquake magnitude an earthquake is felt over a larger area in the east. Although detailed seismicity data are generally unavail- able, the value of these studies is demonstrated by recent results from a telemetered microear~quake array cen- tered in southeast Missouri (Stauder et al., 1977~. The pattern of seismicity (Figure 7.4), which has become evi- dent during 21 months of recording, displays several linear trends in NE-SW and NW-SE directions, which are interpreted as indications of the pattern and extent of cur- rently active faults. STRESS DISTRIBUTION A knowledge of the distribution of stress within the mid- continent should provide valuable insight into We origin of forces responsible for local contemporary tectonic ac- tivity. The regional stress distribution in the eastern United States as measured by several methods (Sbar and Sykes, 197.~3; Haimson, 1976) shows that the maximum compressive stress is nearly horizontal and trends east to

Models for '~idcont~nent Tecton~sm ~ 1 0° 1(30° 608 Boo 40 30' 77 90 80 70 60° 50° I / I I I 1 ~ I I 1/ l \ N: / _ ,4 0 Oot C A. 0 Coo O . ·e ~ . ~ 0 . B°c o 0 O 0 '':& ~ 9 0 ~ O r O ~ ~` \ . ~ ~ - \ ~ A// ~ \ O I'm ,_ ~ c' ~ to ~ ~ · Oo ~ J' `g ~ < al J d s o n~_4 Pl at f orm .. ' . . - , ~ . 0 Pa ~ . 0 ~ - 7 J/~ Hi' 0 ~ of 5~' {, '~ oath °r' · ~of4~ _ 0 0 ° qL-~O,--c~~' ~ ° o '- '' —~— °'O "' -; . . . . . . . ; o or `' Coopts ~ . ~ . o c~'o~N oe\ f-~~ ~ L Too ~ ~ 1 W 9 Aft/; . ~ o Earthquake epicenters 1534-1971 LE<; END Int~nil\Y EP'C.~, '-~v . V-V' O VI-IX O X-XI' O O COOK ~ — - - O ~ ~ ~ ~ ~ 1 80 70° 50° 40° . 30° FIGURE 7.3 Distribution of reported earthquakes (1534 1971) in eastern North America (from York and Oliver 1976) and selected tectonic features (from S bar and Sykes, 1973; Haimson, 1976). Fault plane solutions of earthquakes (solid ~ianglesj, strain relief in situ stress measurements (solid circles), and hydroEracture in situ stress measurements (open triangles) are also shown. Strike of horizontal component of maximum or minimum compressive stress is shown at each locality. northeast. Sbar and Sykes furler suggest that plate mo- tions are responsible for this stress and that earthquake zones in eastern Norm America are controlled by He existence of unhealed fault zones Hat are subject to high deviatonc stress. However, a focal mechanism study by Street et al. (1974) suggests that the stress distribution in the eastern United States can be locally complex. On the other hand, recent microearthquake focal mechanism studies by HerlTnann and Canas (1978) indicate that the two prominent NE-SW trending lines of epicenters in the New Madrid region have focal mechanisms consistent wig right-lateral strike-slip faulting along a NE-SW

78 30 1 l ~00' - ° 1 - R - HI . o "15SOUR1 On Call ~ O ARKANS" / DwM I.: / ECO: O a ~ ~ TIC ~ o O O " - C~ O O 0~ , O ~ O I ~ C _— ' - 3 2 2 1) I J ., ~ I v I :~NTUCKY a,~J~T TENNESSEE r ~ i _) BONG 0 10 20 K40~7ERS I FIGURE 7.4 Epicenter map showing location of all earth- quakes located within t.5° x 1.5° area by the St Louis University seismic network during the 21-month period of July 1974 to March 31, 1976 (from Stauder et al., 1977). Larger earthquakes are indicated by solid circles: 1, June 13, 1975, m. = 4.3; 2, March 23, 1976, m. = 5.0; and 3, March 25, 1976, m`, = 4 5. Generalized fault-plane solution of earthquakes after Hennann and Canas (1978). trending fault plane (Figure 7.4~. This faulting is consis- tent with the previously mentioned regional stress pat- terns. Thus, regional stress field! and intraplate features and forces that may locally perturb or dominate it must be considered. CENOZOIC FAULTING The level of tectonic activity is relatively low; therefore we must use evidence from Me geological record to un- derstand Me contemporary tectonics of Me midcontinent. Although sparse, data on Cretaceous and Cenozoic faulting become particularly important (York and Oliver, 1976). Seismicity data are very valuable, but space~ime magnitude relationships for earthquakes derived from Me relatively short (~200 year) historical record are queshon- able. In Act, the use of the earthquake record to predict filture seismic herds has led Allen (1975) to state: "The very short historic record in Norm Amenca should, there- fore, be used win extreme caution in estimating possible fixture seismic activity. The geologic history of late Qua- WI~LI^M 3. HAZE et al. ternary faulting is the most promising source of statistics on frequencies and locations of large shocks." RECENT VERTICAL MOVEMENTS Long-term, broad-waveleng~, vertical crystal move- ments have an important role in continental tectonics, and their existence in He geological record is readily evi- denced by upliPrs and basins. Quantitative analysis of vertical movements from the geological record} Is difficult because of uncertainties in die age of marker units, inabil- ity to account for possible oscillatory movements or ero- sion cycles, and complicating effects related to glacial rebound. Analysis of precise leveling data provides an important source of information on vertical movements of He earth's Angst. However, since He leveling measure- ments are repeated over relatively short periods of time (tens of years), vertical movements inferred from these data ate indicative of "instantaneous" velocity and may not be representative of long-tenn tectonic modons. An analysis of vertical crustal movements in He eastern United States determined from leveling data (Brown and Oliver, 1976) shows, in general, that modern vertical movements appear to be related to earlier Phanerozoic tectonic trends. However, the rates of modem movements are much larger than average rates over the last 130 m.y., and, ~us, modern movements must be episodic or oscilla- tory. Fur~ennore, Brown and Oliver (1976) suggesta cor- relation between zones of vertical crustal movements and patterns of seismicity. TECTONIC MODELS The geological history and He contemporary geody- namics of the midcontinent region testify to its complex and continuing tectonic development. A wide variety of tectonic models (Figure 7.5) has been suggested to ex- plain the contemporary tectonics of this region, and ~us, it is important to consider the hypotheses leading to these models. RESURGENT TECTONICS Many models assume that much of the contemporary tec- tonic activity is controlled by pre-exisdng geological fea- tures. These models suggest that crustal riRs, zones of weakness and crustal boundaries, and local crystal in- homogeneities serve to localize in a passive manner He defonnation resulting from stresses generated by a variety of tectonic forces. These forces may be and probably are completely alien to those initially responsible for He fea- tures, and, therefore, these models are grouped under the general term "resurgent tectonics." Crustal Rifting Riding of He continental crust and its commonly asso- ciated igneous events are a major source of large-scale

Models for Midcontinent Tectonism FIGURE 7.5 Schematic diagrams of proposed tectonic mechanisms. C. Lithologic Boundary crustal disturbance and are therefore particularly suscep- tible to resurgent tectonics. It has become increasingly clear that noting of the crust has played a major role in the geological history of central North Amenca. This type of deformation, in which vertical movement predominates over horizontal displacement, has been operative for at least 1.6 b.y. since the stabilization of the continental intenor. The feature most widely accepted as being due to rift- ing is the linear midcontinent gravity high, which extends from Kansas to Lake Supenor (Figure 7.1A). Recent studies by King and died (1971) and Ocola and Meyer (1973) indicate that it is associated with a major, deep- seated nit zone of late Precambrian age. Strong arguments have also been made for extension of the nit zone to the southeast into the Michigan basin area (Hinze et al., 79 I. CRUSTAL RIFTING m. LOCAL BASEMENT INHOMOGENITIES \ ~ / As~t~ospbere E. ZONES OF W£AKNESS AND CRUSTAL BOUNDARIES A. Crustal Thickness Variotion ~ . ~ A. St,,,, . .. lIZ. THERMAL EXPANSION AND CONTRACTION A. Heating u lit, B. Coolino-." ~ B. Ancient fault Zone LIZ. ISOSTATIC WARPING . ... .. . A. Intrusion 1975) and possibly farmer south into Ohio, Kentucky and Tennessee. An interesting model for in~aplate riding has been pro- posed by Burke and Dewey (1973), which has several plume-generated, triple junctions in the midcontinent during late Precambrian time. These triple junctions be- came inactive before rifting proceeded to create an ocean basin. Although clearly an oversimplification, this model has the advantage of providing an integrated causal mechanism for this episode of widespread rifting. Sawl~ins (1976) explains that the rifling also may be asso- ciated with continental collision, leading eventually to continental breakup. Triple junctions that did form ocean basins have also been postulated as having an important role in the tec- tonic development of the midcontinent. For example, the

80 Mississippi Embayment and southem Oklahoma aulaco- gen (Figure 7.1B) are interpreted as failed arms of triple junctions (aulacogens) fanned as new episodes of ocean spreading begin. Burke and Dewey (1973) originally sug- gested that die Mississippi Embayment was a Mesozoic failed ann from a triple junction. Ervin and McGinnis (1975) evaluated Ellis proposal in terms of the available geological and geophysical data and concluded that this model was a latest Precambria~Eocambrian failed and of the same origin as the southern Oklahoma aulacogen (Burke and Dewey, 1973) and was thus fanned as a Pre- cambnan land mass broke up prior to the connation of Me Appalachian Ouachita mountain belt. According to Me model of Ervin ant! McGinnis (197~), this older feature (~e Reelfoot RiR) was reactivated in Mesozoic time to form the present-day Mississippi Embayment. Evidence for a rift zone that coincides with the New Madrid seismic zone has been presented by Hildenbrand et al. (1977) in the form of gravity and magnetic data. Kumerapeli and Saud ti966) make a strong case for Me interpretation of the St. Lawrence Valley system as a rift zone that was probably initiated in Mesozoic time but is skill active today. They suggested that this rife zone may extend into the micicontinent and connect with Me New Maciricl seismic zone. The implication of this motley is that major earthquakes could be expected anywhere along a NE~SW trend extending from Arkansas to the St. Lawrence Valley. RiRing has clearly played an important role in the tec- tonic development of Me midcontinent, and Me boundary and intrarift faults of these paleonf;cs are prime candidates for resurgent tectonics. Zones of Weakness and Crustal Boundaries Old zones of weakness are wiclespread in Me m~dconti- nent, and it is well established Mat they may have signif~- cant influence on subsequent structural development. For example, in northwestern Ohio Me nor~h-trending Bowling Green Fault coincides with the subsurface ex- tension of die eastern boundary of Me Grenville erogenic belt (Quick et al., 1976~. The surface fault zones in eastern and central Kentucky (Figure 7.2) exhibit similar behavior in that the east-trencling Kentucky River Fault zone and Me I~ine-Paint Creelc Fault zone appear to be He re- activated northern boundary of the Rome Trough (Ammennan and Keller, in press). These faults have been active as recently as the Pennsylvanian, but the major movement associated with the Rome Trough was Carn- . · . onan in age. The fault complex of western Kentucky, southern Illinois, and southeastem Missouri (Heyl, 1972) is an- other example of resurgent tectonics. These faults outline a complex causal block that encompasses He Rough Creek graben (Moorman syncline), Hicks Dome, He Illinoi - Kentucky mineral distnct, and a concentration of mafic and ultramafic dikes. Many are believed to be reac- tivated Precambrian structures with movement on the WILLIAM J. HINZE et al. major surface faults occurring during late Paleozoic and Cretaceous time. Of particular interest are He inferred nor~east-trending faults at He nor~em extremity of He Mississippi Embayment and Heir extension to the northeast into He Wabash Valley Fault zone. Some of these faults coincide with or parallel geophysical anoma- l~es, suggesting that they reflect buried basement struc- tures (Lidiak and Zietz' 1976) perhaps associated with the nf;r system proposed for the northern portion of He embayment. These examples demonstrate He importance of resur- gent tectonics in the development of many of He struc- tures of the midcontinent. Old zones of weakness appear to exercise control on the location and trend of many younger stnlctures. However, Were is not necessarily a relation between younger faulting and pre-existing struc- t;ures. Some old fault zones have not been reactivated by younger faults (e.g., the southern boundary of the Rome Trough), and younger faults may not reflect old zones of weakness (e.g., the faults along He southern margin of He Moorrnan Syncline). It is clear that the resurgent character of a fault can be neither implied nor ignored. Knowledge of large-scale crustal structure variations is important because these variations may localize or even generate regional stresses. The structure of the earth's crust has generally been assumed to be uncomplicated in the midcontinent region, but our knowledge is limited because few detailed studies have been performed. Re- cent studies suggest that the crystal stn~c~re of this re- gion is more complicated Han generally believed. For example, interpretation of two seismic refraction lines in southern Missouri and northeast Arkansas (McCamy and Meyer, 1966; Stewart, 1968), modeling of long period P- wave spectra near St. Louis, Missoun (Kurita, 1973), surface-wave dispersion measurements, and gravity anomalies suggest He presence of a basal high-velocity crustal layer. The geographical extent of this layer and its geological implications are at present unknown, but its possible spatial correlation with He New Madrid seismic zone suggests Hat it may have an effect on contemporary tectonics. Local Basement Inhomogeneitie.s Local basement inhomogeneities in the form of mafic or ultramafic intrusives and that are the probable source of major gravity and magnetic anomalies have been rec- ognized in the New Madrid seismic zone and other areas of eastern Norm America as being correlative with ear~- quake epicenters (McGinnis and Ervin, 1974; Long, 1976; Kane, 1977; McKeown, 1978~. Generally, the earthquakes occur adjacent to positive gravity gradient areas. The explanations of He origin of these correlations have taken two general forms: (1) the gravity and magnetic anomalies are related to crustal faults Hat are locally re- activated; and (2) He source of He anomalies reflects rigidity variations within the crust, which passively con- trol He swain field causing concentration of earthquakes.

Models for.Uidcontinent Tectonism Although these correlations are potentially significant, these explanations raise several problems. The statistical validity of these correlations still remains to be proven because relative positive gravity and magnetic anomalies occur widely over the midcontinent, but, as discussed previously, earthquakes do not. Even if a statistically valid correlation exists, it is not clear that there is a cause and effect relationship. The origins stated above assume this type of relationship, but, as McKeown (1978) men- tions, We seismicity may be related to reactivated paleo- ntts that are in part inferred from the presence of alkaline mafic rocks Tat produce positive gravity and magnetic anomalies. Thus, the anomalies and seismicity may have a common fundamental origin and not be related cause to effect. If the relationship is cause and effect, it is unclear why only a few positive anomalies are involved with seismicity and whether resurgent tectonics or crustal strength variations are involved. THERMAL EXPANSION AND CONTRACTION ThennaIly induced forces, manifested in a variety of forms and pattems, are generally recognized as the prin- cipal origin of stress within the earth. A major method of translating thermal energy into stress is by thermal expan- sion and contraction. Thus, it is to be expected that this mechanism could be used to explain the geodynamics of plate interiors and contemporary tectonic activity of Me midcontinent. Two general categories of models have been proposed: (1) models based on local thermal vana- tions primarily related to igneous intrusions in the litho- sphere or local heat-flow perturbations and (2) models based on mantle penetrative convection resulting in re- gional tensional and compressional pattems. As an example of the first category of models, Sleep and Snell (1976) proposed that the gradual subsidence of mid- continent basins involves thermal contraction of the lithosphere complicated by time-dependent regional isostatic compensation. They imposed a creep mechanism as well as faulting to relieve isostatic imbalance in the subsiding basin. However, evidence of the thermal heat- ing event that precedes subsidence is unknown for mid- continent basins. The second general class of model is based on mantle penetrative convection. Burke and Dewey (1973) gave numerous examples of continental triple-rift junctions, in- cluding the Mississippi Embayment, which they sug- gested were formed in stationary continental lithosphere over mantle plumes. Presumably, rising material in deep- mantle plumes spreads out in the upper asthenosphere, producing stresses on the overlying plates. Similarly, Hinze et al. (1972) suggested that late Precambrian rifts are related to rising mantle plumes followed by slow cool- ~ng of the upper mantle, which is consistent with continu- ing tectonic activity over long periods of geological time. As Burke and Dewey (1973) point out, reactivation of paleorifts is common. Thus, regional expansion and con- traction associated with surface uplift and subsidence 81 over rising (hotter) and sinking (cooler) mantle may be related to the contemporary tectonics of He midcontinent. ISOSTATIC WARPING Regional variations in loading or unloading of the crest cause isostatic deviations, which lead to crustal warping and the possibility of related cmstal rupture and ear~- guake activity. As a result, isostatic warping of the crust has been related to the contemporary geodynamics of He midcontinent region. Fox (1970>, in discussing the origin of the seismicity of the eastern United States, suggested that rebound from the depression of Me ea. - 's surface by the weight of the Pleistocene glaciation may have trig- gered earthquakes. However, Woollard (1958) finds no relationship between isostatic imbalance and earthquake epicenters. McGinnis (1963) has studied He frequency of ea - - quakes in the New Sladnd seismic zone as a function of river stage along the Mississippi River. He concludes that the most obvious and influential triggering mechanism for earthquakes in this area is the change in surface load caused by the seasonal change in the amount of water held in alluvial valleys. An alternative explanation is that a corresponding increase in pore pressure may lead to decreased friction, which will trigger earthquakes. Simi- larly, Fitch and Muirhead (1974) conclude that the load of water in reservoirs may trigger He release of contempo- rary tectonic stress and result in earthquake activity. Brecke (1964) suggests that dournwarping of He Mis- sissippi embayment under an increasing load of Meso- zoic and Cenozoic sediments produces tectonic activity. McGinnis (1970) relates isostatic warping in the midcon- tinent to the emplacement of high~ensit~r intrusive and extrusive rocks in riRs with sedimentary loading in the rift producing additional isostatic subsidence, which cul- minates in the development of sedimentary basins such as the Illinois Basin and the Mississippi Embayment. A related hypothesis has been proposed by Haxby et al. (1976), who suggest that the Michigan Basin formed by elastic flexure due to conversion of He lower crust from gabbro to heavier eclogite by a hot mantle diapiric plume. As the mantle is cooled by conduction, He basin subsides under the load of eclogite. CONCLUSIONS A summary of data and interpretations that pertain to the tectonic framework of the midcontinent region of the United States has been presented, which should help to delineate critical information that is needed to obtain an understanding of the contemporary tectonism of this area. Accordingly, some general observations on the validity of the tectonic models are appropriate. 1. The New Madrid seismic zone is the focus of He most intense earthquake activity in He midcontinent re-

