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II PLATE- B O UN D ARY TECTON ICS

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INTRO D UCTI ON Complexities of Modern ant! Ancient Subcluction Systems WARREN lIAMILTON U.S. Geological Survey Until the advent of plate-tectonic concepts in the late 1960's, geologists and geophysicists studying the compo- nents of ancient erogenic and magmatic terranes on the continents had no actualistic frarneworlc within which to comprehend the origins and interrelationships of those components. We now see that modern analogs for many features of the ancient terranes form primarily in conti- nental-margin arcs and island arcs, above oceanic litho- spheric plates sliding beneath continental plates or other oceanic plates. These active temnes can be understood in a plate-tectonic framework, and many relationships ex- plained on a genetic basis. Ancient tenants are being increasingly comprehended in teens of analogy with the active ones. The first decade of plate-tectonic explanations of conti- nental geology has been enormously fruitful. Increas- ingly, however, our early explanations appear overly sim- plistic; we have seen the broad relationships but have missed many ofthe detailed ones because we have visual- ized plate interactions as much less complex than has been the actual case. Correspondingly, recent studies of 33 active continental margins and island arcs show them to undergo rapid and complex changes in response to fast- evolving plate interactions. Too little communication takes place in either direction I,etueen the scientists pri- marily studying the modern marine systems and their d~y- land counterparts studying ancient systems. Some pur- portedly plate-tectonic explanations published recently for ancient systems are mere ad hoc conjectures, incom- patible with the known features of active systems. Con- ~ersely, the active-margin students could infer much about their domains by analogy with deeply eroded equivalent tracts exposed on land. A cast amount of work is required to increase over ~~n- derstanding of the products and processes of plate tec- tonics. Much more integration of data already obtained is possible. More data need to be gathered in both field and laboratory, with emphasis on the petrological. structural. and geophysical criteria for identifying, distinguishing, and characterizing the component products of plate inter- actions. on paleontologic and radiometric criteria Or defining their ages, and on paleobiogeographic, paleocli- rnatic, and paleomagnetic studies to constrain their global wanderings.

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34 The brief summary here of complexities of modern arc systems is adapted primarily from my study (Hamilton, 1979) of the onshore and offshore tectonics of the Indone- sian, western Melanes~an, and southern Philippine re- gions. The discussion of ancient arcs in western North America Is derived mostly from another report (Hamilton, 19781. The reader is referred to those works for elabora- tion and documentation of the concepts summarized here and for references to the work, by hundreds of other geo- scientists. from which those concepts are derived. ACTIVE CONTINENTAL.UARGINS Where an oceanic lithospheric plate is subducted beneath a continental plate, a tectonic and magmatic system like WARREN HAMILTON that of modern Sumatra or the Andes is developed. \tost early continental plate-tectonic papers (including my owns, and unfortunately also many current papers. as- sumed erroneously that in such settings the oceanic plate tips abruptly down at a trench to an inclined trajectory, along which the plate dips directly into the mantle. Rather it is now obvious that the usual case is that exem- plif~ed by Sumatra ~ Figure 3. 1). The subducting plate dips very gently beneath the continental slope, landward of a trench with gently sloping sides, for a distance of 100 km or more; only beyond this distance does the plate roll toward the steeper dip of the inclined Benioff zone as deduced from mantle earthquakes. The continental slope typically rises gently to an outer-arc ridge, landward of which is an outer-arc basin. Along some active margins, the basin is full of sedimentary straw and is expressed i>JDIA.N JAVA TER RC C)UT=R-ARC BASIN SULFA . RA Cc =~ TR E.NC~ RIDGE _ Pre-subduct i on s trata, _ beneath teas i n sedi meets, ~ ~ A Water I .04 1 ie on ocean; c crust ~ O / ~ ~ a __ =` ~ .~eiange wedge 2.2 2.4 \~ Edge of trench is ~~ lower continental crust 2.9 1 i mi t of downbow i rig by `me I ange wedge ~ Water . ._ - - O 50 ~ - ._ - c, P.hase-transi tion z~ 5 US D L'C IliNG P 4~ T ~ \\ 0V ERRIDING PLA T E ~ Mantle 3.4 -_ 100 ~ .ASTHE~C)SPHERE Hor i zon ta I and ver t i ca i s ca i e Horizontal and vertical scale 0 50 KM 1 1 ! 1 ~ 1 Sof t trench and pelagic sediments SC raped c>Ff a t toe of wedge 7 Gravitational flow i rub r i ca tes me I ange wedge - .~ Wedge s tands h i ghes t "here me I ange b u t t res sed age i ns t leading edge of i i thosphere p T a te G rev i ta t i one I f I ow deforms outer teas; n S trata \= _ in E o ~ 20 . _ 30~ l 40 - \ - Subduc t i nc ocean i c p 1 ate d rags and th i ckens me I ange wedge FIGURE 3.1 Section through the subduction system of the Java Trench, off southern Sumatra The Cretaceous system of California presumably was similar. The configuration of the melange wedge and the structure of the outer-arc basin are constrained by reflection profiling, and the shape of the subdue tiny plate is (onstrained primanly by the location of the inclined Benioff zone of mantle earthQualces. L'pper diagram: major components; densities in g/cm3. Lower diagram: mechanism of deformation of accretionary wedge `>t melange and imbricated materials.

