International Geology Review, Vol. 44, 2002, p. 479–500.
Copyright © 2002 by V. H. Winston & Son, Inc. All rights reserved.
Crustal-Scale Cross-Section of the U.S. Cordillera, California
and Beyond, Its Tectonic Significance, and Speculations
on the Andean Orogeny
E. M. MOORES,1
Department of Geology, University of California, Davis, California 95616
J. WAKABAYASHI,
1329 Sheridan Lane, Hayward, California 94544-5332
AND
J. R. UNRUH
William Lettis & Associates, 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596
Abstract
A cross-section across northern California from the San Andreas fault to central Nevada exhibits
both major east- and west-vergent structures. East-vergent structures include crustal wedging and
fault-propagation folds in the Coast Ranges, emplacement of the Great Valley ophiolitic basement
over Sierran basement rocks, early east-vergent structures in the latter, displacement along the eastern margin of the Sierra Nevada batholith, and thrust faults in western Nevada. West-vergent structures include faults within the Franciscan complex and “retrocharriage” structures in the Sierra
Nevada
A model of evolution of the U.S. Pacific margin emphasizes the role of ophiolites, island arc–
continental margin collisions, and subduction of a large oceanic plateau. Early Mesozoic subduction
along the Pacific margin of North America was modified by a 165–176 Ma collision of a major intraoceanic arc/ophiolite complex. A complex SW Pacific-like set of small plates and their boundaries
at various times may have been present in southern California between 115 and 40 Ma. Subduction
of an oceanic plateau about 85–65 Ma (remnants in the Franciscan) produced east-vergent tectonic
wedging in the Coast Ranges, possible thrusting along the eastern Sierra Nevada batholith margin,
and development of Rocky Mountain Laramide structures. The “Laramide orogeny” is herein redefined to include all late Cretaceous–Early Tertiary (75–45 Ma) fold-thrust structures from the Pacific
Coast to the Rocky Mountains. A speculative model for collisional involvement in the Andean orogeny is also presented, based upon timing of the onset of the Andean orogeny, the presence of oceanic
terranes along the western margin of the Andes, and the presence along part of the length of the
chain of a remnant marginal basin.
Introduction
A vital lesson of plate tectonics is that there is
no validity to any assumption that the simplest and therefore most acceptable interpretation demands a proximal rather than a
distant origin. (Coombs, 1997, p. 763)
RECENT DEVELOPMENTS IN knowledge of the Eastern
Pacific Cordillera suggest that a re-evaluation of its
tectonic development is appropriate. We begin our
analysis with the U.S. Cordillera in general, and
1Corresponding
author; email: moores@geology.ucdavis.edu
0020-6814/02/598/479-22 $10.00
California in particular. We present here a revised
model for tectonic development of the region of the
U.S. based upon our own work and that of many others. This model is an elaboration and refinement of
the collisional model of orogenic development presented, for example, for California, neighboring
North America, and northern South America by
Moores (1970, 1998), and for California and environs by Ingersoll (2000). Although our scenario is
mostly by consideration of a cross-section of California (south of the Klamath Mountains) and neighboring Nevada, we recognize that strike-slip motion has
affected and continues to affect the western part of
the United States. In addition, we present a re-eval-
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MOORES ET AL.
uation of the Andean orogeny, based upon a brief
summary of the literature.
A key element in our analysis is an emphasis on
the importance of ophiolites. They represent ocean
crust and mantle formed at spreading centers and
emplaced by collision of a continental margin or
island arc with mantle-rooted thrust faults bounding
subduction zones (Moores, 1998, 2002; Moores et
al., 2000). These thrust contacts represent the location of sutures that are major tectonic features of the
region in question.
Western United States
Figure 1 is a generalized map of the western U.S.
margin in California and neighboring regions. Two
maps are shown. Figure 1A shows the position of
selected key elements at present. Figure 1B is a palinspastic sketch incorporating removal of Basin and
Range extension and restoration of approximately
200 km of Mesozoic dextral movement on the Pine
Nut/Mojave–Snow Lake fault (Lewis and Girty,
2001). Figure 2 shows a generalized cross-section of
the region of discussion. The cross-section is based
on work by Wakabayashi and Unruh (1995), Godfrey
and Dilek (2000), and more recent data, as enumerated below. We describe the elements of this crosssection from west to east, approximately along a sector at 39–40° N. Latitude.
The cross-section illustrates major east-directed
thrusts beneath the Coast Ranges, the Central Valley, the Sierra Nevada, and the eastern part of the
Sierra Nevada batholith. These thrust faults vary in
age. Figure 3 tabulates a listing of selected tectonic/
metamorphic/ophiolitic events in the Coast Ranges,
Central Valley, Sierra Nevada, Western Nevada, and
southern California–Baja California. The cross-section conforms with the hypothesis of major eastdirected lithospheric thrusting proposed recently by
Ducea (2001), and a collisional model presented
earlier by Moores (1970).
Franciscan Complex
The Franciscan complex is a series of complexly
folded thrust-nappe structures consisting of intact
(coherent) thrust sheets and mélange zones (e.g.,
Blake et al., 1984, 1988; Wakabayashi, 1992).
North of the San Francisco Bay region, the Franciscan complex traditionally has been divided into
three principal belts (e.g., Blake et al., 1988; Fig. 2).
