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Geological Society of America Bulletin

doi: 10.1130/B26006.1 2008;120;515-530Geological Society of America Bulletin

Michael L. Wells and Thomas D. Hoisch magmatism in the Cordilleran orogen, western United StatesThe role of mantle delamination in widespread Late Cretaceous extension and

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515

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 fl uids, and partial melting, explains many enig-matic yet prevalent aspects of the metamor-phic, 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 Meso-zoic 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 immedi-ately prior to eastward propagation of low-angle subduction of the Farallon plate dur-ing the inception of the Laramide orogeny. Following delamination, extension and ana-texis of the North American crust were aided locally by egress of slab-derived fl uids from the low-angle Farallon slab. We suggest that lithospheric delamination may have aided

in the shallowing of the slab to achieve low-angle 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 pro-posed here for the Sevier-Laramide orogen occurred during protracted plate conver-gence and was synchronous with, and fol-lowed by, continued shortening in the exter-nal part of the orogen.

Keywords: Cordilleran tectonics, Late Creta-ceous, delamination, synconvergent extension, Laramide.

INTRODUCTION

The discovery of synconvergent extension within the interiors of modern convergent oro-gens (e.g., Dalmayrac and Molnar, 1981; Mol-nar 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 fi rst order, by a dynamic interplay between horizontal compressional forces and the gravitational potential energy stored in isostatically com-pensated 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 exten-sion, is evident from numerous ancient exam-ples, including the eastern Alps (e.g., Ratsch-bacher et al., 1989; Wallis et al., 1993), the Apennines (Carmignani and Kligfi eld, 1990; Ferranti and Oldow, 1999), the Calabrian Arc (Wallis et al., 1993; Cello and Mazzoli, 1996), the Himalaya (Hodges et al., 1992a; Burchfi el 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 exten-sion remain controversial.

Mechanisms proposed for synconvergent extension require either (1) a decrease in hori-zontal 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 gravi-tational forces (Platt, 1986; England and House-man, 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–Oligo-cene 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 moun-tain belts is an effective mechanism to increase gravitational potential energy, promoting sur-face uplift and horizontal extension (Bird, 1979; Houseman et al., 1981; England and House-man, 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 pre-dictions that can be used to evaluate the appli-cability 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 heat-ing following decompression; (2) decompres-sion of partial melts of asthenosphere, yielding

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. HoischDepartment of Geology, Box 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA

†E-mail: [email protected]

GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 515–530; doi: 10.1130/B26006.1; 6 fi gures.



Wells and Hoisch

516 Geological Society of America Bulletin, May/June 2008

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 dif-ferent 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.

SEVIER AND LARAMIDE OROGENS

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; Burchfi el et al., 1992b; DeCelles, 2004). The continuous anti-thetic fold-thrust belt (Sevier) and the eastern province of discontinuous basement-involved fold-thrusts (Laramide) (Fig. 1) are generally attributed to Andean-style convergent tecto-nism. 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; Apple-gate et al., 1992; Hodges and Walker, 1992). To address the driving mechanism(s) for Creta-ceous synconvergent extension of the Sevier and Laramide orogens, we fi rst review the structural, metamorphic, and magmatic records of backarc shortening; the timing and magnitude of short-ening; and the plate tectonic setting during the time of extension.

Structural Style, Metamorphism, and Anatexis

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

are within the cratonal stratigraphic section. In the interior (hinterland) of the Sevier orogen, isolated occurrences of greenschist- and amphi-bolite-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 southeast-ern 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 (Burch-fi el 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

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Figure 1. Simplifi ed 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 fi ll: Miller and Bradfi sh, 1980) largely coincides with belt of metamorphic core com-plexes (black fi ll) and inferred axis of maximum crustal thickening in the Cordilleran orogen. The Sevier fold-thrust belt (light gray fi ll, after DeCelles, 2004; leading edge shown by bold line with teeth) converges with the magmatic arc (cross pattern) toward the south in south-eastern 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 specifi c locations discussed in text and keyed to Figures 2–4. CN—Central Nevada thrust belt; ES—Eastern Sierran thrust system; F—Funeral Moun-tains; 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).



