The Role of Mantle Delamination in Widespread Late Cretaceous Extension and Magmatism in the Cordilleran Orogen, Western United States

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 in the shallowing of the slab to achieve low- other studies, causes of synconvergent exten- angle subduction geometry. Delamination sion remain controversial. Extension during plate convergence and has been proposed to be common in areas Mechanisms proposed for synconvergent mountain building is widely recognized, of thickened continental lithosphere in the extension require either (1) a decrease in hori- yet the causes of synconvergent extension terminal phase or late in orogenesis. The zontal compressive stress transmitted across the remain controversial. Here we propose that Late Cretaceous delamination event pro- plate boundary and/or basal décollement to the delamination of lithospheric mantle, aided posed here for the Sevier-Laramide orogen orogen, (2) an increase in lateral contrasts in by decoupling of the crust from the mantle occurred during protracted plate conver- gravitational potential energy, or (3) a reduction via a reduction in the viscosity of the lower gence and was synchronous with, and fol- in rock strength, lessening support against gravi- crust through heating, incursion of fl uids, lowed by, continued shortening in the exter- tational forces (Platt, 1986; England and House- and partial melting, explains many enig- nal part of the orogen. man, 1989; Willett, 1999; Rey et al., 2001). matic yet prevalent aspects of the metamor- Although not mutually exclusive, the diversity phic, magmatic, and kinematic history of Keywords: Cordilleran tectonics, Late Creta- of principal driving mechanisms suggested the Sevier-Laramide orogen of the western ceous, delamination, synconvergent extension, for synconvergent extension in many orogens, United States during the Late Cretaceous. Laramide. including the Himalaya-Tibetan Plateau (e.g., Extension, heating, anatexis, magmatism, England and Houseman, 1989; McCaffrey and and perhaps rock uplift were widespread INTRODUCTION Nabelek, 1998; Vanderhaeghe and Teyssier, during a restricted time interval in the 2001; Kapp and Guynn, 2004), the Andes (e.g., Late Cretaceous (75–67 Ma) along the axis The discovery of synconvergent extension Kay and Kay, 1993; Marrett and Strecker, 2000; of maximum crustal thickening within the within the interiors of modern convergent oro- Allmendinger, 1986), and the Eocene–Oligo- Mojave sector of the Sevier orogen, and gens (e.g., Dalmayrac and Molnar, 1981; Mol- cene Basin and Range (e.g., Coney and Harms, to a lesser extent within the interior of the nar and Chen, 1983) has provided insights into 1984; Platt and England, 1994; Houseman and Idaho-Utah-Wyoming sector to the north; the processes that control crustal thickness and Molnar, 1997; Dickinson, 2002), underscore similar processes may have been active in topography during orogenesis. Topography and their controversial nature. the Peninsular Range, Sierran, western crustal thickness are governed, to a fi rst order, The removal (delamination) of lithospheric Mojave, and Salinian segments of the Meso- by a dynamic interplay between horizontal mantle beneath isostatically compensated moun- zoic Cordilleran arc. These processes are compressional forces and the gravitational tain belts is an effective mechanism to increase viewed as predictable consequences of the potential energy stored in isostatically com- gravitational potential energy, promoting sur- thermal, rheological, and dynamic state of pensated thickened lithosphere (e.g., Molnar face uplift and horizontal extension (Bird, 1979; the overlying crust following delamination and Lyon-Caen, 1988; England and Houseman, Houseman et al., 1981; England and House- of mantle lithosphere beneath isostatically 1989). That the balance is commonly upset, man, 1989). Thermal and numerical modeling compensated mountain belts. The proposed driving parts of convergent orogens into exten- of delamination and its effects on the overlying delamination would have occurred immedi- sion, is evident from numerous ancient exam- lithosphere (e.g., England and Houseman, 1989; ately prior to eastward propagation of low- ples, including the eastern Alps (e.g., Ratsch- Platt and England, 1994) has provided some pre- angle subduction of the Farallon plate dur- bacher et al., 1989; Wallis et al., 1993), the dictions that can be used to evaluate the appli- ing the inception of the Laramide orogeny. Apennines (Carmignani and Kligfi eld, 1990; cability of delamination in orogenic evolution. Following delamination, extension and ana- Ferranti and Oldow, 1999), the Calabrian Arc Delamination of lithospheric mantle may result texis of the North American crust were aided (Wallis et al., 1993; Cello and Mazzoli, 1996), in (1) an increase in Moho temperature and locally by egress of slab-derived fl uids from the Himalaya (Hodges et al., 1992a; Burchfi el geothermal gradient, which may be recorded the low-angle Farallon slab. We suggest that et al., 1992a), the Scandinavian Caledonides by distinctive P-T paths that include either lithospheric delamination may have aided (Gee et al., 1994; Northrup, 1996), and the heating during decompression or isobaric heat- Sevier-Laramide orogen (Wells et al., 1990; ing following decompression; (2) decompres- †E-mail: [email protected] Hodges and Walker, 1992). Despite these and sion 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 ﬁ gures.