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Complexities of Modern and Ancient Subduction Systems bathymetucallY as ~ subhonzontal continental shelf. seismic-reflection profiles and some drilling show that between the continental-slope seafloor and the top of the subducting oceanic plate there is commonly an accre- tionary wedge fondled largely of sediments scraped off the undersliding plate and imbricated ant] converted to melange along shear structures that dip moderately land- `~vard, sharply discordant to the gentle dip of the oceanic plate beneath. Large ;~`retionaIy wedges develop where voluminous elastic sediments are conveyed into the s~b- duction system. Such sediments are deposited mostly longitudinally relative to the trench, as turbidites in the trench or as abyssal fans. Some recent papers have interpreted voluminous com- plexes of melange and sheared rocks in ancient sr~bduc- tion settings to be conned primarily by submarine slump- ing rather than by shearing within the ;ac~cretionary wedge and its tectonic basement. Such conjecture receives no support from studies of modem trench systems, within which large submarine slides are uncommon. Available data snuggest that an outer-arc basin. like that of Figure 3.1, den elops only where the leading edge ofthe continental plate consists of a strip of oceanic angst, either formed in contact w ith the continental crust by a previous spreading event or sutured to the continent following pre- ~ious subduction. The basin is formed by the raising of the leading edge of this strip, not by depression fit the center. The raising is presumably a result of the stoning beneath the strip of accretionary melange. Where the ac- cretionary wedge is buttressed directly against ~ leading edge of cont~riental crust, no basin is present. The active volcanism of the Sumatra system, like that of most s~bd~ction systems, is concentrated in a belt above that part of the seismic Benioff zone that is about 125 km deep. The volcanic rocks are of silicic and intel~`lediate compositions and, in part, fonn calderas developed by Option from large, shallow magma chambers. It is rea- sonable to assume that granitic batholiths moist now be forming beneath the `~,lcanic belt. The magmatic belt is Imposed on ~ regional anticline consisting of pre~ol- canic roclcs bulged broadly upward; apparently the angst has been thickened by ~olr~mino~s magmatic intrusions. BE HAN I()R O F MODERN ISLAND ARCS IsIand arcs do not sit stably fin tenderly ing mantle for an era, constantly Naming inc`'ming manic lith`~phere, although many c`'ntinenta1 geologists `~me them to be- ha`e Ail. Rather, arcs cl~aracteristicall~ migrate relative to the plates behind them; they reverse their s`~bdr~ction ~~>larit`, fir g<, deacl; they c`'llicle with each <'ther and ~ ith c`,ntinents; and tines are del`,rrned by strike-sli~' tilt and `'r`~clinal holds Tale flits <'t these ~'r`~s care e.Y- ceeclinglv c`>mple.x aggregates an(1 s~1~er~siti`'nx `'t airy materials. Oceanic crust ~lisal'pear.s It OCR for page 31
36 New Guinea; west of that intersection, subduction south- west beneath New Guinea is very rapid, whereas east of the intersection subduction is slow. The slow-subduction Mussau arc system trends southward to a trench~ench- trench triple junction against the curving New Ireland- Manus arc, which continues past the junction to another trench~ench-trench junction off the north coast of central New Guinea. Northern New Guinea records the collision dttring Pliocene time of a south-facing island arc (itself likely a composite) with a continental margin, mostly stable since the Jurassic ricing away of another continental mass. The collision progressed eastward with time, and the east, or unblocked, part of the arc advanced past the end of the New Guinea continent to form the Papuan Peninsula. Subduction, previously northward beneath the island arc, reversed to southward beneath the continent, as enlarged by the addition of the arc and un- derlying lithosphere, after the collision; only subse- quently did subduction reverse beneath the peninsular extension. Strike-slip faulting, orocl~nal folding, and other collisions have been additional complications. Large accretionary wedges of melange and imbricated rocks are developed in front of only those island arcs along which voluminous sediments are available on the ocean floor. Abyssal-fan and longitudinal-trench sedimentation can, however, extend several thousand kilometers from a major continental source. Immature oceanic island arcs, represented above the surface, if at all, only by small volcanic islands, lack such sediment supply; these arcs face steep-sided trenches, at which the subducting oce- anic plate tips sharply downward and dips steeply beneath the arc, without any broad melange wedge and underlying gently dipping oceanic plate. Ophiolitic melange~pelagic sediments and mafic oceanic crustal rocks, in a matrix of serpentinite disrupted and hydrated from oceanic mantle rocks~re known in many erogenic assemblages that are now parts of continents, where they are associated with magmatic assemblages analogous to modern island arcs. These ophiolitic melanges may mostly represent the small accretionary wedges fanned along oceanic island arcs, and contest win melanges, dominated by te~Tigenous elastic sediments that fanned along active continental margins. .Uodern arcs provide no support for the various hy- potheses of back-arc abduction, or flake tectonics. Where slabs of oceanic crust and mantle have ridden onto other rock assemblages and are exposed in young arc terranes, dips and transport directions are as required for those slabs to be the fronts of overdying plates. ^\lECHANISM OF SUBDUCTION WARREN HA.MILTON tory. Neither mechanism of subduction can explain the geometry of many modern systems. The Banda Antill~c and Scotia arcs have subducting trenches that are C- shaped in plan and Benioff zones, hence subducted plates, that are spoon-shaped. Opposed arcs are in the process of colliding in the Slolucca Sea and Solomon Sea regions, over subducted plates that have an inverted V configuration in section. Such geometry suggests that the subduction process below a depth of 100 or 150 km may consist primarily of the vertical sinking of the downgoing plate and that the apparent dip of a subducting plate is a Unction of the rate of advance of the plate hingeline over the mantle beneath the subducting plate. This rate would be equal to the convergence rate only where the subducting plate is fixed with regard to the mantle beneath it and where no mar- ginal sea spreading occurs behind the advancing arc. The sinking velocity of the subducted lithosphere must be a function of the density contrast between subducting slab and surrounding mantle and, hence, must increase, at an exponentially decreasing rate, with age ofthat slab. Given a constant rate of hinge advance, the dip of a subducted slab should be a function of its age. . PRE-CRETACEOUS ISLAND-ARC AND MELANGE ASSEMBLAGES OF THE WE STE RN U N ITE D S TATE S The Cretaceous and early Tertiary development of the western conterminous United States was dominated by subduction of large plates of oceanic lithosphere directly beneath the continent. Similar activity of Andean type occurred during parts of Triassic and Jurassic time also, but much ofthe subduction recorded dunng those periods occurred beneath oceanic island arcs. The arcs consist dominantly of submarine basaltic and andesitic lavas, breccias, and tufts; their intrusive and volcaniclastic equivalents; carbonate strata; and diverse accretionary- wedge materials including ophiolitic melanges. These assemblages resemble those that characterize modem oceanic arcs. These arcs were added to the west edge of the continent when intervening oceanic lithosphere was wholly subducted beneath either the continent or arcs advancing toward it. Complexities ire these ancient, ac- creted arc tenanes are still poorly understood but are numerous enough to indicate that complex histories. com- parable with those of modern Indonesia, .~Ielanesia, and the Philippines, are recorded. We must be dealing with the products of consumption of dozens of large and small lithosphenc plates and with complex, jumbled aggregates of arc components that were in part pre-assembled far from their final resting place along the continental mar- gin. After Precambrian rifting and the formation of a new ocean in the direction that is now west, the western mar- gin of North America during early and middle Paleozoic time trended from west-central Idaho to east~entral Cali- The inclined descent of subducted lithospheric plates, down to depths of at least 700 km, is commonly visualized as representing either the injection of a slab down an inclined trajectory that is fixed with regard to the mantle or the gravitational sliding of a slab down such a trajec-