82 Lion and, therefore, deserves special comment as to He possible tectonic mechanisms responsible for He seismic- ity. However, sufficient data are not available to make definite statements on mechanism, and, thus, the follow- ing comments must be regarded as preliminary. The area has been a focus of tectonic activity since Precambnan time, involving noting and igneous activity. Therefore, it is reasonable to presume that significant crustal boundaries are present that may act as zones of weakness for reactivation by the regional stress field. This field appears to be dominated by generally unifonn east to northeast horizontal compressive stress resulting from He relative motion of the Norm Amencan plate. Recent microcar~qualce studies in the New Madrid zone indi- cate significant N~SW trencling zones of epicenters hav- ing a combined length of over 150 km and fault modon consistent win a NEPHEW trending nght-lateral stnice-slip fault. A major crustal feature Hat may be He expression of an ancient not appears to be related to the northern Mis- sissippi Embayment and He New Maclrid seismic zone and is evidenced by NE~SW trending gravity and mag- netic anomalies. Ike implications of these observations are Hat zones of weakness may exist in He crust in the New Madrid region and that ea~qualce activity may be expected anywhere along the extent of these features at least where they are oriented appropriately to the direction of regional com- pressive stress. Because of the contemporary stress field, noting is not likely to be currently active. In He New Madnd region, the relatively consistent ear*'qualce focal mechanisms along linear trends of epicenters suggest a source mechanism involving resurgence of an older fea- ture knit?. Ibis conclusion implies that the potential seis- mic zone can be delineated if die detailed subsurface structure of the region is determined. 2. Any working tectonic model for the midcontinent region should be constructed within the framework of plate tectonics. We know of no major contemporary tec- tonic activity Hat cannot be generally explained by plate- tectonic theory. In fact, except for seismicity induced by man's activities, all major seismic zones similar to the New Madric! area appear to have their origin in plate motions. However, this is not to say that all tectonic activ- ity is currently well understood or completely explained by our present knowledge of plate tectonics. Although the mechanisms of contemporary deformation in the midcon- tinent are unclear, we expect that intraplate tectonism ultimately relater] to the motion of the Now American plate is the cause. We central midcontinent is part of the intenor of He Norm American plate. This area is at present a relatively stable craton. However, it has had a long, complicated history of tectonic activity, which is difficult to unravel. It is obvious Hat He history of deformation and Herman events, largely pre-Mesozoic in age, has leR its imprint on He crust of He midcontinent. Lateral and vertical varia- tions in composition and physical properties, fault zones, WILLIAM J. HINZE et al. and in~aplate or province boundaries are evident in the crust. These features may serve to localize stress, enhance deforrnahon, or act as zones of weakness. ACKNOWLE DGM ENTS This report was supported by U.S. Nuclear Regulatory Commission contact AT(49-24)~323 and is a condensed version of He Commission's Technical Report NUREG- 0382~1977~. The authors express Heir appreciation to 1~. C. Buschbach, N. B. Steuer, and He Nuclear Regula- tory Commission for Heir interest and cooperation. BE FE RENC E S Allen, C. R. (197S). Geological criteria for evaluating seismicity, Geol. Soc. Am. Bull. 86, 1041-1057. Ammennan, M.. L., and G. R Keller (in press). Delineation of the Rome Trough in eastern Kentucky with gravity and deep dnll- ing dam, Am. Assoc. Petrol. Geol. Bull. Baer, A. J. (1976). The Grenville province in Helikian times: a possible model of evolution, Phil. Trans. Roy. Soc. London A280, 49g - 15. Breclce, E. A. (19647. A possible source of solutions of the Illinois- Kentucky Fluorspar District, Econ. Geol. S9, 12~1297. Bristol, H. M., and T. C. Buschbach (1971). Structural features of the Eastern Interior Region of the United States, in Back- ground Matenals for Symposium on Future Petroleum Poten- tial of NPC Region 9, D.C. Bond, chary., Ill. State Geol. Sur?;., Ill. Petrol. 96, 21-28. Brown, L. D., and J. E. Oliver (1976). Vertical crustal movements from leveling data and Heir relation to geologic structure in the eastern United States, Rev. Geophys. Space Phys. 14, 13 35. Brown, W. R. (1970). The Piedmont: investigations of the sedi- mentary record in the Piedmont and Blue Ridge of Virginia, in Studies in Appalachian Geology, Central and Southern, G. W. Fisher et al., eds., Wiley-Interscience, New York, pp. 335 349. Burke, K., and J. F. Dewey ( 1973). Plume-generated triple junc- tions: key indicators in applying plate tectonics to old rocks,J. Geol. 81, 4~433. Denison, R. E., E. G. Lidiak, M. E. Bickford, and E. B. Kisvar- sanyi (in press). Geology and geochronology of Precambrian rocks in the central interior region of the United States, Econ. Geol. Ewin, C. P., and L. D. McGinnis (1975). Reelfoot Rift: re- activated precursor to the Mississippi Embayment, Geol. Soc. Am. Bull. 86, 1287-1295. Fitch, T. J., and K. J. Muirhead (1974). Depths to larger earth- quakes associated with cmstal loading, Geophtys. J. Roy. Astron. Soc. 37, 285~296. Fox, F. L. (1970). Seismic geology of eastern United States, Assoc. Eng. Ceol. Bull. 7, 21~3. Goldich, S. S. (1968). Geochronology in the Lalce Superior re- gion, Can. J. Earth Sci. S. 71~724. Haimson, B. C. (1976). Crystal stress in the continental United States as derived from hydroEracturing tests, in The Earth's Crust, J. G. Heacock, ea., Am. Geophys. Union Geophys. .Monogr. 20, pp. 576~592. Haxby, W. F., D. L. Turcotte, and J. M. Bird (1976). Thermal and

Models for Midcontinent Tectonism mechanical evolution of the Michigan Basin, in Sedimentary Basins of Continental Margins and Cratons, M. H. P. Bott, ea., Tectonophysics 36, 57-75. Herrmann, R. B., and J. A. Canas (1978). Focal mechanisms studies in the New Madnd seismic zone, Bull. Seismol. Soc. Am. 68, 109~1102. Heyl, A. V. ( 1972). The 38th parallel linearnent and its relation- ship to ore deposits, Econ. Ceol. 67, 87~894. Hildenbrand, T. G., A. F. Kane, and W. Stauder ( 1977). Magnetic and gravity anomalies in He northern Mississippi Embayment and their special relation to seismicity, U.S. Geol. Surv. Map MF-914. Hinze, W. l., R. F. Roy, and D. M. Davidson (1972). The origin of late Precambrian rifts, Geol. Soc. Am. Abstr. Programs, 725. Hinze, W. J., R. L. Kellogg, and N. W. O'Hara (1975). Geo- physical studies of basement geology of Southern Peninsula of Michigan, Am. Assoc. Petrol. Geol. Bull. 59, 156~1584. Kane, M. F. (1977). Correlation of major eastern earthquake centers with mafic/ultramafic basement masses, U.S. Geol. Shiv. Prof. Paper 1028~, 19~204. King, E. R., and I. Zietz (1971). Aeromagnetic study of He mid- continent gravity high of central United States, Geol. Soc. Am. Bull. 82, 2187-~08. King, P. B., compiler ( 1969). Tectonic map of North America, U.S. Geol. Sure. Map, scale 1:5,0()0,0()0. Kumarapeli, P. S., arid V. A. Saull (1966). The St. Lawrence valley system: A Now American equivalent of the east African nit valley system, Can. J. Earth Sci. 3, 639~58. Kurita, T. (1973). Regional variations in the stn~cture of He crust in the United States from P-wave spectra, Bull. Seismol. Soc. Am. 63, 1663~1687. Lidiak, E. G., and I. Zietz (1976). Inte~pretahon ot aeromagnetic anomalies between latitudes 37°N and 38°N in the eastern and central United States, Geol. Soc. Am. Spec. Paper 167, 37 pp. Long, L. T. (1976). Speculations concerning southeastern ear~- qualces, magic intrusions, gravity anomalies, and stress amplifi- cations, Earthquake Notes 47, 29 35. McCamy, K., and R. P. kleyer (1966). Crustal results of fixed multiple shots in the Mississippi Embayment, in The Earth Beneath the Continents, J. S. Steinhart and T. J. Smith, eds., Am. Geophys. Union Geophys. Monogr. 10, pp. 37(~381. McGinnis, L. D. (1963). Earthquakes and crystal movement as related to water load in the Mississippi Valley region, 111. State Ceol. Sum. Circ. 344, 20. 83 McGinnis, L. D. (1970). Tectonics and gravity field in the conti- nental intenor,J. Ceophys. Res. 75, 317~31. McGinnis, L. D.. and C. P. Ervin (1974). Earthquakes and block tectonics in He Illinois Basin, Geology 2, 517~19 McKeown, F. A. (1978). Hypothesis: many ear~qualces in the central and southeastern United States are casually related to mafic intrusive bodies,J. Res. U.S. Geol. Sun;. 6, 41 50. Nuttli, O. W. (1974). Magnitude-recurrence relation for central Mississippi valley earthquakes, Bull. Seismol. Soc. Am. 64, 1 189~1207. Ocola, L. D., and R. P. Meyer (1973). Central Norm American nit system: 1. Structure of the axial zone from seismic and gravi- metric data,J. Geophys. Res. 78, 5173~5194. Quick, R. C., E. F. Pawlowicz, and W. J. Hinze (1976). The Bowling Green Fault—a case of resurgent tectonics? (abstr.), in American Association of Petroleum Geologists, Eastern Sec- tion Meeting, Lexington, Kentucky, p. 20. Rankin, D. W. (1976). Appalachian salients and recesses: late Precambrian continental breakup and He opening of the Iapetus Ocean,J. Geophys. Res. 81, 5~)~5619. Sawkins, F. J. (1976). Widespread continental nfting: some con- siderations of timing and mechanism, Geology 4, 427~30. S bar, M. L., and L. R. Sykes (1973). Contemporary compressive stress and seismicity in eastern Norm Amenca, an example of intraplate tectonics, Geol. Soc. Am. Bull. 84, 1861-1882. Sleep, N. H., and N. S. Snell (1976). Thennal contraction and flexure of midcontinent end Atlantic marginal basins, Ceophys. J. Roy. Astron. Soc. 45, 125~154. Stauder, W., M. K=mer, G. Fischer, S. Schaefer, and S. Momssey (1977). Seismic characteristics of southeast Missoun as indi- cated by a regional telemetered microear~quake array, Bull. Seismol. Soc. Am. 66, 1953 1964. Stewart, S. W. (1968). Crystal structure in Missoun by seismic refraction methods, Bull. Seismol. Soc. Am. 58, 291~23. Street, R. L., R. B. Heranann, and O. M. Nuttli (1974). Eard~- quake mechanics in the central United States, Science 184, 1285~1287. Woollard, G. P. (1958). Areas of tectonic activity in the U.S. as indicated by earthquake epicenters, Eos Trans. Am. Ceophys. Union 39, 1135~1150. York, E., and J. E. Oliver ( 1976). Cretaceous and Cenozoic fault- ing in eastern Now Amenca, Geol. Soc. Am. Bull. 87, 1105~1114.

Problems of Intra plate Extensional Tectonics, Western United States 8 INTROD UCTION GRE GO RY ~ . DA V I S University of Southern California Intraplate extensional tectonics within continents has tra- ditionally been viewed from the standpoint of high-angle normal faulting. Other extensional mechanisms (dikes, low-angle normal faults, zones of ductile flow) have re- ceived much less attention. Normal faults within conti- nental plates occur in widely varying settings, both oro- genic and anorogenic. The spatial association of normal faulting with erogenic belts is so common that many geol- ogists have considered that block faulting is a necessary late or postorogenic phase of a geotectonic cycle (e.g., Roberts, 1972~. Nevertheless, support for the concept of a geotectonic cycle has justifiably waned with the advent of plate tectonics (Coney, 1970), and there is little in the disparate examples of nonnal faulting within erogenic belts to indicate a single underlying cause for them. One of the most widespread categories of intraconti- nental normal faults is the type described by Burke (Chapter 40nonnal fault systems that lead, unless ar- rested in their development, to Me complete rifting and separation of continental lithosphere. Zones of conti- nental plate attenuation and riPring appear to develop in- 84 discriminately across both erogenic and cratonic areas. Rifts will not specifically be treated here, although else possibility cannot be dismissed entirely that some ex- amples of Cordilleran normal faulting such as in the Basin and Range province might, from a causal stanclpoint, be atypical expressions of continental riding of the Atlantic- Bed Sea-East African type. Normal faults in the GulfCoast area of the southeastern United States are the dominant Cenozoic tectonic fea- tures of that region, but their origin as growth structures accompanying continental-margin sedimentation appears well documented. The Cordillera of the western United States offers many examples of structures that do not ap- pear to fall in the categories cited above. These form the basis for this summary of the problems of continental, intraplate extensional tectonics. What are the problems of intraplate extension within the western portion of the North American continent? They encompass all aspects of the formulation of struc- tural elements—geometric, kinematic, and dynamic. What, for example, is the geometry at depth of normal faults in the Basin and Range province? What is the total

Problems of lntraplate Extensional Tectonics, Western United States amount of extension represented by faulting in the Great Basin area of that province? Geometrically, how is brittle distension in the uppermost part of the lithosphere ac- commoclated at deeper structural levels? Have patterns of Cenozoic extension been influenced by pre-existing crystal anisotropies? If so, how, and by what controls? What are the kinematics of Basin and Range faulting, and have they changed in time? How are diverse styles of coeval late Cenozoic extension in the northwestern, west- ern, and southwestern United States related geometri- cally and Cinematically? Does such extension represent the response of the North American plate to transform motion along its western edge, to back-arc "spreading," to the inception of subplate asthenospheric plumes, to rift- ing of Atlantic type, or to undetermined factors? In what ways are stress fields in upper plates along convergent plate boundaries determined by the geometry of subduc- tion and the rate of plate convergence? This list of ques- tions is hardly conclusive, but it serves to illustrate the great diversity and scope of uncertainties regarding the nature of intraplate extension in just one part of the North American continent. EARLY TERTIARY EXTENSION IN THE PACIFIC lNORTHWEST A large region of the Pacific Northwest experienced a poorly understood extensional event in early Tertiary time with the reinitiation of arc volcanism following a Late Cretaceous-early Tertiary magmatic hiatus (Figure 8.1~. Volcanic activity began between 55 and 50 million years (m.y.) ago and was spatially accompanied by re- gional N-S block faulting across the entire U.S.~anadian border area from northwestern Washington to northwest- ern Montana (12~116° W longitude). Eocene uplift ofthe Northem Cascade Mountains was accompanied by di~slip reactivation of earlier strike-slip faults (Hope-Straight Creek and Chewack-Pasayten), for- mation of the Chiwaukum graben, and intrusion of the Teanaway basalts in N-S-striking dike swarms. The Chi- waukum graben contains interbedded fanglomerates and fluviatile sediments, interpreted by Whetten (1976) to represent deposition during block faulting. There is also a clear spatial relationship between block faulting and regional andesitic magrnatism in areas east of the North Cascades. Formation of the Republic graben in the Okanogan region was accompanied by andesitic vol- canism (52A3 m.y. ago; Fox et al., 1977) and deposition of graben sediments. To the east, the southern Rocky Moun- tain Trench, a half-graben at the time, came into being during the Eocene or early Oligocene as recorded by syn- tectonic sedimentation (Clague, 1974~. Unfortunately, the southward extent beneath the Columbia Plateau basalts of the steep bounding faults of the Chiwaukum, ,Methow- Pasayten, and Republic grabens is not known. The total amount of coastal extension during this early Tertiary event was probably not great, since the major 85 ~ ~ ~ ^ ~ r 160 320 km FIGURE 8.1 Location map of Eocene~ligocene (550~).1 m.y. ago) volcanic arc in the Pacific Northwest and contemporaneous block faulting. Position of the patterned arc from Snyder et al. (1976). SC, Straight Creek Fault; C, Chiwaukum graben; P. Pasayten Fault; R. Republic graben; RMT, Southem Rocky Mountain Trench. faults dip steeply, but reasons for the int~a-arc setting of the block-faulted area and its great E-W breadth are un- clear. Shallow dipping subduction of an oceanic plate could explain the unusual breadth of the arc, but the con- sequences of such subduction beneath the Andes of central Peru are a shutoff of arc volcanism and an upper- plate compressional stress state oriented more or less per- pend~cularly to the trench (Stauder, 1975~. Miocene crustal extension in the Pacific Northwest (dike develop- ment and normal faulting) occuned in areas east of the contemporaneous Cascade volcanic arc, not spatially coincident with it as in the case of the Eocene events. MIOCENE EXTENSION IN THE WE STE RN UN ITE D STATE S The Miocene was a time of profound tectonic change for the western United States in areas extending from south- eastern Washington to southeastern Califomia. This en- tire region experienced diverse but strikingly synchro- nous extensional deformation beginning approximately 16 or 17 m.y. ago. In the Pacific Northwest, crustal dilation was represented by extensive, primarily NNW-trending basaltic dike swarms in central and northeastern Oregon, southeastern Washington, and western Idaho. The noted western portion of the Snake River Plain was also formed at this time, but the amount of distension represented by this feature is controversial. Crustal extension was pronounced in the Great Basin area of Nevada, Utah, and adjoining states, where wide- spread nonnal faults characterize the deformation and control the topography. Volcanic activity that accom-