Within and south of the greater San Francisco Bay
region, however, the picture is more complex, and
distinctive belts are not discernible. Generally, the
structural position, metamorphic grade, and age of
incorporation of units (individual nappes inferred
from metamorphic or clastic rock ages) decreases
from east to west, consistent with progressive offscraping and accretion in the accretionary complex
(Wakabayashi, 1992), but this relatively simple pattern is complicated by displacement on the San
Andreas fault and associated transpressional folding. In the cross-section, the three northern belts are
shown modified after the reconstruction of Wakabayashi and Unruh (1995).
Major events of the Franciscan shown in the
table of Figure 3 include the ages of formation and
arrival of pelagic sediments in the “Central Belt,” as
well as periods of major metamorphism and exhumation, and timing of clastic sedimentation. Generally the ages of deposition and incorporation of
Franciscan rocks are not the same, as indicated in
Figure 3. Some mélanges may have originated as
olistostromes and have been subsequently subducted, whereas other mélanges may be solely of
tectonic origin (e.g., Cowan, 1985).
The Coastal Belt rocks represent the youngest
Franciscan rocks, accreted from Paleocene to
Eocene time (Blake et al., 1988). Rocks include
variably deformed sandstone and shale, subordinate
mélange, and minor basalt, limestone, and chert.
Field relations indicate that this belt is thrust
beneath the Central Belt to the east. Two fresh
peridotites, the Leggett and the Cazadero bodies
along the eastern margin of the Coastal Belt, may
represent the offscraped remnants of high-standing
domes formed near ridge-transform intersections
(Coleman, 2000).
The Central Belt Franciscan comprises a belt of
shale-matrix mélange units with blocks of diverse
lithologies and metamorphic grades. These blocks
include the major pelagic units listed on Figure 3.
The Central Belt mélange matrix exhibits both
prehnite-pumpellyite facies (e.g., Blake et al.,
1988), and blueschist facies (Terabayashi and
Maruyama, 1998) metamorphism. Fossils from
clastic rocks in this belt range from Tithonian to
Campanian age, but radiolaria from cherts are as
old as Pliensbachian (Blake et al., 1988). The distribution of Tithonian to Valangian fossils within the
matrix clastic rocks indicate considerable tectonic
recycling (by mélange plucking and recirculation,
e.g., Cloos, 1986; generation of a strike-slip
mélange, e.g., McLaughlin et al., 1988), or exhuma-
U.S. CORDILLERA
481
FIG. 1. Tectonic sketch map of part of the U.S. Pacific margin, showing selected principal tectonic features, major
ophiolite complexes (dark shading), the Great Valley ophiolite (light shading), and the Salinian block (cross-hatched).
Abbreviations: B = Bear Mountains ophiolite; C = Catalina schist; CRO = Coast Range Ophiolite; EF = Excelsior fault;
F = Franciscan; FRP = Feather River Peridotite; GM = Grizzly Mountain thrust; GV = Great Valley sequence; GVO =
Great Valley ophiolite; JO = Josephine ophiolite; HC = Humboldt complex; K = Klamath Mts; KK = Kings Kaweah
ophiolite; LFT = Luning Fencemaker thrusts; M = Mojave block; MS = Mojave Sonora megashear; MSLF = Mojave Snow
Lake Fault; PNF = Pine Nut fault; PrP = Preston Peak ophiolite; S = Salinian block; SC = Santa Cruz Is.; SCC = Smartville, Slate Creek, and Jarbo Gap ophiolites; T = Trinity ophiolite. A. Present configuration. B. Palinspastic map with
displacement on PNF-MSLF removed (Lewis and Girty, 2001). Diagrams modified after Dilek and Moores (1992, 1993)
and Moores (1998).
tion and resedimentation. Most Central Belt accretion probably occurred from Cenomanian to
Campanian time.
The Eastern Belt Franciscan, structurally the
highest and most uniformly recrystallized of the
Franciscan units, comprises coherent thrust sheets
of mostly metaclastic and metavolcanic rocks, and
subordinate mélange (Worrall, 1981), exhibiting
blueschist– and blueschist-greenschist–grade metamorphism (Blake et al., 1988). Metamorphic cooling
482
FIG. 2. Crustal-scale cross-section of the western United States along the approximate line indicated in Figure 1, just prior to San Andreas development. Abbreviations are the
same as in Figure 1 plus: CB = Sierra Nevada Central Belt; EB = Sierra Nevada Eastern Belt; F = Coastal Belt Franciscan Coastal belt; F CenB = Franciscan Central belt; GT =
Golconda thrust; J = Jurassic rocks; PP = Franciscan Picket Peak unit; S.N. = Sierra Nevada; WL = Walker Lane; XB = North American Precambrian crystalline basement; YB =
Franciscan Yolla Bolla unit. Modified after Godfrey and Dilek (2000).
MOORES ET AL.
ages of the structurally highest and most recrystallized part of the belt (Pickett Peak terrane) are 110–
146 Ma (e.g., Wakabayashi, 1992, 1999). The metamorphic age of part of the structurally lower Yolla
Bolly terrane is probably slightly younger than 120
Ma (reviewed in Wakabayashi, 1999). Cenomanian
rocks of the Hull Mountain area, probably the structurally lowest part of the Eastern Belt, were accreted
shortly after 95 Ma.
The 159–163 Ma high-temperature/high-pressure metamorphism of the high-grade blocks of
Eastern and Central Belt mélanges (Fig. 3) probably
occurred during the inception of subduction (Wakabayashi, 1990). The most significant exhumation of
coherent blueschist-facies Franciscan rocks took
place from 100 to 70 Ma (Tagami and Dumitru,
1996).