Cretaceous extension and delamination in the Cordilleran orogen

Geological Society of America Bulletin, May/June 2008 517

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Figure 2. Simplifi ed 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; pC–—Precambrian; volc—volcanic rocks; sed—sedimentary rocks. Figure modifi ed from Wells et al. (2005).



Wells and Hoisch


with the Sevier fold-thrust belt in the southeast-ern Mojave Desert.

Exposures of metamorphic rocks in the interior of the Sevier-Laramide orogen in both the Mojave Desert and Great Basin are com-monly associated with peraluminous granites of Late Cretaceous age, inboard of the extinct coastal magmatic arc (Fig. 1) (Miller and Brad-fi sh, 1980; Miller and Barton, 1990). Isotopic compositions of these Late Cretaceous Cor-dilleran-type peraluminous granites (usage of Patiño Douce, 1999) are consistent with vari-able sources in Precambrian continental base-ment 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 crystalli-zation 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 (All-mendinger, 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 out-of-sequence deformation interpreted in terms of dynamic orogenic wedge mechanics (DeCelles and Mitra, 1995; DeCelles et al., 1995; Camil-leri 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 shorten-ing in the Laramide foreland province (Dickin-son et al., 1988).

In contrast, shortening terminated earlier in the Mojave Desert region than to the north. Frontal thrusts in the northeastern Mojave Des-ert 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

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 vari-able along strike and fraught with uncertain-ties, 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) sug-gests cumulative Late Jurassic to early Eocene shortening of the backarc region of Nevada-Utah-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 (Burchfi el and Davis, 1981).

Plate Tectonic Setting of the Laramide Orogeny

Paleomagnetic plate reconstructions of the Mesozoic Pacifi c 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). Far-allon–North American and Kula–North Ameri-can 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 signifi cant reduction in the convergence rate at 50–45 Ma (Engebretson et al., 1985; Kelley and Engebretson, 1994). The Late Cretaceous exten-sion and magmatism of the Sevier-Laramide orogen occurred during accelerated conver-gence 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 tran-sition 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-affi nity 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 (Jacob-son et al., 1988), and are exposed in a SE-trend-ing 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 modifi ed 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 underplat-ing 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 sub-ducting oceanic slab was segmented, with the fl at-slab segment confi ned to the Laramide “deformation corridor” southeast of the seg-ment boundary (Fig. 1) and projecting under the southernmost Sierran arc, Mojave Desert, Colo-rado Plateau, and Laramide foreland province of the Rocky Mountains (Saleeby, 2003). A steeper slab segment is interpreted to underlie the cen-tral 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 oro-gen, as defi ned 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-con-strained pressure-temperature (P-T) paths. Addi-tionally, 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 signifi cant 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





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.

Iron MountainsPorphyritic monzogranite of the Late Creta-

ceous Cadiz Valley batholith in the Iron Moun-tains is overlain by a roof zone composed of screens of Precambrian rocks, separated by Cre-taceous sills (Miller and Howard, 1985; Wells et al., 2002). Within the roof zone a solid-state deformation fabric comprises a >1.3-km struc-tural thickness of mylonitic rocks. Foliation in

the mylonite dips shallowly to the west, and min-eral 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 monzo-granite a magmatic foliation is concordant with the overlying wall-rock screens and solid-state foliation. Ion microprobe U-Pb analyses of zir-con of two phases of the batholith yield similar

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Figure 3. Cooling curves for selected Late Cretaceous plu-tons in the eastern Mojave Desert region derived from U-Pb geochronology (zircon), 40Ar/39Ar thermochronology (hornblende, biotite, and mul-tiple diffusion domain [MDD] modeling of K-feldspar), and apatite fi ssion 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 (Fos-ter et al., 1992); apatite fi ssion track ages vary by location. (C) Monzogranite from south-west Granite Mountains (Kula, 2002). (D) Mid Hills monzo-granite of the New York Moun-tains 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). K-feldspar MDD cooling enve-lopes represent 90% confi dence interval for the median of the distribution of modeled cool-ing curves (Lovera et al., 1997). Numbers in the lower right cor-ners of the panels refer to loca-tion numbers shown in Figures 1 and 2. GARB-GASP refers to the garnet-biotite geothermom-eter of Holdaway (2000) and GASP refers to the geobarom-eter of Holdaway (2001).