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Complexities of.~fc~dern ctnd.Ancient Subduction Systems Doria. This margin was stable tectonically and on it de- ~eloped an oceanward-thickening wedge of continental shelf strata. In Late Devonian and Early Mississippian time, creep-water strata from the west, with some of their underlying oceanic crust, were shoved onto the shallow- water shelf strata, and both were pushed eastward above the continental basement in the Nevada sector (Figure 3.2). This event may record the collision of an island arc with the continent; but the arc itself did not remain at- tached to the continent. tor another period of stable- margin sedimentation followed, with deep water again to the west. Presumably the polarity of subduction reversed beneath the collided arc, which migrated away from the continent, opening an ocean basin behind it. Another similar collision occurred against Nevada near the middle of Triassic time. Again, dee~water sedimentary rocks and oceanic basement were driven eastward upon shallow- water strata, and both moved eastward over the conti- nental crust; but this time, the causative island arc re- mained attached to the continent. The Permian and Triassic marine faunas of this island arc differ only moder- ately from those of mainland North America, so the pre- collision wandering of the arc need not have been at vast distances from the continent. The collision was followed by a reversal of subduction polarity, and Upper Triassic arc magmatic rocks were erupted through the old conti- nental crust, above the subduction system dipping east- ward from the Pacific edge of the added island arc. Still greater motions of lithospheric plates are required by juxtapositions of other arc and melange assemblages that yield fossil or paleomagnetic evidence for distant sites of origin. One belt of such assemblages occurs in the tectonic-accretion terrane from the southern Sierra Ne- ~ada of Califomia to the Yukon Ternto~y in Canada; it is characterized by shallow-water Upper Permian lime- stones bearing tropical Asian fossils. Particularly distinc- tive among these fossils are fusulinids aIgal-grazing, calcareous-test protozoa~of the verbeekinid family. These are widespread in correlative deposits of paleo- tropical Eurasia yet are completely unknown in paleo- tropical mainland North America. (The equator of the time trended through Texas, in the direction now north- eastward, so appropriate habitats were available; but the American fusulinid assemblages are quite different from the Eurasian ones.) These fossils occur in limestone blocks in melanges, in reef limestones capping atolls, in the scraped-off complexes along the paleomargin of North America, and in part between island-arc belts. Subduction of more than 10,000 km of oceanic lithosphere beneath North America during Late Triassic and Jurassic time, and beneath island arcs that were themselves added to North America in Jurassic time by subduction of intervening oceans, is required. Oceanward of this tract in British Columbia and southern Alaska is still another tract of isIand-arc and related rocks, characterized by southem- hemisphere paleomagnetic latitudes. The Cretaceous and early Tertiary pattern of relatively simple subduction of oceanic lithosphere beneath the continent, with minimal .~_ ,~ ~ involvement of island arcs. followed the addition of these various terranes to North America. C R ETAC E O ~ S AN' D C E NO ZO I C CALI FORN {A SU BDUCTION CONFIGURATION Both the geology of Califomia and the seafloor-spreading history of the northeast Pacific Ocean require that during Cretaceous and early Tertiary time oceanic lithosphere was being subducted beneath Califomia. The Cretaceus components of most of Califomia fit the model of Figure 3.1, when palinspastic restoration is made to reverse the disruption and offset by late Cenozoic strike-slip faults and other structures. In the west, forming most of the Coast Ranges (Figure 3.~, is the accretionary u edge, the Franciscan complex of melange and imbricated rocks- mostly of abyssal uppermost Jurassic, Cretaceous, and lower Tertiary elastic sediments but including fragments of oceanic crust and mantle and of seamounts, and masses of high-pressure, low-temperature metamorphic rocks. Shear structures in the chaotic Franciscan complex typi- cally dip steeply to moderately eastward, yet gravity stud- ies indicate that the oceanic lithosphere beneath the wedge dips only gently eastward. Along the east side of the Coast Ranges, coeval outer-arc-basin strata, the Great Valley facies, lie with depositional contact upon oceanic crust and upper mantle, which in turn is in thrust contact with the tectonically underlying Franciscan accretiona~y wedge. Lower Great Valley strata were deposited at abyssal depth on oceanic cmst, a narrow strip of which remained attached to the continent while Cretaceous sub- duction proceeded. The strata lap eastward across the oceanic crustal strip and in the subsurface butt against what was in Cretaceous time the moderately sloping edge of continental crust. The leading edge ofthe oceanic strip was rotated upward concurrent with Cretaceous subduc- tion. Farther inland, the SielTa Nevada batholith, granitic rocks mostly of Cretaceous age, followed synchronously with the formation of the Franciscan accret~onary wedge and with the development of the Great Valley outer-arc basin. The batholith represents the subjacent equivalent of a volcanic arc such as that of modern Sumatra. Batho- lithic rocks of any one age display a cross-strike increase in the ratio of potassium to silicon. characteristic of action e volcanic terranes foxed above subducting plates. and the distribution of the rocks defining the gradient indi- cates moderately steep subduction dips compatible with the Franciscan and Great Valley geometry in a system like that illustrated in Figure 3.1. The magmatic record of Cretaceous and early Tertiary time In the western states indicates that the dip of the subducting slab was moderately steep and relatively con- stant during Early and early Late Cretaceous time but that the dip was markedly gentler during late Late Cretaceous

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38 WARREN H~ILTO! 120 l ~ ~ ~ ~ volcanic ~ coLuMl~la PLATeAi} . )- ~ (~} ~ ~ ~ arc ,,:._ ~ '' I'd ~ 1 (_ `~_~c~~ ~ ,,, ~ ~ / Us ! 'I 9 .r-~,o.~ == // ~~: An eland of . cane c / ~ ,; ~~Devor'ion to To,. .~7orc ~ i= ''''a ~ ~ . ~ ~ . _ . . I ~ ~ ~ ~ / ITS. 1 At- it_ . ~ J Carboniterous- ~ ~ ~ ~~~ ( Jura.ssic ore, , ~ Earty \ | oph~olite, ond ~ , Pa's02sic \ : f \me'~nge /f me~coge \ mater~ole, ~ / ~ \ J idt' :: ,_ : :? -. -. Jurassic ana Eari, Crctaccous:, ~ `~; ~~ ''~43 pl u tons in nor t h east Oregon . ~_ ;' `',: k1 ~_' postdcte most subduction ~ ~t i accretion \'( >"~-"~ ~Al :2`.. .::. NEOGENE BASALT Fills th~nned and r~fted crust I t ^~\~ ?~;~ !~.ndecino~ ~ ic,'; 40 ~ ~.i~, ~ ~ ~ ~ ~ ~ nm~Cr9tm~rt.t ~ ~ - tit ~ c ~ , . ~, , ' .'~k ~, ~ `^, I 's / PROv'NCE 1 l ~i~.\,\ ~ 0tOut 173 of wl~th I of tasin-rOnge proYinCe i repreSents Neogene crustal extens~on, northwesteard re~ct~ve tO croton P~poroz imCt. rt~ngel~ne De,~een /j P~160201C She1~/ and crotCnic ~ j \ st~atc ~ _ ~ `/ i ~Scn Gregorio fcult ~r,,cssic o, ~urc,s,c r,~t ot old continental morg'n I ~ angul ~ aC b~ you n ger p I u rons 200 1< M 0 f00 FIGI:RE 3.2 Selected tectonic elements ot the west-central United States. The present dis~iblltion of these elements was atTected greatly by disruption during middle and late Cenozo~c time.