The widespread presence of aragonite in Franciscan blueschists and the lack of greenschist-facies
overprints indicate that east-dipping subduction
was continuous throughout Franciscan accretionary
history (160 to 20 Ma; cessation dependent on latitude; e.g., Wakabayashi, 1992). Continuous subduction is consistent with uninterrupted but variablerate pluton emplacement in the Sierra Nevada arc
until ~80 Ma (e.g., Ducea, 2001).
Structurally, the Franciscan rocks display surficial dips generally to the east, but with a significant antiform/synform structures affecting the
easternmost rocks and, as shown on Figure 2, the
underlying Central Belt Rocks (Maxwell, 1974).
The age of this regional-scale folding may be
equivalent to the “exhumation” and formation of
the Coast Range fault between 70 and 100 Ma (Fig.
3), or alternatively to formation of regional-scale
folds of ≤ 70 Ma of the Great Valley group,
described below.
In offshore southern California, garnet amphibolite and amphibolite metamorphism in the Catalina
schist occurred at 112 Ma (Mattinson, 1986), and
likely marks the inception of subduction there
(Platt, 1975). Continued subduction produced blueschist/greenschist– and blueschist-facies metamorphic rocks structurally beneath the hightemperature rocks. All Catalina schists cooled to
temperatures between 200 and 300°C by 90–100
Ma (Grove and Bebout, 1995). The Pelona-Orocopia
schists of southern California and Arizona, a unit
that includes blueschist-greenschist, epidote
amphibolite, and amphibolite metamorphism, was
metamorphosed at 60–65 Ma (Jacobson, 1990). The
higher-temperature metamorphism of the Pelona-
U.S. CORDILLERA
483
FIG. 3. Generalized time-space diagram of features along North American Cordilleran margin. Abbreviations are the
same as those in Figures 1 and 2, plus: am = amphibolite-facies metamorphism; bls = blueschist-facies metamorphism;
BH = Burnt Hills unit of Franciscan; “CRF” = Coast Range fault; dr = dextral-reverse shear; FFS = Foothill fault system
of Sierra Nevada; H.C. = Humboldt complex; HM = Hull Mountain unit of Franciscan; MH = Marin Headlands terrane;
PT = Permanente terrane; SC = Slate Creek complex; SFM = South Fork Mountain schist; SI = Stikine-Intermontane
superterrane; sr = sinistral-reverse shear; WI = Wrangell-Insular superterrane; WM = white mica; wr = whole rock; YRP =
Yuba Rivers pluton.
Oropia schists may be related to the inception of
another subduction zone, or to shallowing of eastdipping subduction beneath an active arc (Jacobson,
1997). In Baja California, 160–170 Ma amphibolite
metamorphism in tectonic blocks may mark the
inception of subduction, and blueschist assemblages yield dates of 95–115 Ma (Baldwin and Harrison, 1992).
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MOORES ET AL.
Coast Range/Great Valley Ophiolite
Great Valley Group
This ophiolite sequence underlies the Great Valley group throughout the Coast Ranges (the Coast
Range ophiolite), as well as in the subsurface of the
Great Valley itself (the Great Valley ophiolite)
(Godfrey et al., 1997; Godfrey and Klemperer,
1998). The magmatic age of the Coast Range ophiolite is ~165–170 Ma (Hopson et al., 1996), and a
“sedimentary hiatus” between formation of the ophiolite and deposition of overlying volcanopelagic
sediments apparently occurred from 165 to 155 Ma
(Pessagno et al., 2000).
In outcrop, faults are present everywhere below,
and in some places above, the Coast Range ophiolite. The fault beneath the ophiolite is commonly
called the Coast Range fault (Worrall, 1981; Jayko
et al., 1987), and north of San Francisco, along the
eastern margin of the Coast Ranges, the fault at the
base of the Great Valley Group is the Stony Creek
fault (Lawton, 1956; Chuber, 1962). Godfrey and
Klemperer (1998) and Godfrey et al. (1997) interpreted their results to reflect extensive boudinage in
the ophiolite along the western side of the GreatValley, an interpretation that agrees with our own observations of outcrop relations and data from the
western Sacramento Valley.
Most of the Coast Range ophiolite lacks penetrative deformation and displays negligible burial
metamorphism (Hopson et al., 1981). In southern
California, however, the ophiolite on Santa Cruz
Island exhibits a pronounced foliation and greenschist-facies burial metamorphism (Hopson et al.,
1981), as discussed below.
The geophysical data of Godfrey and Klemperer
(1998) and Godfrey et al. (1997) indicate that
beneath the Central Valley the ophiolite includes a
thick crustal and mantle sequence that in turn overlies continental crust and mantle. Thus the Central
Valley possesses a double Moho, which is congruent
with its low-lying status surrounded by the rapidly
rising Coast Ranges and Sierra Nevada, and may
explain the long-standing enigma of the Great Valley’s existence as an intermontane basin surrounded
on all sides by rising regions — the Coast Ranges,
the Klamath Mountains, Sierra Nevada, and Tehachapis. We adopt the thrust interpretation for this
Moho duplication, an interpretation favored by Godfrey and Klemperer (1998), Godfrey et al. (1997),
and Godfrey and Dilek (2000), and which is consistent with the isotopic evidence of Ducea (2001).
