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The role of mantle delamination in widespread Late Cretaceous extension
and magmatism in the Cordilleran orogen, western United States
Michael L. Wells†
Department of Geoscience, University of Nevada–Las Vegas, Las Vegas, Nevada 89154, USA
Thomas D. Hoisch
Department of Geology, Box 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA
ABSTRACT
Extension during plate convergence and
mountain building is widely recognized,
yet the causes of synconvergent extension
remain controversial. Here we propose that
delamination of lithospheric mantle, aided
by decoupling of the crust from the mantle
via a reduction in the viscosity of the lower
crust through heating, incursion of fluids,
and partial melting, explains many enigmatic yet prevalent aspects of the metamorphic, magmatic, and kinematic history of
the Sevier-Laramide orogen of the western
United States during the Late Cretaceous.
Extension, heating, anatexis, magmatism,
and perhaps rock uplift were widespread
during a restricted time interval in the
Late Cretaceous (75–67 Ma) along the axis
of maximum crustal thickening within the
Mojave sector of the Sevier orogen, and
to a lesser extent within the interior of the
Idaho-Utah-Wyoming sector to the north;
similar processes may have been active in
the Peninsular Range, Sierran, western
Mojave, and Salinian segments of the Mesozoic Cordilleran arc. These processes are
viewed as predictable consequences of the
thermal, rheological, and dynamic state of
the overlying crust following delamination
of mantle lithosphere beneath isostatically
compensated mountain belts. The proposed
delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the Farallon plate during the inception of the Laramide orogeny.
Following delamination, extension and anatexis of the North American crust were aided
locally by egress of slab-derived fluids from
the low-angle Farallon slab. We suggest that
lithospheric delamination may have aided
E-mail: [email protected]
†
in the shallowing of the slab to achieve lowangle subduction geometry. Delamination
has been proposed to be common in areas
of thickened continental lithosphere in the
terminal phase or late in orogenesis. The
Late Cretaceous delamination event proposed here for the Sevier-Laramide orogen
occurred during protracted plate convergence and was synchronous with, and followed by, continued shortening in the external part of the orogen.
Keywords: Cordilleran tectonics, Late Cretaceous, delamination, synconvergent extension,
Laramide.
INTRODUCTION
The discovery of synconvergent extension
within the interiors of modern convergent orogens (e.g., Dalmayrac and Molnar, 1981; Molnar and Chen, 1983) has provided insights into
the processes that control crustal thickness and
topography during orogenesis. Topography and
crustal thickness are governed, to a first order,
by a dynamic interplay between horizontal
compressional forces and the gravitational
potential energy stored in isostatically compensated thickened lithosphere (e.g., Molnar
and Lyon-Caen, 1988; England and Houseman,
1989). That the balance is commonly upset,
driving parts of convergent orogens into extension, is evident from numerous ancient examples, including the eastern Alps (e.g., Ratschbacher et al., 1989; Wallis et al., 1993), the
Apennines (Carmignani and Kligfield, 1990;
Ferranti and Oldow, 1999), the Calabrian Arc
(Wallis et al., 1993; Cello and Mazzoli, 1996),
the Himalaya (Hodges et al., 1992a; Burchfiel
et al., 1992a), the Scandinavian Caledonides
(Gee et al., 1994; Northrup, 1996), and the
Sevier-Laramide orogen (Wells et al., 1990;
Hodges and Walker, 1992). Despite these and
other studies, causes of synconvergent extension remain controversial.
Mechanisms proposed for synconvergent
extension require either (1) a decrease in horizontal compressive stress transmitted across the
plate boundary and/or basal décollement to the
orogen, (2) an increase in lateral contrasts in
gravitational potential energy, or (3) a reduction
in rock strength, lessening support against gravitational forces (Platt, 1986; England and Houseman, 1989; Willett, 1999; Rey et al., 2001).
Although not mutually exclusive, the diversity
of principal driving mechanisms suggested
for synconvergent extension in many orogens,
including the Himalaya-Tibetan Plateau (e.g.,
England and Houseman, 1989; McCaffrey and
Nabelek, 1998; Vanderhaeghe and Teyssier,
2001; Kapp and Guynn, 2004), the Andes (e.g.,
Kay and Kay, 1993; Marrett and Strecker, 2000;
Allmendinger, 1986), and the Eocene–Oligocene Basin and Range (e.g., Coney and Harms,
1984; Platt and England, 1994; Houseman and
Molnar, 1997; Dickinson, 2002), underscore
their controversial nature.
The removal (delamination) of lithospheric
mantle beneath isostatically compensated mountain belts is an effective mechanism to increase
gravitational potential energy, promoting surface uplift and horizontal extension (Bird, 1979;
Houseman et al., 1981; England and Houseman, 1989). Thermal and numerical modeling
of delamination and its effects on the overlying
lithosphere (e.g., England and Houseman, 1989;
Platt and England, 1994) has provided some predictions that can be used to evaluate the applicability of delamination in orogenic evolution.
Delamination of lithospheric mantle may result
in (1) an increase in Moho temperature and
geothermal gradient, which may be recorded
by distinctive P-T paths that include either
heating during decompression or isobaric heating following decompression; (2) decompression of partial melts of asthenosphere, yielding
GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 515–530; doi: 10.1130/B26006.1; 6 figures.
For permission to copy, contact [email protected]
© 2008 Geological Society of America
515
Wells and Hoisch
RH
PE
POR *3 2
0
4
PR
8
1
lt
P
Be
9
Colorado
Plateau
F
ide
The Idaho-Utah-Wyoming sector of the Sevier
fold-thrust belt is generally north striking and
thin skinned and lies well east of the magmatic
arc (Fig. 1). Thrusts carry thick packages of late
Proterozoic to Triassic rocks of the Cordilleran
miogeocline eastward over thinner Cambrian
and younger sedimentary rocks deposited on the
Precambrian basement of the North American
craton. Thrust duplications in the frontal part
*SN ES5
11
an
ce
O
Structural Style, Metamorphism, and
Anatexis
6
ra m
37°N
SS
c
ifi
ac
P
The Sevier and Laramide orogens (e.g.,
Armstrong, 1968; Tweto, 1975) of the western
United States (Fig. 1) are Mesozoic to early
Cenozoic backarc belts of crustal shortening
that are segments of the larger Cordilleran
orogen (Allmendinger, 1992; Burchfiel et al.,
1992b; DeCelles, 2004). The continuous antithetic fold-thrust belt (Sevier) and the eastern
province of discontinuous basement-involved
fold-thrusts (Laramide) (Fig. 1) are generally
attributed to Andean-style convergent tectonism. Unlike the Andes, mid-crustal levels of
a noncollisional orogen have been exhumed by
extensional and erosional denudation, which
allow the study of deeper exposures of the
products of synconvergent extension.
Studies over the past two decades have led to
the recognition that Cretaceous to early Tertiary
extension in the interior of the Sevier-Laramide
orogen was widespread and locally of large
magnitude (e.g., Wells et al., 1990, 2005; Applegate et al., 1992; Hodges and Walker, 1992). To
address the driving mechanism(s) for Cretaceous synconvergent extension of the Sevier and
Laramide orogens, we first review the structural,
metamorphic, and magmatic records of backarc
shortening; the timing and magnitude of shortening; and the plate tectonic setting during the
time of extension.
R
W 7
LB CN
10
SEVIER AND LARAMIDE OROGENS
516
42°N
La
distinctive magma chemistries; (3) surface uplift
and erosion; and (4) extension (Houseman et al.,
1981; Kay and Kay, 1993; Platt and England,
1994). We use the term delamination here to
describe the removal of mantle lithosphere by
density-driven foundering. Delamination by different mechanisms (e.g., peeling or convective
removal; Bird, 1979; Houseman et al., 1981;
Houseman and Molnar, 1997) produces similar
responses of heating and increased buoyancy
of the remaining lithosphere. In this paper we
marshal available petrologic and structural data
to show that delamination is the likely cause for
synconvergent extension in the hinterland of the
Cretaceous Sevier-Laramide orogen.
400 km
N
POR
108°W
120°W
Figure 1. Simplified tectonic map of the western Cordillera, showing selected Mesozoic to
early Cenozoic tectonic features and location of Figure 2. Belt of muscovite granites (dark
gray fill: Miller and Bradfish, 1980) largely coincides with belt of metamorphic core complexes (black fill) and inferred axis of maximum crustal thickening in the Cordilleran orogen.
The Sevier fold-thrust belt (light gray fill, after DeCelles, 2004; leading edge shown by bold
line with teeth) converges with the magmatic arc (cross pattern) toward the south in southeastern California. Dashed line shows position of inferred segment boundary in Laramide
slab between shallower angle (south) and steeper angle (north) subduction, after Saleeby
(2003). Numbers 1–11 refer to specific locations discussed in text and keyed to Figures 2–4.