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 MountainsThe 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 Meso-zoic 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 asso-ciated 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 1-km-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 Moun-tains shear zone; Carl et al., 1991), have also been interpreted as extensional. Thermochronometric studies from the footwall of the shear zone indi-cate rapid cooling from intrusion temperatures (~750 °C) at 74 Ma to fi ssion 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).

Granite MountainsJurassic and Cretaceous plutonic rocks (How-

ard 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 pres-sures (Anderson et al., 1989; Kula, 2002; Kula et al., 2002). The presence of screens of Paleo-zoic rock in these midcrustal intrusions sug-gests intrusion through tectonically buried rocks. Barometry, geochronology, and thermochronol-ogy of 72–76 Ma granites indicate intrusion at ~4.5 kbar, followed by rapid cooling through hornblende 40Ar/39Ar closure interpreted as con-ductive 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 MountainsThe Pinto shear zone records Late Cretaceous

extensional unroofi ng 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 green-schist–lowermost amphibolite facies to catacla-stic conditions. The geometry and kinematics, hanging-wall deformation style, progressive changes in deformation temperature, and dif-ferences 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 thermochronol-ogy 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 emplace-ment 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 cur-rently 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 gran-ites of the Great Basin defi ne the northward con-tinuation 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 geo-chronology, resulting in an unclear relationship between Cretaceous intrusion and extension in the Great Basin.

Funeral MountainsThe Funeral Mountains, located in Death

Valley National Park (Fig. 1), preserve a Bar-rovian metamorphic fi eld gradient in Protero-zoic metasedimentary rocks that resulted from differential Mesozoic thrust burial; metamor-phism 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 fi eld metamorphic gradient (Applegate and Hodges, 1995). The age of the early extensional deformation is con-strained by U-Pb dating of synkinematic peg-matite (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 con-temporaneous large-displacement Pequop nor-mal fault (Camilleri and Chamberlain, 1997). Similar to the Funeral Mountains, these ranges collectively preserve a Barrovian metamorphic fi eld gradient, with meta morphism increasing toward the northwest owing to increased tectonic burial, with local metamorphic grade increases near Jurassic, Cretaceous, and Tertiary plutons





(Snoke et al., 1997; Camilleri and Chamberlain, 1997). Thermobarometric and geochronologic studies of the deepest level rocks in the East Humboldt Range defi ne a decompressional pressure-temperature-time (P-T-t) path from >9 kbar and 800 °C to ~5 kbar and 630 °C meta-morphic conditions (Fig. 4A; McGrew et al., 2000), bracketed between a Late Cretaceous intrusion of leucogranite (84.8 ± 2.8 Ma, Pb-Pb zircon) at peak metamorphic conditions and Oligocene extensional shearing. Of the total decompression, >2.5 kbar is interpreted to pre-date 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 thin-ning, cutting out an inferred older thrust (Thor-man, 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 green-schist 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 Chamber-lain, 1997). Muscovite 40Ar/39Ar ages of 75 Ma from uppermost greenschist facies rocks in the Pequop Mountains may also record such cool-ing; alternatively they may record thermal reset-ting (Thorman and Snee, 1988). The signifi cant imprint of Eocene and Oligocene high-grade metamorphism and plutons, dikes, and sills, and Oligocene mylonitic deformation has made it diffi cult 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 south-ern Idaho (Fig. 1), two distinct but supporting observations record Late Cretaceous exten-sion: (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 Ordovi-cian carbonate rocks and crops out discontinu-ously 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 indi-cates 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 Ste-vens 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 peri-ods of thrusting separated by marked decom-pression 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% confi dence interval) and thus provide a lower bound on the earlier period of exhumation and heating interpreted from decom-pression P-T paths and geologic structures.