Deposits in the Central Valley intermontane
basin include the Tithonian-Maastrichtian Great
Valley group (e.g. Moxon, 1988) and overlying Cenozoic basin fill (e.g. Bartow, 1990). Jurassic and
Cretaceous rocks are deep-sea fan turbidites deposited on oceanic basement represented by the Coast
Range ophiolite (described above). Sedimentation
in Late Jurassic and Early Cretaceous time may
have coincided with subsidence and boudinage of
the Coast Range ophiolite and subsidence of the
ancestral forearc basin. A pronounced unconformity
in the subsurface affects pre-Albian rocks, with
Upper Cretaceous strata lapping eastward over
deformed, west-dipping, lower Great Valley rocks
and ophiolitic basement. The mid-Cretaceous angular unconformity visible in seismic reflection profiles from the Sacramento Valley may be due to: (1)
west-down subsidence along the eastern margin of
forearc basin; or (2) westward tilting of Late Jurassic
and Early Cretaceous strata due to shortening
accommodated by east-vergent thrusting (as
depicted on Figure 2). Reflector geometries in data
from the northern Sacramento Valley are consistent
with the former, whereas deformed (ophiolitic?)
basement visible in data from the southwestern Sacramento Valley seem better explained by the latter.
Franciscan, Coast Range ophiolite and lower
Great Valley rocks along the eastern Coast Ranges
are deformed in a series of NW-trending folds that
are not present in the upper Great Valley rocks (see
Fig. 4). Given the lack of Late Cenozoic cover in the
Coast Ranges, interpreting the origin of these folds
is problematic. They may be pre–Late Cretaceous in
age. Alternatively, they may be Pliocene-Recent,
and reflective of folding of similar orientation and
style in the northern Coast Ranges that has affected
rocks as young as Pleistocene. If pre-Late Cretaceous, the period of deformation would coincide in
time with the onset of the Sevier orogeny to the east,
and approximately with a major east-directed lithosphere-scale thrusting postulated on the basis of
geochemistry of the Sierra Nevada batholith and its
inclusions by Ducea (2001).
A major east-vergent thrust fault that was rooted
in the ancestral subduction zone displaces Franciscan, Coast Range ophiolite, and Great Valley
Group rocks to the east (Figs. 2 and 3). As discussed
by Wakabayashi and Unruh (1995), this thrusting
(antithetic to the subduction zone) began in latest
Cretaceous–Early Tertiary time, just after a period
485
U.S. CORDILLERA
Sierra Nevada/Klamaths (SK) :
F IG. 4. Generalized map of Great Valley–Franciscan,
Coast Range ophiolite relations along western side of the Sacramento Valley, California. Note that folds in the Coast Range
fault, Coast Range ophiolite, Stony Creek fault, and lower
Great Valley sequence do not affect upper Cretaceous Great
Valley rocks. After Jennings (1977).
of thick accumulation in the Great Valley forearc
basin, and it corresponds to a major unconformity in
the Great Valley group (Peterson, 1967a, 1967b).
Main-stage exhumation of blueschist-facies rocks of
the Franciscan (100–70 Ma) was likely completed
prior to the inception of this east-vergent thrusting,
because significant differential exhumation of the
Franciscan relative to the unmetamorphosed Great
Valley Group and Coast Range ophiolite cannot
occur with this fault geometry (Wakabayashi and
Unruh, 1995). The east-vergent thrusting was reactivated in Pliocene–Pleistocene time, resulting in
steep dips in the Plio-Pleistocene strata along the
western Central Valley margin, and significant seismicity including the 1892 Winters-Vacaville and
1983 Coalinga earthquakes (Unruh and Moores,
1992; Unruh et al., 1995).
The Sierra Nevada and Klamath mountains
together exhibit a complex set of pre-batholithic
rocks, generally loosely correlated with each other
and with the Stikine-Intermontane superterrane of
the northern Cordillera (e.g., Moores, 1998).
Although various units of the Sierra Nevada and
Klamath mountains have been correlated with one
another, in detail, significant differences exist
between the correlated units and their tectonic histories. One possible explanation for some of the differences is that there was a trench-trench transform
fault between the Sierra and the Klamaths during
part of Mesozoic (e.g. Dilek and Moores, 1992). We
restrict our comments to the Sierra Nevada, where
pre-batholithic rocks are grouped into four generally
recognized belts.
The Western Jurassic belt consists of a belt of
andesitic and related extrusives, shallow intrusives,
and plutonic rocks that extends some 200 km southward from the northern end of the Sierra Nevada
(e.g., Schweickert, 1981). In the north it chiefly consists of the Smartville complex, a rifted volcanic arc
constructed on 200–220 Ma oceanic basement
(Bickford and Day, 1988, 2001; Dilek, 1989a,
1989b; Saleeby et al., 1989). In the Smartville complex itself, folded pillow lavas and oceanic andesite
volcaniclastic deposts are cut by dikes that in turn
intruded and are intruded by plutons ranging in age
from 164 to 152 Ma (Beard and Day, 1987; Bickford
and Day, 1988, 2001; Saleeby et al., 1989).
The Central belt consists of the 225–175 Ma
Jarbo Gap and Slate Creek ophiolites, the youngest
units of which are correlative with the oldest Smartville rocks, and a Mesozoic chert-argillite chaotic
unit containing blocks of Carboniferous–Permian
Panthalassa limestones (Edelman et al., 1989).
Ophiolitic rocks sit in thrust contact above the
chert-argillite sequence in places, but elsewhere the
latter are intruded by ophiolitic lithologies. The
thrust contact between the ophiolitic rocks (Slate
Creek and related terranes) and chert-argillite
sequence is intruded by a 165 Ma pluton (Edelman
and Sharp, 1989). Although ophiolitic remnants are
common within and along the western margin of the
Central Belt, a tectonic root zone is not present on
the eastern margin of the belt. We interpret this to
mean that the Central belt rocks at least in part represent an accretionary prism that was formed in a
west-dipping subduction zone. Bickford and Day’s
(2001) data indicate that Smartville magmas incor-
486
MOORES ET AL.
porated Precambrian zircons, indicating a continent-derived source component for this oceanic arc,
consistent with a west-dipping subduction zone.