CN—Central Nevada thrust belt; ES—Eastern Sierran thrust system; F—Funeral Mountains; LB—Luning thrust belt; P—Panamint Range; PE—Pequop Mountains; POR—
Pelona-Orocopia-Rand schist; PR—Peninsular Range; R—Raft River–Albion–Grouse
Creek; RH—Ruby Mountain–East Humboldt Range; SS—Sierra de Salinas; W—Wood
Hills. Stars indicate localities of xenolith studies: central Sierra Nevada (Big Creek; Lee et
al., 2000, 2001a), east Mojave (Cima Dome; Leventhal et al., 1995; Lee et al., 2001b).
are within the cratonal stratigraphic section. In
the interior (hinterland) of the Sevier orogen,
isolated occurrences of greenschist- and amphibolite-facies Barrovian metamorphic rocks (Fig.
1), locally recording up to 8–10 kbar pressures,
attest to substantial thrust burial and subsequent
exhumation (Hoisch and Simpson, 1993; Lewis
et al., 1999; McGrew et al., 2000; Hoisch et al.,
2002; Harris et al., 2007).
The Mesozoic shortening belt in southeastern California and southwestern Arizona differs
from the northern belt in structural style, timing,
and in distance from the coeval magmatic arc.
The belt departs from the miogeocline-craton
transition, wraps to the east-southeast (Figs. 1
and 2), and deforms the thin Phanerozoic strata
and underlying Precambrian basement of the
craton. The belt of shortening is coincident with
the eastern fringe of the Mesozoic magmatic arc
and exhibits marked plastic deformation (Burchfiel and Davis, 1981). Thrusts commonly carry
Proterozoic crystalline rocks to the southwest
and south over thin Paleozoic and Mesozoic
cratonal rocks (Reynolds et al., 1986; Howard
et al., 1987a). Ductile fold nappes, thrusts, and
4–6 kbar pressures from Mesozoic plutons and
their cratonal wall rocks mark the discontinuous
belt of substantial localized Mesozoic structural
burial (Howard et al., 1987a; Anderson et al.,
1989; Foster et al., 1992; Boettcher and Mosher,
1998) (Fig. 2). Shortening localized along the
east side of the Sierran magmatic arc to the
north (Eastern Sierran thrust system, Dunne and
Walker, 2004) (Fig. 1) may converge southward
Geological Society of America Bulletin, May/June 2008
Cretaceous extension and delamination in the Cordilleran orogen
CA
Q-T volc
NV
T plutons
T volc & sed
Late K plutons
Cla
thru rk Mo
st c unta
om
i
ple n
x
K sed
New York
Mtns
Cima Volcanic
Field
100 Ma volc
4 Pinto
shear
a
toni
Teu olith
h
Bat
Mid K plutons
NV
CA
Jr volc
zone
Colorado
3
Granite Mtns
Jr plutons
Pz-TR sed
pC
Mylonitic gneiss
CM
PM
r
ve
CHM
Ri
an
om s.
W n
ld t
O M
2
KH
Cadiz
Valley
Batholith
1
WM
Breakaway to
Colorado River
extensional corridor
Iron
Mtns.
LMM
N
BMM
Dome Rock
Mtns.
0
50 km
Figure 2. Simplified geologic map of eastern Mojave Desert region with location of areas discussed in text numbered 1–4. Cooling curves
for locations 1–4 are presented in Figure 3. Note discontinuous nature of Mesozoic thrusts (bold lines with teeth), interaction between
thrusts and Jurassic and middle Cretaceous plutons, and distribution of Late Cretaceous plutons. BMM—Big Maria Mountains; CHM—
Chemehuevi Mountains; CM—Clipper Mountains; LMM—Little Maria Mountains; KH—Kilbeck Hills; PM—Piute Mountains; WM—
Whipple Mountains. Lithologic abbreviations: Q—Quaternary; T—Tertiary; K—Cretaceous; Jr—Jurassic; TR—Triassic; PZ—Paleozoic;
– —Precambrian; volc—volcanic rocks; sed—sedimentary rocks. Figure modified from Wells et al. (2005).
pC
517
Wells and Hoisch
with the Sevier fold-thrust belt in the southeastern Mojave Desert.
Exposures of metamorphic rocks in the
interior of the Sevier-Laramide orogen in both
the Mojave Desert and Great Basin are commonly associated with peraluminous granites
of Late Cretaceous age, inboard of the extinct
coastal magmatic arc (Fig. 1) (Miller and Bradfish, 1980; Miller and Barton, 1990). Isotopic
compositions of these Late Cretaceous Cordilleran-type peraluminous granites (usage of
Patiño Douce, 1999) are consistent with variable sources in Precambrian continental basement and are widely attributed to be products
of crustal anatexis (Farmer and DePaolo, 1983;
Patiño Douce et al., 1990; Miller and Barton,
1990; Wright and Wooden, 1991), but with a
juvenile component (Patiño Douce, 1999; Kapp
et al., 2002). Geobarometry of both intrusions
and country rock suggests midcrustal crystallization depths (Anderson et al., 1989; Foster et
al., 1992). These plutons are especially abundant
in the eastern Mojave Desert region (Kapp et al.,
2002), although the belt continues northward
into the Canadian Cordillera (Fig. 1) (Miller and
Barton, 1990).
Timing and Magnitude of Shortening
Initial shortening in the Idaho-Utah-Wyoming
sector of the Sevier orogen progressed eastward
from Middle Jurassic to early Eocene time (Allmendinger, 1992; DeCelles, 2004). The foreland
fold-thrust belt is principally Early Cretaceous to
early Eocene in age (DeCelles, 1994; Lawton et
al., 1997; Yonkee et al., 1997), with episodic outof-sequence deformation interpreted in terms of
dynamic orogenic wedge mechanics (DeCelles
and Mitra, 1995; DeCelles et al., 1995; Camilleri et al., 1997). The hinterland underwent older
Middle–Late Jurassic shortening (Allmendinger
and Jordan, 1984; Hudec, 1992; Elison, 1995)
and also substantial Late Cretaceous shortening
(Miller et al., 1988; Camilleri and Chamberlain,
1997; McGrew et al., 2000; Harris et al., 2007).
Late Cretaceous to early Eocene deformation of
the Sevier belt temporally overlapped shortening in the Laramide foreland province (Dickinson et al., 1988).
In contrast, shortening terminated earlier
in the Mojave Desert region than to the north.
Frontal thrusts in the northeastern Mojave Desert region (Clark Mountain area) (Figs. 2 and 3)
are bracketed by 100 and 90 Ma igneous rocks
(Fleck et al., 1994; Smith et al., 2003). Farther
south in the westernmost Maria tectonic belt,
terminal shortening may be bracketed between
84 and 74 Ma (Barth et al., 2004). In the Late
Cretaceous to early Tertiary, most of the backarc
shortening in the southern Cordillera north of
518
central Arizona may have been accommodated
far to the northeast in the Laramide foreland
province east and north of the Colorado Plateau
(Dickinson et al., 1988).
Estimates of Late Jurassic to early Eocene
shortening across the backarc of the orogen
(Allmendinger, 1992; DeCelles, 2004) are variable along strike and fraught with uncertainties, resulting in part from extreme Cenozoic
extensional fragmentation, in particular for the
hinterland. Shortening estimates, principally
from palinspastic restoration of balanced cross
sections, are most robust across the fold-thrust
belt and vary from 100 km at the latitude of Las
Vegas, Nevada (Wernicke et al., 1988), to 220–
240 km in central Utah (DeCelles and Coogan,
2006). Additional shortening estimated across
the western hinterland of Nevada (Elison, 1991)
and the Laramide foreland (Brown, 1988) suggests cumulative Late Jurassic to early Eocene
shortening of the backarc region of NevadaUtah-Wyoming (east of the Luning-Fencemaker
belt) as much as 310 km. Shortening estimates
in the Mojave Desert region south of Las Vegas
(line of section of Wernicke et al., 1988) are
hindered by spatial overlap of the eastern fringe
of the Mesozoic magmatic arc with the belt of
shortening (Burchfiel and Davis, 1981).
Plate Tectonic Setting of the Laramide
Orogeny
Paleomagnetic plate reconstructions of the
Mesozoic Pacific basin provide constraints on
the relative plate motions between the Farallon
and Kula plates and the North American plate
during the Laramide orogeny (80–50 Ma). Farallon–North American and Kula–North American plate convergence was generally NE-SW
and at high rates (100–150 km/m.y.) during the
Laramide orogeny (80–50 Ma), followed by a
significant reduction in the convergence rate at
50–45 Ma (Engebretson et al., 1985; Kelley and
Engebretson, 1994). The Late Cretaceous extension and magmatism of the Sevier-Laramide
orogen occurred during accelerated convergence between the Farallon-Kula plates and
North America.