86 panted nonnal faulting was bimodal predominantly basaltic but with subordinate rhyolite and rhyodacite (see Chapter 14~. Farmer south in the Colorado River area between California-Nevada and Arizona, block faulting of classic Basin and Range type generally did not occur, but extreme high-level crystal distension appears to be repre- sented by an enigmatic terrane of low-angle Miocene faults of still unknown extent and origin. DIKE SWARMS, COLUMBIA RIVER BASALT The dike swarms ofthe Columbia River basalt occur prin- cipally in the Snake River area between northeastern Ore- gon and Idaho (Figure 8.2) and constitute the feeders for one ofthe most voluminous outpourings of basaltic lava in the geological record. Within a brief 3 m.y. period (ca. 16.~13.4 m.y. ago) more than 200,000 km3 of Columbia River tholeiitic basalts were erupted as three geochemi- cally distinct groups_Picture Gorge, Imnaha, and Lower Yalc~a (McDougall, 1976~; the latter is by far the most voluminous and areally extensive. Watkins and.Baksi (1974) conclude that this rate of extrusion exceeds any w ;R~ Swam _ ~ 100 200 m, 0 - 160 320 km C-R hi\ FIGURE 8.2 Location map of Miocene (2~10.1 m.y. ago) vol- canism in the Pacific Northwest Position of Western volcanic are Dam Snyder et al. (1976). Distribution ofColumbiaRiverbasalts (stippled pattern) and limits of Grande Ronde Comucopia dike swarm (DS) from Snavely et al. (1973). CR, Cascade Range; M, Monument dike swann; WSR, Western Snake River graben; C-R, dike swarTns in Cortez and Roberts Mountains (Stewart e! al., 1975~; WLPC, western limit of Precambrian continental crust based on initial 87Srl88Sr isotopic ratios (Armstrong et al., 1977). GREGORY A. DAVIS witnessed in historical times and is four to six times greater than that which occurs at typical midocean spread- ing centers. Swanson and Wright (1977) disagree. They have calculated that yearly-averaged flow rates for the basaltic province were 0.07 km3/year, roughly comparable to Hawaiian lava production in the last 3 m.y. (0.08 krn3/year) and greater than the rate of Icelandic volcanism (0.05 km3/year) during the last 10,000 years. On the aver- age, Columbia River flows were erupted in a given area every 10,00(~20,000 years and with average volumes of 1~20 kIl}3. Some single flows with eastern Plateau feeders were so voluminous (up to 600 km3) and were erupted in so brief a time (a few days) that they reached the Pacific Ocean near the present mouth ofthe Columbia River (Figure 8.2~. It is notable that the eastern Oregon-westem Idaho dike swarms from which most of the basalts were appar- ently extruded roughly coincide with the north-south contact between eastern Prec~nbnan continental crust and younger "eugeosynclinal" units in the Oregon-Idaho border region (Figure 8.2~. It is suggested that this Meso- zoic tectonic boundary, or suture, represented a funda- menul zone of crystal weakness that was reactivated in Miocene time during ENE-WSW regional extension. Similar and contemporaneous basaltic andesite dike swanns are present in the Cortez and Roberts Mountains of north~en~al Nevada (Stewart et al., 1975~; they also strike northerly (ca. N 20° W) and occur near the postu- lated westward limit of Precambrian crust in the underly- ing basement (Figure 8.2~. Zoback and Thompson (1978) have proposed continuity of Miocene rifling along a 700- long linear zone that includes the Nevada dike swarms described above, the graben of the western Snake River Plain, and the feeder dikes of the Columbia River basalts. The amount of Mio- cene crusted extension clearly duninishes northward from the Basin and Range province (major nonnal faultings to the eastern Columbia Plateau-Blue Mountains region (vertical fissunng). Therefore, it is not surprising that evidence for Miocene extension is not seen still farther north, for example in northeastern Washington. GREAT BASIN EXTENSIONAL TECTONICS The extent and general topographic and stmc~ral charac- teristics of the Great Basin area of the Basin and Range province are well known (e.g., Hamilton and Myers, 1966; Stewart, 19711. Nevertheless, many fundamental geolog- ical aspects of the Great Basin area are not well under- stood. No attempt is made to review the geophysical char- actenstics of the Basin and Range province, as they are treated in a companion paper by Eaton (Chapter 9~. Geometry of Range-Front Faults in the Great Basin In most areas of the Great Basin (Figure 8.3) range-front faults are observed to dip 50~0° [although Donath (1962) reports that near-vertical faults are dominant in south-

Problems of Intraplate Extensional Tectonics, Western United States J ~ N... .. .' aid 200 400 mi 0~ . ~ . , 200 600 k m FIGURE 8.3 Distribution of extensional tectonics in the south- westem United States. Areas affected by normal faulting of basin-range type are indicated by NEPHEW ruled pattern. Areas of low-angle detachment faulting are stippled. The inferred distn- bution of crystal low-velocity layer (Smith, 1978) is shown by NW~E ruled pattern. Geographic localities from north to south: SRP, Snake River Plain; Y. Yenngton; S. Snake Range; F. Funeral Range; P. Pahranagat Shear System; G. Garlock Fault; LV, Las Vegas; E, Eldorado Mountains; W. Whipple Mountains; P. Phoenix; T. Tucson. central Oregon]. Many workers (e.g., Stewart, 1971) have projected graben-bounding faults downward at constant angles to determine the depths below the surface where pairs of such faults would intersect and define the level of transition between shallow extension by nonnal faulting and extension by other, deeper mechanisms. Most es- timates fall in the range of 1~17 km. However, other workers (e.g., Wright and Troxel, 1973; Pro ffett, 1977) have supported the early suggestion of Longwell ( 1945) that at least some basin-range faults must flatten at depth in ortler to explain rotational tilting of hanging-wall strata toward the faults. Deep seismic re- flection studies across selected grabens in the Great Basin confirm a downward flattening of some range faults (A. W. Bally, Shell Oil Company, personal communica- tion, 1978~. The "bottoming~out" of basin-range faults may, therefore, occur at shallower levels than those depths determined by projecting faults downward at con- stant dip. Wright and Troxel (1973) believe that nonnal faults east of Death Valley, California, may flatten to the horizontal at depths of only ~10 km, although Proffett 87 (1977) postulates that curviplanar range-front faults in the Yenngton area of western Nevada did not become hon- zontal until depths of at least 1~16 km. Amount of Great Basin Extension Estimates of the amount of extension across the Great Basin vary widely and are highly influenced by the as- sumptions made regarding the geometry of range-front faults. At 4(P N latitude the Great Basin is 750 km wide. Stewart (1971) assumed average dips for range-front faults of 60° and calculated 2.5 km of average extension across each major graben in the Great Basin at this latitude, for a total E-W extension in the province of 75 km (a~ proximately 10 percent). Extrapolating from a single basin in northern Nevada (Dixie Valley), Thompson and Burke (1974) postulate 100 km of total Great Basin extension. Alternatively, Proffett (1977) estimates 16(L180 km of overall extension (by his calculation, approximately 3~ percent) based on an assumed downward flattening of normal faults in some portions of the Great Basin and on the observed patterns of crustal thinning beneath the re- gion. Hamilton's most recent estimate of total extension across the Great Basin is from 50 to 100 percent depend- ing on the extent to which the tectonically thinned crust has been thickened by Cenozoic magnetism (Hamilton, 1978, and personal communication, 1978~. There are certainly localized areas of extreme extension within the Great Basin. Wright and Troxel (1973) calcu- lated that a 30 50 percent extension has occurred} along downward-flattening nonnal faults between Death Valley and the Nopah Range, California Davis and Burchfiel (1973) concluded from a geometric analysis of a larger area that included the area studied by Wright and Troxel that E-W crystal extension north of the Garlock Fault between the Nopah Range and the Spangler Hills has been approximately 100 percent. Finally, ProffeK (1977) estimates more than 100 percent extension in the Yering- ton district near Reno as the consequence of multiple, superposed generations of curviplanar normal faults. A1- though these examples are not thought~by the writer to be characteristic of the entire Great Basin, they are instruc- tive in indicating that local or regional estimates of limited extension (10 percent or less) based on unproven assumptions of steep faults and constant dips may be highly erroneous and much too conservative. Relations Between Extension and St~ke-Sl~p Faulting Not all portions of the western United States have ex- penenced the same amounts of Cenozoic extension. The boundaries between areas of differential extension are geometrically interesting and of considerable importance in defining the kinematics of intraplate deformation. Davis and Burchfiel (1973) and Lawrence (1976) have defined the southwestern and northwestern boundaries of the Great Basin, respectively, as intraplate transform or strike-slip faults that separate the distended Great Basin

88 from southern and northern terranes that lack basin-range structure. The ENE- to E-striking Garlock Fault (Figures 8.3 and 8.4) is the most impressive ofthe transfonn bound- aries. It represents a major lithospheric structural element that abruptly separates two regions (Great Basin and Mo- jave l:)esert) with dissimilar crustal thicknesses and seis- mic velocity characteristics (Davis and Burchf~el, 1973~. An unresolvecl, fundamental problem of Great Basin genesis is why lithospheric extension terminated south- ward so abruptly along an ENE- to E-striking boundary that was to become the Garlock Fault. Significant varia- tions in initial strontium isotopic ratios from Mesozoic granitic rocks in southern California indicate to Kistler and Peterrnan (1978) that the Garlock Fault developed along a major pre-Mesozoic boundary in the crust. Thus, mechanical or compositional costar anisotropies may have influenced the pattern of Cenozoic extension in this area. A similar general conclusion was tentatively reached above for the localization of Columbia River basaltic dike swanns in eastern Oregon and western Idaho. Vanable amounts of distension within the Basin and Range province may also be accommodated by internal stnlce-slip or transfonn fault zones. The Pahranagat shear system of southeastern Nevada (Liggett and Ehrenspeck, 1974) is ~ leR-lateral displacement that appears to connect nonaligned northern and southern areas of major normal faulting (Figure 8.3~. The transform zone strikes approxi- mately N 60° E and thus defines the relative direction of crystal dilation in this part of the Great Basin at the time of its connation. A kinematic analysis of the entire Great Basin might be made by looking for other examples of stnke-slip faults that developed synchronously with normal faulting as the consequence of differential exten- sion. Hi! \ — ~ KQ \ \ ~ SAN \ GARLoCX FAuLt \ ~ ANDREA S \ \\ FA V ~ r — ~ .~ 140JAVE DESERT E| FIGURE 8.4 Northward diagrammatic view of Garlock Fault, sounder California, as a boundary between a northern, dis- tended crustal block (Basin and Range province) and a southern, nondistended crustal block (Mojave Desert). Topographic rela- tions are highly generalized and are shown only north of the Garlock Fault Geographic localities: SJV, San Joaquin Valley; SN, Siem Nevada; PV, Panamint Valley; DV, Death Valley; NR, Nopah Range; OR, Kingston Range. (From Davis and Burchfiel, 1973, reprinted from the Bulletin of the Geological Society of America, with permission). GREGORY .~. DAvlS Mechanisms of Crustal Extension at Depth The Great Basin is characterized by a crust considerably thinner than that of surrounding regions (Smith, 1978 Thompson and Burke, 1974), a relation best explained as the consequence of Cenozoic tectonism. Attenuation of the 18- to 30-km-thick Great Basin crust cannot be due solely to nonnal faulting because Famous estimates for the depths to which nonnal faulting occurs fall in the range `'f `17 km. An area of possible exception is the west side `'f the Great Basin, where Smith (1978) reports crustal thick- ness as little as 18 km, and Proffett ( 1977) proposed nonnal faulting to depths of at least 1~16 km. In general, how- ever, mechanisms over than normal faulting must ire sought for extension of the lower Great Basin crust. Thompson and Burke (1974) and ~'nght and Trowel (1973), among others, have proposed Mat extension Icy nonnal faults in Me upper crust may be accommodated wholly or in part by Me intrusion of igneous dikes and plutons at depth. The spatial and temporal relations b~- tween basin-range faulting and volcanism, however. are so inconsistent as to cast doubt on the overall utility of this mechanism. Igneous intIusion could account for deep crystal extension, but there is no a prion reason to believe Mat it would produce attenuation of the lower Angst. Extension and Winning could, altematively, be the con- sequence of the penetrative, duchle flow of lower crustal rocks, a phenomenon made more probable in light of Me high heat flow of the province and Me occurrence in west- em Utah and eastern Nevada of metamorphic and mylo- nitic ter~anes that experienced metamorphic tempera- tures as recently as Me Miocene (Compton et al., 1977~. Smith (1978) reports that there is evidence for Me exis- tence of a crustal low-velocity zone in this eastern Great Basin region today. The zone extends from a depth of ~15 km and might conceivably coincide with a transition from highly brittle defonnation to ductile deformation. Proffett (1977) has recently proposed a geometric model for ex- plaining Me occurrence of such a transition between downward flattening basin-range faults and deeper levels of laminar flow. LOW-ANGLE DETACHMENT FAULTS Nearly 20 years ago Misch (1960) described an extensive terrane in Me border area between nor~em Nevada and Utah that is, he believed, underlain by a low-angle re- gional fault (Figure 8.3). He Darned this fault Me Snake Range decollement, and the area in which it occurs Me "Nor~westem Nevada structural province." The decolle- ment typically separates complexly faulted, unmetamor- phosed upper-plate rocks from stratigraphically older, lower-plate crystalline complexes, the latter now known to include bow Mesozoic and TertiaIy metamorphic as- semblages. Misch (1960) noted Mat while some mountain ranges in the stIuc~ra1 province were fault blocks off

Problems of Intraplate Extensional Tectonics, We.stern United States basin and range type, others were elongate-domal uplifts with only subordinate block faulting. In recent years decollement structures and domal ranges of the type rec- ognized by Misch have also been discovered in more southerly areas of the Cordillera (Figure 8.3), in the Death Valley area `~E Califomia (Reynolds, 19~4), and, more ex- tensively, in southernmost Nevada, southeastern CaIifor- nia, and Arizona. Many ofthe low-angle faults in We Nevada-Utah region studied by Misch are found in areas that lack Tertiary rocks (or direct evidence for the involvement in faulting of Tertiary units) and are complicated by the superposed disruptive effects of basin-range faulting. As a conse- quence' the age and origin of these low-angle faults are highly controversial. Nevertheless, Armstrong (1972) has presented a strong case that the Snake Range decollement and higher, related faults (1) are Tertiary in age or, at the least, show Tertiary reactivation; (2) formed in an exten- sional regime; and (3) result from Dinning of supracrustal rocks by normal faulting above a basal detachment surface (denudational tectonics). In contrast, Misch (1971) con- siders the Snake Range decollement to be Mesozoic in age and compressions] in origin. Hose and Danes (1973) offer still another interpretation—Me decollement re- sulted from regional eastward gravity sliding during Mesozoic time The case for a Tertiary origin for the low-angle faults of the northern Nevacla-Utah area is greatly enhanced by geological relations in the closely similar (and compar- ably enigmatic) terrane of low-angle faults that begins near Hoover Dam in the Las Vegas area, follows the Colo- rado River trough as far south as Parker, then swings southeastward and extends across southern Arizona into areas near Tucson (Figure 8.3~. On the flanks of several ranges that characteristically owe their elevation to dom- ing or arching rather than block faulting, basal detach- ment faults are found that separate allochthonous upper- plate units from crystalline rocks in the cores of the ranges. Tertiary strata as young as Middle Miocene are widely involved in the low-angle faulting, as are Precam- brian gneisses, Paleozoic and Mesozoic metasedimentary rocks, and Precambrian and Mesozoic plutons. Lower- plate rocks in the terrane include Precambrian gneisses and platonic rocks, Mesozoic and Tertiary plutonic rocks, and strongly lineated mylonitic gneisses of Cretaceous(?) and Tertiary age. Ongoing studies by I. Lawford Anderson and the wnter, and their students at the University of Southern Califor- nia, in the Colorado River portion of the Nevada- California-Anzona dislocationa1 terrane are reviewed below, but with no assurance that conclusions drawn from these studies are applicable to portions of the terrane farther east, e.g., in the Tucson area (Davis, 1975; 1977), or in the .Nevada-Utah terrane first described by Misch (i9601. Carr and Dickey (1977), Lucchit~ and Suneson (19/7), and Rehrig and Reynolds (197~; in presto have also investigated areas within the terrane described here. 89 COLORADO RIVER TROUGH SOUTH OF LAKE MEAD Low-angle faults cutting rocks of Precambrian through Miocene age were mapped in 1960, by geologists of the Southern Pacific Land Company in the mountain ranges surrounding Needles, California (Figure f3.5). They con- cIuded that the faults were late Tertiary thrust faults, but their important studies were never published. Tend (1972) recognized an extensive subhorizontal fault in the Whipple Mountains, California, 60 km south of Needles, and interpreted it to be a thrust fault of Cretaceous age: Anderson's (1971) study of low-angle faulting in the Eldo- rado Mountains, Nevada, 100 km north of Needles, was the first to attribute the low-angle faults of the Colorado River area to noncompressional tectonics. He described the Eldorado Mountains as an area of major imbricate nonnal faulting accompanied by pronounced rotational tilting of Tertiary strata. Miocene nonnal faults that dip westward were mapped as flattening downward and merging with a subhorizontal basal fault surface below which extension by normal faulting has not occurred. Dis- placement of Tertiary rocks above the basal surface was thus westward (S 70° W) relative to lower-plate autochtho- nous units. Remapping of the Whipple Mountains Fault rec- ognized by Terry ( 1972) confirms that it also is of Tertiary age and that it underlies a 3000-km2 normal faulted ter- rane that includes the Wh~pple, Buckskin, and Raw- hide Mountains of western Arizona (Shackelford, 1976a, 1976b, 1977; L~ngrey et al., 1977; Davis et al., 1977~. The surface may be coextensive with a similar basal disloca- tion surface mapped by Southern Pacific geologists on the flanks ofthe Chemuheuvi, Sacramento, Homer, and Dead Mountains to the north (Figure 8.51. As in the Eldorado Mountains, the basal detachment or dislocation surfacers) separates an allochthonous upper-plate assemblage of Tertiary sedimentary and volcanic units (and their crystal- line basement) from lower-pl~te crystalline rocks that are mylonitic in some ranges. NoIthwest-striking, typically northeast-dipping, normal faults occur throughout the upper plate in the area of Figure 8..o but are absent from lower-plate autochthonous rocks. The normal faults (not shown on the figure) both merge with and are cut by the basal dislocation surfacers), indicating a complex move- ment history for the upper plate. Kinematic relations (fault striae, rotational tilting of Tertiary strata, and drag folds) indicate that tectonic trans- port of upper-plate units in the area of Figure 8..5 `~.~s predominantly northeastward (N 50+10° E, opposite to that in the Eldorado Mountains). This striking consis- tency of upper-plate movement direction clearly indicates that the basal dislocation surfacers) and higher related faults fanned prior to the regional doming that produced the present ranges. No evidence has been found that al- lochthonous units moved radially away fro-m uplifted areas, in contrast to the conclusions drawn by Coney

go \ to + GREGORY A. DAVIS H ~ ~ ~ Cod ~ _——I,. - - ~ J J ,_ tills r %X ~d,, 543 +35° ~ ,, to +34°N A FIGURE 8.5 Tertiary low-angle Cult complex, southeastern California and west central Arizona. Map shows location of regional low-angle faultts) (hatchured contacts) separating undifferenhated allochthonous upper-plate units (stippled pattern) from autochtonous lower-plate rocks. Lower-plate lithologies include undifferentiated metamorphic and intrusive rocks (short-lined paetem) and their ~nylonitic equivalents (long-wavy-lined pattern). The nvo lower-plate assemblages are separated by a "mylonitic front" (mf) in the Whipple Mountains. Heavy dashed lines represent the axial traces of broad antiforTnal (tnangles point outward) and synformal (triangles point inward) folds that warp the basal dislocation fault(s). Communities: N. Needles; LH, Lalce Havasu City; P. Parlcer. Mountain ranges, from NW to SE: H. Homer; D, Dead; S. Sacramento; M, Mojave; C, Chemehuevi; W. Whipple; B. Buckskin, R. Rawhide, A, Artillery. Geological relations north of the Whipple Mountains first ascertained by geologists of the Southern Pacific Land Company (unpublished studies) and confirmed and supplemented by E. Frost and He wnter. Geological relations in the Rawhide Mountains from Shackelford ( 1976a). (1974) for units above the Snalce Range decollement in the Snake Range and by Davis (1975) for folded rocks above the Catalina Fault in the Rincon Mountains near Tucson. Figure 8.6 is a diagrammatic cross section of geological relations in the Whipple Mountains. Mapping in that range indicates that the basal dislocation surface dies out southwestward, that it experienced at least two stages of movement separated by a major interval of erosion and deposition of Miocene sedimentary and volcanic rocks (E. Frost, Univ. of Southern California, personal communica- tion, 1978), and that it has been warped by postfaulting domal uplift ofthe Whipple Mountains. An earlier sugges- tion (Davis et al., 1977) that low-angle faults in the Colo- rado River trough may everywhere have developed