The Feather River complex and associated Devils
Gate ophiolite contain mafic and ultramafic rocks
that formed in at least two magmatic events, a poorly
defined one of Devonian age, and a later one at
about 300–320 Ma (Saleeby et al., 1989). Hornblendes, which date amphibolite-grade metamorphism, range in age from 240 to 390 Ma (reviewed in
Hacker and Peacock, 1990). This complex is a 6–10
km wide zone that extends over 100 km from the
north end of the Sierra to the south, where it may
merge with the Calaveras–Shoo Fly thrust (Schweickert, 1981).
The Eastern belt is a thick, polydeformed
sequence of Paleozoic–Jurassic rocks. The basal
unit is the Lower Paleozoic Shoo Fly complex, containing fault-bounded units of quartzose turbidite,
mafic volcanics, a 15-km long serpentinite lens discontinuously developed in the northern Sierra, and a
mélange containing exotic blocks of chert and
Ordovician limestone (Hannah and Moores, 1986;
Harwood, 1992). Early ENE-trending isoclinal folds
are present in this sequence (Varga and Moores,
1981). The serpentinite lens is only a few hundred
meters wide in outcrop, but gravity and magnetic
data indicate that it becomes several kilometers
thick at depth (Griscom, in Blake et al., 1989).
These rocks are interpreted as an off-continent turbidite sequence deposited upon unknown, but presumably oceanic, crust. The mélange and
serpentinite may reflect an ophiolite emplacement
event in mid–Late Devonian time (Varga and
Moores, 1981). Three volcanic-arc complexes
unconformably overlie the Shoo Fly rocks : (1)
Devonian–Mississippian units of the Sierra Buttes
and Taylor, Elwell, Keddie, and Peale formations,
interpreted as an oceanic volcanic arc; (2) Permian–
Triassic units of the Robinson, Reeve, Arlington,
and possibly Cedar formations, also possibly representing an oceanic arc; and (3) a Jurassic sequence
of the Mount Jura and Milton sequences that is
interpreted to represent a continental or near-continental arc (e.g., Hannah and Moores, 1986). For
much of the length of their exposure, the rocks are
primarily east-facing and overturned to the east.
Near the northern end of the Sierra Nevada, however, NW-trending, tight to isoclinal, overturned to
recumbent east-vergent folds and thrust faults
deform the rocks. Along the western margin of the
Eastern Belt, an enigmatic sequence of Devonian
(?)–Jurassic rocks crop out that show affinities with
sequences in the Klamath Mountains (Harwood,
1992; Jayko, 1988, 1990; Hannah and Moores,
1986).
Three major fault blocks separated by east-vergent thrust faults are present in the Eastern belt
(Fig. 1). A cleavage fan in the northern Sierra passes
westward to the south and incorporates the Feather
River peridotite. In this cleavage fan, NE-dipping
foliation and axial surfaces of folds in the west pass
eastward into SW-dipping foliation and axial surfaces. We interpret this structure to reflect the “retrocharriage” that has affected rocks of the northern
Sierra Nevada.
Deformation in the Eastern belt also increases in
complexity from east to west. In the east, post–Shoo
Fly rocks are generally unfoliated and only slightly
metamorphosed, but become progressively isoclinally folded and finally multiply folded as one
approaches the contact with the Feather River
peridotite.
The general structural relations described by
Day et al. (1985) suggest that the main Early Mesozoic structures in the Sierra Nevada consist of eastvergent thrust faults and folds, subsequently modified by west-dipping faults. Accordingly, structures
are depicted on the cross-section as both east-dipping and west-dipping (Fig. 2). The early east-vergent faults are interpreted to dip westward beneath
the Central Belt, Western Belt, and Great Valley,
consistent with geophysical and geochemical data
(Dilek and Moores, 1993; Godfrey et al., 1997; Godfrey and Klemperer, 1998; Godfrey and Dilek, 2000;
Bickford and Day, 2001; Ducea 2001). These fault
geometries also conform with those displayed by a
COCORP traverse across the northern Sierra
Nevada that indicated both west- and east-dipping
reflections (Nelson et al., 1986).
The Sierra Nevada batholith intrudes the rocks
described above. On Figure 3, the batholith is
shown modified from Godfrey and Dilek (2000).
Significant tectonic and magmatic events for the
Sierra Nevada on Figure 3 include: the age of the
Slate Creek and Smartville complexes, as well as
the Yuba River “stitching” pluton (Bickford and
Day, 2001); gold mineralization (Böhlke, 1999); the
Humboldt complex ages (Dilek and Moores, 1993);
age of Eastern Belt thrusts (Hannah and Moores,
1986); plutonic activity and lithosphere-scale
thrusting (Ducea 2001); apparent times of arrival of
the Smartville/Slate Creek (i.e., the Stikine-Intermontane terrane; Moores, 1998); backfolding and
U.S. CORDILLERA
487
FIG. 5. Generalized (A) sketch map and (B) cross-section illustrating possible configuration in Middle Jurassic time,
just prior to collision of oceanic island arc with the North American continent. Abbreviations: CGO = Coast Range–Great
Valley ophiolites; EK = Eastern Klamath belt; ESK = Eastern Klamath, Eastern Sierra Nevada belts; FRP = Feather
River peridotite; GU = Guerrero terrane, Mexico; HW = Walker-Humboldt basin; JG = Jarbo Gap ophiolite; NA = North
American continent. Mescalera plate after Dickinson and Lawton (2001).
formation of the Foothill fault system; arrival of
Wrangellia; and the arrival of Salinia (Wakabayashi
and Moores, 1988). Arrival of the Smartville/Slate
Creek ophiolites, widespread penetrative deformation and folding, as well as Eastern belt thrusting,
together constitute the “Nevadan” orogeny of Schweickert (1981), although they occurred earlier
than originally envisaged (Edelman and Sharp,
1989). The well-known Foothill fault system formed
later in the early–mid Cretaceous, associated with
gold mineralization (e.g., Böhlke, 1999; Ducea,
2001).