distinctive magma chemistries; (3) surface uplift and erosion; and (4) extension (Houseman et al., 1981; Kay and Kay, 1993; Platt and England, 42°N 1994). We use the term delamination here to R describe the removal of mantle lithosphere by W 7 density-driven foundering. Delamination by dif- RH ferent mechanisms (e.g., peeling or convective 6 Lara removal; Bird, 1979; Houseman et al., 1981; PE Houseman and Molnar, 1997) produces similar responses of heating and increased buoyancy LB CN mid of the remaining lithosphere. In this paper we marshal available petrologic and structural data eBelt to show that delamination is the likely cause for 37°N F Colorado synconvergent extension in the hinterland of the SS 11 Cretaceous Sevier-Laramide orogen. *SN ES Plateau P 5 P a 10 SEVIER AND LARAMIDE OROGENS c i fi 9 4 0 400 km The Sevier and Laramide orogens (e.g., c POR *3 Armstrong, 1968; Tweto, 1975) of the western 2 O 1 N United States (Fig. 1) are Mesozoic to early c e PR Cenozoic backarc belts of crustal shortening a that are segments of the larger Cordilleran n 8 orogen (Allmendinger, 1992; Burchfi el et al., POR 1992b; DeCelles, 2004). The continuous anti- 120°W 108°W thetic fold-thrust belt (Sevier) and the eastern province of discontinuous basement-involved Figure 1. Simplifi ed tectonic map of the western Cordillera, showing selected Mesozoic to fold-thrusts (Laramide) (Fig. 1) are generally early Cenozoic tectonic features and location of Figure 2. Belt of muscovite granites (dark attributed to Andean-style convergent tecto- gray fi ll: Miller and Bradfi sh, 1980) largely coincides with belt of metamorphic core com- nism. Unlike the Andes, mid-crustal levels of plexes (black fi ll) and inferred axis of maximum crustal thickening in the Cordilleran orogen. a noncollisional orogen have been exhumed by The Sevier fold-thrust belt (light gray fi ll, after DeCelles, 2004; leading edge shown by bold extensional and erosional denudation, which line with teeth) converges with the magmatic arc (cross pattern) toward the south in south- allow the study of deeper exposures of the eastern California. Dashed line shows position of inferred segment boundary in Laramide products of synconvergent extension. slab between shallower angle (south) and steeper angle (north) subduction, after Saleeby Studies over the past two decades have led to (2003). Numbers 1–11 refer to specifi c locations discussed in text and keyed to Figures 2–4. the recognition that Cretaceous to early Tertiary CN—Central Nevada thrust belt; ES—Eastern Sierran thrust system; F—Funeral Moun- extension in the interior of the Sevier-Laramide tains; LB—Luning thrust belt; P—Panamint Range; PE—Pequop Mountains; POR— orogen was widespread and locally of large Pelona-Orocopia-Rand schist; PR—Peninsular Range; R—Raft River–Albion–Grouse magnitude (e.g., Wells et al., 1990, 2005; Apple- Creek; RH—Ruby Mountain–East Humboldt Range; SS—Sierra de Salinas; W—Wood gate et al., 1992; Hodges and Walker, 1992). To Hills. Stars indicate localities of xenolith studies: central Sierra Nevada (Big Creek; Lee et address the driving mechanism(s) for Creta- al., 2000, 2001a), east Mojave (Cima Dome; Leventhal et al., 1995; Lee et al., 2001b). ceous synconvergent extension of the Sevier and Laramide orogens, we fi rst review the structural, metamorphic, and magmatic records of backarc are within the cratonal stratigraphic section. In craton. The belt of shortening is coincident with shortening; the timing and magnitude of short- the interior (hinterland) of the Sevier orogen, the eastern fringe of the Mesozoic magmatic arc ening; and the plate tectonic setting during the isolated occurrences of greenschist- and amphi- and exhibits marked plastic deformation (Burch- time of extension. bolite-facies Barrovian metamorphic rocks (Fig. fi el and Davis, 1981). Thrusts commonly carry 1), locally recording up to 8–10 kbar pressures, Proterozoic crystalline rocks to the southwest Structural Style, Metamorphism, and attest to substantial thrust burial and subsequent and south over thin Paleozoic and Mesozoic Anatexis exhumation (Hoisch and Simpson, 1993; Lewis cratonal rocks (Reynolds et al., 1986; Howard et al., 1999; McGrew et al., 2000; Hoisch et al., et al., 1987a). Ductile fold nappes, thrusts, and The Idaho-Utah-Wyoming sector of the Sevier 2002; Harris et al., 2007). 4–6 kbar pressures from Mesozoic plutons and fold-thrust belt is generally north striking and The Mesozoic shortening belt in southeast- their cratonal wall rocks mark the discontinuous thin skinned and lies well east of the magmatic ern California and southwestern Arizona differs belt of substantial localized Mesozoic structural arc (Fig. 1). Thrusts carry thick packages of late from the northern belt in structural style, timing, burial (Howard et al., 1987a; Anderson et al., Proterozoic to Triassic rocks of the Cordilleran and in distance from the coeval magmatic arc. 1989; Foster et al., 1992; Boettcher and Mosher, miogeocline eastward over thinner Cambrian The belt departs from the miogeocline-craton 1998) (Fig. 2). Shortening localized along the and younger sedimentary rocks deposited on the transition, wraps to the east-southeast (Figs. 1 east side of the Sierran magmatic arc to the Precambrian basement of the North American and 2), and deforms the thin Phanerozoic strata north (Eastern Sierran thrust system, Dunne and craton. Thrust duplications in the frontal part and underlying Precambrian basement of the Walker, 2004) (Fig. 1) may converge southward