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Complexities of .~odern arid Ancient Subduction Sy.stem.s time, perhaps as the resmelt of an increased convergence rate between the .Nrorth American and subducting plates. Nlagmatic-arc patterns changed complexly during early Tertiary time, indicating corresponding variation in sub- duction dips. In Oligocene time, for example, the sub- ducting slab was apparently segmented: the southern seg- ment decreased in dip northward Prom moderately gentle to very gentle. there was an abrupt change across a line trending northeast Tom southern Califomia to northern Colorado, and the northern segment decreased in dip northward from moderately gentle to moderately steep (see Chapter 14). .~ch ofthe Tertiary complexity may be due to the subduction of oceanic lithosphere of varying age and density as the East Pacific Rise. offset along trans- ~nn faults, approached the continental margin and young lithosphere of variable temperatures was being subducted. Conversely, the spreading history and geom- etm of eastern Pacific plates, since wholly s~bd~cted beneath North America. can perhaps be inferred from the magmatic-arc migrations. In the Klamath Contains region of northwest Califor- nia and southwest Oregon, the Franciscan melange wedge lies directly beneath crystal assemblages that were part `~t the continental crust in Cretaceus time (Figure 3.~). No `~ter-arc-basin strata lie to the east. Here, the leading edge of the continental plate was ~~..'ed of conti- nental cutest; the narrow strip of attached oceanic crust is missing, and no o~ter-arc basin developed. LATE CENOZO}C DISRUPTIO.N During Cretaceus and early Tertiary time, the Pacific margin ot Califomia was ~ relatively simple system of parallel tectonic and magmatic belts, controlled by the igoro~s s~bd~ction then occurring beneath all of Califior- nia The subduction was more rapid than the spreading of the East Pacific Rise offsl?ore to the west, so the rise missed progressively closer to the trench despite the ac- ti~e spreading. The oceanic plate east of the rise was mov ing northeastward relative to North .\merica, whereas the plate west of the rise was moving northwestward. Then the rise itself hit the s~bd~ction system oboist 30 million years (m.~.) ago. the western oceanic plate came into direct contact with ~ segment of the continental mar- Kin. and strike-slip m<,tion resmelted. The strike-slip bo~ndarv lengthened faith time. and its north limit mi- gr;Ated northwestward, as spreading ridge and trench col- lided along progressively more of the c<,ntine~ta1 margin. Tile manic lithosphere. `` hich had liven Updated shortly before the transition. `~as ~ng and hilt. sit the change from .~bd~cti`'n to strike-slip regimes Erred `~hen the continental margin `~`I'; l~nderl.lin b~ sort. ~ eak mantle. The results `~t these changes and c`,n~liti<,ns elan be seen in s`~them California and north est \~.YiC() ( Figure 3.3). Tale Cretace`~s m`~gmatic ``rc (S it rra .N ex `~1~. Peninsr~l.~r Ranges and Baja C.~lit`~rni`~ t'~th`,lith~ii. `~ter-~`rc basin. 39 and Franciscan melange wedge are all present blat their distribution is scrambled. The simplest and ingest major disruptive structure is the San Andreas Fault. which is now the dominant break along which the western Pacific plate is moving northwest past the continent, carrying Baja California and coastal California along. The San Andreas offset of a well-documented 300 km and the concurrent opening of the Gulf of California hare oc- curred within the past 5 m.y. Lotion patterns during the transition period. between 30 m.y. and 5 m.y. ago, from subduction to strike-slip regimes were much more complex. In the southern Coast Ranges (northwest part of Figure 3.3), strike-slip faults underwent pre-San Andreas offsets of at least 200 km; yet these faults probably do not cut the Transverse Ranges, w hich trend across them in the south. In the Coast Ranges, the westward progression from magmatic arc to `'uter-arc basin to Franciscan melange wedge is present, variably truncated by Cenozoic faults; but then more outer-arc-basin strata were present beyond the Francis- can, in the Santa Ynez and Santa Monica Mountains of the western Transverse Ranges. The complete progression is present also in the Vizcaino Peninsula sector of Baja Cali- tomia. Along southern California south of the Transverse Ranges, however, the sequence is partly repeated: off- shore in the San Nicolas Island~anta Rosa Island sector, outer-arc basin strata are again present, out of place to the west of Franciscan; and Franciscan is present west of these strata' apparently in proper succession. One expla- nation possible for some of these and other relationships is that a western sliver of outer-arc basin and Franciscan terraces slid northwestward from Baja California. along a right-lateral strike-slip fault, around the Franciscan ter- rane now enclosed to the east; the leading edge of the sliver impinged on the continent, and the rest rolled clockwise past it, forming the western Transverse Ranges by rotation. The required rotation and northward transla- tion are consistent with the paleomagnetic orientations of lower Miocene basalts in the Santa Monica .\Iountains and islands to the west. Other styles of crystal disruption affected the western states further inland. Distributed crustal extension pro- d~ced the Basin and Range province (see Chapters 8 and 9) of blocks bounded by downward-flattening nodal tilts. The amount of total late Cenozoic extension is in dispute; my interpretation, more extreme than most. is that bet`` een 30 and o0 percent of the present width of the turbine represents extension. The NetWare of the transition from brittle fracturing and block rotations ot the tipper crust to hypothetical uniform extension in the deep. hid- den loner angst is also in dispute. Steadies in progress of deeply eroded basin-range terranes in southeast Calitor- nia and southern Arizona snuggest that the level of detor- mati`,n beneath the horizontal bases of rotated fault blocks is characterized by giant bodies or mullions, hating lengths <~l as milch as several tens of kilometers. and thus b` delonnation that is partly ductile and partly brittle. In

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40 3so 30 WARREN H~!~{ILTON COAST RANGES 120 "S~ ~~\ ElASliV- RANGE 115 ~)PROV 1 N C I \aclo: 5: ! ~ N \~: He \ ~ \ ~ W~ ~: i ~ I, - l 1 _~. ~` -_ ~ ~ __ \ ~00%~ a~e SON ~ ~ ~'t ;o~<,~ '^ ^~\ \ G :1\(: ~ ~.\ 6~A \ ` .: \ FIG~'RE 3.3 Selected tectonic elements of the southwestem t;nited States and northwestern \1exico.