Various observations support the contention
that the subducting Farallon slab evolved from a
moderate dip to a low-angle dip during the transition from the Sevier to the Laramide orogeny
in the Late Cretaceous (ca. 80–75 Ma) (Coney
and Reynolds, 1977; Dickinson and Snyder,
1978; Bird, 1984, 1988; Usui et al., 2003). Direct
evidence for a low-angle subducted Farallon
slab exists in the occurrence of intermediate to
high pressure oceanic metamorphic rocks of the
Franciscan-affinity Pelona, Orocopia, Rand, and
related schists in southeastern California and
southwestern Arizona. The schists lie in tectonic
contact beneath the Precambrian to Mesozoic
continental basement and Mesozoic arc (Jacobson et al., 1988), and are exposed in a SE-trending belt of tectonic windows from the Tehachapi
Mountains–western Mojave Desert, along the
Transverse Ranges, into southwestern Arizona
(Fig. 1; Jacobson et al., 1988; Grove et al.,
2003b). Similar rocks are present in the Sierra de
Salinas of the Salinian block (SL, Fig. 1; Barth
et al., 2003; Grove et al., 2003b). Although
the upper contact bounding these windows of
underplated schist has been locally modified by
younger faults and retrograde metamorphism
(e.g., Jacobson and Dawson, 1995; Jacobson et
al., 1996), the broad regional distribution of the
schists (Grove et al., 2003b) suggests underplating of oceanic metasediments during low-angle
subduction and removal of the subcontinental
mantle lithosphere and the lower crust.
It has recently been suggested that the subducting oceanic slab was segmented, with the
flat-slab segment confined to the Laramide
“deformation corridor” southeast of the segment boundary (Fig. 1) and projecting under the
southernmost Sierran arc, Mojave Desert, Colorado Plateau, and Laramide foreland province of
the Rocky Mountains (Saleeby, 2003). A steeper
slab segment is interpreted to underlie the central and northern Sierran arc and the interior of
the Sevier-Laramide orogen in the Great Basin
(Saleeby, 2003). This model has important
implications for the geodynamic development
of the overriding plate and is discussed later.
LATE CRETACEOUS
SYNCONVERGENT EXTENSION IN
THE SEVIER-LARAMIDE OROGEN
Here we describe evidence for extensional
structures and exhumation during the restricted
time interval from 75 to 67 Ma from seven areas
(Figs. 1 and 2) along the axis of the Sevier orogen, as defined by the belt of metamorphic core
complexes (Coney and Harms, 1984). These
areas were selected on the basis of illustrative
relationships between extension and intrusion,
well-bracketed timing constraints, or well-constrained pressure-temperature (P-T) paths. Additionally, we consider the record of exhumation
from the Mesozoic batholithic belt to the west.
Taken together, these examples indicate that
extensional exhumation, and locally erosional
exhumation, was widespread along a significant
segment of the orogen between 75 and 67 Ma.
Eastern Mojave Desert
The record of Late Cretaceous extension
and plutonism in the Sevier belt is most clear
Temperature °C
900 E. Iron Mountains
800 granodiorite Zircon
4.8 kbar
700 GARB-GASP
Deformation
temperatures
600
Hornblende
400
Biotite
Tectonic
exhumation
200
200
A
1
70
80
Granite
800 Mountains
700 SW monzogranite
Zircon
900
500
Hornblende
B
0
50
Apatite
2
70
60
90
80
Zircon
Zircon
(late dike)
700
600
500
400
Tectonic
exhumation
K-feldspar
MDD
Deformation
temperatures
K-feldspar
MDD model
300
200
200
100
K-feld.
New York Mountains
800 Mid Hills monzogranite
400
300
Tectonic
exhumation
Biotite
900
Conductive
cooling
600 4.5 kbars
Al-in Hbl
90
100
Conductive
cooling
Hornblende
400
300
60
Al-in Hbl
GARB-GASP
500
300
0
50
C
0
50
3
60
70
90
80
600 East-Central Peninsular
Ranges batholith
500
Temperature °C
700
600
500
100
Temperature °C
Conductive
cooling
900 Old Woman pluton
Zircon
800 4.5 kbar
4
60
70
90
80
600
K-feldspar
MDD models
300
D
0
50
900 Salinia
Zircon U/Pb
Coast Ridge Complex
800
Garnet Sm-Nd
700
Biotite Ar-Ar
400
100
500
400
200
Biotite K-Ar
300
200
100
E
0
50
8
60
70
80
Age (Ma)
90
in the eastern Mojave Desert region, where Late
Cretaceous granites are abundant and numerous
extensional shear zones have been shown to be
coeval with granite intrusion. Here we outline
evidence of the magnitude and timing of Late
Cretaceous extension and of the synextensional
emplacement of some Late Cretaceous granites
from the Iron, Old Woman, Granite, and New
York Mountains.
100
F
0
50
10
Sedimentation
60
70
80
Age (Ma)
Iron Mountains
Porphyritic monzogranite of the Late Cretaceous Cadiz Valley batholith in the Iron Mountains is overlain by a roof zone composed of
screens of Precambrian rocks, separated by Cretaceous sills (Miller and Howard, 1985; Wells
et al., 2002). Within the roof zone a solid-state
deformation fabric comprises a >1.3-km structural thickness of mylonitic rocks. Foliation in
Figure 3. Cooling curves for
selected Late Cretaceous plutons in the eastern Mojave
Desert region derived from
U-Pb geochronology (zircon),
40
Ar/39Ar
thermochronology
(hornblende, biotite, and multiple diffusion domain [MDD]
modeling
of
K-feldspar),
and apatite fission track. (A)
Granodiorite from the eastern
Iron Mountains; biotite cooling
ages vary systematically, with
younger ages to the east (Wells
et al., 2002, unpublished data).
(B) Old Woman pluton (Foster et al., 1992); apatite fission
track ages vary by location.
(C) Monzogranite from southwest Granite Mountains (Kula,
2002). (D) Mid Hills monzogranite of the New York Mountains footwall to the Pinto shear
zone (Wells et al., 2005). (E)
Four representative K-feldspar
MDD cooling envelopes (Grove
et al., 2003a) from east-central
Peninsular Ranges batholith.
(F) Coast Range Complex,
Salinian block framework
rocks (Kidder et al., 2005). Kfeldspar MDD cooling envelopes represent 90% confidence
interval for the median of the
distribution of modeled cooling curves (Lovera et al., 1997).
Numbers in the lower right corners of the panels refer to location numbers shown in Figures
1 and 2. GARB-GASP refers to
the garnet-biotite geothermometer of Holdaway (2000) and
GASP refers to the geobarometer of Holdaway (2001).
90
the mylonite dips shallowly to the west, and mineral and stretching lineations plunge westward.
Kinematic indicators demonstrate top-to-the-east
noncoaxial shear. Beneath the shear zone and in
the uppermost 200 m of the porphyritic monzogranite a magmatic foliation is concordant with
the overlying wall-rock screens and solid-state
foliation. Ion microprobe U-Pb analyses of zircon of two phases of the batholith yield similar
519
Wells and Hoisch
ages of 75.4 ± 2.1 and 75.6 ± 1.7 Ma (Wells et
al., 2002), within 2σ analytical error of the age
(73.9 ± 1.9 Ma) reported by Barth et al. (2004).
Biotite 40Ar/39Ar analyses from 3-km intervals
along a 9-km lineation-parallel transect show a
consistent progression of isochron ages that are
younger toward the east—from 66.6 ± 0.3 Ma
(west) to 59.1 ± 0.7 Ma (east) (Fig. 3A). These
data, together with (U-Th)/He apatite and zircon
ages, which are consistent with westward tilt of
the range in Miocene time (Brichau et al., 2006),
support an initial component of east dip to the
shear zone. Downdip top-to-the-east kinematics
and bracketing U-Pb zircon and 40Ar/ 39Ar biotite
ages indicate extensional shearing between 75
and 67 Ma.