516 Geological Society of America Bulletin, May/June 2008 Cretaceous extension and delamination in the Cordilleran orogen

NV CA Q-T volc

T plutons

T volc & sed

Late K plutons Clark Mountain thrust comple K sed

x New York 100 Ma volc Mtns Mid K plutons NV Cima Volcanic Pinto CA Field Jr volc 4 shear zone Teutonia Colorado Jr plutons Batholith Pz-TR sed 3 pC Granite Mtns Mylonitic gneiss

PM CM

oman River CHM Old WMtns. 2

WM Cadiz KH Valley Breakaway to Batholith Iron Colorado River extensional corridor Mtns. N 1

LMM BMM

Dome Rock Mtns.

0 50 km

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).

Geological Society of America Bulletin, May/June 2008 517 Wells and Hoisch with the Sevier fold-thrust belt in the southeast- central Arizona may have been accommodated southwestern Arizona. The schists lie in tectonic ern Mojave Desert. far to the northeast in the Laramide foreland contact beneath the Precambrian to Mesozoic Exposures of metamorphic rocks in the province east and north of the Colorado Plateau continental basement and Mesozoic arc (Jacob- interior of the Sevier-Laramide orogen in both (Dickinson et al., 1988). son et al., 1988), and are exposed in a SE-trend- the Mojave Desert and Great Basin are com- Estimates of Late Jurassic to early Eocene ing belt of tectonic windows from the Tehachapi monly associated with peraluminous granites shortening across the backarc of the orogen Mountains–western Mojave Desert, along the of Late Cretaceous age, inboard of the extinct (Allmendinger, 1992; DeCelles, 2004) are vari- Transverse Ranges, into southwestern Arizona coastal magmatic arc (Fig. 1) (Miller and Brad- able along strike and fraught with uncertain- (Fig. 1; Jacobson et al., 1988; Grove et al., fi sh, 1980; Miller and Barton, 1990). Isotopic ties, resulting in part from extreme Cenozoic 2003b). Similar rocks are present in the Sierra de compositions of these Late Cretaceous Cor- extensional fragmentation, in particular for the Salinas of the Salinian block (SL, Fig. 1; Barth dilleran-type peraluminous granites (usage of hinterland. Shortening estimates, principally et al., 2003; Grove et al., 2003b). Although Patiño Douce, 1999) are consistent with vari- from palinspastic restoration of balanced cross the upper contact bounding these windows of able sources in Precambrian continental base- sections, are most robust across the fold-thrust underplated schist has been locally modifi ed by ment and are widely attributed to be products belt and vary from 100 km at the latitude of Las younger faults and retrograde metamorphism of crustal anatexis (Farmer and DePaolo, 1983; Vegas, Nevada (Wernicke et al., 1988), to 220– (e.g., Jacobson and Dawson, 1995; Jacobson et Patiño Douce et al., 1990; Miller and Barton, 240 km in central Utah (DeCelles and Coogan, al., 1996), the broad regional distribution of the 1990; Wright and Wooden, 1991), but with a 2006). Additional shortening estimated across schists (Grove et al., 2003b) suggests underplat- juvenile component (Patiño Douce, 1999; Kapp the western hinterland of Nevada (Elison, 1991) ing of oceanic metasediments during low-angle et al., 2002). Geobarometry of both intrusions and the Laramide foreland (Brown, 1988) sug- subduction and removal of the subcontinental and country rock suggests midcrustal crystalli- gests cumulative Late Jurassic to early Eocene mantle lithosphere and the lower crust. zation depths (Anderson et al., 1989; Foster et shortening of the backarc region of Nevada- It has recently been suggested that the sub- al., 1992). These plutons are especially abundant Utah-Wyoming (east of the Luning-Fencemaker ducting oceanic slab was segmented, with the in the eastern Mojave Desert region (Kapp et al., belt) as much as 310 km. Shortening estimates fl at-slab segment confi ned to the Laramide 2002), although the belt continues northward in the Mojave Desert region south of Las Vegas “deformation corridor” southeast of the seg- into the Canadian Cordillera (Fig. 1) (Miller and (line of section of Wernicke et al., 1988) are ment boundary (Fig. 1) and projecting under the Barton, 1990). hindered by spatial overlap of the eastern fringe southernmost