Problems of Intraplate Extensional Tectonics, Western United States within a brief, 1- to 2-m.y.-long episode (1~15 m.y. ago) was much too simplistic and is erroneous. Mapping by E. Frost in the southeastern Whipple Mountains reveals that Oligocene or Lower Miocene sedimentary and volcanic strata (Gene Canyon Fonnation) have been rotatecl~more steeply along northeast-clipping normal faults than uncon- formably overlying Miocene strata (Copper Basin Forrna- tion). Both Frost and I~ucchitta and Suneson (1971), the latter in studies between the Whipple and Rawhide Mountains(Figure 8.5), also report seditions of rotated Ter- tiary strata in which the dip of tilted beds decreases pro- gressively upward. These geometric relations are strongly indicative of growth faulting, i.e., Tertiary sedimentation and volcanic activity occurred synchronously with upper- plate normal faulting and the lateral displacement of upper-plate units along the Whipple Mountain basal dis- location surface. Rehrig and Reynolds (in press) reached a similar conclusion in their western Arizona investiga- tions. The origin of the Colorado River area structures and their relationship, if any, to conventional basin-range faulting in the western United States is unclear. Mountain ranges in the area of Figure 8.5 are definitely not outlined by range-front faults of the type seen in the Great Basin. Significant, but unknown amounts of Miocene extension of upper-plate units apparently occurred throughout the CO10BdO River trough, but no regional and coeval exten- sional phenomena have yet been identified in the crystal- line rocks of the lower platers). Anderson (1971) postu- lated that extreme "thin-skin distension" at shallow crustal levels in the Eldosdo Mountains was compen- sated at depth by the intrusion of Miocene plutons. But, although present in the Eldosdo Mountains, Miocene plutons have not been recognized in the lower platers) of the dislocational terrane south of the E ldorado Mountains and, if present, cannot be extensively developed. SW ~.~11 91 It is tempting to speculate that the low-angle fault stn~c- tures of the Colorado River area may represent the "bottoming~ut," as discussed earlier, of typical basin- range faults into a transitional zone between an upper level of brittle deformation and deeper levels of crustal flow. However, field relations do not support this specula- tion. Mylonitic gneisses occur throughout some lower- plate areas south of the Eldorado Mountains and possess a penetrative NE-SW lineation indication of mylonitic flow in that direction (parallel to upper-plate extension by normal faulting). Field relations in Me Whipple-Buck- skin-Rawhide Mountains portion of the dislocational ter- rane clearly indicate that regional mylonitization pre- dated significantly the Miocene dislocational events; tilted and folded mylonitic gneisses are cut with pro- nounced discordance by the basal dislocation surface in the eastern and central Whipple Mountains. They may have been subject to major erosion prior to Oligocene(?) (or Miocene) deposition and faulting and certainly were being eroded between phases of Miocene faulting (Figure 8.6~. Mylonitic cobbles and boulders of lower- plate rock types occur abundantly in some of the Ol~gocene(?) and Miocene fanglomerate units of the southern and northern Whipple Mountains (both Gene Canyon and Copper Basin Formations). Sphenes from a mylonitic gneiss in one such boulder have yielded an 82.9 + 3-m.y.-old fission track age (Dokka and Lingrey, in press). A mylonitic gneiss from the lower plate of the Rawhide Mountains has yielded semiconcordant hom- blend~e and biotite K/Ar ages of 57.4 and 52.3 m.y. ago, respectively (Shackelforcl, 197 7~. Furthermore, low-angle faulting and NEPHEW extension of upper-plate rocks were not restricted to areas of deeper mylonitic deformation. This is documented in the central Whipple Mountains, where nonmylonitic lower-plate gneisses and plutonic rocks are separated from structurally lower mylonitic NE T1,2 ~ ~~~ ~ - ~,/~W,~—7//i b, /"~~~ A// mxIn , . .,, ~ ,, ~ , ~ ·, ,, , ~ , ~ , ^~ ,,,,,,,, ,~ %~% ~ mxln mi 0 1 lam 8 FIGURE 8.6 Diagrammatic cross section across the Whipple Mountains, southeastern Califomia, illustrating Middle Miocene geology ica1 relations prior to domal uplift and warping of the Whipple .\Iountains basal dislocation surface (WMBDS). The cross section illustrates evidence for two phases of rotational, nonnal fault displacement along the basal dislocation surface. T., older Tertiary sedimentary and volcanic rocks; T2, younger Tertiary sedimentary and volcanic rocks deposited across the basal dislocation surface prior to their involvement in renewed fault displacement. ME is a "mylonitic front," the abrupt nonfault contact between undifferentiated lower-plate metamorphic and intrusive rocks (xln) and their largely mylonitic equivalents (mxln); br, breccias developed below the basal dislocation surface.

92 equivalents by an abrupt "mylonitic front" (Figures 8..o and 8.61; in the Chemeheuvi Mountains (Figure 8.5), where a nonmylonitized Mesozoic(?) quartz dionte comprises most of the lower plate; and in the Eldorado Mountains as described above. The probable level of erosion in the Colorado River area appears to be much too shallow (<5 km) to have exposed a brittle-ductile transitional zone of the Great Basin type postulated by Proffett (1977) and others (e.g., Eaton' see Chapter 9~. Several lines of evidence suggest that the basal dislocation surface or detachment fault in the Whipple Mountains fanned at very shallow crustal levels: (1) the stratigraphic thickness of rotated fault blocks (Tertiary strata plus Precambrian crystalline basement) does not exceed 5 km; (2) Miocene erosion that occurred between two phases of movement on the Whipple basal dislocation surface locally exhumed and breached that fault before deposition of younger, now partly allochthonous strata; and (3) Middle Miocene fan- glomerate deposits and interbedded basalt flows (Os- bome Wash Formation) partly buried the tilted fault blocks within 3 or 4 m.y. (maximum) after their last move- ments. These posttectonic surficial deposits locally lie only several hundred meters above the basal dislocation surface and relations (both field and tempos!) indicate that the amount of erosion between faulting and fan- glomerate deposition was slight. The origin of the low-angle faults of the Colorado River trough remains obscure. Our studies indicate the detach- ment of thin upper crustal slabs beneath the area and their synchronous internal distension by nonnal faulting. Re- gional gravity sliding of the detached rocks is indicated (a conclusion first reached by Shackelfonl, 1976a), since lower-plate geological relations are quite diverse and no synchronous lower-plate extensional phenomena of re- gional extent have been recognized. Lower-plate my- lonitic gneisses in portions of the dislocational terrane attest to an earlier phase of NE-SW crystal extension by penetrative, semiductile flow. The tectonic significance and even the agefs) of these puzzling mylonitic roclcs re- main unresolved; problems of their origin have recently been reviewed by Davis (1977) and Rehng and Reynolds (1977; in press). It is likely Mat extensional faulting in the Colorado River trough predated, perhaps significantly, crustal extension by nonnal faulting in Great Basin areas to the north. A genetic relationship between the two faulted! te~-~nes cannot, at present, be demonstrated. PLATE-TECTONIC SPECULATIONS ON THE CAUSE(S) OF LATE CENOZOIC EXTENSION IN THE WESTERN UN ITE D STATE S Throughout essentially its entire length, from south- eastern Washington to southern Nevada, the interior por- tions of the western United States experienced variable amounts and styles of E-W to ENE-WSW extension GREGORY A. DAVIS in.. beginning 17-14 m.y. ago. This coincidence in timing applies constraints on plate-tecton~c models proposed for the origin of the phenomena. For example. the areas af- fected by Great Basin normal faulting and the dike swarms that fed Columbia River basalts lay completely north of the Mendocino triple junction at this time (Snyder et al.. 1976~. Thus, extensional tectonics cannot be related simply to northward migration of the Mendo- cino triple junction as Atwater (1970) originally sug- gested, a conclusion also reached by others for the tTansi- tion in the western Unitecl States from calc-alkaline mainly andesitic volcanism to basaltic and subordinate silicic volcanism (Snyder et al., 1976; Cross and Pilger, 1978; Lipman, see Chapter 14~. Atwater (1970) proposed that extension in the Basin and Range province was related to oblique (NW-SE) riding or "stretching" within a broad transform zone that de- veloped south of the Mendocino triple junction and east of the Pacific-North American tensions plate boundary. Two lines of evidence refute this suggestion: (1) the rela- tion already mentioned that at the time of inception of normal faulting in the Great Basin (ca. 17 m.y. ago) almost all of the region lay north of the triple junction and, there- fore, east of a convergent plate boundary; and (2) increas- ing kinematic evidence that initial extension across normal faults and dikes in the Great Basin occurred in an E-W to ENE-WSW direction, not along the NW-SE trend as postulated by Atwater. E-W to ENE-WSW distension is defined by the orientation ofthe Garlock transfonn fault at the southern end of the Great Basin (Davis and Burch- fiel, 1973), by the orientation of the Pah~nagat shear system in southeastern Nevada, and by the orientations of a 17- to l~m.y.~old dike swank in north~entral Nevada and a small orthogonal transfonn Cult associated with it (N 70° E; Zoback and Thompson, 1978~. At present, much of the western Great Basin does ap- pear to be extending in a NW-SE direction parallel to the San Andreas plate boundary to the west. N-S faults in affected areas have typically exhibited oblique slip (slip with both nonnal, or dip-slip, and right-slip components) during historic seismic events (Gumper and Scholz, 19711. The eastern Great Basin, however, does not ap- parently "feel" a superposed transfonn boundary stress regime and continues to extend in an E-W direction (Smith and Sbar, 1974~. Recent workers favor the idea that initial extension and magmatism within the Great Basin area is a back-arc phe- nomenon (e.g., Scholz et al., 1971; Snyder et al., 1976; Zoback and Thompson, 1978), perhaps related to the dia- piric rise and lateral spreading of North American plate asthenosphere that was heated at depth above a subduc~t- ing plate. Similarly, the location of the Columbia Riser basalts east of the Cascade Range (Figure 8.2) and their temporal equivalency to early phases of Basin-Range faulting in areas to the south have suggested to most workers that they, too, are a back-arc response to subduc- tion of oceanic lithosphere beneath the western edge of North America (e.g., Snyder et al., 1976; McDougall

Problems of Intraplate Extensional Tectonics, Western United States 1976; and Christiansen and Lipman, 1972~. McBimey et al. (1974) have presented data in support of an association of the Columbia River basaltic eruptions with Cascade arc activity. The former coincides impressively, at least in the central Oregon Cascades, with the most intense pulse of andesitic volcanic activity in the Cenozoic period (16. 14.2 m.y. ago). McBimey et al. (1974) state that a coeval mid-Miocene pulse of arc volcanism may also be present in the Indonesian and other circum-Pacific areas. Accel- erated global rates of plate convergence in mid-.Miocene time may be indicated, and it is perhaps in this general plate context that the Columbia River dike swarms and basalt outpouring should be viewed. An additional or alternative explanation for Miocene back-arc extension in the western United States is pro- vided by the fact that this extension followed a probable steepening of the Farallon-juan de Fuca plate approxi- mately 20 m.y. ago. Snyder et al. (1976) presented data indicating an east-west narrowing of the volcanic arc in the Pacific Northwest at this time. A more pronounced narrowing occurred in areas to the south. Since the width of a volcanic arc is related to the dip of the subducting plate beneath it, the wider the arc, the lower the inclina- tion of the underthrusting slab. Changes in dip of a sub- ducting plate may govern, or at least influence, stress state in the overlying plate [other factors, e.g., convergence rate, absolute motions of upper plate, and age of descend- ing oceanic plate, are discussed by Cross and Pilger (197811. Subduction relationships between the Nazca plate and the South American plate are instructive in this regard. At present, the Nazca plate is being subducted eastward beneath nor~em and central Peru at a low angle (1~15°; Stauder, 1975; Megard and Philip, 19761. Above this plate the Andean volcanic arc is inactive and the stress in the South American plate is characterized by east-west compression. In contrast, beneath southern Peru and northern Chile the Nazca plate dips more steeply (ca. 30°), the upper plate is affected by predomi- nantly extensional tectonics acting perpendicularly to the trench, and the An dean calc-alkaline volcanic chain is active. It is thus conceivable that back-arc extension in the western United States was triggered by (1) an accelerated rate of plate convergence and (2) by steepening of the Farallon-Juan de Fuca plate approximately 20 m.y. ago, with the resultant initiation of an upper-plate stress field favoring ENE-WSW dilation in the Columbia Plateau and Basin and Range provinces. PERSPECTIVES 93 size `'why" we must understand the basic geometries and kinematics of ~ how" extension took place. flinch more field work is needed in the extensional terranes of the western continent to resolve these first-order problems. The recognition in only the past 8 years (beginning with Anderson, 1971) of an entire province in southern Ne- vada, southeastern California, and western and southern Arizona, of late Cenozoic low-angle nodal faults with an as-yet cryptic origin, is just one indication of how farfield studies hare lagged behind plate-tectonics speculations. The recently published study of the Yerington district of western Nevada by Proffett (1977) is an admirable exam- ple of how much new data and f~eld-based interpretation can come from detailed mapping studies in the western states. Proffett's conclusions were not based solely on field mapping but were supplemented by a total of 30,000 m of drill-hole data, cores, and cuttings. His work indi- cates the importance of subsurface infonnation in identi- f~ing the geometry of normal faulting within the Basin and Range province. The need for deep seismic reflection data in the province is obvious, particularly for resolving the problems of normal fault geometry at depth. And fi- nally, the possible existence of a newly discovered low- velocity zone In the upper crust ofthe eastern Great Basin is but one impressive indication of how much funda- mental geophysical data are still needed in order to un- derstand the physical state of the lithosphere in western North America. ACKNOWLE DGM E NTS J. L. Anderson, A. W. Bally, Gordon Eaton, Eric Frost, and Warren Hamilton kindly suggested improvements in several drafts of this paper. Portions of this summary con- cerning extensional tectonics in the Pacific Northwest were originally prepared for the Washington Public Power Supply System (wPPSs). David Tillson, of the wPPss, is thanked for his interest in and support of these studies. Research by the writer and his associates in the Whipple-Buckskin-Rawhide Mountains area described herein has I'een supported by the National Science Foundation, Earth Science Grants GA43309 and EAR77-09695. REFERENCES Anderson, R. E. (1971). Thin skin distension in Tertiary rocks of southeastern Nevada Ceol. Soc. Am. B?~11. 82, 4~58. It should be evident from this brief review of Cenozoic , _ A'..~stron~ R. L. (}972). Low-an~le (denudation) faults, hinter- intrap ate extension in the western United states t eat ~ _ land of the Sever orogen~c belt, eastern Nevada and western much work remains to be done and more data neec ! to he Utah, Geol. Soc. Am. Bull. 83, 192~1954. gathered before even the structural phenomena are well understood. The problems are not just why extension oc- cu~ed in the intraplate setting and which plate-tectonics explanations are most satisfying. Before we can hypothe- Annstrong, R. L., W. H. Taubeneck, and P. O. Hales ( 1977). Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho, Geol. Soc. Am. Bull. 88, 397011.