Old Woman–Piute Mountains
The Old Woman Mountains, Kilbeck Hills,
and Piute Mountains (Fig. 2) are underlain by
greenschist- to amphibolite-facies Cambrian to
Triassic cratonal metasedimentary rocks and
Proterozoic crystalline basement intruded by
Jurassic and Late Cretaceous granitoids. The
Jurassic and older rocks exhibit extensive Mesozoic ductile deformation, most prominently
within the NE-striking Scanlon thrust and its
associated basement-cored nappes (Miller et al.,
1982; Howard et al., 1987a). Thermobarometry
of Cretaceous granodiorite and pelitic schist in
its contact aureole indicates intrusion pressures
of 4–5 kbar, following crustal thickening associated with initial nappe emplacement (Foster
et al., 1992; Rothstein and Hoisch, 1994). The
most pervasive penetrative deformation in the
Old Woman Mountains apparently records a
younger extensional overprint. Synmagmatic
deformation within the Old Woman pluton
(74 ± 3 Ma; Foster et al., 1989) and solid-state
mylonitic deformation within the pluton and
wall rock screens along its tabular margins have
been interpreted as extensional, recording ~E-W
extension and vertical shortening (McCaffrey
et al., 1999). A top-to-the-southwest fabric that
overprints the nappes within the Scanlon thrust
zone (Nicholson, 1990; Owens, 1995), and a 1km-thick top-to-the-southwest mylonitic shear
zone that deforms the Cretaceous plutons and
wall rocks along the western margin of the Old
Woman Mountains (western Old Woman Mountains shear zone; Carl et al., 1991), have also been
interpreted as extensional. Thermochronometric
studies from the footwall of the shear zone indicate rapid cooling from intrusion temperatures
(~750 °C) at 74 Ma to fission track closure in
apatite (~100 °C) at 66 Ma (Fig. 3B). In light of
the recognition of the synextensional nature of
the Old Woman pluton (McCaffrey et al., 1999),
extension was active from intrusion at 74 Ma to
<68 Ma (Carl et al., 1991; Foster et al., 1992).
520
Granite Mountains
Jurassic and Cretaceous plutonic rocks (Howard et al., 1987b) in the Granite Mountains of the
Mojave National Preserve (Fig. 2) were emplaced
at midcrustal depths; barometry of Cretaceous
and Jurassic granitoids indicates 4–6 kbar pressures (Anderson et al., 1989; Kula, 2002; Kula
et al., 2002). The presence of screens of Paleozoic rock in these midcrustal intrusions suggests intrusion through tectonically buried rocks.
Barometry, geochronology, and thermochronology of 72–76 Ma granites indicate intrusion at
~4.5 kbar, followed by rapid cooling through
hornblende 40Ar/39Ar closure interpreted as conductive cooling to country rock temperatures
(Fig. 3C) (Kula et al., 2002). Continued rapid
cooling from 350 to 180 °C from 73 to 69 Ma, as
shown by K-feldspar multiple diffusion domain
(MDD; Lovera et al., 1991, 1997) is interpreted
as having resulted from extensional exhumation
(Fig. 3C) (Kula, 2002; Kula et al., 2002).
New York Mountains
The Pinto shear zone records Late Cretaceous
extensional unroofing of the Teutonia batholith
in the New York Mountains (Fig. 2) (Miller et
al., 1996; Beyene, 2000; Wells et al., 2005). The
shear zone deforms the middle Cretaceous (90
Ma ± 2, U-Pb zircon) Mid Hills monzogranite
and late (ca. 75 Ma) porphyry dikes and records
downdip, top-to-the-southwest shear during
decreasing temperatures from uppermost greenschist–lowermost amphibolite facies to cataclastic conditions. The geometry and kinematics,
hanging-wall deformation style, progressive
changes in deformation temperature, and differences in hanging wall and footwall thermal
histories allow a clear interpretation of the zone
as due to extension, not shortening (Wells et al.,
2005). Deformation is well constrained between
ca. 74 and 68 Ma by 40Ar/39Ar thermochronology of the exhumed footwall, including MDD
modeling of K-feldspar, which shows cooling
rates of 62–76 °C/m.y. (Fig. 3D). The magnitude
of exhumation along the shear zone remains
unclear owing to uncertainties in the emplacement depth of the Mid Hills monzogranite and
in the paleogeothermal gradient.
Great Basin
The record of Late Cretaceous extension
of the Sevier belt is more cryptic in the Great
Basin region than in the Mojave Desert, but
it may have been more widespread than currently recognized. This event in the Great Basin
is currently best understood in three areas: the
Funeral Mountains of southeastern California;
the East Humboldt Range, Wood Hills, and
Pequop Mountains of northeastern Nevada;
and the Raft River, Albion, and Grouse Creek
Mountains of northwestern Utah and southern
Idaho (Fig. 1). Although Late Cretaceous granites of the Great Basin define the northward continuation of the Cordilleran-type peraluminous
granite belt (Barton, 1990; Miller and Barton,
1990), in comparison with the Mojave there
are fewer recognized surface exposures of the
granites and Cretaceous extensional structures.
There is also a less extensive modern U-Pb geochronology, resulting in an unclear relationship
between Cretaceous intrusion and extension in
the Great Basin.
Funeral Mountains
The Funeral Mountains, located in Death
Valley National Park (Fig. 1), preserve a Barrovian metamorphic field gradient in Proterozoic metasedimentary rocks that resulted from
differential Mesozoic thrust burial; metamorphism increases from subgreenschist facies in
the SE to upper amphibolite facies and 7.5–9
kbar pressures to the NW across a distance of
~40 km (Labotka, 1980; Hodges and Walker,
1990; Hoisch and Simpson, 1993; Applegate
and Hodges, 1995). Although the metamorphic
rocks were exhumed during Cenozoic motion
along the Boundary Canyon detachment fault
(Wright and Troxel, 1993; Hoisch and Simpson,
1993), an earlier history of partial exhumation
in the Late Cretaceous has also been suggested
(Applegate et al., 1992). Shear zones record
top-to-NW shear down the field metamorphic
gradient (Applegate and Hodges, 1995). The
age of the early extensional deformation is constrained by U-Pb dating of synkinematic pegmatite (72 ± 1 Ma, zircon) and postkinematic
pegmatite (70 ± 1 Ma, zircon) (Applegate et
al., 1992). In addition, the Harrisburg fault in
the Panamint Range has been interpreted as the
southern continuation of Late Cretaceous shear
zones of the Funeral Mountains (Hodges et al.,
1990; Andrew, 2000).
East Humboldt Range, Wood Hills, Pequop
Mountains
Cretaceous exhumation of metamorphic rocks
in the East Humboldt Range core complex and
the adjacent Wood Hills and Pequop Mountains
(Fig. 1) is indicated by dated decompressional
P-T paths (Hodges et al., 1992b; McGrew and
Snee, 1994; McGrew et al., 2000) and the contemporaneous large-displacement Pequop normal fault (Camilleri and Chamberlain, 1997).
Similar to the Funeral Mountains, these ranges
collectively preserve a Barrovian metamorphic
field gradient, with metamorphism increasing
toward the northwest owing to increased tectonic
burial, with local metamorphic grade increases
near Jurassic, Cretaceous, and Tertiary plutons
11
A
10
6
9
B
8
ma
50 Ma
5
n
3
a
30 M
22–
2
1
Cores
Cores
20 Ma
0
Rims
tio
30–40 Ma
4
0
hu
6
Ex
Upper
Zone
7
7
Rims
85 Ma
200
400
600
800
1000
3
400
450
500
550
600
Lower
Zone
650
Figure 4. P-T paths from Great Basin core complexes. (A) P-T-t path envelope determined from an array of
individual P-T determinations from metabasite and metapelite of the East Humboldt Range, from McGrew
et al. (2000). Interpreted decompression is corroborated by decompressional metamorphic reaction textures.
(B) P-T paths from schist of Stevens Spring from the northern Grouse Creek Mountains, northwestern Utah
(location R, Fig. 1). Paths from Hoisch et al. (2002) are shown as solid lines, and paths from Harris et al. (2007)
are shown as dashed lines. Samples came from two zones within the schist of Stevens Spring, upper and lower,
as shown, corresponding to different garnet growth reactions. Al2SiO5 polymorphic transformations are shown
in solid lines (Pattison, 1992); Ky—kyanite; Sil—sillimanite; And—andalusite. Paths were determined from
simulations of garnet growth zoning, using the Gibbs method on the basis of Duhem’s theorem (e.g., Spear et
al., 1991). Numbers in the upper right corners of both panels refer to location numbers shown in Figure 1.