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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.



Wells and Hoisch


Magmatic Arc

The Mesozoic magmatic arc of the south-west Cordillera underwent cooling and exhu-mation associated with removal of lower crust and mantle lithosphere in the Late Cretaceous at the onset of Laramide tectonism and fl at-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 refrig-eration, depending on location (Dumitru et al., 1991; George and Dokka, 1994; Grove et al., 2003a; Saleeby, 2003).

Northern Peninsular Range BatholithIn the northeastern Peninsular Range batho-

lith, indistinguishable apatite fi ssion 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 pos-sible contribution from subduction refrigera-tion (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 unrecog-nized extensional structures.

Sierra Nevada, Western Mojave, and SaliniaThe southernmost Sierran, western Mojave,

and Salinian segments of the Mesozoic Cordil-leran arc, segments adjacent to the Mojave sec-tor of the Sevier orogen in pre-late Tertiary res-torations (Grove et al., 2003b; Saleeby, 2003), show evidence for both removal of lower crust and mantle lithosphere and widespread exten-sion in the Late Cretaceous (Saleeby, 2003). The Sierra Nevada batholith exhibits a south-ward-increasing depth of exposure, from ~2 kbar in the south-central Sierra to ~9 kbar meta-morphic conditions in the Tehachapi Range to the south (Ague and Brimhall, 1988; Pickett and Saleeby, 1993; Saleeby, 2003). The deep-est exhumed rocks are in low-angle fault con-tact 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). Thermochronol-ogy indicates a Late Cretaceous age for denuda-tion and northward tilt of the southern Sierras

(Wood and Saleeby, 1998), interpreted as the isostatic response to removal of lower crust and mantle lithosphere and to mid- to upper-crustal 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 cool-ing 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) sug-gest 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 low-angle 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-Orocopia-Rand 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 Sm-Nd (76 ± 1 Ma), biotite K-Ar (75 ± 4 Ma), and apatite fi ssion track (71 Ma), suggesting cool-ing 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). Metamor-phic framework rocks are overlain by Maastrich-tian 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 evi-dence for removal of all but a thin remnant of ancient mantle lithosphere and for its replace-ment 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 stratifi ed with shallow (<45–60 km) and cold Proterozoic mantle showing evidence for heat-ing, 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 accre-tion 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 diffi cult to date precisely, thermal arguments of Lee et al. (2000, 2001a),

based on preserved cation diffusion profi les in orthopyroxene, suggest removal of mantle more recently than ca. 150 Ma, and prior to late Mio-cene eruption of the host basalt. Arc-like mantle wedge compositions of the accreted mantle sug-gest 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 thermochronom-etry 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) signifi cant Late Cretaceous exhumation of up to 14 km; (3) the common association of extension with plutonism, usually of peraluminous composi-tion; (4) extension being synmagmatic to post-magmatic, 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 Cre-taceous plutons are absent, at middle crustal depths (~4 kbar) following signifi cant exhuma-tion; (7) extension and magmatism occurring at the onset of slab fl attening 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 prov-ince 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 mecha-nisms that result in orogen-parallel extension (e.g., Ave Lallemant and Guth, 1990; McCaf-frey 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 Cre-taceous 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 steep-ening. An increase in horizontal compressive stress is predicted from both an increased sur-face area of plate contact and increased conver-gence rate (Dickinson and Snyder, 1978; Bird, 1988; Livaccari and Perry, 1993).