94 Atwater, T. ( 1970). Implications of plate tectonics for the Cenozoic evolution of western North Amenca, Geol. Soc. Am. Bull. 81, 351~6. Carr, W. ]., and D. D. Diclcey (1977). Cenozoic tectonics of eastern Modave Desert, U.S. Geol. Sure. Prof. Paper 1~0, 7S PP. Christiansen. R. L., and P. W. Lipman (1972). Cenozoic volcanism and plate-tectonic evolution of the western United States. II. Late Cenozoic, Phil. Trans. Roy. Soc. London A271, 24~284. Clague, J. J. (1974). The St. Eugene Formation and the de- velopment of the southern Rocky Mountain Trench, Can. ]. Earth Sci. 11, 916~38. Compton, R. R., V. R. Todd, R. E. Zar~nan, and C. W. Naeser (19T7~. Oligocene and Miocene metamorphism, folding and low-angle faulting in northwestern Utah, Geol. Soc. Am. Bull. 88, 1237-1250. Coney, P. ]. (1970). The geotectonic cycle and the new global tectonics, Geol. Soc. Am. Bull. 81, 73~747. Coney, P. J. (1974). Structural analysis of the Snalce Bange "decollement," east central Nevada, Geol. Soc. 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Abstr. Program* 3, 164~166. Proffett, I. M., Jr. (1977). Cenozoic geology ofthe Yenogton dis- trict, Nevada. and implications for the nature and origin of Basin and Range faulting, Geol. Soc. Am. Bull. 88, 247-266. Rehng, W. A., and S. J. Reynolds (1977). A northwest zone of metamorphic core complexes in Arizona, Ceol. Soc. Am. Abstr. Programs9, 1139. ReLng, W. A., and S. I. Reynolds (in press). Geologic and geo- chronologic reconnaissance of a northwest-trending zone of metamorphic complexes in southern Arizona, Ceol. Soc. Am. Mem. Reynolds, \£. W. (1974). Geology of the Grapevine ~fountains, Dead Valley, Califomia: a summary, in Guidebook: Death Valley Region, California and Nevada, Death Valley Publish- ing Co., Si}oshone, Calif., pp. 91-96. Roberts, R. J. (1972). Evolution of the Cordilleran fold belt, Ceol. Soc. Am. Bull. 83, 198~2004. Scholz, C. H., .~1. Barazangi, and M. L. Sbar (1971~. Late Ceno- zoic evolution of the Great Basin, western United States, as an ensialic interarc basin, Geol. Soc. Am. Bull. 82, 2979~2990. Shackelford, T. J. (1976~). Structural geology of the Rawhide Mountains, Mohave County, Arizona, U. Southern Calif., un- publ. PhD dissertation, 175 pp. Shackelford, T. J. (1976b). Juxtaposition of contrasting structural and lithologic terranes along a major Miocene gravity detach- ment surface, Rawhide Mtns., Arizona, Geol. Soc. Am. Abstr. Programs 8, 1099. Shackelford, T. J. (1977). Late Tertiary tectonic denudation of a \£esozoic(?) gneiss complex, Rawhide Mountains, Arizona. Geol. Soc. Am. Abstr. Programs 9, 1169.

Problems of Intraplate Extensional Tectonics, Western United States Smith, R. B. (1978). Seismicity, crustal structure and intraplate tectonics of the western Cordillera, in Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, R. B. Smith and G. P. Eaton, eds., Geol. Soc. Am. Mem. 152, 111-144. Smith, R. B., and M. L. Sbar (1974). Contemporary tectonics and seismicity of the western United States with emphasis on the Intermountain Seismic Belt, Geol. Soc. Am. Bull. 85, 120~1218. Snavely, P. D., Jr., N. S. NIacLeod, and H. C. Wagner (1973). Miocene tholeiitic basalt of coastal Oregon and Washington and their relation to coeval basalt of the Columbia Plateau, Geol. Soc. Am. Bull. 84, 387~24. Snyder, W. S., W. R. Dickinson, and M. L. Silbennan (1976). Tectonic implications of space-time paKems of Cenozoic vol- canism in the western United States, Earth Planet. Sci. Lett. 32, 91-106. Stauder, W. (1975). Subduction of the Nazca plate under Peru as evidenced by focal mechanism and by seismicity,J. Geophys. Res. go, 105~1064. Stewart, J. H. (1971). Basin and Range structure: a system of horsts and grabens produced by dee~seated extension, Geol. Soc. Am. Bull. 82, 101~1044. Stewart, J. H., G. W. Walker, and F. J. Kleinhampl (1975). Oregon-Nevada lineament, Geology 3, 26~268. 95 Swanson, D. A., and T. L. Wright (1977). The Columbia River basalt, product of a major thermal event in the western Cordil- lera, Geol. Assoc. Can. Program Abstr. 2, 50. Terry, A. H. ( 1972). The geology ofthe Whipple Mountains thrust fault, southeastern California, Calif. State U. at San Diego, unpubl. M.S. thesis, 90 pp. Thompson, G. A., and D. B. Bulge (1974). Regional geophysics of the Basin and Range Province, Ann. Rev. Earth Planet. Sci. 2, 21~238. Watkins, N. D., and A. K. Baksi (1974). Magnetostratigraphy and oroclinal folding of the Columbia River, Steens, and Owyhee basalts in Oregon, Washington, and Idaho, Am. J. Sc:. 274, 14~189. Wketten, J. T. (1976). Tertiary sedimentary rocks in the central part of the Chiwaukum graben, Washington, Geol. Soc. Am. Abstr. Programs 8, 420. Wright, L. A., and B. W. Troxel (1973). Shallow-fault interpreta- tion of Basin and Bange structure, southwestern Great Basin, in Gravity and Tectonics, K. A. De Jong and R. Scholton, eds., John Wiley, New York, pp. 397~07. Zobaclc, M. L., and G. A. Thompson (1978). Basin and Range riding in northern Nevada: clues from a mid-Miocene rifle and its subsequent offsets, Geology 6, 111-116.

Geophysical and Geological Characteristics of the Crust of the Basin and Range Province 9 GORDON P. EATON U.S. Geological Survey INTRODUCTI ON Selected geophysical and geological characteristics of the Basin and Range province of Me western United States are examined here. They represent the effects of nearly 30 million years (m.y.) of crustal extension, preceded by subcluction-related compressional deflation and magma genesis. The effects of this compressionaI regime appear to have exerted a significant influence on the me- chanics and locus of late Cenozoic extension. In its present state, the crust of We Basin and Range province is atypical of most continental crust, being ap- preciably thinner, warmer, and more highly fractured and permeable. Some of what is seen or measured geophysi- cally at We surface in the province is influenced by proc- esses or phenomena in the shallow angst. Faulting, for example, appears to be a condition of the upper 10 to 15 kin, and some of the phenomena associated with it sense to screen or distort infonnation from deeper levels. As an example, convection of groundwater in the fractured, shallow crust complicates the determination of heat flow from deeper crustal levels and is apparently `~so capable of creating regional geoelectric anomalies. 96 One of We remarkable features of the Basin and Range province is the broadly distributed nature of its extension. Most other regions of continental extension consist of singular or branching large ribs, like the contemporary, neighboring Rio Grande rift in New .Me.xico or the Rhine graben of Europe. Although marginal seas bordering con- tinents, like the Seas of Okhotsk or Japan [both of which are at present inactive with regard to cmstal spreading but nevertheless have high heat flow (Karig, 197411, have ~ similarity to the Basin and Range province, they have significant contrasts as well. The same is true for actively spreading intTaoceanic, back-arc basins, like the Bonin, Manana, New Hebndes, and Lau-Havre troughs. lithe Great Basin consists of continental crust, stands nearly 2 km above sea level, has a very broad, but well- developed, bilateral symmetry in its geophysical char- acteristics (Eaton et al., 1978), and has a perimeter marked by active cmstal seismicity and young volcanism. In contrast, marginal seas near the Asian continent and intraoceanic spreading basins consist of oceanic crust, the upper surface of which is submerged belong sea level. They show little or no distributed or peripheral crusty

Characteristics of the (?rust <if the Basin and Range Province !j~ismicity (~e Barazangi and Donnan 1969~. No uell- developed `~r e.xtensi`e pattern of symmetry nor any per- si~itent peripheral volcanism has been described f<,r them. ([though paired linear magnetic an<'malies exist locally, they are relatively incoherent compared with those at spreading ocean ridges. Ad their amplitudes are much smaller. Extension in the Great Basin began within and behind an andesitic volcanic arc, typical of where back-arc basins form and exist. Although much of that arc is now dead, extensional deformation continues. Finally, active conti- nental volcanic arcs persist today in the Cordillera of Central and South America but have no Basin-Range structure behind them. According to Uyeda and Kanamori (1979), this is probably the result of a compressionaI state of stress in the overriding American plate; it follows from a relatively low angle of subduction and rapid rate of plate convergence. A fundamental distinction is made between what they term a "Chilean," or convergence-related com- pressional mode, and a 4'Mananas," or convergence- related extensional mode, of subduction. Interpretations of crustal thickness of different parts of the western United States vary somewhat (compare Pro- dehl, 1970; Warren and [Iealey, 1973; and Smith, 1978), but it appears that Me Great Basin crust Is no more than 25 30 km Dick. The crust of Me rest of the Basin and Range province ranges from 20 to 30 km thick. By contrast, Me crust of Me unextended Colorado Plateaus and Great Plains provinces to Me east is 40 and 50 km thick, respec- tively. Upper-mantle velocities beneath the Great Basin are anomalously low (Smith, 1978) and the lithosphere is anomalously thin (c65 km). It is one of the few conti- nental areas of the globe beneath which a low-velocity zone is known to be present in the mantle. Not all of the Basin and Range province is tectonically active at present. The Great Basin section is extending, the western Modave Desert is in horizontal dextral shear, and Me rest ofthe province is tectonically inactive and has been for several millions of years. PRESENT AND PAST CRUSTAL EXTE NS ION Nonnal faults, the surface evidence of crustal extension, are widespread in the western United States. Their distri- bution in the Basin and Range province is broad and per- vasive, but in the Rio Grande riPc [see Figures 9.5(a) and 9.6 for identification of the principal geographic features mentioned in Me text] they define a narrow band typical of continental grabens. On the basis of Me geomorphology of the faulted region, crustal spreading is a continuing process, still active in some areas, but inactive in others. The Sonoran Desert section ofthe Basin and Range prov- ince has a general elevation and topography characteristic of profound erosion and tectonic inactivity (Fenneman, 1931; Lobeck, 1939; Hunt, 1967~. In sharp contrast, Qua- ternary faults of Nevada are amenable to detailed age 97 classification based on the degree of erosion, the slope angle of the scary, and the width of the crestal break in slope (Slemmons, 1967; Wallace, 19771. Figure 9.1 is a map ofthe contemporary fault-displace- ment field of the United States. Active normal-fault dis- placements are restricted almost exclusively to the Great Basin and Rio Grande rift. Local, scattered thrusting events in the West are seen to the northwest, east, and southwest, but displacements of this type are found mostly in the middle and eastern United States. The azimuths of He P (pressure) and T (tension) axes cannot be equated with those of He principal stress direc- tions, ~~ and ~3, with certainty. McKenzie (1969) demon- stTated Hat in the general case of Biaxial stress the only restriction on the spatial relation between the Ho is that the stress directions must lie in quadrants containing He related displacement axes but may diverge from them by many tens of degrees. He argued, as did Brace (1972a), that because most crustal earthquakes occur on pre- existing fault planes, they are unrepresentative of He ideal situation in which initial failure takes place in vir- gin, homogeneous material. Only in this case will stress directions necessarily bear direct correspondence to dis- placement. Because of the long history of normal faulting in the Great Basin we may assume that those earthquakes whose fault-plane solutions are shown in Figure 9.1 took place on pre-existing faults and that He orientation of these faults with respect to the stress field determined both the direction and nature of displacement. To investigate He potential discrepancy between dis- placement and stress direction, in site measured direc- tions of cr. for the eastern United States were plotted in Figure 9.1. Together, the two kinds of data suggest that on a regional basis P and A, have much the same onenta- tion—northeast to east. In a related plot (Figure 9.2), a comparison is made between locally paired, measured directions of T and ~3 in He West. Most of the directions of Cr3 determined from in situ stress measurements are not shown in Figure 9.1 because of He density of over date. They were taken from sources listed in the caption. The data selection process required only that the earthquake and in situ stress mea- surement locality in each pair be within 200 km of one another. Figure 9.2 indicates moderately good agreement be- tween extensional stresses and displacement directions, despite McKenzie's ( 1969) well-reasoned caution. On this limited basis we may assume that the displacement field ofthe normal faults is a measure of He direction of current extension. On average, it is east-west. The strike of these faults in relation to that of the displacements is, in much of the region, orthogonal or nearly so, hence the spreading is generally in consonance with that of continental spread- ing elsewhere in the world (Ranalli and Tanczyk, 1975) and with Hat of seafloor spreading (Moore, 19731 Figure 9.1 shows two other significant characteristics of He region of active continental spreading: (1) it stands high, having been uplifted to an elevation of 1100 m and

98 ~ 1100 meter elevation contour (1° average elevation) =7 Area of heat flow ~ 1.5 ~ cal cm 2 sec A GORDON P. EATON E X P L A ~ A ~ I O N · ~ Direction of T. normal fault mechanism Direction of P. thrust fault mechanism —~ Direction of ore . In sits stress measurement ~3 Battle Mountain heat flow high ~ Direction of ~3 . In sits stress measurement i,": ,:,:'2.! ~ . . , ._. ~ a. - , : , , -? 1%, ~ ! .,'' _~, in a. v - .,~ · 4-- ~ . _ ! I , 5~ - :.= . \ : ~ , .._, . J —. ~~ ._.. O 900 km ~ ~ 1 ~ I I ~ \ \.f I: 1~:: '.~- J. ~..: W~t i. en, }a FIGURE 9.1 Horizontal components of fault displacements and maximum principal stress directions. Compiled from data in Couch et al. ( 1976), Haimson (197 7), Jalcsha et at. (1977), Lachenbmch and Sass ( 1977), Malone et al. (1975), Mott ( 1976), Rogers ( 19T7) Sanford et al. (1977), Sbar and Sykes (1973, 1977), Smith (1978), Smith and Lindh (1978), and Strange and Woollard (1964~. Thrust faulting and horizontal compression dominate in Me eastem United States, nonnal faulting and extension, in the west. more; and (2) it is hot; heat-flow values nearly everywhere are greater than 1.1 local cm2 sect, the average value for stable continents. We return to these observations below. Figure 9.3(a) is a map of most of the major faults in western Norm America that have steep dips, regardless of age. The longer ones are stnke-slip faults, die shorter ones, normal faults. The latter are much more abundant and widespread. Active strike-slip faults that have major displacements of several tens of kilometers are today re- stncted largely to a 500 km-wide corridor paralleling Me west coast of the United Sates. Nose farmer inland, in Canada, have not been active in late Cenozoic time We period of crustal spreading wad which we are concemed. Because the azimuthal range ofthese faults is great, it is difficult to see well-defined patterns from which we might draw immediate conclusions about variations in spreading. To do this one must filter We data. Figure 9.3(b) is temporally filtered, showing only Nose faults active in the past 1~15 m.y. for wllich some Quatemary movement is suspected. It distinguishes the Great Basin section ofthe Basin and Range province from the Sonoran Desert section. It also clearly defines the Rio Grande nR. We fault data, like the geomorphic data, suggest that ex- tensional spreading in We Sonoran Desert region ended some time ago. Eberly and Stanley (1978) alleged that block faulting began to wane about 10.5 m.y. ago in We Sonoran Desert and that throughgoing drainage of We Gila River was well established by 6.0 m.y. ago. The ces- sabon of crustal spreading at these latitudes may be related to We opening of the Gulf of California on the west. If so, it represents die continental equivalent of ~ kind of ridge jum~from distributed extension on the east to narrowly focused extension on We west. Figure 9.4 shows that the orientation of basin ranges in

Charactenst~cs of the Crust of the Basin and Range Province <o 120 IX 40 20 ~ 1~ 120 a3 AZIMUTH, IN DEGREES a./ . ·/ ~ /~ 1 1 1 1 of . , . O 20 48 . 140 160 180 FIGURE 9.2 Azimuthal relations between horizontal compo nents of minimum principal stress (~3) and T axes oinormal-fault earthquake mechanisms. Localities in each set of paired measurements (stress measurement and fault displacement) were less than 200 km apart. Data from sources listed in caption of Figure 9.1. Solid line defines locus of stress directions and fault displacements having the same horizontal direction. the Sonoran Desert is significantly different from that of the Great Basin and Rio Grande nit. If the topography is a reflection of block faulting, this difference, in conjunc- tion with die data of Figures 9.1 and 9.2, suggests that extensional stresses had a markedly different orientation in the Sonoran Desert region dian Hey do in the Great Basin today. They were generally southwest to west- southwest, in contrast to later east-west to west-nor~- west extension in the rest of the western United States. THE THERMAL REGIME Heat lost at the surface of the earn may reflect any of several phenomena: (~) conduction of heat into ant! through the lithosphere, the base of which may approx- imate an isotl~ennal surface marking the upper bound of a region of partial melt in the mantle; (2) upward mass transport of heat through the lithosphere by ascending magma, a penetrative fonn of convection; (3) production of heat in the crust itself by radioactive decay of U. Th, and OK and/or by conversion from mechanical energy, as in faulting; (4) transient cooling ofthe lithosphere; and (5) convection of heat in the shallow crust by circulating groundwaters. The last phenomenon is especially trou- blesome in permeable rocks. It can greatly disturb the heat flow associated with deeper crystal regimes. Many heat-flow measurements have been made in the extended regions of We western United States (Lachen- bruch and Sass, 1977~. The average value is roughly twice that of the tectonically stable interior and eastern part of the North Amencan continent, after accounting for crustal 99 radioactivity. Although heat-flow values in the Pacific Northwest are also higher than those in the continental interior, the reduced values (those for which the contribu- tion of heat from heat procluction within the crust has been subtracted) appear to be somewhat lower than those of the Great Basin (compare Figures 1 and 14 of Lachen- bruch and Sass, 19771. At present, there are an insuf6~- cient number of heat-flow measurements in the Sonoran Desert to be certain of what its thermal regime is, but thennal lag may keep crustal temperatures moderately high if extension there ceased as recently as 7 m.y. ago. The Rio Grande rid has heat-flow values as high as those of the Great Basin (Reiter et al., 1975), and they decay systematically outward, much as they do at spreading ocean edges. Lachenbruch and Sass (1977; 1978) examined He thermal regime of the U.S. continental crust, evaluated venous factors affecting crustal temperature, and pro- posed a model in which the high, but vanable, heat flow in the Great Basin is accounted for by (1) regionally dis- tnbuted basaltic intrusions of the lithosphere, accom- panied by lithospheric thinning and possibly magmatic underplating and (2) spatial variations of He extensional strain rate that controls access of this basalt to the litho- sphere. The lithosphere is pulled apart and basalt wells up into it from the asthenosphere. Basaltic intrusions in this mode} are viewed as vertical, dike-like or bleb-like bodies that accommodate extension while preserving the continuity of die lithosphere. In- stead of pulling open at a single place, as it does at most spreading edges, the lithosphere pulls open in a broadly distributed fashion at a great many places. Spatial and temporal variations in rates of extension control the in- tensity of the upward flux of basalt and, hence, that of surface heat flow. The model is mechanically somewhat like one proposed by Thompson ( 1959; 1966), who postu- lated a Great Basin lithosphere dilated by intrusive dikes. It provides a rationale for the local occurrence of shallow magmatic systems that convey heat from the astheno- sphere to the surface by mass transport. CRUSTAL MAGMAS The Mesozoic and Cenozoic history of the western United States is one of repeated intrusion of the shallow crust by magrnas. Many broke through to the surface, creating large volcanic fields. Figure 9.5 shows the spatial distri- bution of igneous rocks of post-Paleozoic age. From earliest Mesozoic Trough early Miocene time, rocks of intermediate, calc-alkaline composition were formed, probably as a result of subduction of the Farallon plate beneath the region (Lipman et al., 1972; see Chapter 14~. From Miocene time on, however, they were dominantly bimodal (basaltic and rhyolitic) in composition, reflecting extension of the lithosphere that followed cessation of subduction (Christiansen and Lipman, 1972; Chnstiansen and McKee, 19781. Figure 9.5(a) shows He regional limits of Mesozoic igneous rocks and individual areas of outcrop