(Snoke et al., 1997; Camilleri and Chamberlain,
1997). Thermobarometric and geochronologic
studies of the deepest level rocks in the East
Humboldt Range define a decompressional
pressure-temperature-time (P-T-t) path from >9
kbar and 800 °C to ~5 kbar and 630 °C metamorphic conditions (Fig. 4A; McGrew et al.,
2000), bracketed between a Late Cretaceous
intrusion of leucogranite (84.8 ± 2.8 Ma, PbPb zircon) at peak metamorphic conditions and
Oligocene extensional shearing. Of the total
decompression, >2.5 kbar is interpreted to predate 40Ar/39Ar hornblende ages of ca. 50–63 Ma
and intrusion of 40 Ma (±3 Ma, U-Pb zircon)
diorite sills with Al-in-hornblende pressure
estimates of 4.5–5.5 kbar (Wright and Snoke,
1993; McGrew and Snee, 1994). In the Peqoup
Mountains the west-rooted Pequop low-angle
normal fault juxtaposes a nonmetamorphosed
over a metamorphosed miogeoclinal section
and caused an estimated 10 km of vertical thinning, cutting out an inferred older thrust (Thorman, 1970; Camilleri and Chamberlain, 1997).
The Pequop fault cuts a prograde metamorphic
fabric dated at 84.1 Ma (±0.2 Ma, U-Pb) on
metamorphic titanite from uppermost greenschist facies rocks and is overlain by 41 Ma
volcanic rocks (Brooks et al., 1995; Camilleri
and Chamberlain, 1997). Slip on the Pequop
fault is interpreted to have caused cooling at
75 Ma, as evident from U-Pb dating of titanite
(75 ± 1 Ma) in diopside zone metacarbonate
from the Wood Hills (Camilleri and Chamberlain, 1997). Muscovite 40Ar/39Ar ages of 75 Ma
from uppermost greenschist facies rocks in the
Pequop Mountains may also record such cooling; alternatively they may record thermal resetting (Thorman and Snee, 1988). The significant
imprint of Eocene and Oligocene high-grade
metamorphism and plutons, dikes, and sills,
and Oligocene mylonitic deformation has made
it difficult to unravel potential Late Cretaceous
exhumational structures from the deep levels of
the core complex (Snoke et al., 1997).
Raft River, Albion, and Grouse Creek
Mountains
In the Raft River, Albion, and Grouse Creek
Mountains of northwestern Utah and southern Idaho (Fig. 1), two distinct but supporting
observations record Late Cretaceous extension: (1) normal faults, including the Mahogany
Peaks fault (Wells, 1997; Wells et al., 1998); and
(2) P-T paths that require marked exhumation in
between two periods of thrusting (Hoisch et al.,
2002; Harris et al., 2007).
The Mahogany Peaks fault separates the
Neoproterozoic schist of Mahogany Peaks
and quartzite of Clarks Basin from Ordovician carbonate rocks and crops out discontinuously throughout the Raft River, Grouse Creek,
and Albion Mountains (Wells et al., 1998).
The Ordovician rocks of the hanging wall are
metamorphosed at upper greenschist–lower
amphibolite facies (475–510 °C) (Wells et al.,
1998). In contrast, mineral assemblages and
geothermometry of the Neoproterozoic pelitic
schist of Mahogany Peaks in the footwall indicates pressures and temperatures >6.5 kbar and
590 °C, respectively. The metamorphic grade
discordance of ~80–110 °C across the fault is
consistent with the postmetamorphic omission
of 3–4 km of rock. Because the fault places
younger strata over older, and shallower (colder)
over deeper (hotter) rocks, the fault is interpreted
as extensional in origin.
Cretaceous exhumation is also evident in the
P-T paths recorded in the pelitic schist of Stevens Spring, which lies in the footwall of the
Mahogany Peaks fault. Garnets in two zones of
the schist of Stevens Springs from Basin Creek
in the northern Grouse Creek Mountains grew by
different reactions at different times and record
different segments of a composite P-T path (Fig.
4B). The P-T path provides a record of two periods of thrusting separated by marked decompression and heating (Hoisch et al., 2002), the
latter interpreted to record tectonic exhumation.
The youngest thrust burial event has been dated
by Th-Pb dating of monazite inclusions in garnet
(Hoisch and Wells, 2004). Garnet growth and
inferred renewed thrust burial initiated at 69.5
± 5.2 Ma (95% confidence interval) and thus
provide a lower bound on the earlier period of
exhumation and heating interpreted from decompression P-T paths and geologic structures.
521
Wells and Hoisch
Magmatic Arc
The Mesozoic magmatic arc of the southwest Cordillera underwent cooling and exhumation associated with removal of lower crust
and mantle lithosphere in the Late Cretaceous
at the onset of Laramide tectonism and flat-slab
subduction (George and Dokka, 1994; Grove et
al., 2003a; Saleeby, 2003). Interpretations for
cooling vary between erosional and extensional
denudation, with or without subduction refrigeration, depending on location (Dumitru et al.,
1991; George and Dokka, 1994; Grove et al.,
2003a; Saleeby, 2003).
Northern Peninsular Range Batholith
In the northeastern Peninsular Range batholith, indistinguishable apatite fission track ages
and track length distributions over a 2.3 km
crustal section indicate rapid cooling between
80 and 74 Ma (George and Dokka, 1994). Late
Cretaceous rapid cooling (≤80 °C/m.y., 76–72
Ma) is further indicated by K-feldspar MDD
analysis (Grove et al., 2003a) (Fig. 3E). This
rapid cooling has been interpreted as having
resulted from erosional denudation induced
by the isostatic response to tectonic removal
of mantle lithosphere–lower crust, with a possible contribution from subduction refrigeration (George and Dokka, 1994; Grove et al.,
2003a). Thermochronology of detrital minerals
of the forearc sediments supports a component
of basement cooling owing to erosion (Lovera
et al., 1999). However, the high rates of cooling
may indicate a contribution from yet unrecognized extensional structures.
Sierra Nevada, Western Mojave, and Salinia
The southernmost Sierran, western Mojave,
and Salinian segments of the Mesozoic Cordilleran arc, segments adjacent to the Mojave sector of the Sevier orogen in pre-late Tertiary restorations (Grove et al., 2003b; Saleeby, 2003),
show evidence for both removal of lower crust
and mantle lithosphere and widespread extension in the Late Cretaceous (Saleeby, 2003).
The Sierra Nevada batholith exhibits a southward-increasing depth of exposure, from ~2
kbar in the south-central Sierra to ~9 kbar metamorphic conditions in the Tehachapi Range to
the south (Ague and Brimhall, 1988; Pickett
and Saleeby, 1993; Saleeby, 2003). The deepest exhumed rocks are in low-angle fault contact with underlying schist associated with the
Pelona-Orocopia-Rand schist belt, consistent
with delamination of mantle lithosphere and
lower crust and detachment along the base of
the felsic arc (Saleeby, 2003). Thermochronology indicates a Late Cretaceous age for denudation and northward tilt of the southern Sierras
522
(Wood and Saleeby, 1998), interpreted as the
isostatic response to removal of lower crust
and mantle lithosphere and to mid- to uppercrustal extension (Saleeby, 2003). Numerous
extensional allochthons are distributed about the
western Mojave, southern Sierra–Tehachapi,
and the Salinian block (Wood and Saleeby,
1998; Saleeby, 2003). K-Ar and 40Ar/39Ar cooling ages in other areas of the western Mojave
Desert (Evernden and Kistler, 1970; Armstrong
and Suppe, 1973; Kistler and Peterman, 1978;
Miller and Morton, 1980; Jacobson, 1990) suggest the possibility of widespread exhumation
of the western Mojave Desert at 75–67 Ma,
although the relative contributions of extension
and erosion remain unclear.
Similar relations are evident in the Salinian
block. The Sierra de Salinas schist is in lowangle fault contact beneath Cretaceous arc and
older metamorphic framework rocks (Barth et
al., 2003; Kidder and Ducea, 2006), similar to
the relations between the Pelona-OrocopiaRand schists and the southern Sierran–western
Mojave arc and framework rocks (Saleeby,
2003). Following attainment of metamorphic
conditions of ~7.5 kbar and 800 °C between 81
(±3) and 76 (±1) Ma, the framework rocks in
the Coast Ridge belt cooled through garnet SmNd (76 ± 1 Ma), biotite K-Ar (75 ± 4 Ma), and
apatite fission track (71 Ma), suggesting cooling rates during exhumation between 120 and
75 °C/m.y. (Kidder et al., 2005) (Fig. 3F). These
results are consistent with 40Ar/39Ar cooling
ages of biotite and hornblende from Cretaceous
granitoids in the Sierra de Salinas (Kistler and
Champion, 2001; Barth et al., 2003). Metamorphic framework rocks are overlain by Maastrichtian marine and terrestrial deposits, interpreted
by Grove (1993) as deposited in grabens.