Orogenic wedge mechanics (Platt, 1986; Wil-lett, 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 hin-terland of the Idaho-Utah-Wyoming sector of the Sevier fold-thrust belt, but it is diffi cult 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 thicken-ing during evolution of the orogenic wedge. In contrast, the contractile belt in eastern Califor-nia lacked an initial wedge-shaped sedimentary prism, resulting in discontinuous thrusts involv-ing crystalline rocks commonly not of décolle-ment style (Burchfi el and Davis, 1971; Reyn-olds et al., 1986). We conclude that the dynamic and geometric models of orogenic wedges (e.g., Platt, 1986; Willett, 1999), including underplat-ing 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 evi-dence 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 Des-ert 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 Cali-fornia by this time. Diffi culties in corroborating

this hypothesis include uncertainties in the abso-lute convergence rate between the Farallon and North American plates (Engebretson et al., 1985; Stock and Molnar, 1988), the timing of termina-tion 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 fl attening (Cross and Pilger, 1982; Henderson et al., 1984). A suffi cient 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 synmag-matic are more supportive of asthenosphere, rather than the Farallon plate, directly underly-ing North American lithosphere prior to and per-haps during the initial stages of extension. Fur-thermore, 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 eclog-itic lower crust (Saleeby et al., 2003) owing to delamination and/or subduction erosion beneath the arc (southern Sierra, Salinia, northern Pen-insular Ranges), and removal of mantle litho-sphere owing to delamination in the backarc (Sevier hinterland and eastern Mojave Desert), resulted in isostatic uplift, upwelling of astheno-sphere, incursion of decompression basaltic par-tial 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 fl ow of the lower crust and decoupling between the upper-middle crust and the remaining subcon-tinental mantle lithosphere and allowing the upper-middle crust to respond to lateral gradi-ents in potential energy and to extend.

Secondly, fl attening and dehydration of the Farallon slab, thought to be most relevant to areas south of the inferred slab infl ection (Mojave Desert), but perhaps also a lesser

contributing factor to the north (Great Basin), followed replacement of lithosphere by asthe-nosphere. Fluid fl ux 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 similar-ity 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 fl attening 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 con-sequences of delamination of mantle litho-sphere—an increase in Moho temperature, decompression partial melting of astheno-sphere, 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 petrol-ogy 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 gran-ites are widely viewed as representing crustal melts with only a minor mantle contribution; however, the mechanism of production of the



Wells and Hoisch


granites remains controversial. Crustal thicken-ing, fl uid infi ltration from a shallow Laramide slab, increased mantle heat fl ux, 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,

as best exemplifi ed 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 frac-tion produced by muscovite dehydration melt-ing may also have been insuffi cient for separa-tion, migration, and accumulation (Clemens and Vielzeuf, 1987; Patiño Douce et al., 1990). It is debated whether in situ heat production follow-ing crustal thickening was suffi cient to induce biotite dehydration partial melting, which could

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Figure 5. Tectonic cartoons illustrating Late Cretaceous removal of mantle lithosphere and its consequences for the Mojave region (left col-umn, A–C) and the Great Basin region (right column, D–F). (A, D; 95 Ma) A signifi cant 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 thicken-ing 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 counterfl ow or driven whole-sale 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 fl uids 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.





have produced the larger melt fractions required for migration and accumulation of melts in plu-tons (Patiño Douce et al., 1990; Barton, 1990).

Alternatively, the gradual heating of the lower crust as a consequence of asthenospheric coun-terfl ow 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 Cre-taceous. Delamination, and the resulting incur-sion 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 fi eld observations of interaction between felsic and mafi c magmas (e.g., Robinson et al., 1986; Kapp et al., 2002). Furthermore, mid-oce-anic-ridge basalt (MORB)–source lower crustal xenoliths of Late Cretaceous age from the Cima volcanic fi eld (Fig. 2) suggest asthenospheric upwelling beneath the eastern Mojave Desert (Leventhal et al., 1995). All lower-crustal xeno-liths 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 evi-dent from the P-T path from the Grouse Creek Mountains in northwest Utah, which shows con-tinuous heating during a sequence of compres-sion, decompression, and renewed compression in the Late Cretaceous (Hoisch et al., 2002; Har-ris et al., 2007). The initial heating during thick-ening 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 fol-lowing exhumation would be diffi cult 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.