100 6 ,`' GORDON P. EATON - {a) O. , , . 1000 km IBM art_ _\ - _._ 1 ..~ FIGURE 9.3 (a) Steeply dipping faults in western North America (from King and Beikman, 1974; King, 1969; and Cohee, 1961). Faults are shown without regard to age of initiation or last movement. Longer ones are generally strike-slip faults; shorter ones, nonnal faults. Dashed lines in southern pant of map mark long axes of block ranges whose boundary faults are obscured by upper Cenozoic alluvium. (b) Faults active in the past 1~15 m.y. for which some Quatemary movement Is suspected (after Howard et al., 1978). of Paleocene Trough Oligocene igneous rocks. Clearly, the Great Basin had abundant igneous activity in Meso- zoic and early Tertiary fume. Igneous rocks of Miocene, Pliocene, and Quaternary age are shown in Figures 9.5(b), 9.5(c), and 9.5(d). An outward restriction of magmatic activity win time toward the margins of die Great Basin is illustrated by these Tree diagrams. Areas immediately adjacent to Me Great Basin had a generally similar history of magmatism, as die four illustrations show. Magmatism alone does not set the Great Basin or Rio Grande nit apart Tom Me rest of Me region. Figure 9.6 shows the spatial distribution of known Cenozoic plutons and ash flow-related calderas. The cal- deras represent large magma chambers at crustal levels shallow enough to permit catastrophic eruptions of great volumes of pyroclastic material Such magma chambers are in the upper 5 to 10 km of the crust and give up large amounts of heatthere. More than 50 of these features have been identified thus far in the Great Basin and die region immediately norm of it. Another 22 are seen in a broad nor~-sou~ combos that includes the Rio Grande nor and its western environs. The distribution of these shallow crustal heat sources coincides generally with that of normal faults active dur- ing the past 10 m.y. to 15 m.y. [Figure 9.3(b)~. Although the fanner span Me entire Cenozoic Era, Hey are spatially distributed more or less like the regions of young exten- sional faulting, suggesting some soft of genetic relation- ship. Mackin (1960) proposed direct cause and eject:

Characteristics of the Crust of the Basin and Range Province FIGURE 9.4 Rose diagrams of the orientations of range fronts. (a) 54S range-front segments, 50 km long, in the Great Basin section of the Basin and Range province and the Rio Grande rift (Nevada, Utah, and north- ern and central New Mexico); (b) 548 range-front segments, 50 km long, in the Xtojave Desert. Sonoran Desert, and Mexican H ighlands section of the Basin and Range pros ince (south- eastem Califomia, southern Arizona, and southern New Mexico). Note dif- ference in maximum Frequency values of the outermost rings of the two dia- grams and the greater scatter of range- front directions in (b). t8, withdrawal of large volumes of magma to the surface al- lowed collapse and spreading of the overlying slab. I do not subscribe to this view. HYDROTHERMAL CONVECTION Heated groundwaters appear at the surface as hot springs and warm springs, the surface manifestations of convect- ing hydrothermal systems. In some places they are related directly to shallow bodies of magma, as at Yellowstone National Park (Eaton et al., 1975; Smith and Shaw, 1975~. They mark sites where the regional conductive heat flow is perturbed. The areal distribution of thermal springs is shown in Figure 9.7(a), where boundaries have been drawn about regional clusters of springs. The guideline followed in drawing the boundaries required that indi- vidual members of a cluster should be no more than 100 km from one of its neighbors in Me same cluster. The hydrothermal systems in Figure 9.7(a) are active today, yet, with two exceptions, their distribution seems generally to mimic that of extensional faults of the last 15 m.y. [Figure 9.3(b)~. Areas of prominent exception are (1) a broad comdor along the San Anclreas Fault, in coastal California [see Lachenbruch and Sass (1973) for an expla- nation of the high-heat flow there I; and (2) a corridor along the Cascade Range of Washington, Oregon, and northern California, a belt of active, subduction-related andesitic volcanoes. These hot springs are thus associated with re- gions of (1) crystal extension and basaltic volcanism, (2) active calc-alkaline volcanism, and (3) transform plate mo- tion. Hydro~errnal systems in the extensional regime doubtless use the high regional permeability provided by faults and related fractures. Fossil equivalents of these hydrothermal systems are shown in Figure 9.7(b). They are sites of Tertiary and \tesozo~c (and some still older) epigenetic ore deposits, many of hydrothermal origin. To a limited extent their distribution may reflect accidents of erosion. In some re- gions, however, such as the Colorado Plateaus, subsurface data are abundant enough to indicate ~ general scarcity, if not presence, of such deposits on ~ regional scale, both 101 to' / ADO (bl S _ ~ - ,, / loo ,' 100 now and in the past. The boundaries drawn in Figure 9.7~) are repeated in Figure 9.7(b) to illustrate the fact that although some of the older hydrothe~'al systems are in locations quite different from those active today. ~ great many are in the same areas. The Great Basin hot springs, Hose north of the Snake River Plain (in central Idaho and southwestern Montana), and those in and near the Rio Grande riR (in Colorado and New NIe.xico) have ancestors in very much the same places, dating back tens of millions of years. Clearly, hydrothermal convection has been a feature of the shallow crest in these areas during times of both crustal compression and extension, whether of subduction-related or postsubduction origin. What can be said of the exceptions, those clusters of springs or areas of spring-absence identified in Figure 9.7(b) by letters? Significantly, the most numerous excep- tions are older deposits and hydrothermal systems that have not persisted to the present, rather than younger ones that sprang up anew, as they have in virgin areas B. G. and K. Deposits in areas C ant! D occur largely in oceanic crust accreted to the continent. Areas E and F are of special interest because they tell us something about significant contmsts within the Basin and Range province, such as He strong difference be- tween the Great Basin and Sonoran Desert sections. Hy- drotherrnal convection is dying out in the Sonoran Desert region, just as crustal extension has. Figures 9.~9.7 show that the Great Basin and Rio Grande rift have been perturbed thermally for a very long time and that crustal temperatures were high in these areas long before extension began. High temperature probably influenced extensional deformation from its out- set; at the very least it thennally weakened the cmst and thereby helped to determine where extension and thin- ning would ultimately take place. THE DISTRIBUTION OF EARTHQUAKES The spatial distribution of earthquakes in the western United States is shown in Figure 9.8. Many are concen- trated in a broad corridor parallel to the San Andreas Fault

102 GORDON P. EATON :'2: ~ \ N~ ~~ ', ,',, ','.', '-;, ,~~/,/ ! ~ , ~ ~ ~ ~ \ Ye- <~/ - -it/, /// j ~ _ L-\ I ~ ~ ~ \ :~ ~~V~/~'~' I are' ~ \ 1 ~ lit - -my - By 'I' - / 'off - ~ - Am/ ~A ? 44. ~ ~ `~ \ i ~ - < ~ i' ~ ~^ ~~ v ~ `' ~/ //, V A . ~ ___! t _ ~ ~ ~ ~ (a) \: TO ~~> (b) :: >. Rd~s in easy sad middle 11ioe~ time R\ Rdations In I leso20.c and ear' Tenor, time Pi ~,//~^~' )~~'`)'^'\~_ _ Rations In Upper Miocene and Pl~e time N _ , Relet,ons In Qwtem~ toot AN Specific antes of ma~matiSm [= btesezo~e ma~matc newtons ~ lda~mabc arc bonder of 8;ts~n Ott Range oro~nace (Fenoeman 1931) hlatmabc arc 40-20 m.r. ate ant northern Rio Grande rift (Chapin and Seater. I97S) 65 40 m.r. ago ~ -_ - Location of trench at comment Pete boundary o- ~ ~ ~ - 30undanes of vdun~c arcs (centinental regloss of calc alkaline ma~metism) FIGURE 9.5 Post-P eozolc magmatisrr.t in the western United States (data from Kin and Beikm g an. 19 ~ 47 Snyder et al., 19 / 6). Th p day border ofthe Basin and Range province, after Fer,neman (1931), with addition ofthe northern Rio Grande

Characteristics of the Crust of the Basin and Range Province ~# ~ . . At_ ~3'Co,'~ .',: ~ , - '1'' SOllORlN DESERT a ~ 'A ! . . ~ ~ ' Oo I \ \ .. _ ..~ ,~ I ~ al 1 \ ~1- 1 '% 1 \ _ O.` ..'— ~ r ~ ~ ~~ ~ ~ I 1 1 i i - *\ `,. I '1 o o\ R10 GRANGE Rln ' · _,4~ ° \: _, _ _ ·,_ _ ~ I - ~.q FIGURE 9.6 Shallow Cenozoic intrusive masses and ash-flow related calderas in the western United States (data from King and Beilanan, 1974; E. H. McKee, uses, unpublished data; T. A. Steven, Uses, unpublished data; and Eaton et al., 1978). Small irregular black areas and tiny dots are exposed intrusive rocks; open circles are sites of ash-flow related calderas. The calderas represent large bodies of magma at crustal levels shallow enough for their roofs to founder. They mark sites where shallow local crustal temperatures were, or still are, anomalously very high. The heavy line is Fenneman's (1931) boundary ofthe Basin and Range province, plus Chapin and Seager's (1975) boundary for the Rio Grande rift, the southem part of which is dashed. The dash~ot~ot line marks the midpoint of a steep, regional gravity gradient between the Great Basin and Sonoran Desert sections of the Basin and Range province (after Eaton et al., 1978). system in coastal California, but many are well inland, some as far as 1700 km from the coast. Atwater (1970) considered Me San Andreas to be the key element in the transform boundary between the Pacific and Norm Amer- ican plates. She viewed tectonic activity inland, at least to We eastern edge of the Great Basin, as reflecting deforma- tion in a broad, soft zone functioning as part of Mat boundary. The smoothly curving line in Figure 9.8 marks the in- land edge of the most abundant earthquakes. Although Smith (1978) used a lower threshold magnitude for plot- ting California earthquakes, the line I have drawn is, for the most part, within California, hence represents a real boundary. This line approximates the present inboard limit of pronounced nght-lateral shearing motion be- tween the Pacific and North American plates. East of this line, strike-slip first motions in the southern Great Basin are lef;c-lateral in displacement sense, with the active 103 nodal plane striking west to southwest (see the compila- tion of Smith and Lindh, 1978~. Strike-slip motion has not been recorded or observed in the eastern Great Basin, but in west-central Nevada, in a zone 100 to 150 km wide east of the boundary shown in Figure 9.8, oblique-slip faults with dextral components of shear are observed. The nature of displacements has been confirmed by fault-plane solution, by geodetic measure- ment, and by direct observation in the field (Thompson and Burke, 1974~. Slemmons (1967) observed strike-slip faults across the entire Nevada section of the Great Basin, but those east of He zone in question are of much smaller displacement, suggesting that the direct effects of dextral shear related to plate interaction die out inland in the western Great Basin. The total seismic-strain energy re- leased in the eastern two thirds of the Great Basin is less, in the aggregate, than that related to strike-slip faulting in the curve-bounded region to the west (Ryall et al., 1966; Crampin et al., 1976~. Figure 9.9 shows statistical and geographical variations in the focal depths of earthquakes. Hypocenters are dis- tributed from He near surface to a depth of 20 hen but are seldom found at depths greater than 15 km. In the eastern part of the Great Basin (histograms A, D, G. and H) ear~- quakes are limited to the upper 15 km of the crust and concentrated in the upper 10 km, with some of the modal values (e.g., sites D and G) in the upper ~ km. Sites D and G also show a systematic <lecrease in the numbers of earthquakes downward. Western Great Basin earthquakes (histograms I through N) show a tendency toward some- what greater depth, and at sites K and L there is a system- atic increase clownward, but, as just noted, this is an area of profound strike-slip faulting, hence the Reformational regime is clifferent. These depth characteristics provide clues to die nature of crusted extension. Because sudden instabilities that create ear*~qualces in the shallow crust are probably re- lated to stack-slip displacements on pre-existing faults (Brace, 1972a), one may interpret their depths as depths of instantaneous faulting. Extensional faulting accompanied by abrupt stress drop appears to continue in places scarcely deeper Man 10 km. If the faults themselves con- tinue to deeper levels their displacements are charac- terized by stable sliding or fault creep rather than stick- slip (Byerlee, 1968; Brace and Byerlee, 1968; Brace, 1972b). Almost certainly the extensional faults do not continue to deeper levels. Thompson (1966) and Hamilton and Myers (1966) pointed out that if the normal faults in the Great Basin continued as planes having dips like those observed at the surface to depths where facing pairs inter- sected, the fault blocks could be no thicker Man 1~15 km. Many students of the region, beginning with LongwelI (1933; 1945), have described features of these faults that suggest they flatten with depth (see Moore, 1960; Mackin, 1960; Hamblin, 1965; Anderson, 1971; Wright and Trox- el, 1973; and Proffett, 19 l 7), leading to fault blocks even thinner than 1~15 km. In addition, regional-gravity data ( Eaton et al., 1978) indicate that the local mountain ranges

104 (w31..N . \J i `~-N .~- ~- N - -~Q FIGURE 9.7 Active and fossil hydrothermal systems in the western United States Dato ~~ we (1967). (a) Active wane c~- _~ ~ . - - ---- - ---- --~~~~~ -- -~ ~-~-- GORDON P. EATON' Eat _ _. F ~ generally are not compensated isostatically, suggesting, in tum, that faults serving as surfaces of adjustment do not pass through the lithosphere. All of these data tend to suggest that the depth limit of earthquakes may be Me depth limit of faulting. Stewart (1978) illustrated altema- tive interpretations of how such faults may tenninate downward. Figure 9.10 compares the aggregate ear~quake~epth distribution for the entire region with Basin ant! flange widths. The two histograms are generally similar in fonn, bow having intermediate values between O and 5 km peaking between 5 and 10 km, and having relative low levels of occurrence for values greater than 20 km. Of We earthquakes, 97 percent occur in the upper 15 km of the crust, which in We Great Basin Is We upper half of We crust. Of Basin and Range block widths, 88 percent are no wider than 20 km. Theoretical and experimental analyses of the depth and spacing of fractures torTned in extension (both tensile fractures and extensional shear fractures) suggest that the two values should be similar at least within an order of magnitude (LachenbIuck, 1961 Sowers, 19721. In a mechanical analysis of block gliding in which horsts and grabens formed, Voight (1973) denved an am proximate equation for the width of a block formed by extensional faulting of an initially continuous slab. It is or wretch are discussed in text W = 2T(45° - ¢/2), where W is the width of the block, T the thickness of the faulted slab; and ¢, the coefficient of Internal *iction for effective stresses. Application of this equation to the Basin and Range province requires We assumption of a surface or zone of translators sliding at the base of the fragmenting upper crust. It will be shown below that the Great Basin crust may have such a zone. In order to solve Voight's equation we must have a value for ¢. Byerlee's (1968) experimental study of the bnttle~uc~le transition in rocks indicated that Fiction is Independent of composition. Rocks under confining pres- sures of from O to 5.2 kilobars (~e depth equivalent of 0 to 16~5 km) revealed a remarkably systematic variation between increasing nonnal stress (a) and shear stress (T) for friction. The limiting slopes, eta = tan ¢, of Byerlee's finchon data curve are 36° and 46°. Substitution of these values in Voight's equation yields tl~e following wimps for fault blocks fanned in this manner: for shallow crystal slabs initially 10 km thick, widths of 8.1-10.2 km, for slabs 15 km thick, 12.1-15.3 km; and for slabs 20 km Wick 16.~20.4 km. These results are in good agreement win the observed Basin and Range block width~arthquake depth relation (Figure 9.10), hence Voight's model of ex- tensional sliding may be judged to have potential rele- vance to an understanding of the mechanics of Basin- Range faulting.