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Cvmplexitie.s of Modern and Patient Subduct:<'n System.s the northwest states. Mom northwest Nevada and south- en, Idaho northwestward' continental crust older than middle Tertiary was thinned by extension in some areas and completely rifled in others, the volumes has ing been filled subsequently mostly by middle and upper Ceno- zoic basalts (Figure 3.21. Other aspects of Cenozoic dis- ruption are discussed in Chapter 8. ~1 REFERE!.'CES Hamilton. lo. (1978). Mesozoic tectonics of the western ~ noted States. in Pacific Cubist Pale<~raphy Symp`'s2um A. Pacific Section. Society yists and Mineralogists. pp. 33~70 Hamilton At. t19~9). Tectonics of the Indonesian region C.S. Geol. Surr. Proof. Paper 10~-8, .~34.o pp.

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52 FIGURE 5.2 Diagrammatic exam- ples of outer highs on divergent conti- nental margins. very irregular. For example, areas with extensive volt canics, such as the Faeroe Island area (Talwani and Eldholm, 1977) and eastern Greenland (Hailer, 1971) con- trast with areas having virtually no extrusive volcanics in the area of the outer high, such as the west Africa exam- ples discussed later in this chapter. The distribution of volcanics in presently rifts is similarly irregular. For example, the Rhine Graben has two large volcanic centers, Vogelsberg and Kaisers~hl, separated by areas of limited volcanic activity (Rhine Graben Research Group for Explosion Seismology, 1974~. In general, divergent margins have a characteristic pro- gression of sediment types that vary as an area progres- sively undergoes rifting and then driRing. Rifts that form in areas of thick continental crust, such as cratonic shields and areas that have undergone an erogenic event, are in most cases first filled with nonmarine sediments that grade upward to shallow marine sediments that com- monly include evapontes, as shown in the West Africa example discussed later in this chapter. RiRs that form In areas of thinned continental crust, such as older inactive rids, commonly contain marine or interbedcled marine and coastal deposits. Figure 5.3 Tom offshore east Green- land (Hailer, 1971) is an example of divergent margin evolution in which the crust has been thinned by earlier riding. Post-pull-apart or driR sediments are usually marine, except where large prograding delta complexes are pres- ent. In many areas, the marine sediments of the drift phase are prograding carbonate banks and reefs. Marine temgenous elastics commonly overlie the carbonates or extend throughout the entire driR phase, where car- bonates are absent. GEOPHYSICAL EVIDENCE FOR OUTER HIGHS Outer highs are best recognized on reflection seismic data by looking for the relatively high area in between the MARTIN A- SCHUEPBACH and PETER R. VAIL _ - _ ~ - 83 U4TE Rl~ SEDIMENTS - _ ~ il ~NLAPPNG 0N10 DRIFT SEDIU0iig ~ __== , " n ~ Elton u=u I ~ OCEANIC . ' CRUST _ / \ '\ ~ . / ! 0~ 8~ - SALT\ 7D ~ 1 ~ seems ted 'I LATE RlfT SEDIIUENTS / , I, _: ~6 ONTO oUrER ~ 5 ~ ()it ORFT SEDIMENTS MINES ___ ~ ~ TWO WAY O ;Alri /~ /oUT~8 HIGH _ ~ _ /, /, / ' / - , ' t, Il CRUST' / / \ / \, i\ `// ~ \ _ ~ 1 - ,, seismic expression of oceanic crust and seaward onlap of the reflectors over the predrift unconformity. The reflec- tor at the top of the outer high may be continuous or highly faulted. In addition, reflectors and faults may or may not be observable below the reflector at the top of the outer high. The diagrams shown in Figure 5.2 are patterned after seismic examples and thus illustrate typical reflec- tion patterns of outer highs on divergent margins. An example ofthe seismic expression of oceanic crust is shown in Figure 5.4. Note the high amplitude with nu- merous diffractions that characterize this reflection. In this example, no good reflectors occur below it. This is the common case. In other examples, however, dipping re- flectors occur below the top of the oceanic crust. Areas where these reflectors occur ale commonly paleotopo- graphic highs, which may indicate local thickening of oceanic crust by volcanic extrusives. Two multichannel seismic lines hum offshore West Af- rica (A, Figure 5.5, and B. Figure 5.7) are presented to illustrate the seismic expression of outer highs. Both lines extend from the present-day shelfto near the continental- oceanic crustal boundary. This paper only discusses the interpretations in the vicinity of the outer highs. A de- tailed discussion of seismic line A (Figures 5.5 and 5.6) is presented in Vail et al. (in press). A dashed line indicates the interpreted boundary between rift and drift reflectors' and a heavy solid line indicates the top of the outer highs On both seismic lines the interpreted presalt early-riD reflectors are bordered on the seaward side by outer highs consisting ofblocks bounded by steep faults. Seismic line A (Figure 5.5) shows a relatively smooth surface at the top of the outer high with underlying faults and reflectors Seismic line B (Figure S.7) shows large tilted fault blocks Late-rift salt reflectors onlap or pinch out against the outer highs. Onlap patterns on seismic line A indicate that the paleotopographic high point of the outer high was on its eastern side and the top of the interpreted oceanic crust actually was higher than the western portion of the outer