Mantle xenolith studies from the central
Sierra Nevada (locality 11, Fig. 1) provide evidence for removal of all but a thin remnant of
ancient mantle lithosphere and for its replacement by asthenosphere (Lee et al., 2000, 2001a;
cf. Saleeby et al., 2003). Petrologic and Re-Os
isotopic studies of xenoliths in late Miocene
basalt show the mantle lithosphere as vertically
stratified with shallow (<45–60 km) and cold
Proterozoic mantle showing evidence for heating, whereas deeper (~45–100 km), younger
mantle lithosphere shows evidence for cooling
(Lee et al., 2000, 2001a). These observations
are interpreted to record removal of mantle
lithosphere, followed by upwelling and accretion of conductively cooled asthenosphere to the
base of the thinned lithosphere, in turn causing
heating of the relict mantle lithosphere (Lee et
al., 2000, 2001a). Although the timing of the
implied events is difficult to date precisely,
thermal arguments of Lee et al. (2000, 2001a),
based on preserved cation diffusion profiles in
orthopyroxene, suggest removal of mantle more
recently than ca. 150 Ma, and prior to late Miocene eruption of the host basalt. Arc-like mantle
wedge compositions of the accreted mantle suggest removal during or shortly following arc
production (Lee et al., 2000, 2001a). Mesozoic
removal of mantle lithosphere is consistent with
support of high elevations inferred for the Sierra
Nevada from low temperature thermochronometry and paleoelevation studies (House et al.,
1997; Mulch et al., 2006; Cecil et al., 2006).
DISCUSSION
Based on our current understanding, any
model to explain Late Cretaceous (75–67 Ma)
extension of the Sevier hinterland must account
for (1) the localization of extension along the
axis of prior crustal thickening; (2) significant
Late Cretaceous exhumation of up to 14 km;
(3) the common association of extension with
plutonism, usually of peraluminous composition; (4) extension being synmagmatic to postmagmatic, but apparently not premagmatic;
(5) a deep crustal source for Late Cretaceous
Cordilleran-type peraluminous granites, but with
a juvenile component; (6) the local development
of elevated temperatures (~600 °C), where Cretaceous plutons are absent, at middle crustal
depths (~4 kbar) following significant exhumation; (7) extension and magmatism occurring at
the onset of slab flattening during an increase
in plate convergence rate; (8) extension and
anatexis north and south of the inferred segment
boundary of the Farallon slab; and (9) extension
and magmatism synchronous with continued
shortening in both the Laramide foreland province and the Sevier fold-thrust belt northeast of
the Mojave Desert.
Rejection of Alternative Mechanisms for
Synconvergent Late Cretaceous Extension
Mechanisms that may be applicable to the
backarc Sevier-Laramide orogen, analogous in
tectonic setting to the modern subandean thrust
belt and its internal plateau (Jordan et al., 1983;
Allmendinger et al., 1997), are considered
below. As currently understood, Late Cretaceous
extension in the Sevier orogen was dominantly
perpendicular to the trend of the orogen, and
therefore we do not consider explicitly mechanisms that result in orogen-parallel extension
(e.g., Ave Lallemant and Guth, 1990; McCaffrey and Nabelek, 1998; Murphy et al., 2002).
Late Mesozoic to early Cenozoic plate
velocities and subducting plate geometry along
the western margin of North America preclude
a reduction in convergent velocity and slab
rollback as viable mechanisms for Late Cretaceous synconvergent extension of the Sevier
orogen. The orthogonal component of the plate
convergent velocities increased over this time
interval (Engebretson et al., 1985), and this was
a period of slab shallowing rather than steepening. An increase in horizontal compressive
stress is predicted from both an increased surface area of plate contact and increased convergence rate (Dickinson and Snyder, 1978; Bird,
1988; Livaccari and Perry, 1993).
Orogenic wedge mechanics (Platt, 1986; Willett, 1999), including the response to localized
crustal shortening via underplating and duplex
faulting, may have been a contributing factor in
causing Late Cretaceous extension in the hinterland of the Idaho-Utah-Wyoming sector of
the Sevier fold-thrust belt, but it is difficult to
apply to the Mojave sector. Duplex faulting to
produce culminations at the craton-miogeocline
transition and farther east in the hinterland of the
Idaho-Utah-Wyoming sector thickened the rear
of the foreland wedge (Yonkee, 1992; DeCelles
and Mitra, 1995; DeCelles et al., 1995; Mitra
and Sussman, 1997). Farther west, culminations
marking the location of the core complexes of
the Great Basin may have served a similar role
in accomplishing continued internal thickening during evolution of the orogenic wedge. In
contrast, the contractile belt in eastern California lacked an initial wedge-shaped sedimentary
prism, resulting in discontinuous thrusts involving crystalline rocks commonly not of décollement style (Burchfiel and Davis, 1971; Reynolds et al., 1986). We conclude that the dynamic
and geometric models of orogenic wedges (e.g.,
Platt, 1986; Willett, 1999), including underplating as a framework for driving extension, are
probably not viable for the Mojave sector of the
Sevier fold-thrust belt.
Saleeby (2003) proposed that synconvergent
extension and westward “breachment” of the
southern Sierran, Mojave, and Salinian arc
segments developed in the passing wake of
an underthrust aseismic ridge (Henderson et
al., 1984; Barth and Schneiderman, 1996) in
response to the progressive increase in density
of trailing abyssal oceanic lithosphere relative
to the subducted aseismic ridge. Although the
observations summarized here bolster the evidence presented by Saleeby (2003) for Laramide
extensional fragmentation of the forearc, arc,
and eastern arc fringe, the proposed mechanism
for extension faces challenges in application to
the eastern arc fringe in the eastern Mojave Desert region, and to regions north of the inferred
slab segment boundary. Initiation of extension
as shown here is well constrained at 73–75 Ma,
requiring the aseismic ridge to be east of California by this time. Difficulties in corroborating
this hypothesis include uncertainties in the absolute convergence rate between the Farallon and
North American plates (Engebretson et al., 1985;
Stock and Molnar, 1988), the timing of termination of arc magmatism and its causes (e.g., 76–
82 Ma; Mattinson, 1990; Kistler and Champion,
2001; Ducea, 2001), and in the causes of slab
flattening (Cross and Pilger, 1982; Henderson
et al., 1984). A sufficient slab rollback effect is
required at the trailing edge of the aseismic ridge
to generate extension in the overriding plate
without invoking slab breakoff. Slab breakoff
is precluded because shortening deformation of
the Rocky Mountain foreland and underplating
of the Pelona-Orocopia-Rand schist continued
into the early Tertiary (Dickinson et al., 1988;
Grove et al., 2003b). The thermal, chemical, and
xenolith evidence for Late Cretaceous invasion
of basalt into the lower crust (see below) and
the observation of initial extension as synmagmatic are more supportive of asthenosphere,
rather than the Farallon plate, directly underlying North American lithosphere prior to and perhaps during the initial stages of extension. Furthermore, if we accept the notion of a segmented
slab at face value, the model cannot be applied
to localities that overlie the inferred steeper slab
segment in the northern Great Basin.
Proposed Mechanisms: Delamination
Followed by Slab Dehydration
Based on the observations from the Mojave
Desert and Great Basin summarized above,
we propose the following two-stage evolution
during the inception of the Laramide orogeny
in the southwestern Cordillera (Fig. 5). Firstly,
the removal of mantle lithosphere and eclogitic lower crust (Saleeby et al., 2003) owing to
delamination and/or subduction erosion beneath
the arc (southern Sierra, Salinia, northern Peninsular Ranges), and removal of mantle lithosphere owing to delamination in the backarc
(Sevier hinterland and eastern Mojave Desert),
resulted in isostatic uplift, upwelling of asthenosphere, incursion of decompression basaltic partial melts and their associated heat into the lower
crust (most profound in the eastern Mojave),
surface erosion, and extension. Heating of the
lower crust led to anatexis and the reduction of
lower crustal viscosity, promoting channel flow
of the lower crust and decoupling between the
upper-middle crust and the remaining subcontinental mantle lithosphere and allowing the
upper-middle crust to respond to lateral gradients in potential energy and to extend.
Secondly, flattening and dehydration of
the Farallon slab, thought to be most relevant
to areas south of the inferred slab inflection
(Mojave Desert), but perhaps also a lesser
contributing factor to the north (Great Basin),
followed replacement of lithosphere by asthenosphere. Fluid flux into the lower crust from
a dehydrating Farallon slab may have further
promoted rheological weakening and crustal
melting, leading to magmatism and extension.