Characteristics of the Crust of the Basin and Range Province Thompson (1959; 1966) was the first to suggest that extension in the deeper bas~n-range crust takes place via plastic stretching or injection of dikes. Hamilton and Myers (1966), Stewart (1971), and Proffett (1977) all ac- cepted the first of these concepts, viewing Basin and Range structure as fragmentation of ~ shallow crustal slab riding on a plastically extending substratum. Lateral dila- tion of We lithosphere by magma from below is implicit in the thermomechan~1 model of Lachenbruch and Sass (1978~. \\right and Trowel (19~.3) c`~11ed upon both mechanisms (plastic stretching and intrusion) to extend Me deeper crust beneath Me fault-fragmenting surface slab of Me western Great Basin. NORMAL FAULTING AND BASAL SLIDING Laboratory-scale models of extensional faulting Mat use sand or dry mortar as Me deforming media (Hubbert, FIGURE 9.8 Seismicity of the west- em United States (from Smith, 1978). Lower threshold magnitudes were used in plotting California data. Heavy line, based on seismic data, marks in- board limit of highest earthquake event frequency and areal density in California, high cumulative seismic- strain energy release, and major strike- slip faulting related to dextral shear of the western plate boundary in Ho- locene time. Major faults within 150 km east of this line show oblique slip, with active stnke-slip components, but farther east, the dominant mechanism is simple extension. (Reprinted from Geological Society of America Memoir 152, with permission.) 105 1951; Stewart, 19~1) yield structures similar to single, simple grabens and distributed arrays of alternating grabens and horsts. Stewart's model, which was designed specifically to resemble Great Basin structure, had a sig- nificant feature—~ constructed surface of translatory slid- ing at its base. In order to predestine the spacing and plan of individual horsts and grabens, Stewart placed seg- mented sheets of paper beneath dry mortar. These sheets constituted a basal dislocation between the locally frag- menting mortar above and a sheet of uniformly extending rubber (the model's analog of a plastic substrate) beneath. Translatory sliding took place between the paper and the rubber sheet. In Hubbert's (1951) model, horizontal slid- ing took place at the base of the sand section, and it is not difficult to imagine striations developing on the floor of the deformation lynx parallel to the direction `'f e.xtensi<,n af;rer repeated runs. In a real earth, such a dislocation could take one of two forms: (1) a simple surface of sliding or (2) a Win zone of . . . . . . .. O _ .. , ~ . .. -I. . . · . . · - - . . _ . . . .:' ~ . . . . - . lo. . · . . ._ . C'~t' - -,\- ~= .~:_ a. · t~\ · . .. J A. . ..CC id- _ ,dz., · _ ~ · .: .- . · ·: · . . en do .. ' - - ~ EF~C£NTER MAP Of WEST thy UNITED STARS . ; . . . 45 ; . . ·. Cat . . . . .: By- 4~ . .... .

106 ~ ~ _ DI '°~ W--~ ~ , ~ i- ~ I,lo. ~ /20 E ~.° 0 , ~ 0~~ °r 1° '50 , 20 1 J 0 so on 10~ 'art 20^ Q so FIGURE 9.9 Histograms of ear~qualce focal depths (in kilometers) for 14 areas within the region of active extension in the western United States. Data Mom Gumper and Scholz (1971); Jaksha et al. (1977); Ryall and Savage (1969); Shuleski et al. (197 7); and Smith (19783. listributed shear or decollement. A subhonzontal striated surface, a thin zone ofmylonite, or a thick layerofdynamo- thennally metamorphosed rock might Bus be anticipated, depending on kept, effective pressure, temperature, composition, and shear stress. The Turnagain Heights translatory slide in Alaska may be cited as an example of such clefonnation. It was de- scnbed and illustrated by Hansen (1965) and mechani- cally analyzed by Voight (19731. It mimics He Basin and Range province in structure. The ongina1 Sickness of He Augmenting slab was only 20 m, hence Here was little tendency for normal faults to flatten appreciably wig depth. Extension and sliding of He originally intact sur- face slab was possible because of an absence of lateral support on one side and a perceptible (2.2°) slope of the basal surface. Genetically, the structure was a gravity slide. It resulted from a loss in strength of material in He vicinity of He sliding surface as a result of excitation by a major earthquake. Owing to the backward retreat of its headwall, as more and more of the fragmenting surface slab slid away laterally, Voight termed He feature a "ret- rogressive block-glide." I do not suggest Hat the driving force of Basin-Range faulting is the same, only that kine- matics and resultant structures are grossly similar and, for this reason, instructive. Evidence suggests that the Great Basin may be growing laterally, i.e., that it may be consuming neighboring re- gions on both west and east (Smith et al., 1976; Eaton et al., 1978~. Both margins show transitional regions Hat GORDON P. EATON have geophysical anomalies characteristic of the Great Basin extending tens of kilometers into the neighboring provinces (see Ryall and Stuart, 1963; Shucy et al., 1973; Smith and Sbar, 1974; Keller et al., 1975; and Eaton et al., 1978~. The eastern margin of the Basin and Range prov- ince aIso has geological charactenshcs suggestive of tran- sition (Best and Hamblin, 1978; Howard et al., 1978; Luedke and Smith, 1978~. This state may relate, at least superficially, to the retrogressive aspect of He Turnagain Heights slicle. If so, the Simian and Wasatch fronts (opposed headwalls) may be retreating from each over as a result of plastic stretching at depth, Bus effecting a grown of He province at He expense of adjoining re- "ions. This could account for the obse~vecl outward re- striction of magrnatism with time. Uplift in the regions of these headwalls is probably a thermal phenomenon that is essentially contemporaneous wig the extensional fault- ing itself, just as it is in He oceans. Although heat flow in the Sier a Nevada is anomalously low, it must, in part, reflect the appreciable thermal time constant of He crust, for He mass deficiency characteristic of the Great Basin continues beneath the Sierra Nevadla (Eaton et al., 1978~. The Great Basin has a well-developed bilateral sym- met~y in certain aspects of its geology, but far more ob- viously in its geophysical fields (Proffett, 1977; Eaton et al., 1978~. In Proffett's model the western half of He shal- low, fragmenting, causal slab translates eastward relative to the extending substrate beneath (~e middle and lower crust), and Hat of the eastem half, westward. According to Voight (1973) retrogression cannot take place if die glicle blocks are need (as opposed to internally deformable) unless fluid pressures within fractures are sufficiently him to perform We fi'nchon of plastic wedges. Basin ~ Range blocks are sufficiently f~ctu red at We space ~ suggest internal deformation. We signifi- cant defonnationa] model thus appears to be lateral spreading win deformable block gliding. GEOPHYSICALLY ANOMALOUS LAYERS IN THE SHALLOW CRUST The possibility of a surface of sliding or a zone of ductile flow (mylonite or over metamorphic rocks) beneath He fault-fragmented surface slab of the Basin and Range province raises He question of Heir detectability by geo- physical means. We examine this issue only briefly, but in the last section of the chapter offer a tentative crustal model, based primarily on geology, heat flow, and ear~- quake dam, to which the other geophysical observations are fit by }~ypod~esis. The hypothesis needs specific test- ing. In He past decade and a half an increasing number of reports of a seismic low-velocity layer in He shallow crust have been published. One example is In He eastern Great Basin, near its boundary win the Colorado Plateaus (Mueller and ~ndisman, 1971; Landisman et al., 1971; Braile et al., 1974; Keller et al., 1975; Smith et al., 1975;

Characteristics of the Cmst of the Basin and Range Province and Braile, 19771. Because such a feature is also asso- ciated with the Rhine graben (Landisman et al., 1971) it is tempting to conclude that it is a feature characteristic of extensional regimes. Shurbet and Cebull (1971) suggested that the crustal low-velocity layer in the Great Basin is a zone of de- creased rigidity that provides a means of absorbing the displacements of Basin-Range faults. According to them, the top of this zone is at levels of 5 km or so, and the base, at ~9 Icm. Braile et al. ( 1974) suggested that surface exten- sion by normal faulting at the surface is absorbed in a soft, plastically extending region of lowered seismic velocity. Braile (1977) later published the following additional information: (1) the layer has a compressional wave veloc- ity perhaps as low as 5.5 km sect (compared with veloci- ties above and below of 6.0 and 6.5 km see-i, respectively); and (2) it has a heightened Poisson's ratio; (3) an anoma- lously low Q (quality factor) for the transmission of com- pressional seismic waves; and (4) a top at 9.5 km and base at 15 km. Although the depth and thickness of this layer are different in the Shurbet and Cebul1 model, the values were derived in very different ways. The figures of Braile (1977) are preferred. Both sets of authors agree on the existence and gross mechanical properties of the layer, and on its role in accommodating extension at the surface. Neat as this picture is, it is marred by the fact that crustal low-velocity layers are not peculiar to extensional re- gimes. Two collections of papers on the physical properties and conditions of the continental crust (Heacock, 1971, 1977) reveal that the phenomenon is widespread, found in areas of young extension as well as in stable Precambrian shield areas (Berry and Mair, 1977~. Such features have been observed in the crust of all tl~e continents except Antarctica. Mueller (1977) has incorporated it as a key element in a generalized model of the continental crust. According to him, it is found fairly consistently at depths of ~15 km. Some investigators, however, place it as deep as 20 lam, e.g., Landisman and Chaipayungpun (1977~. Its origin has been ascribed to high temperatures (Smith et al., 1975), to Me presence of a zone of granitic intrusions (Mueller, 1977~, to high pore-fluid pressures (Berry and Mair, 1977), or to some combination of any or all of these factors (Mueller, 1977~. Laboratory experiments (Nur and Simmons, 1969; Todd and Simmons, 1972; and Brace, 1972b) demonstrate that as pore pressures rise toward li~ostatic values (thereby reducing effective pressure) seismic velocities fall toward those observed at exceedingly shallow levels in the crust. If a consensus as to the origin of the crystal seismic low- velocity layer is emerging, it is high pore pressure. In the Great Basin, high regional heat flow, which implies high crusty temperatures, probably plays an important sup- portive role. Pore water at high pressures in a closed system is capa- ble of lowering seismic velocity, Q values, and rock strength, conditions that appear to occur in the Great Basin crust. As Berry and Mair (1977) point out, however, 0 — s — 10 — 15— ye — 20 — 25 — 30 — 35— (A) 40— 10— IS— At 20— of 2S— 30— 35— (b) a_ 107 0 10 fREQUEl~Y. IN PEACES 20 30 So so 0 10 fREQUEI=. ~ ~~ 20 30 40 50 · I l l l ~ ~ , ~ ~ , . . . FIGURE 9.10 Earthquake focal depths and widths of Basin and Range blocks. (a) Histogram offocal depths of 2,4~o earthquakes in the region of extension; (b) widths of individual basins and ranges in the Great Basin scaled from the map of King and Beik- man (1974). Both characteristic dimensions (depth and width) show modal values in the range ~10 km, with most values (>85 percent) in the range ~20 km.

108 this explanation requires that rocks in the layer in ques- tion have finite porosities at depths of ~15 km, while those in the zone immediately above it must be free of permeability because the hydraulically pressured layer must have some sort of impermeable cap. This cap in the Great Basin may be a layer of rock extended by ductile flow, the one whose upper surface may mark the base of the region of brittle faulting. If so, the seismic low- velocity zone may not coincide with this layer but is per- haps beneath it. Depths to surfaces or zones of Tertiary sliding in the Great Basin have been estimated by Arm- strong (1972) to be at least 8 km on geological grounds, but Me Wick, ductile zones must be deeper still, because in many places the glide faults do not rest directly on ductile rocks. Because depths of somewhat more than 8 km are in reasonable agreement with Braile's (1977) seismic es- timate of 9.5 km to the top of We seismic low-velocity layer, I tentatively regard the ductile layer (if it tally exists) as a possible cap. Pore fluids could be trapped at high pressure in fractures in rock immediately beneath such a ductile layer. Internal displacements or structural adjustments within rocks at this level would take place by stable sliding (Brace, 1972a; 1972b), and earthquakes would not be common at such depths. As we have already seen, they are not. Pore water in closed-rock systems will also lower elec- trical resistivity, as will increasing temperature' which elevates the ionic mobility of such fluids. At very him temperatures the onset of partial melting could do much the same thing; the melting temperature of the rocks is lowered by the presence of water. For these reasons one might anticipate the presence of electrical conductors in the Great Basin crust, and, in fact, they are observed. Some investigators (e.g., Landisman and Cha~payung- pun, 1977; Lienert ant! Bennett, 1977) have equated the crustal low-velocity layer with a low resistivity layer in tectonically active or high heat-flow areas. Much remains to be done to substantiate this equivalence, and also to establish equivalence between a subhonzontal low- velocity layer in the crust and a porous zone capped by a stratum of ductile impermeable rock. The data in hand are permissive, but thus far hardly conclusive. The electrical data are reviewed briefly to provicle an idea of what is currently known. I am indebted to my fnend and col- league, I. N. TowIe, for providing the information sum- mary that follows. Schmucker's (1970) geomagnetic variometer investiga- tions in California indicated the presence of an electrical conductor in the western Cordillera at the eastern base of the Sierra Nevacla, at and near its boundary with the Great Basin. Stanley et al. ( 1976b) identified a shallow (2~7 'lan), highly conductive layer in the crust beneath the Carson sink in western Nevada by means of magnetotellunc soundings. Stanley et al. (1976a) also studied the elec- frical structure of the Long Valley geothermal system in the western Great Basin by means of direct current and electromagnetic techniques, concluding that hydro- ~ertnal activity is reflected in discrete conductive zones GORDON P. EATON in the crust, which are controlled, in turn, by regional faulting. Lienert and Bennett (1977) have identified ~ crystal conductor in the western Great Basin at a depth of 90 lam using controlled-source geomagnetic vanomet~y. Reitzel et al. (19703 and Porath and Cough (1971) ob- served generally reduced vertical geomagnetic field vari- ations in the eastern Great Basin Mat Hey interpreted as reflecting a shoaling of Me mantle. Ambiguities in Weir interpretation of crustal thickness will doubtless be re- solved as the evidence mounts both for a shallow, strongly conduchng, crustal layer in the Great Basin as a whole and for local shoaling of the asthenosphere. Studies by W. D. Stanley and colleagues at the U.S. Geological Survey (personal communication, 19~8) have revealed the presence of ~ conductive crustal laser near the boundary between the Great Basin and Snake River Plain on the norm. Depths to its top range from 2 to 10 km; its thickness may be as great as 10 km. Stanley et al. (1977) have also identified a conductor beneath the Snake River Plain region at depths of only 5 lam in the Yellowstone caldera, but deepening to 20 km on Me southwest. In the vicinity of the Raft River geothermal area, in the northeastern Great Basin, it is 7 km deep. This conductor may be related directly to the presence of magma, at least in the Yellowstone area, and, therefore, may or may not be directly related to the crustal low- velocity layer under discussion. On the basis of these limited data it appears that an electrically conductive layer is a common feature of He shallow Great Basin cmst. Depth estimates place its top between 2 and 20 kin, and in several areas, at less than 10 km. Possibly this conductor coincides with the crustal low-velocity layer, but too little is known about it to be certain. Coincidence might be anticipated simply be- cause some of the factors that lower seismic velocity also raise electrical conductivity (high temperature, high porosity, the presence of a pore fluid, or the presence of a silicate melt). An electrically conductive layer by itself does not require abnormally high pore pressures and, hence, does not require the presence of an impermeable cap to keep the system closed. The presence of conduc- tive minerals such as metallic sulfides or those having high ion-exchange capacity, like clays or zeolites, can also lower rock resistivities without affecting seismic velocity. The low-resistivity layer in the crust could just as well be above the low-velocity layer, reflecting some combination of high porosity, temperature, pore-fluid salinity, or hy- drothertnal alteration in the lower part of the shallow crust. CRUSTAL MODEL FOR THE BASIN AND RANGE PROVINCE: A SUMMARY AND INTERPRETATION The crust of the Great Basin section of the Basin and Range province (and its immediate environs) is higher in elevation, thinner, warmer, more highly fractured. and

Characteristics of the Crust of the Basin and Range Province more well endowed with hot springs than that of sur- rounding regions, excluding the area immediately north of Me Snake River Plain. The fractures (mostly faults) extend a third of the way to halfway through Me crust. They are loci of abundant shallow earthquakes and vigor- ous hydrothermal circulation. Cnlstal extension takes place by faulting near the surface, but probably takes place by other modes at depth, most likely by dike intru- sion and stretching of Me lower crust and lithospheric mantle, and by ductile shear flow (distributed decolle- ment) in a relatively thin layer at some intermediate level. Repeated magmatic invasions of the crust have taken place during the past 100 million years. Some of these magmas broke the surface, but some have come to rest within the crust' giving up their heat Mere. Shallow mag- matic systems serve to drive hydro~ennal convection in the shallow crust, as does high regional heat flow from Me deeper crust. The phenomena have been long lived. At present, extension is taking place in an east-west or west-northwest direction; earlier, it was directed sou~- west or west-southwest (Eaton et al., 1978; Zoback and Thompson, 1978~. The Sonoran Desert was included in the initial episode of extension but is not included in the present one. Because the Sonoran Desert is deeply eroded, it exposes Me effects of crustal extension at deeper levels. A large part of the Sonoran Desert in south- western Arizona reveals evidence of subhonzontal, unidi- rectional plastic strain of middle Tertiary age (Davis, 1977; Davis et al., 1977; Davis, see Chapter 8; Rehrig and Reynolds, 1977) that initially developed before block faulting began but that may have served as the base of We faulted' shallow slab. The mylonitization and metamor- phism probably took place at lithostatic pressures of several kilobars. Armstrong ( 1972) reviewed evidence of transIatory dis- placements in the Basin and Range province and argued Mat some of the subhonzontal surfaces of sliding in the Great Basin are certainly Tertiary in age, Mat many of them may be Tertiary, and that they are more likely related to Basin and Range faulting Man to the Sevier orogenic event' the youngest episode of pre-extension thrusting. Most of these dislocations place younger strata over older. He noted that some of the structures are of relatively deep-seated origin (at least 8 km). It is possible that these subhorizontal zones of sliding first developed as thrust soles during crustal compression, later to evolve into extensional decollements. These observations and speculations lend themselves to the interpretation that prolonged thermal conditioning of die crust plus hori- zontal shearing simply may have continued earlier ini- tiated dynamotherrnal metamorphism of rocks Mat now serge as a boundary layer between parts of Me lithosphere extending by fundamentally different mechanisms. As young nominal faults developed near the surface in the regime of crustal extension, they became Iistric to (they came to sole on) older thrust zones. The mechanical model of Kehle (1970), in which ~ decollement is distributed through the middle (relatively 109 m`'re ductile) leaver Elf `~ cn~.st`~1 fir lith`~;pheric triad. my I'e applicable. Shearing in such a layer would lead to the mechanical generation of heat. Its magnitude w ould be controlled by the rate of shearing, which, judging from rates of extension measured at the surface, should be lower than that generated along the San Undress Fault (see Lachenbluch and Sass, 19 `31. S`'me part <'f ~e high heat loss in the Great Basin could be due to shearing, however. The greater part has been ascribed to penetra- tive convection of Me lithosphere by basaltic magma (Lachenbruch and Sass, 1978~. Part is ascribable to con- vective groundwater circulation in the shallow crust and locally, to young, hot, volcanic systems residing in the upper crust. If this model, based largely on geological, heat flow, and earthquake data, is generally correct, it has implications for the surface patterns of deformation, the regional distribution of geological resources, and some of the effects of earthquakes. The model is shown in sche- matic fore` in Figure 9.11. To summarize its implications: 1. The location and extent of the Basin and Range prov- ince may have been largely predetermined by Me loca- tion and extent of early Tertiary and Mesozoic magmatism that preheated (thermally weakened) the crust and augmented a regime of compressiona1 thrusting in which subhonzontal dislocation surfaces fir ductile zones (dis- tributed decoIlements) first developed at middle to shal- low crustal levels. Such zones could be used later as basal dislocations for normal faulting and might also serve (at depth) as crustal membranes impermeable to the deeper circulation of groundwaters but allowing the upward passage of magma by intrusion. Normal faults at the sur- face probably are listric to Me deepest of these zones, inasmuch as the shallowest ones are exposed in Me uplifted fault blocks themselves. 2. Zones of translatory, ductile shear would constitute near-horizontal surfaces of mechanical decoupling or in- e~cient coupling within Me crust. As a result, deforrna- tions en cl kinematic motions in the lower crust would not always be clearly or faithfully reproduced at We surface. 3. The regional maintenance of long-continued high temperatures and high permeability assures the continua- tion of vigorous hydrothermal circulation and attendant epigenetic deposition of minerals through both compres- sional and extensional regimes. They may' on the other hand, be responsible for what appears to be a regional scarcity of oil and gas in the Basin and Range province. Where unfavorably situated, such fluids could be driven to the surface, where they could escape, except for local conditions of entrapment. For the same reason, considera- tion of Great Basin sites for the isolation and storage of radioactive wastes cames with it Me requirement of a critical evaluation of the local hydrologic and seismo- tectonic regime. 4. Seismic energy traversing the crust of the extended region is probably absorbed to a somewhat greater degree Man it is in the relatively less intensely fractured, shallow crust of the central and eastern United States, hence the