Fluid Flux From Laramide Slab

In the Mojave region, melting of the lower crust may have been promoted further by the infi ltration of fl uids derived from the shallowing Farallon slab, perhaps from water-rich under-plated metasediments (Pelona-Orocopia-Rand schist) in the subduction channel (Hoisch and Hamilton, 1990; Malin et al., 1995). Zircon sat-

uration temperatures from many Mojave Late Cretaceous Cordilleran-type peraluminous gran-ites with abundant inherited zircon indicate low magma temperatures (725–825 °C) requiring signifi cant 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 fl uid fl ux 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 metamor-phism, fl uid-to-rock ratios of >17:1 are required to produce the observed quantity of wollastonite (Hoisch, 1987). The minimum quantity of fl uid required is much too large to be derived from

the crystallization of hydrous melts beneath the level of exposure. The infl ux of fl uids 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 rheo-logical weakening of the lower crust, which are required in the production of Cordilleran-type peraluminous granites, may have played a role in facilitating synconvergent extension.

P (

kb)

T (°C)

1 2

3

4

f

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 reac-tions 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 materi-als (Patiño Douce, 1999). Dark gray polygon shows the region of biotite dehydra-tion 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.



Wells and Hoisch


A reduction in strength is predicted by thermal softening through the temperature sensitivity of plastic fl ow 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 overly-ing crust. If extension initiated and/or continued after the low-angle slab made contact with east-ern Mojave continental lithosphere, fl uid infi ltra-tion 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 fl ow in response to pressure gradients between the topographically elevated orogen and the lower elevations to the east, and fl ow 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 fl ow under the Tibetan Plateau and the marginal belts of crustal shortening (e.g., Roy-den, 1996; Royden et al., 1997).

Precondition for Delamination

Mass balance considerations permit the litho-spheric mantle beneath the Sevier hinterland and the Mesozoic arc to have been substantially thickened and subsequently removed. The short-ening estimates of up to 310 km are calculated from upper-crustal strain; transpressional accre-tion of terranes at the continental margin in the U.S. segment of the Cordillera is insuffi cient to balance shortening in the fold-thrust belt (Oldow et al., 1989). Therefore, to maintain strain com-patibility, an equivalent magnitude of shorten-ing must have been accommodated in the lower crust and mantle lithosphere, leading to develop-ment 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 alterna-tive 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

Molnar, 1997). Because backarc shortening dur-ing the Sevier-Laramide orogeny was protracted for up to 110 m.y., delamination during rather than after mountain building and plate conver-gence is permitted. The recognition of a thin and fertile Archean mantle lithosphere beneath the eastern Mojave from Re-Os isotopic stud-ies of mantle xenoliths in Pliocene basalts of the Cima volcanic fi eld (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 astheno-sphere have been responsible for the removal of mantle lithosphere and lower crust, rather than subduction erosion by traction, then the forma-tion of eclogites in the deep levels of the arc (Ducea, 2001; Saleeby et al., 2003) may have been signifi cant in the arc region.

CONCLUSIONS

The Mesozoic arc and backarc belt of short-ening 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 signifi cantly between localities, extension was nonetheless prevalent north and south of the inferred seg-ment 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 magma-tism 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 fab-rics (John and Mukasa, 1990; Applegate and Hodges, 1995; kinematic reactivation of Hold-sworth et al., 1997). We predict that many more of the metamorphic core complexes, with fur-ther study, will reveal evidence for Cretaceous

extensional exhumation. Furthermore, we argue that the exhumation of mid-crustal rocks record-ing 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; Hur-low et al., 1991; Wells et al., 2000).

Mechanisms to trigger extension during plate convergence require either a reduction in hori-zontal 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 metamor-phic, 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 Foun-dation 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 benefi ted 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 benefi ted 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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