We suggest that delamination may have aided
in the shallowing of the slab by removal of a
physical impediment. Otherwise, attainment
of the shallow-angle geometry would require
“mechanical removal” or “subduction erosion”
by traction (e.g., Bird, 1984, 1988; Hamilton,
1988; Spencer, 1996). Removal of lithospheric
mantle would further facilitate the underplating
of weak, water-rich metasedimentary protoliths
for the Pelona-Orocopia-Rand schist. We note
that the complete (Bird, 1984, 1988) or partial
(Spencer, 1996) removal of mantle lithosphere
by lateral displacement, and its replacement
by subducted oceanic lithosphere, may result
in a similar response of increased buoyancy of
the overriding plate, leading to uplift, but will
lack the thermal and magmatic effects resulting
from replacement by asthenosphere (Dumitru
et al., 1991; English et al., 2003). Expulsion of
asthenosphere during subsequent shallowing of
the slab, perhaps not complete northeast of the
exposures of the Pelona-Orocopia-Rand schist,
occurred as the position of the shallowly dipping
Farallon plate propagated northeastward beneath
North America. The removal of lithosphere may
have been piecemeal or wholesale and variable
along orogenic strike (Fig. 5), but the similarity in timing of magmatism and extension (e.g.,
the eastern Mojave Desert region) suggests that
removal occurred over a short time interval for
>400 km along strike. Similarly, lithospheric
delamination during progressive slab flattening
has been invoked for the Andes (Kay and Kay,
1993; Whitman et al., 1996; Kay and Abbruzzi,
1996), where the geophysical signature of a
thinned mantle lid is preserved.
Below we explore why the geologic consequences of delamination of mantle lithosphere—an increase in Moho temperature,
decompression partial melting of asthenosphere, surface uplift, and extension—are most
compatible with observations (1) through (9)
from the selected areas as above, as well as
observations from mantle xenoliths and petrology of crustal metamorphic and igneous rocks
from elsewhere in the southwest Cordillera.
Heating and Deep Production of
Cordilleran-Type Peraluminous Granites
The Cordilleran-type peraluminous granites are widely viewed as representing crustal
melts with only a minor mantle contribution;
however, the mechanism of production of the
523
Wells and Hoisch
Mojave
95 Ma
A
95 Ma
D
Shortening
ES
Shortening
Great Basin
SFTB
LB
CN
SFTB
Shortening
Moho
Moho
End load
Isotherm
Isotherm
Topography 2X vertical
0
100
200 km
75 Ma
B
LFP
Extension
bp
Shortening
75 Ma
E
Extension
Shortening
Melting
bp
Uplift
SFTB
Shortening
LFP
Melting
Uplift
dm
dm
Delamination
Delamination
70 Ma
70 Ma
C
Extension
LFP
Shortening
F
SFTB
Shortening
Shortening
LFP
Melting
POR schist
Fluid flux
Figure 5. Tectonic cartoons illustrating Late Cretaceous removal of mantle lithosphere and its consequences for the Mojave region (left column, A–C) and the Great Basin region (right column, D–F). (A, D; 95 Ma) A significant root of mantle lithosphere developed during Sevier
orogenesis—mass balance (strain compatibility) requires shortening of mantle lithosphere equivalent in magnitude to shortening of crust in
fold-thrust belts of the Mesozoic backarc. Prior to delamination, increases in gravitational potential energy resulting from crustal thickening may have been offset by compensatory thickening of the mantle lithosphere. (B, E; 75 Ma) Instability of thickened mantle lithosphere
leads to delamination, causing decompression partial melting of asthenosphere, heating of lower crust resulting from basalt intrusion and
conductive heating of thinned lithosphere, crustal anatexis and magmatism, buoyancy-driven uplift, and gravitationally driven extension.
Delamination may have been piecemeal, driven by density inversion and possible traction from asthenosphere counterflow or driven wholesale by these factors plus end loading (Mojave). Detachment along low-viscosity crust at base of felsic batholith in arc after Saleeby (2003).
(C, F; 70 Ma) Continued extension in the Mojave Desert region, perhaps aided by egress of fluids from Laramide slab. Renewed shortening
in the hinterland region of the Great Basin. Abbreviations: dm—decompression melting; bp—basalt ponding; CN—Central Nevada thrust
belt; ES—Eastern Sierran thrust system; LB—Luning thrust belt; LFP—Laramide foreland province; SFTB—Sevier fold-thrust belt.
granites remains controversial. Crustal thickening, fluid infiltration from a shallow Laramide
slab, increased mantle heat flux, magmatic
underplating, decompression, and delamination
have all been invoked to explain melting of the
lower crust (Foster and Hyndman, 1990; Hoisch
and Hamilton, 1990; Patiño Douce et al., 1990;
Miller and Barton, 1990; Hodges and Walker,
1992). Dehydration partial melting owing to
rapid decompression is unlikely to have been of
widespread importance because most granites,
524
as best exemplified in the Mojave Desert
region, are pre- or synextensional rather than
postextensional, and intersection of P-T paths
with dehydration melting curves requires rapid
decompression (Fig. 6). Muscovite dehydration
melting resulting from crustal thickening has
been locally invoked for the Ruby Mountains
and East Humboldt Range of the Great Basin
(Lee et al., 2003). However, the production of
large quantities of melt requires the presence of
large volumes of a muscovite-rich metapelite
source, such as the Neoproterozoic siliciclastic
rocks of the miogeocline of the Great Basin,
and these rocks are absent in the eastern Mojave
Desert region. Furthermore, the low melt fraction produced by muscovite dehydration melting may also have been insufficient for separation, migration, and accumulation (Clemens and
Vielzeuf, 1987; Patiño Douce et al., 1990). It is
debated whether in situ heat production following crustal thickening was sufficient to induce
biotite dehydration partial melting, which could
Fluid Flux From Laramide Slab
In the Mojave region, melting of the lower
crust may have been promoted further by the
infiltration of fluids derived from the shallowing
Farallon slab, perhaps from water-rich underplated metasediments (Pelona-Orocopia-Rand
schist) in the subduction channel (Hoisch and
Hamilton, 1990; Malin et al., 1995). Zircon sat-
1
f
2
3
P (kb)
have produced the larger melt fractions required
for migration and accumulation of melts in plutons (Patiño Douce et al., 1990; Barton, 1990).
Alternatively, the gradual heating of the lower
crust as a consequence of asthenospheric counterflow in the mantle wedge (Armstrong, 1982;
Barton, 1990) would not produce the rapid
heating and increase in buoyancy apparently
required for the Cordilleran crust in the Late Cretaceous. Delamination, and the resulting incursion of basaltic asthenospheric melts produced
by decompression partial melting, could have
provided the necessary rapid heating to produce
crustal melts of similar age across the Sevier
hinterland (Barton, 1990; Kay and Kay, 1993;
Platt and England, 1994; Annen and Sparks,
2002). Incursion of basaltic melt is consistent
with experimental studies that suggest a basaltic
chemical contribution to the Cordilleran-type
peraluminous granites (Patiño Douce, 1999),
and field observations of interaction between
felsic and mafic magmas (e.g., Robinson et al.,
1986; Kapp et al., 2002). Furthermore, mid-oceanic-ridge basalt (MORB)–source lower crustal
xenoliths of Late Cretaceous age from the Cima
volcanic field (Fig. 2) suggest asthenospheric
upwelling beneath the eastern Mojave Desert
(Leventhal et al., 1995). All lower-crustal xenoliths at Cima are consistent with partial melts of
the asthenosphere, suggesting that invasion of
basalt into the lower crust was widespread.
Late Cretaceous heating of the crust is evident from the P-T path from the Grouse Creek
Mountains in northwest Utah, which shows continuous heating during a sequence of compression, decompression, and renewed compression
in the Late Cretaceous (Hoisch et al., 2002; Harris et al., 2007). The initial heating during thickening recorded by the path (marked upper zone,
Fig. 4B) is typical and expected in thrust settings
(e.g., England and Thompson, 1984) owing to
thermal relaxation and radiogenic heating of the
thickened pile; however, continued heating following exhumation would be difficult to explain
without invoking an additional heat source.
Because the additional heating took place in the
absence of exposed age-correlative plutons in
the nearby region, delamination accompanied
by asthenospheric upwelling seems necessary
to explain the continued heating in this area.