Sierran arc, Mojave Desert, Colo- of the Mesozoic magmatic arc with the belt of rado Plateau, and Laramide foreland province of Timing and Magnitude of Shortening shortening (Burchfi el and Davis, 1981). the Rocky Mountains (Saleeby, 2003). A steeper slab segment is interpreted to underlie the cen- Initial shortening in the Idaho-Utah-Wyoming Plate Tectonic Setting of the Laramide tral and northern Sierran arc and the interior of sector of the Sevier orogen progressed eastward Orogeny the Sevier-Laramide orogen in the Great Basin from Middle Jurassic to early Eocene time (All- (Saleeby, 2003). This model has important mendinger, 1992; DeCelles, 2004). The foreland Paleomagnetic plate reconstructions of the implications for the geodynamic development fold-thrust belt is principally Early Cretaceous to Mesozoic Pacifi c basin provide constraints on of the overriding plate and is discussed later. early Eocene in age (DeCelles, 1994; Lawton et the relative plate motions between the Farallon al., 1997; Yonkee et al., 1997), with episodic out- and Kula plates and the North American plate LATE CRETACEOUS of-sequence deformation interpreted in terms of during the Laramide orogeny (80–50 Ma). Far- SYNCONVERGENT EXTENSION IN dynamic orogenic wedge mechanics (DeCelles allon–North American and Kula–North Ameri- THE SEVIER-LARAMIDE OROGEN and Mitra, 1995; DeCelles et al., 1995; Camil- can plate convergence was generally NE-SW leri et al., 1997). The hinterland underwent older and at high rates (100–150 km/m.y.) during the Here we describe evidence for extensional Middle–Late Jurassic shortening (Allmendinger Laramide orogeny (80–50 Ma), followed by a structures and exhumation during the restricted and Jordan, 1984; Hudec, 1992; Elison, 1995) signifi cant reduction in the convergence rate at time interval from 75 to 67 Ma from seven areas and also substantial Late Cretaceous shortening 50–45 Ma (Engebretson et al., 1985; Kelley and (Figs. 1 and 2) along the axis of the Sevier oro- (Miller et al., 1988; Camilleri and Chamberlain, Engebretson, 1994). The Late Cretaceous exten- gen, as defi ned by the belt of metamorphic core 1997; McGrew et al., 2000; Harris et al., 2007). sion and magmatism of the Sevier-Laramide complexes (Coney and Harms, 1984). These Late Cretaceous to early Eocene deformation of orogen occurred during accelerated conver- areas were selected on the basis of illustrative the Sevier belt temporally overlapped shorten- gence between the Farallon-Kula plates and relationships between extension and intrusion, ing in the Laramide foreland province (Dickin- North America. well-bracketed timing constraints, or well-con- son et al., 1988). Various observations support the contention strained pressure-temperature (P-T) paths. Addi- In contrast, shortening terminated earlier that the subducting Farallon slab evolved from a tionally, we consider the record of exhumation in the Mojave Desert region than to the north. moderate dip to a low-angle dip during the tran- from the Mesozoic batholithic belt to the west. Frontal thrusts in the northeastern Mojave Des- sition from the Sevier to the Laramide orogeny Taken together, these examples indicate that ert region (Clark Mountain area) (Figs. 2 and 3) in the Late Cretaceous (ca. 80–75 Ma) (Coney extensional exhumation, and locally erosional are bracketed by 100 and 90 Ma igneous rocks and Reynolds, 1977; Dickinson and Snyder, exhumation, was widespread along a signifi cant (Fleck et al., 1994; Smith et al., 2003). Farther 1978; Bird, 1984, 1988; Usui et al., 2003). Direct segment of the orogen between 75 and 67 Ma. south in the westernmost Maria tectonic belt, evidence for a low-angle subducted Farallon terminal shortening may be bracketed between slab exists in the occurrence of intermediate to Eastern Mojave Desert 84 and 74 Ma (Barth et al., 2004). In the Late high pressure oceanic metamorphic rocks of the Cretaceous to early Tertiary, most of the backarc Franciscan-affi nity Pelona, Orocopia, Rand, and The record of Late Cretaceous extension shortening in the southern Cordillera north of related schists in southeastern California and and plutonism in the Sevier belt is most clear