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54 .UARTIN A. SCHUEPBACH and PETER R. VAIL Am_ ~ _, OCEANIC CRUSTS , , __ !~ FIGURE 5.4 Example of the seismic expression of oceanic crust from the Blake-Bahama Basin (from Shipley asked Buffler, 1978, courtesy American Association of Petroleum Geologists). high in the early drifting phase. Unfortunately, seismic line B did not extend across the continental~ceanic crystal boundary, but the magnetic quiet zone, inter- preted as oceanic crust, is known to exist just west of this seismic line. The interpreted salt interval thins, but does not completely pinch out at the western end of this line. The outer high on seismic line A is bordered by what we interpret to be oceanic crust. The boundary is placed between reflection patterns similar to those characteris- tics of oceanic crust and to the outer high. Both seismic lines are controlled by drilling on the shelf and by regional outcrop geology. Based on these data, line A rift sediments include Triassic and Lower Jurassic (Hettangian) red beds and Lower Jurassic carbonates and anhydrites. Based on its stratigraphic position, we inter- pret the thick salt on line A to be Early Sinemurian in age, but it could be significantly older or younger. The rift sediments are overlain by drift sediments consisting of a Middle Jurassic to Cretaceous Berriasian prograding car- bonate bank and a Cretaceous-Tertiary prograding elastic wedge (see Vail et al., 19771. Wells penetrating the shelfnear seismic line B indicate that the rift sediments are Neocomian-Aptian elastics overlain by late Aptian salt. DriR sediments are post- Aptian Cretaceous carbonates and elastics and Tertiary elastics. No good evidence for volcanics is shown on these sec- tions in the vicinity of the outer highs. However, on seismic sections from other areas, the reflection paKem of volcanics is common and quite variable. Volcanics exhibit considerable variation in paleotopography at the top and/ or base of the unit as evidenced by onlap. Interval reflec- tions within the unit commonly dip at a greater angle than the top or base. In many cases these dipping reflections are high amplitude and continuous for short distances. In other examples, extrusives appear as lens-shaped lobes with a relatively high-amplitude reflection at the top and little to no reflections within the lobe. Interval velocities it, tend to be intermediate to high in the first example and high in the second. We interpret the first example to be largely interbedded volcanics and the second example to be massive flows. Intrusives commonly appear on seismic data as irregular high-amplitude reflectors, sometimes crossing and sometimes subparallel to reflections from stratal surfaces. Reflections originating Mom intnlsives commonly occur sporadically in the vicinity of outer highs. Only limited magnetic and gravity data were available over the outer highs described in this study. Where magnetic data were available, the presence of magnetic stripe anomalies correlates well with the typical seismic expression of oceanic crust discussed earlier. However, the steep magnetic gradients oRen associated with the continental~ceanic crustal boundary commonly occur well landward of the outer high. In areas where extensive volcanics are present, the relation of the magnetic pat- terns to the continental-oceanic crust boundary was vari- able. In some cases, where volcanics were present, the steep magnetic gradient was seaward of the outer high. Gravity data commonly show positive isostatic gravity anomalies extending along the interpreted outer edge of the continental margin (Emery et al., 1970; Talwani and Eldholm, 1972; Rabinowitz, 1972~. Figure 5.8 is an exam- ple. A gravity low is commonly present landward of the outer high' indicating a deep sedimentary basin. The as- sumed shallow basement of the outer high causes a rather strong gravity anomaly but not a large magnetic anomaly (Talwani and Eldholm, 1972~. SUMMARY OF EVOLUTION OF DIVERGENT (PAS S IVE ) CONTINENTAL MARGIN S If the formation of outer highs during the late rid stage is as prevalent as we believe, it would add another phase to the evolution of divergent (passive) continental margins.

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56 AGES ~= TO RTONIAN SER RAVALLlAN BU RDIGALIAN CHATTIAN RUPELIAN PR IA80 N IAN 8ARTONIAN LUTETIAN YPR ESIAN THANETIAN OAN IAN MAASTRICHTlAN CAMPANIAN SANTONIAN _ CONIACIAN TU RONIAN CENOMANIAN ALBIAN APTIAN BAR REMIAN HAUTER IVIAN VALANGINIAN z BER RIASIAN _ PO R r LAN D IAN TITH O N - , ~ ~, KIMMERIOGIAN ~ JAI' OXf O RDIAN CAL LOVIAN BATHONIAN BAJOCIAN AAUENIAN TOARCIAN PLI ENS8ACHIAN Sl NEMU RIAN HEUANGIAN R HAETIAN NORIAN ~ Il . -=-~ T. ~ . T~T2 T Te _ :T02.2 Td TO 1 Tc _ E2 2 T TE3; Ta T92j 1: _ ~,-~ _ j K2 ~ j ~ _ ~/'~ ~ K~ _ Q K~ I Ka - _ J22 -t50 TR2 3 T R 208 c FIGURE 5.6 Stratigraphic col~nn represented in seismic line A (Figure 5.~). MARTIN A. SCHUEPBACH and PETER R. VAIL This section summarizes the evolution of divergent conti- nental margins as a basis for placing the formation of the outer high as an evolut~onary phase in divergent continental-margin development A vast amount of literature has contributed to the un- derstanding of clivergent margins. Just a few publications are citecl here: Drake et al. (1959~; Belmont' et al. (1965~; Burk (1968~; Emery et al. (1970~; Talwani asld Eldholm ( 1972, 1977~; Rabinowitz ( 1974~; Boeuf and Ooust ( 19751; Exon and W~llcox (19781. A summary of the evolution of divergent (passive) con- tinental margins (Figure S.9) is presented by the IPOD Committee on Passive Continental Margins (CulTay, in press). In that paper, the evolution of divergent margins is subdivided into the following phases: (1) variable degrees of doming, (2) niting, and (3) driflting. Doming of graben margins, such as occurs in the East African riR (Pilger and Rosler, 1976) and the Rhine Graben (Illies, 1977) might not always be present and in some cases may occur while the riRing was taking place (Carey, 19581. RiRing occurs as a single graben or as a complicated subparallel system of grabens and horsts. The driRing is characterized by the fonnation of oceanic crust, seafloor spreading, and subsidence. TECTONIC-SEDIMENTARY MODEL FOR THE EVOLUTION OF OUTER HIGHS ON DIVERGENT MARGINS The tectonic-sedimentary model presented here (Figure 5.10) for the evolution of the outer high differs fiom the Curray (in press) model for divergent margins by adding a late-riR phase consisting of a longitudinal predriR upliR within the rifle graben or graben complex. Our model shows two phases of rifting and two phases of clriPcing. The initial nflcing phase fonns a single graben in ~is model [Figure 5.10(a)~. However, as discussed in the previous section, the initial nPring may be an intricate graben system. The initial grabening phase may be caused by crustal thinning over a zone several times wider than the graben itseIf (Rhine Graben Research Group for Explosion Seismoiogy, 1974~. Figure 5.11, re- produced rom the Rhine Graben Research Group report, shows a cross section of the Rhine Graben. This section is an example of the relationship between early ri:cing and crus~l thinning. Whether the sediments deposited during the early-riR phase are manne or nonmarine depends primarily on ~e thickness of the continental crust in which the nflc forms. Where the crust is thick, the early rifle tends to be topo- graphically high and nonmarine sediments tend to be deposited. Where the crust is thinner, ~e level of the rifle floor tends to be close to or below sea level, resulting in marine or interbedded marine and nonmarine sediments. Sea-level fluctuations as described in Vail et al. (1977) and access to marine waters are important factors in con- trolling the sediments deposited in these types of rifts.