Parts of the western Sierra Nevada were
affected by penetrative deformation associated
with a sinistral-reverse sense of shear (east over
west) from 151 to 123 Ma (Paterson et al., 1987;
Tobisch et al., 1989), possibly as a consequence
of partitioning of strain associated with oblique
Franciscan subduction (Wakabayashi, 1992).
Dextral-reverse (east over west) ductile shear
488
MOORES ET AL.
FIG. 6. Schematic map of eastern Pacific basin at 180 Ma (Middle Jurassic). Abbreviations: C = Cuba; CC = Cordillera Central of Colombia; ESK = Eastern Sierra and Klamath belts; FRP = Feather River peridotite; G = Guerrero
terrane; GCO = Great Valley-Coast Range ophiolites; H = Hispaniola; HW = Humboldt-Walker basin; PR = Puerto Rico;
SI = Stikine Intermontane superterrane; SK = Sierra Nevada central and western belts and related rocks in Klamath
Mountains; VC = Venezuelan Coast Ranges; WI = Wrangell-Insular superterrane. Modified after Moores (1998, Fig. 10).
See text for discussion.
zones that cut the Sierra Nevada batholith
(approximate deformation age 100–85 Ma; e.g.,
Renne et al., 1993; Tobisch et al., 1995) may also
be related to strain partitioning of oblique Franciscan subduction.
Basin and Range
At the latitude of the cross-section, the Sierra
Nevada batholith extends into the Basin and Range.
The eastern side of the batholith is shown involved
in overthrusts, which we associatie with west-dipping crustal-scale reflectors of Allmendinger et al.
(1987). West-dipping thrust faults along the Walker
Lane and other parts of western Nevada are modified after Dilek and Moores (1993), Dilek et al.
(1988), and Godfrey and Dilek (2000). The timing of
these thrusts is not clear, although they must be
post–Jurassic, pre–Late Tertiary.
Archipelago Style of Orogeny
Moores (1998) proposed a model for an “archipelago” style of orogenic development, involving
convergence and collision of already complexly
deformed oceanic island arcs during Mesozoic and
Cenozoic time along the western margins of North
America and northern South America. Figure 5
shows a generalized tectonic sketch map and crosssection for Early Jurassic time, just prior to this collision. A volcanic arc on the North American craton
gives way to the northwest to an Alaska Peninsula–
like extension in the northern Sierra Nevada and
western Nevada, separated from the craton by the
oceanic Walker-Humboldt basin in western Nevada.
This configuration accounts for the total inferred
displacement on the Mojave–Snow Lake fault
(Lewis and Girty, 2001) and the Lower Mesozoic
basinal sediments in thrust contact beneath the
Humboldt complex. An oceanic island arc repre-
U.S. CORDILLERA
489
FIG. 7. Schematic map of the eastern Pacific basin at 160 Ma (Late Jurassic) after collision of SI and SK terranes
along western North America, and development of west-facing Franciscan subduction zone (outboard of GCO). Abbreviations are the same as those in Figure 6.
FIG. 8. Schematic map of the eastern Pacific basin at 140 Ma (Early Cretaceous). Abbreviations are the same as in
Figure 6, plus: C = Catalina schist.
490
MOORES ET AL.
FIG. 9. Schematic map of the eastern Pacific basin at 100 Ma (later Early Cretaceous) showing possible positions of
WI superterrane and hypothetical oceanic plateau. Abbreviations are the same as in Figures 6 and 7 plus: CH = Chortis
block; P = Piñon sequence of western Colombia and Ecuador; PE = Franciscan Permanente terrane; PO = Pelona and
Orocopia basins; V = Venezuelan basin; Y = Yucatan basin.
sented by the Jarbo Gap, Smartville–Slate Creek
ophiolites, Guerrero terrane, and intervening ophiolitic/island-arc rocks is separated from the North
American margin by a plate that was consumed on
both its margins—the Mescalera plate of Dickinson
and Lawton (2001). This plate has essentially disappeared.
We interpret the thrust-fault soles of these island
arc-ophiolite complexes to represent major sutures.
The polarity of the colliding arc was along a westdipping subduction zone (east-facing arc), because
of the geometry of the associated structures and the
internal pseudostratigraphy of the collided blocks.
The deformed and partly chaotic metasedimentary
rocks of the Central Belt of the northern Sierra
Nevada may partly represent material derived from
this plate.
The Feather River peridotite may either be a
remnant of the Mescalera plate preserved by out-ofsequence thrusting during the island arc–continen-
tal arc collision, or part of the North American plate.
The Feather River peridotite must also represent a
major suture of an as-yet unknown nature or age.
West of the oceanic arc is an entirely oceanic
plate called the Americord plate (this is the same
plate referred to by Moores [1998] as the Cordilleria
plate). The name has been changed because of prior
usage of the term “Cordilleria” by Chamberlain and
Lambert, 1985, and Lambert and Chamberlain,
1988). This model for plate evolution is similar to
that proposed by Ingersoll (2000).
