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OCR for page 63
III
#
INTRAPLATE
TECTONICS
OCR for page 64
OCR for page 65
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-
OCR for page 66
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-
OCR for page 67
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
OCR for page 68
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
OCR for page 69
Tectonics of Noncollis~onal Regimes
Aft::
_' , ~ . .
~_~ .
=_....
. . . . .
~.~ . ~ . . .
, ~ ... `~ . . . .
c'- , . . ... . . .
..... ~
`. ~ ^.
'I
_\ \.
~ '
~ if.
I, . ~ . . . .
I:-: ~ . An.- , . 2
>,-, -__ ~ ~ ~
~,:':
ll
-rat ~ ~ ~
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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.
OCR for page 70
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.
COLORADOWYOMING 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
OCR for page 71
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
OCR for page 72
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.
OCR for page 73
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
OCR for page 104
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.
OCR for page 105
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
. . .
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..
O
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.. -I.
. . · .
. · - - .
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. .:' ~
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- . lo.
. · . . ._ .
C'~t' - -,\-
~= .~:_ a.
· t~\
· . .. J A.
. ..CC id-
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· .: .-
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en do ..
' -
- ~
EF~C£NTER MAP Of WEST thy
UNITED STARS
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; . . . 45
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·. Cat
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OCR for page 106
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;
OCR for page 107
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.
OCR for page 108
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
OCR for page 109
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
OCR for page 110
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.
OCR for page 111
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.
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Representative terms from entire chapter:
western united