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:. 1 ' : l : 1 ' ! en SON003S - 3W11 o 1 1 1 1 o ~ o lo. en c" ~ In SON003S- ~W11 , _ ~ . . . _ . . , , , o o o C" ~ HA sONO03S- 3W11 _ E Y U.

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58 5a BURIED RIDGE ,~\ 50 ~INTOUR INTERVAL 20mgal Ad\\ ~ JO , . , , , I'\ \ , to 10 15 FIGURE 5.8 Isostatic gravity anomaly map (from Rabinowitz, 1972) of Me Angola Continental margin showing the outer high as a positive anomaly. Dunng the late-nRing phase [Figure 5.10(b)], a narrow irregular longitudinal uplift fortes within the graben. In cross-sectional view, the narrow uplifted strip has the am pearance of a basement high. Also, during the late-nE phase, the area of thinned continental crust on both sides of the basement high continues to subside. Our data in- dicate that the late-nh phase sedimentary sequences in these two subsiding basins commonly consist of facies varying from moderately dee~water sediments to evade orites. In general, erosion removes the early-rifle-phase nonmarine sediments Dom the uplifted median basement high, and late-rifle-phase marine strata onlap it. Our geo- physical studies indicate that the outer high is commonly injected by basic intrusions and is in many cases covered by basic extrusives. The upward movement ofthe median basement high appears to us to be caused by ~ thermal- MARTIN A. SCHUEPBACH and PETER R. VAIL DON15116 R~6 ~ - ~5~ 1 ~EDU~ l l |2,W'~ ~ ~ MAN FIGURE 5.9 Passive margin model Tom Curray (in press). ]5D mechanic event Mat immediately precedes We fo~a- tion of oceanic crust. The process of the therrnal- mechanical event is not well understood and is an area recommended for further study. At the beginning of the drift phase [Figure 5.10(c)], the outer high splits longitudinally into two outer highs sepa- rated by oceanic crust The outer highs subside rapidly from close to sea level to We presently depths following the subsidence curves for thinned continental crust and oceanic crust cliscussed by Parsons and Sclater (1977~. Cooling ofthe mantle below the thinned continental crust and the newly formed oceanic crust and sedimentary C ~ ~ LEO - PROGRADHG SWAMPS OF DREG PHASE i, ~ MARE ~M~ 2- ~ / _ ', --, / `'' \ _ I,' BROW ~ - Rem OF MOW ~ _' ~ ~~ 1__ _\ _ ~U RR ~ I -.-1 SEDIMENTS OF EARLY '' '---"aid-,-' ~ PRERH~M~ FIGURE 5.10 Tectonic sedimentary model showing the evolu- tion of outer lights on divergent margins.

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Evolution of Outer Highs on Divergent Continental Margins w FIGURE 5.11 Crustal cross section through Rhine Graben (from Rhine Graben Research Group for Explosion Seismology, 1974). Thinning of the continental crust and mantle upwelling occur in a zone several times wider than the width of the graben. loading results in tilting and subsidence of Me continental margin. In the proper climate, the rapid subsidence and initially small Drainage areas for elastics provide a deposi- tional site ideally suited for the development of Dick pro- HYPOTHEllCAL DEPTH CONTOUR UN~ \ MEDLAN OCEANS m=1 rat 4. + 1 EAflLY RlR WAGE ONMM.E SWIMS) LATE B" n"E MARIIIE SEDINIEIJ'S) PROGRAD.16 MARIIIIE SE0 - Em FIGURE 5.12 Schematic evolution of divergent margin open- ing in a pivotlike fashion. All stages of evolution, from initial rifting to the final drift stage, may occur simultaneously along a margin opening in a pivotlike fashion. The marine basins of the late-rift stage may be restricted and completely separated From the early-riPc and driR phase by the outer high and transform faulting. 59 grading carbonate banks and reefs. As subsidence slows down througl, time, the drainage area caused by subsi- dence expands. This tends to cause tenigenous elastics to build seaward, commonly covering the carbonate bank margins. Full development of the features associated with riding is in part dependent on the total extension across the rift. A divergent margin that undergoes a pivotlilce opening (Figure 5.12) will fully develop its characteristic features sooner at large distances from the pivot point than at small distances, because large extensions are achieved sooner there. Thus, even though extension along tlie length of the opening is synchronous, basin formation and filling are diachronous. Also, the early-driR basin may be sub- divided into subbasins by highs caused by transform fault- ing. Subbasins thus formed can restrict the circulation within the late-rift- and early-driR-phase basins, thus causing conditions favorable to the deposition of black shales and/or evapontes. PROBLEMS AND RECOMMENDATIONS The combination of regional geological studies and multi- channel seismic profiles used in conjunction with other geophysical data has provided the information for the definition of some key problems related to the evolution of outer highs on divergent continental margins. We rec- ommend that a long-term program consisting of geo- physical studies, emphasizing regional multichannel seismic lines, and regional geological studies, including outcrop, piston, and dredge samples, be undertaken to locate optimum drill sites for the best possible confinna- tion ofthe geophysical interpretations and understanding of the geological problems. Such work is proposed for the 1980's by the JOIDES program (White, 1979~. The key geo- logical problems that we have defined concerning the outer highs on divergent continental margins are the following: 1. The character, age, and location of the continental- oceanic crusted boundary; 2. The existence, nature, size variations, and evolution of median highs within grabens to outer highs on diver- gent continental margins; 3. The nature and evolution of the overthickened oceanic crust and volcanics commonly present in We area adjacent to the continental-oceanic crustal boundary; 4. The type, age, and controls of late-riR-phase sedi- mentation, particularly organic-nch shales and evap- orites; 5. The relation of subsidence and heat flow to crustal type and thickness; 6. The paleoenvironmental history of the sedimentary section through the drift phase including age, paleontol- ogy, hiatuses, unconformities, lithofacies, paleobathym- etry depth, physical properties, and other significant parameters.