Tectonic Reconstructions of
North American–Northern
South American Margins
We present here a series of approximate cartoons
(schematic maps) modified after Moores (1998) that
elaborate on the “archipelago” style of deformation.
Figure 6 shows a possible reconstruction at ~180
U.S. CORDILLERA
491
FIG. 10. Schematic map of the eastern Pacific basin at 60 Ma (Paleocene), showing development of the Laramide
orogeny from the Pacific coast to the Rocky Mountains, re-attached Salinian block. Abbreviations are the same as in
previous figures plus: PR = Peninsular Ranges terranes.
Ma. From west to east, the features are the Farallon
plate, the Wrangell-Insular (WI) superterrane with
an active island arc in the approximate position at
this time given by Debiche et al. (1987), the Americord plate separating the Wrangell-Insular from the
Stikine/Intermontaine (SI) terrane and its possible
continuations to the south, the Mescalera plate, and
the North American plate. Note that two plates separate North and South America from the Farallon
plate. The spreading center producing the Great
Valley/Coast Range ophiolite within the Americord
plate may have extended to the north or south.
Figure 7 shows a possible scenario at 160 Ma.
The SK terranes have attached to the north, but is
still outboard of the continent to the south, thereby
possibly setting up a transform fault between two
trenches of opposite polarity. Subduction is present
off northern South America.
At 140 Ma (Fig. 8), the Guerrero terrane has
attached (Dickinson and Lawton, 2001), but its
southern equivalents are still oceanward of the continents. The future position of the Catalina schist
(i.e., future C) is shown east of the WI superterrane.
At 100 Ma (Fig. 9), a possible scenario has a subduction zone along the entire margin of western
North America. The WI terrane has possibly arrived
in a “compromise” position (Stamatakos et al.,
2001), although its position could be further to the
west as shown, based upon the lack of paleolongitude constraint from paleomagnetic data. A westdipping subduction zone has formed to produce the
Catalina schist (C); this zone begins at 115 Ma and
492
MOORES ET AL.
FIG. 11. Schematic map of the eastern Pacific at 40 Ma (Middle Eocene). Abbreviations are the same as in Figures
6, 7, and 8.
collides at 90–100 Ma. An oceanic plateau west of
WI contains the Permanente Terrane (PE). Subduction of this oceanic plateau beginning in latest Cretaceous–Early Tertiary time (75–45 Ma) will cause
the Laramide orogeny (Henderson et al., 1984), as
here understood to include all crustal shortening of
the same age west to the continental margin. Location of possible future rifting of the Salinian block
and development of the Pelona-Orocopia (PO)
basins are shown.
By 60 Ma (Fig. 10), subduction of the oceanic
plateau is inferred to have produced a flattening of
the slab (now the Farallon plate). Increased traction
from this flattening would have produced compressional structures from the continental margin to the
Rocky Mountains, including thrust wedging in the
Coast Ranges, possibly thrusting along the eastern
edge of the Sierra and Klamaths, possibly renewed
movement on Sevier thrusts in Nevada-Utah, and
development of the Laramide structures in the
Rocky Mountains. At a 5–10 cm/year subduction
rate, the front of a subducted wide oceanic plateau
would take approximately 10–20 million years to
sweep eastward 1000 km from the coast.
By 40 Ma (Fig. 11), subduction has been reestablished along part of the North American margin. The Pelona-Orocopia schists have been
emplaced along an east-vergent thrust, and Salinia
may have moved slightly northward by pre-San
Andreas strike-slip faulting.
Figures 9–11 reflect our interpretation that this
time in eastern Pacific history was exceptionally
complex, more than commonly envisioned.
Although we acknowledge that these figures are only
schematic, they point toward a complexity that is
reminiscent of contemporary reconstructions of the
Southeast Asia/Southwest Pacific region (e.g. Hall,
1996).
U.S. CORDILLERA
493
FIG. 12. Generalized map of Andes, showing possible allochthonous terranes and marginal basins. Symbols: light
shading = allochthonous terranes of Colombia and Ecuador, inferred oceanic arc in western Chile; intermediate shading
= aborted marginal basins in Chile; dark shading = Rocas Verdes ophiolites of southern Chile; cross ruling = Pampean
ranges of western Argentina. Note location of cross-sections of Figure 13. After Moores and Twiss (1995, Fig. 12.12),
Mégard (1989), Mpdozis and Ramos (1989).
Discussion:
The North American Cordillera and a
Speculative Reconstruction of the Andes
Advances yet to be made in geology at first
will be regarded as outrages.
– W. M. Davis, 1926
The Andes of South America have long been considered as a type example of subduction beneath the
continental margin paired with antithetic thrusting,
in part associated with “flat slab subduction”(i.e.,
“Andean-style orogeny). Gutscher et al. (2000) convincingly demonstrated, however, that modern areas
of flat slabs correspond to the subduction of aseismic
ridges. Here we present a short synopsis of a possible
revision in some of the ideas on tectonic development of the Andes. This is meant not as a comprehensive review, but as a provocative essay in the
context of our model for western North America.
Figure 12 shows a generalized sketch map of the
Andes (modified after Moores and Twiss, 1995, Fig.
494
MOORES ET AL.
FIG. 13. Generalized cross-sections of Andes. Symbols: light shading = accreted terranes in sections A, B, C, D, E,
and G; intermediate shading = aborted marginal basin in sections C, and D; dark shading in section G = Rocas Verdes
ophiolites. After Moores and Twiss (1995, Fig. 12.13), Roeder (1988), Vicente (1989), and Mpodozis and Ramos (1989).