110 GORDON P. EATON 1~.'~ If. i,,, At' W: I L-1 ,~L 1 .-...,,__.' L-3 L-4 FIGURE 9.11 Interpretive model of possible crustal structure dynamothermally metamorphosed Miocene and older rocks of a of the Great Basin (simplified, schematic, and not to scale); based wide varied of original compositions (medium stippling). At one on surface geology, heat flow, and earthquake distribution. (See extreme, the layer is a vanishingly thin stratum of mylonite, 1-10 Stewart, 1978, for alternative interpretations of the near-surface mm thick; at the other, a layer of granitic augen gneiss, schist, or structure.) The crust is composed of three layers (L-1, L-2, L-3) amphibolite, 1-3 km thick. This layer is locally or regionally having different lithologies and physical properties. Each fails or lineated and extends by laminar plastic flow. It is generally im- yields in extension by a different physical mode. Erosion in the permeable to groundwater circulation except where later Sonoran Desert region generally has cut down to the level of L-2. uplifted and fractured in the brittle regime, but at depth it may be LO is lithospheric mantle. Characteristics of these layers are as cut by dikes of Tertiary igneous rocks (solid block). It developed follows: L-1, Fault-fragmented, surface layer, ~15 km thick, first as a regional thrust sole (heavy dashed line) during earlier composed of rocks of a great variety or origins, compositions, and crustal compression. L-3, lower crustal layer, 10-20 km thick. ages, all exposed at the surface somewhere in the region; diagram composed near its top of igneous and metamorphic basement shows Cenozoic continental sedimentary and volcanic rocks at rocks like those of layer ~l but grading downward into increas- the surface (patterns of dense stippling and solid black, ing proportions of old granites, migmatites, gneisses, amphi- respectively)overlyingoldersedimentaryrocks(openstippling) bolites, and felsic to mafic granulites, in approximately that and granitic and metamorphic basement rocks (plain white). Al- order. This layer extends by a combination of diking (by basalts though the diagram does not show it, stratified rocks extend well from the asthenosphere, solid black) and solid-state convection into L-2 and probably into L-3 (as in Arizona). All of these rocks (stretching and underplating). It is rigid at relatively high and are highly fractured, as indicated by the plexus of fine, irregular intermediate strain rates. These modes of penetrative convection lines. The layer fails in semibriKle fashion by normal faulting, are responsible for the mass transport of heat from the deepe fault-block rotation, pervasive fracturing, and slumping. The mantle, causing the anomalously high heat flow observed in the deformation creates high fracture porosity and permeability, al- province. The seismic low-velocity zone may coincide with the lowing convechve circulation of groundwater (curved arrows) uppermost part of this layer as a result of anomalously high pore driven both by the high heat flow from the deeper crust and by pressure in a system capped by the impermeable layer, L-2, and local, young intrusions (black, dike-like bodies). The upper part high temperature. Let, lithospheric mantle. 25-35 km thick, of the crustal low-resistivity layer may coincide with the lower composed of ultramaf~c rock devoid of finely disseminated melt. part of this layer. The base of the layer generally marks the man- This layer, like the lower crustal layer above it, extends by diking imum depth of earthquakes. L-2 ductile intermediate layer, 0-3 (perhaps via rising, bleb-like bodies) and solid-state convection. km thick, composed of pervasively sheared, mylonitized, and/or It is immediately underlain by asthenospheric mantle.

Characteristics of the Crest of the Basin arid Range Province geographical extent of isoseismal boundaries for an ear~- quake of given magnitude is generally less in the West than it is in the M~clwest ant! East. ACKNOWLEDGM ENTS Preparation of this review was aided substantially by dis- cussions win, and/or data con~ibudons from, M. D. Cnt- tenden, fir., G. A. Davis, Spiel Friedman, W. Hamilton, A. H. Lachenbruch, P. W. Lipman, E. H. McKee, S. S. Oriel, A. R. Sanford, B. B. Smith, T. A. Steven, and J. N. Towle. The manuscript benefited from constructive re- views by K. A. Howard and H. M. Iyer. None of the above should be regarded as subscribing to the interpretive crustal model presented here, however, nor to its stated implications. I am indebted to members of the technical staffofthe Hawaiian Volcano Observatory, Ron Hanatani, Rick Hazlett, Jenny Nakata, and Maunce Sako, as well as summer student aide Kevin Cuff for graphical comp~la- tion of many of the regional geological and geophysical data presented here. REFERENCES Anderson, R. E. (1971). Thin skin distension in Tertiary rocks of southeastern Nevada, Geol. Soc. Am. Bull. 82, 43~8. Armstrong, R. L. (1972). Low-angle (denudation) faults, hinter- land of the Sevier erogenic belt, eastern Nevada and western Utah, Geol. Soc. Am. Bull. 83, 172~1754. Atwater, T. ( 1970). Implications of plate tectonics for the Cenozic tectonic evolution of western Now America, Ceol. Soc. Am. B?~11. 81, 3513~536. Ba~angi, M., and J. Dorrnan (1969). World seismicity maps compiled from ESSA, Coast and Geodetic Survey, epicenter data, 1961-1967, Bull. Seismol. Soc. Am. 59, 369 380. Berry, M. J., and J. A. Mair ( 1977). 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Late Cenozoic vol- canic and tectonic evolution of d~e Great Basin and Columbia intermountain region, in Cenozoic Tectonics and Regional Geophysics of He Western Cordillera, R. B. Smith and G. P. Eaton, eds., Geol. Soc. Am. Mem. 152, 2~312. Cohee, G. V. (compiler) (1961). Tectonic map of the United States, exclusive of Alaska and Hawaii, U.S. Geol. Surv. and Am. Assoc. Petrol. Geol., scale, 1:2,500,000. Couch, R. W., G. Thrasher, and K. Keeling ( 1976). The Deschutes Valley ear~qualce of April 12, 1976, Ore Bin 38, 151-161. Crampin, S., C. J. Eyfe, D. P. Bickmore, and R. H. W. Linton (1976). Atlas of seismic activity 1909 to 1968, Inst. Geol. Sci. Seismol. Bull. 5, 1-29. Davis, G. A., K. V. Evans, E. G. Frost, S. H. Lingrey, and T. 1. Shackleford (1977). Enigmatic Miocene low angle faulting, southeastern California and west-central Anzona: suprastluc- tural tectonics?, Geol. Soc. Am. Abstr. Programs 9, 943. Davis, G. H. (1977). 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Microseismicity and tec- tonics of the Nevada seismic zone, Bull. Seismol. Soc. Am. 61, 141~1432. Haimson, B. C. (1977). Crustal stress in He continental United States as derived from hydrofrac~nng tests, in The Earth's Crust, J. G. Heacoek, ea., Am. Geophys. Union Geophys. Monogr. 20, pp. 576 592. Harnblin, W. K. (1965). Origin of reverse drag" on the down- thrown side of normal faults, Ceol. Soc. Am. Bull. ~ 6, 114~1164. Hamilton, W., and W. B. Myers ( 1966). Cenozoic tectonics of die western United States, Ret;. Geophys. 4, 5~549. Hansen, W. R. (1965). Effects of doe earthquake of March 27, 1964, at Anchorage, Alaska, in The Alaska Earthquake, March 27, 1964~Effects on Communities, U.S. Geol. Sure;. Prof. Paper 542-A, 68 pp. Heacock, J. G., ed. (1971). The Structure and Physical Properties of the Earth's Crust, Am. Geophys. Union Geophys. Monogr. 14, 348 pp.

112 Heacock. J. G., ed. (1977). The Earth's Crust, Its Nature and Physical Properties, Am. Geophys. Union Geophys. Monogr. 20, 754 pp. Howard, K. A., I. HI. Aaron, E. E. Brabb, M. R. Brock, H. D. Cower, S. J. Hunt, D. J. Milton, W. R. ~luehlberger, J. K. Nakata, G. Planter, D. C. Prowell, R. E. Wallace, and I. J. Witkind (1978). Preliminary map of young faults in the United States as a guide to possible fault activity, U.S. Ceol. Sun;. Map MF-916, 1 5,000,000. Hubbert, M. K. (1951). Mechanical basis for certain familiar geo- logic structures, Geol. Soc. Am. Bull. 62, 35~372. Hunt, C. B. (1967). Physiography of the {Jnited States, W. H. Freeman, San Francisco, 4430 pp. Jaksha, L. H., l. Locke, J. B. Thompson, and A. Garcia (1977). Albuquerque basin seismic network, U.S. Geol. Sure. Open- File Rep. 77~65. Jerome, S. E., and D. R. Cook (1967). Relation of some metal mining districts in the western United States to regional tec- tonic environments and igneous activity, Nevada Burl Mines Bull. 69, 1~5. Karig, D. E. (1974). Evolution of arc systems in the western Pacific, Ann. Rev. Earth Planet. Sci. 2, 51-75. Kehle, R. O. ( 1970). Analysis of gravity sliding and erogenic tTans- lation, Geol. Soc. Am. Bull. 81, 1641-1663. Keller, G. R., R. B. Smith, and L. W. Braile (1975). Crystal struc- ture along He Great Basin~olorado plateau transition from seismic refraction studies,J. Geophys. Res. 80, 109~1098. King, P. B., compiler ( 1969). Tectonic map of Norm Amenca, U.S. Ceol. Suro. Map, scale 1:5,000,000. King, P. 8., and H. M. Beilanan (1974). Geologic map of the United States, U.S. Geol. Sure. Map, scale 1:2,SOO,OOO. Lachenbn~ch, A. H. ( 1961). Depth and spacing of tension cracks, ]. Ceophys. Res. 66, 4273~292. Lac},enbruch, A. H., and J. H. Sass (1973). Therrn~mechanical aspects of the San Andreas Fault systems, in Proceedings of the Conference on the Tectonic Problems of the San Andreas Fault, Stanford U. Press, pp. 19~205. Lachenbn~ch, A. H., and J. H. Sass (1977). Heat flow in the United States and the thermal regime of the crust, in The Earth's Cmst, J. G. Heacock, ea., Am. Geophys. Union Geo- phys. Monogr. 20, pp. 626 675. Lachenbn~ch, A. H., and J. H. Sass ( 1978). Models of an extend- ing lithosphere and heat flow in the Basin and Range province, in Cenozoic Tectonics and Regional Geophysics of the West- ern Cordillera, R. B. Smith and G. P. Eaton, eds., Geol. Soc. Am. Mem. 152, 2~250. Land~sman, M., and W. Cha~payungpun ( 1977). First results from electrical and seismic studies of low resistivity, low velocity mete nal beneath eastern Colorado, Geophysics 42, 804 810. ndisman, M., S. Mueller, and B. 1. Mitchell (1971). Review of evidence for velocity inversions in the continental crust, in The Structure and Physical Properties of the Earth's Cmst, J. G. Heacock, ea., Am. Geophys. Union Geophys. Monogr. 14, pp. 11~34. Lienert, B. R., and D. I. Bennett (1977). High electrical con- ductvities in the lower crust of the northwestern Basin and Range: an application of inverse theory to a controlled-source deep-magnetic-sounding experiment, in The Earth's Crust, J. G. Peacock, ea., Arn. Geophys. Union Geophys. .\Ionogr. 2O, pp. 531~52. Lipman, P. W., H. J. Prostka, and R. L. Chnstiansen ( 1972). Ceno- zoic volcanism and plate-tectonic evolution of the western United States, I. Early and middle Cenozoic, Phil. frays. Roy. Soc. London A271, 217-248. GORDON P. EATON Lobeck, A. K. (1939). Ceomorpholo~y, an Introduction to the Study of Landscapes, `McGraw-Hill, New York. 731 pp. Longwell, C. R. tl933). Rotated faults in the Desert Range, southern Nevada (abate.), Geol. Soc. Am. Bull. 44 93. Longwell, C. R. ( 1945). Long-angle normal faults in the Basin and Range province, Eo.s Trans. Am. Ceophy.~. Union 26, 107-118. Luedke, R. G., and R. L. Smith ( 1978). Map showing distnbution, composition, and age of late Cenozoic volcanic centers in Anzona and New Mexico, U.S. Geol. Sun;. Map 1-1091A, scale 1: 1.000.000. Catkin, l. H. (1960). Empffve tectonic hypothesis for origin of Basin-Range structure (abstr.), Geol. Soc. Am. Bull. 71, 1921. Malone, S. O., G. H. Rothe, and S. W. Smith ( 1975). Details off microcarthqualce swarms in the Columbia Basin, Washington' Bull. Seismol. Soc. Am. 65, 85~. McKenzie, D. P. (1969). The relation between fault plane solu- tions for ear~qualces and He directions of the principal stresses, Bull. Seismol. Soc. Am. o9, 591 601. Moore, G. W. (1973). Westward tidal lag as the caving force of plate tectonics, Geology 1, 99 100. Moore, l. G. ( 1960). Curvature of normal faults in the Basin and Range province of the western United States, U.S. Ceol. Sun;. Prof. Paper 400-B, B40~B411. Mott, R. P., Jr. ( 1976). lithe relationship of microearthquake activ- ity to structural geology for the region around Soco~o, New Mexico, unpubl. M.S. thesis, New Mexico Institute of Mining and Technology, 64 pp. Mueller, S. (1977). A new model of the continental crust, in The Earth's Crust, J. G. Heacock, ea., Am. Geophys. Union Geo- phys. Monogr. 20, pp. 28~317. Mueller, S., and \1. Landisman (1971). An example of He unified method of interpretation for crustal seismic data, Geophys. 1 Roy. Astron. Soc. 23, 36~371. Nur, A., and G. Simmons (1969). The eject of saturation on veloc- ity in low porosity rocks, E:arth Planet. Sci. Lett. 7, 183 193. Porath, H., and D. I. Gough ( 1971). Mantle conductive structures in the western United States from magnetometer array studies, Geophys. J. Roy. Astron. Soc. 22, 261-275. Prodehl, C. ( 1970). Seismic refraction study of crystal structure in the western United States, Ceol. Soc. Am. Bull. 81, ~2!~2646. ProffeK, J. M., Jr. ( 1977). Cenozoic geology of the Yenngton dis- tnct, Nevada, and implications for He nature and origin of Basin and Range faulting, Ceol. Soc. Am. B?~11. 88, 2A7-266. Ranalli, G., and E. I. Tanczyk (1975). .Mendional orientation of grabens and its bearing on geodynamics,J. Geol. 83, 596 531. Rehrig, W. A., and S. J. Reynolds (1977). .\ northwest zone of metamorphic core complexes in Anzona, Geol. Soc. Am. Ab.str. Programs 9, 1139. Reiter, M., C. L. Edwards. H. Ha`~..,an, and C. Weidman (197~). Terrestrial heat flow along He Rio Grande rift, .New Mexico and Southern Colorado, Ceol. Soc. Am. Bull. 86, 811~18. Reitzel, J. S., D. I. Cough, H. Porath, and C. W. Anderson [II (1970). Geomagnetic deep sounding and upper mantle stulc- mre in the western United States, Geophys.J. Roy. Astron. Sot . 19, 21~235. Rogers, G. C. (1977). Two recent Georgia Strait earthquakes (abstr.), Ceol. Assoc. Can. Ann. Mtg. Program Abstr. ~ 45. Ryall, A., and W. U. 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Characteristics of the Crust of the Basin and Range Province Ryall, A., D. B. Slemmons, and L. D. Gedney ( 1966). Seismicity, tectonism, and surface faulting in the western United States during historic time, Bull. Seismol. Soc. Am. 56, 110.~1135. Sanford, A. R., and R. P. Elliott, Jr., P. J. Shuleski, E. J. Rinehart, F. I. Caravella, R. M. Ward, and T. C. Wallace (1977). Geo- physical evidence for a magma body in the crust in the vicinity of Socorro, New Mexico, in The Earth's Crest J. G. Heacock, ea., Am. Geophys. Union Geophys. Slonogr. 20, pp. 385~03. Sbar, HI. L., and L. R. Sylces (1973). Contemporary compressive stress and seismic in eastern North Amenca: an example of intraplate tectonics. Geol. Soc. Am. Bull. 84, 1861-1882. Sbar, A. L., and L. R. Sykes (1977). Seismicity and lithospheric stress in New York and adjacent areas, J. Geophys. Res. 82, 5771~786. Schmucker, U. (1970). Anomalies of geomagnetic variations in the southwestern United States, Bull. Scripps Inst. 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