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60 Venfication of the model relies on extensive geophys- ical surveys involving multichannel seismic, magnetic, and gravity data and testing of optimum sites by drilling. If the mode! is verified, it will better explain many of the geological problems associated win divergent conti- nental margins, including a better understanding of the nature of the oceanic~ontinental crusted transition, asso- ciatec3 deposition including salt and organic-nch shales, and Me relation of volcanism to the onset of seafloor spreading. SUMMARY Observation of a large number of geological and seismic profiles across divergent continental margins has demon- strated the widespread occurrence of structural highs that were formed during the late-nflc phase of divergent mar- gin evolution and are now the boundary between conti- nental and oceanic crust. This feature originates as a median high, which divides the rift zone into two subsid- ing basins. This median high later evolves into an outer high along doe newly forrneil condnental~ceanic bound- ary during the drift phase. The outer high commonly ex- tends the whole length of a divergent margin but can be segmented by such factors as transform faults andJor vol- canics. In many cases, the outer highs are complicated by extensive intrusives and extrusive volcanics. Early-riP: sediments are commonly nonmarine, but in areas of Tinned continent crust they may be manne. Manne sediments or volcanics are almost Sways deposited in the late-nflc basins on opposite sides of the median high. Salt and organic-rich shales are commonly deposited during this phase. This chapter has presented a mode! for the develop ment of an outer high on clivergent continental margins that relates the major structural episodes with the asso-. ciated sedimentation. It consists of (1) early-nflc-phase graben formation, ~a' late-nR-phase development of a median high subdividing the graben complex into two subsiding basinal areas, and (3) driflc-phase Connation of oceanic crust and He outer high as a remnant of the median high and subsidence. ACKNOWLE OGM ENTS The writers are indebted to E. B. Nelson of Esso Explora- tion, Inc., anil I. Viellart of Esso aEP for Heir seismic interpretations; to I. P. Verdier and R. I~ehmar~n of Esso Production Researc~European Laboratones for their paleontological contributions; to H. R. Gould of Esso Pro- duction Research Company for his guidance; to C. G. Zinkan, D. R. Seely, R. W. Murphy, I. K. Davidson, D. B. Macurda, K. T. Biddle, R. C. Vierbuchen, R. P. George, Ir., all of Exxon Production Research Company, who re- viewed the manuscnpt; and to Esso Exploration, Inc., for permission to publish He seismic lines. MARTIN A. SCHUEPBACH and PETER R. vAIL RE FE RE NC I: S Belmonte, Y., P. Hirtz, and R. Wenger (1965). The salt basins of the Gabon and He Congo (Brazzaville): a tentative paleogeo- graphic interpretation, in Salt Basins Around Africa, Elsevier, Amsterdam, pp. 5~74. Boeuf, M. G., and H. Doust (1975). Stnlcture and development of the southern margin of Australia, Aust. Petrol. Explor. Assoc. J. 15, 3~43. Bulk, C. A. (1968). Buried ridges within continental margins, N.Y. Acad. Sc'. Trans. al:), 135 160. Carey, S. W. (1958). A tectonic approach to continental driR, in Continental Drift, a Symposium, S. W. Carey, ea., Geology Department, University of Tasmania, Hobart, Tasmania, Australia, pp. 177~49. Curray, J. R. (in press). The I~D P=g~me on Passive Conti- nental Margins, in The Evolution of Passive Continental Mar- gins in the Light of Becent Dnlling Results, Philosophical Transactions of the Royal Society, London, England. D'alce, C. L., M. Ewing, and G. H. Sutton (1959). Continental mans and geosynclines: the east coast of North Amenca, north of Cape HaKeras, in Physics and Chemistry of the Earth, Vol. 3, L. H. Ahrens, ea., Pergamon, New Yorlc, pp. 11~158. EmeIy, K O., E. Uchupi, J. D. Phillips, C. O. Bowin, E. T. Bunce, and I. T. Knott (1970). Continental rise oReasten~ North Amer- ica, Am. Assoc. Petrol. Geol. Bull. 54, 99~108. Exon, N. F., and T. B. Wilicox (19~78). Geology and petroleum potential of Exmouth Plateau and off western Australia, Am. Assoc. Petrol. Ceol. Bull. 641, 4~72. Hailer, J. (1971). Geology of the East Greenland Caledonides, Interscience, New York, 413 pp. Illies, J. H. (1977~. Ancient and recent niting in the Rhine graben, Geol. Midnbouux 56, 32~. . Pawns, B., and J. G. Sclater (1977). Analysis of *me variation of ocean floor bathymetry and heat flow with age, J. Ceophys. Res. 82, 80~7. Pilger, A., and A. Roster (1976). Temporal relationships in die tectonic evolution of the Afar Depression (Ethiopia) and the adjacent Arabian riR system, in Afar Between Continental and Oceanic Rifting, Interunion Commission on Geody- namics, Rep. No. 16, E. Schweizerbart'sche Verlagsbuchhand- lung, Stuttgart, pp. 1-25. Rabinowitz, P. D. (1972). Gravity anomalies on the continental margin of Angola, Afiica,J. Geophys. Res. 77, 6327 6357. Rabinowitz, P. D. (1974). The boundary between oceanic and continental crust in the we stem North Atlantic, in The Geology of Continental Margins, C. A. Burls and C. L. Dralce, eds., Spnuger-Verlag, New York, pp. 67~84. Rhine Graben Research Group for Explosion Seismology (1974). lithe 1972 seismic refraction experiment in the Rhine graben first results, in Approach to Taphrogenesis, J. H. Illies and K. Fuchs, eds., Schweizerbart'sche Verlagsbuchbandlung, Stuttgart, pp. 122 137. Shipley, T. H., and R. T. Buffler (1978). Seismic stratigraphy and geologic history of Blalce Plateau and adjacent western Atlantic continental margin,Am. Assoc. Petrol. Geol. Bull. 62, 792~19. Talwani, M., and O. Eldholm (1972). Continental margin offNor- way: a geophysical study, Geol. Soc. Am. Bull. 83, 3S75~606. Talwani, \1., and O. EIdholm (1977). Evolution of Norwegian Greenland Sea, Ceol. Soc. Am. Bull. 88, ~999. Vail, P. R., R. M. Mitchum, Jr., R. G. Todd, I. M. Widmier, S. Thompson III, J. B. Sangree, Jr., J. N. Bubb, and W. G. Hatlelid ( 1977). Seismic stratigraphy and global changes of sea

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Evolution of Outer Highs on Divergent Continental Margins level, in Seismic Stratigraphy~pplications to Hydrocarbon Exploration, C. E. Payton, ea., Am. Assoc. PetTol. Geol. Stem. 26, Tulsa. Oklahoma, pp. 4~212. Vail, P. R., R. M. Mitchum, Jr., T. H. Shipley, and R. T. Buffler (in press). Unconformities of Me North Atlantic, in The E?;olu- tion of Passive Continental Margins in the Light of Recent Drilling Results, Philosophical Transactions of the Royal Society, London, England. 61 Veevers, J. J. ( 1977). Paleobathymetry of the crest of spreading edges related to the age of ocean basins, Earth Planet. Sci. Lett. 34, 100 106. White, 5. M. (1979). Deep-sea drilling, Ceotimes, January 1979,

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