4
T (°C)
Figure 6. P-T diagram illustrating possible causes for melting of lower crust. Wet
melting of muscovite granite in the system K2O-Na2O-Al2O3-SiO2-H2O is shown by
the dotted-dashed line (labeled 1, from Thompson and Tracy, 1979). Melting reactions in the system Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O (NaKFMASH)are shown
as dashed lines (Spear et al., 1999). Al2SiO5 polymorphic transformations are shown
in solid lines (Pattison, 1992). Light gray polygon shows the region of muscovite
dehydration partial melting as determined from experiments on natural materials (Patiño Douce, 1999). Dark gray polygon shows the region of biotite dehydration partial melting as determined from experiments on natural materials (Patiño
Douce, 1999). NaKFMASH reactions: muscovite dehydration partial melting in the
presence of quartz and albite (labeled 2); two biotite dehydration partial melting
reactions (labeled 3); and wet melting of pelitic schist (labeled 4). Arrow shows the
adiabatic decompression path of Harris and Massey (1994). As—Al2SiO5; Ab—
albite; And—andalusite; Bt—biotite; Cd—cordierite; Gt—garnet; Ilm—ilmenite;
Kf—K-feldspar; Ky—kyanite; L—liquid; Mus—muscovite; Opx—orthopyroxene;
Pl—plagioclase; Qz—quartz; Sil—sillimanite.
uration temperatures from many Mojave Late
Cretaceous Cordilleran-type peraluminous granites with abundant inherited zircon indicate low
magma temperatures (725–825 °C) requiring
significant water at the melting site, more than
can be ascribed to in situ dehydration reactions
in the lower crust (Miller et al., 2003). Evidence
for Late Cretaceous fluid flux is also provided by
metamorphism of calcite-cemented sandstone
zones in the Supai Formation to massive (>90%)
wollastonite in the Big Maria Mountains (Fig.
2) (Hoisch, 1987). At the upper greenschist to
lower amphibolite facies conditions of metamorphism, fluid-to-rock ratios of >17:1 are required
to produce the observed quantity of wollastonite
(Hoisch, 1987). The minimum quantity of fluid
required is much too large to be derived from
the crystallization of hydrous melts beneath the
level of exposure. The influx of fluids expelled
from the Farallon plate and hydration of the
overlying North American lithosphere have also
been suggested to have occurred over a broad
region of the western United States to account
for anomalous elevation, low seismic velocities,
and melt production as far east as the Colorado
Mineral Belt (Humphreys et al., 2003).
Weakening of Lower Crust and Decoupling
Widespread melting and attendant rheological weakening of the lower crust, which
are required in the production of Cordillerantype peraluminous granites, may have played
a role in facilitating synconvergent extension.
525
Wells and Hoisch
A reduction in strength is predicted by thermal
softening through the temperature sensitivity of
plastic flow laws (Kohlstedt et al., 1995), and by
melt-induced weakening through anatexis and
production of migmatites (Jamieson et al., 1998;
Handy et al., 2001). Experimental petrology and
petrochemical modeling of the Cordilleran-type
peraluminous granites indicate that the crustal
source was deep (e.g., Patiño Douce, 1999; Kapp
et al., 2002), consistent with weakening of lower
crust. The site of melting was probably diffuse
and sheetlike, because it was controlled by the
thermal and lithologic structure of the lithosphere
(e.g., Sawyer, 1998), facilitating mechanical
decoupling between the upper mantle and overlying crust. If extension initiated and/or continued
after the low-angle slab made contact with eastern Mojave continental lithosphere, fluid infiltration from a devolatilizing Farallon slab would
have further weakened the lower crust (Hoisch
et al., 1988; Malin et al., 1995; Humphreys et
al., 2003). This weakened lower crust may have
undergone channel flow in response to pressure
gradients between the topographically elevated
orogen and the lower elevations to the east, and
flow may have been dynamically related to uplift
of the Colorado Plateau and shortening in the
Laramide belt (Livaccari, 1991; McQuarrie and
Chase, 2000), similar to the linkage between
crustal flow under the Tibetan Plateau and the
marginal belts of crustal shortening (e.g., Royden, 1996; Royden et al., 1997).
Precondition for Delamination
Mass balance considerations permit the lithospheric mantle beneath the Sevier hinterland
and the Mesozoic arc to have been substantially
thickened and subsequently removed. The shortening estimates of up to 310 km are calculated
from upper-crustal strain; transpressional accretion of terranes at the continental margin in the
U.S. segment of the Cordillera is insufficient to
balance shortening in the fold-thrust belt (Oldow
et al., 1989). Therefore, to maintain strain compatibility, an equivalent magnitude of shortening must have been accommodated in the lower
crust and mantle lithosphere, leading to development of a lithospheric root (Oldow et al., 1990).
Similar arguments have been put forth for the
development of thickened mantle lithosphere
beneath the Bolivian Altiplano (Sheffels, 1993)
and the Argentine Puna (Kay and Abbruzzi,
1996) (see Hyndman et al., 2005 for an alternative view of Cordilleran orogenesis). Numerical
simulations have shown that thickened mantle
lithosphere can become convectively unstable
and undergo convective removal tens of millions
of years after development of a lithospheric root
(England and Houseman, 1989; Houseman and
526
Molnar, 1997). Because backarc shortening during the Sevier-Laramide orogeny was protracted
for up to 110 m.y., delamination during rather
than after mountain building and plate convergence is permitted. The recognition of a thin
and fertile Archean mantle lithosphere beneath
the eastern Mojave from Re-Os isotopic studies of mantle xenoliths in Pliocene basalts of the
Cima volcanic field (Lee et al., 2001b) shows a
dense mantle lithosphere capable of foundering
and a thickness compatible with prior removal.
Should delamination in the presence of asthenosphere have been responsible for the removal of
mantle lithosphere and lower crust, rather than
subduction erosion by traction, then the formation of eclogites in the deep levels of the arc
(Ducea, 2001; Saleeby et al., 2003) may have
been significant in the arc region.
CONCLUSIONS
The Mesozoic arc and backarc belt of shortening of the southwest Cordilleran orogen
underwent widespread and large-magnitude
exhumation in the Late Cretaceous at the onset
of the Laramide orogeny. Although the relative
contribution of extension and erosion to the
total exhumation probably varied significantly
between localities, extension was nonetheless
prevalent north and south of the inferred segment boundary (Saleeby, 2003) in the Laramide
slab. The record of extensional exhumation is
most clear in the eastern Mojave Desert region,
where, in comparison with the Great Basin,
abundant Late Cretaceous plutons provide age
constraints, and subsequent Late Cretaceous to
early Tertiary shortening was partitioned east
of the inactive fold-thrust belt. Additionally, the
eastern Mojave Desert region is less overprinted
by subsequent Cenozoic extension and magmatism than the core complexes of the Great Basin
(Wells, 1997; Snoke et al., 1997), central Mojave
Desert (Fletcher et al., 1995), and Colorado
River Extensional Corridor (Howard and John,
1987), allowing unambiguous interpretation of
the age and kinematics of the Late Cretaceous
deformation. The propensity for reactivation of
continental faults–shear zones (e.g., White et
al., 1986) played a role in masking Cretaceous
extensional fabrics in areas of large magnitude
Cenozoic extension. Overprinting Cenozoic
fabrics in some metamorphic core complexes
of the western United States exhibit the same
foliation and lineation orientations and the same
sense-of-shear as the Late Cretaceous rock fabrics (John and Mukasa, 1990; Applegate and
Hodges, 1995; kinematic reactivation of Holdsworth et al., 1997). We predict that many more
of the metamorphic core complexes, with further study, will reveal evidence for Cretaceous
extensional exhumation. Furthermore, we argue
that the exhumation of mid-crustal rocks recording 6–9 kbar Mesozoic pressures (e.g., Hoisch
and Simpson, 1993; Lewis et al., 1999; McGrew
et al., 2000; Hoisch et al., 2002; Harris et al.,
2007) requires multiple exhumation events, as
Cenozoic detachment fault systems typically
exhumed pre-Cenozoic crustal levels no deeper
than 10–15 km (e.g., Richard et al., 1990; Hurlow et al., 1991; Wells et al., 2000).
Mechanisms to trigger extension during plate
convergence require either a reduction in horizontal compressive stress or an increase in forces
resulting from lateral contrasts in gravitational
potential energy. The initiation of extension in
the Late Cretaceous of the western United States
was associated with anatexis of the lower crust
and pluton emplacement at mid-crustal levels,
suggesting a common root cause. The metamorphic, magmatic, thermal, and kinematic histories
of the arc and backarc lead us to propose that
delamination of mantle lithosphere, aided by
decoupling of the crust from the mantle through
a reduction in the viscosity of the lower crust,
provides the most plausible explanation (Fig. 5).
ACKNOWLEDGMENTS
Financial support for our research in the Mojave
Desert was provided by U.S. National Science Foundation grant EAR 96-28540 (M.L.W.), and in the
Great Basin by NSF grants EAR-9805076 (T.D.H.)
and EAR-9805007 (M.L.W.). We have benefited from
discussions of Mojave geology and Laramide tectonics
with D. Foster, M. Grove, K. Howard, E. Humphreys,
C. Jacobson, J. Kula, C. Miller, D. Miller, T. Spell,
and P. Gans. This manuscript benefited from informal
reviews by M. Nicholl and A. Simon. We would like
to thank C. Jacobson and an anonymous reviewer for
their detailed reviews and thoughtful comments that
helped us to improve the paper.
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