U.S. CORDILLERA
495
FIG. 14. Generalized time-space diagram showing principal depositional and tectonic events for the Andes. Broad
line indicates onset of “Andean orogeny.” After Moores and Twiss (1995, Fig. 12.14) and Mpodozis and Ramos (1989).
Abbreviations: bls-ec = blueschist-eclogite–facies metamorphism (after Feininger, 1980).
12.12 and Mpdozis and Ramos, 1990), together
with possible allochthonous features. The latter
include the “western terranes” of Cretaceous and
Jurassic age in western Ecuador and Colombia and
northwestern Peru, which include blueschist and
eclogite (Feininger, 1980, 1987; Aspden and Litherland, 1992), the Paracas arc of Colombia, the
volcanic-rich “Western Mesozoic Series” of Peru
(Mégard, 1989), and possibly the allochthonous
Precambrian Arequipa massif of southern Peru
(Vicente, 1989) and the very thick Jurassic–Lower
Cretaceous oceanic-affinity island-arc rocks of the
Coast Range of Chile (Vergara, et al., 1995, Buchelt and Cancino, 1988). These oceanic-affinity
rocks possibly developed when separated from
South America by a marginal basin represented by
the “aborted” marginal basin of Chile (Mpdozis
and Ramos, 1990, Ramos and Aleman, 2000,
Dalziel, 1986), the West Peruvian Trough (Dalziel,
1986), and the “Rocas Verdes” ophiolite basin of
Patagonia and Tierra del Fuego (e.g., Dalziel,
1986; Stern and deWit, 1997).
Figure 13 shows cross-sections of the Andes for
the various sectors. These cross-sections display the
various rocks to the west, and the presence (or
absence) of the Andean fold belt to the east. Noteworthy is the fact that Sector F lacks thrust-related
deformation. Only orogen-parallel strike-slip faulting is present.
Figure 14 is a time-space diagram, modified
after Moores and Twiss (1995, Fig. 12.14), that
shows the main lithologies in the various sectors and
the onset of the Andean orogeny. This onset is
marked by a transition from basinal and/or arc
development and major crustal shortening. In particular, this crustal shortening involves east-vergent
folds and thrusts in the north and south (sectors A
and G) where a recognized suture is present, as well
in other parts of the mountain belt where a suture
has not been identified. The similarity of structures
along the length of the Andes implies a similar tectonic history in sectors B through E, but not F.
Note that the timing of onset of the Andean orogeny is diachronous, being early Late Jurassic in the
north and south, and becoming progressively later to
the center. This variation in onset of orogeny argues
against an orogenic event driven by far-field effects
of spreading from the Mid-Atlantic Ridge or by
increase in movement in a so-called “absolute”
(hotspot) frame of reference.
496
MOORES ET AL.
FIG. 15. Hypothetical configuration of the western margin of South America in Late Mesozoic time, showing inferred
preorogenic edge of South America, and proposed offshore island arc and inferred components. See text for discussion.
These data and the orogen-wide prevalence of
east-vergent thrusting suggest that the Andes orogen
also may include a role for Mesozoic collision of an
offshore island arc, perhaps previously rifted from
the South American continent by back-arc rifting,
reversal of subduction, “collapse” (subduction) of
the marginal basin, and collision. Figure 15 is a
schematic tectonic sketch for Late Mesozoic time,
showing an offshore island arc composed of the possibly allochthonous units described above. The
“quiet zone” (C. Mpdozis, pers. commun., 1989) of
sector F is represented by a gap in the arc. The plate
between South America and the postulated offshore
island arc may have been equivalent to the Mescalera plate of Dickinson and Lawton (2001).
This reconstruction may seem outrageous, but it
fits a number of intriguing and heretofore unexplained features of the Andes. Of course, we
acknowledge the major continental-arc nature of
much of the Late Mesozoic and Cenozoic history of
the Andes (e.g., Lamb, et al., 1997). At least one
suture is present from northern Colombia to southern Ecuador (e.g., Aspden and Litherland, 1992)
and from Tierra del Fuego to the South Patagonian
ice field (S. Harambour, pers. commun., 1989).
Blueschist and eclogite (132 Ma white mica cooling
age) in southern Ecuador is certainly associated
with a suture (Feininger, 1980).
We are aware that no suture has been identified
along much of the rest of the length of the Andes. If
U.S. CORDILLERA
one is present, its lack of exposure or recognition
may possibly have resulted from subsequent shortening, strike-slip faulting, juxtaposition of volcanic
rocks of similar character across the suture, or
because the suture is overlain by younger rocks or
obscured by younger intrusions. We suggest that
the Andean fold-thrust belt had its inception with
the arc-continent collision, and has been reactivated
today because of major South American–Cocos–
Nazca–Antarctic plate interactions. Thus, the
revised tectonic history of the Mesozoic western
United States suggested in this paper may find its
counterpart in the Andes.
Acknowledgments
EMM thanks I. W. D. Dalziel, C. Mpdozis, F.
Hervé, and V. Ramos for introducing him to the
Andes, W. G. Ernst and S. Klemperer for organizing
the Thompson Symposium, and George Thompson
for fruitful discussions about many topics. We have
also benefited from helpful discussions with J. F.
Dewey, R. J. Twiss, Y. Dilek, and W. D. Sharp. Comments from J. F. Dewey and Y. Dilek on an earlier
version of this manuscript helped to improve it.
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