518 Geological Society of America Bulletin, May/June 2008 Cretaceous extension and delamination in the Cordilleran orogen

900 E. Iron Mountains 900 Old Woman pluton 800 granodiorite Zircon 800 4.5 kbar Zircon 4.8 kbar Conductive Al-in Hbl Conductive Figure 3. Cooling curves for 700 GARB-GASP cooling 700 GARB-GASP cooling selected Late Cretaceous plu- Deformation 600 temperatures 600 tons in the eastern Mojave Hornblende Desert region derived from 500 Hornblende Tectonic 500 exhumation U-Pb geochronology (zircon), Biotite Tectonic 40Ar/39Ar thermochronology 400 400 Biotite exhumation (hornblende, biotite, and mul- 300 300 K-feld. tiple diffusion domain [MDD] 200 200 modeling of K-feldspar), Apatite and apatite fi ssion track. (A) 100 100 A 1 B 2 Granodiorite from the eastern 0 0 Iron Mountains; biotite cooling 50 60 70 80 90 5060 70 80 90 ages vary systematically, with younger ages to the east (Wells 900 900 Granite Zircon New York Mountains et al., 2002, unpublished data). 800 Mountains 800 (B) Old Woman pluton (Fos- Mid Hills monzogranite Zircon Zircon ter et al., 1992); apatite fi ssion 700 SW monzogranite 700 Conductive (late dike) track ages vary by location. 4.5 kbars 600 cooling 600 (C) Monzogranite from south- Al-in Hbl west Granite Mountains (Kula, 500 Hornblende 500 2002). (D) Mid Hills monzo- Deformation granite of the New York Moun- 400 400 temperatures K-feldspar tains footwall to the Pinto shear Tectonic 300 K-feldspar 300 MDD model zone (Wells et al., 2005). (E) Temperature °C MDD exhumation 200 200 Four representative K-feldspar MDD cooling envelopes (Grove 100 100 et al., 2003a) from east-central CD3 4 Peninsular Ranges batholith. 0 0 (F) Coast Range Complex, 5060 70 80 90 5060 70 80 90 Salinian block framework 600 East-Central Peninsular 900 Salinia rocks (Kidder et al., 2005). K- Ranges batholith Coast Ridge Complex Zircon U/Pb feldspar MDD cooling enve- 800 500 Garnet Sm-Nd lopes represent 90% confi dence interval for the median of the 700 distribution of modeled cool- 400 Biotite Ar-Ar K-feldspar 600 ing curves (Lovera et al., 1997). MDD models 500 Numbers in the lower right cor- 300 ners of the panels refer to loca- erature °C400 Temperature °C tion numbers shown in Figures Biotite K-Ar 200 300 1 and 2. GARB-GASP refers to Temp the garnet-biotite geothermom- 200 100 eter of Holdaway (2000) and 100 GASP refers to the geobarom- 8 F 10 eter of Holdaway (2001). 0 E 0 Sedimentation 50 60 70 80 90 50 60 70 80 90 Age (Ma) Age (Ma) in the eastern Mojave Desert region, where Late Iron Mountains the mylonite dips shallowly to the west, and min- Cretaceous granites are abundant and numerous Porphyritic monzogranite of the Late Creta- eral and stretching lineations plunge westward. extensional shear zones have been shown to be ceous Cadiz Valley batholith in the Iron Moun- Kinematic indicators demonstrate top-to-the-east coeval with granite intrusion. Here we outline tains is overlain by a roof zone composed of noncoaxial shear. Beneath the shear zone and in evidence of the magnitude and timing of Late screens of Precambrian rocks, separated by Cre- the uppermost 200 m of the porphyritic monzo- Cretaceous extension and of the synextensional taceous sills (Miller and Howard, 1985; Wells granite a magmatic foliation is concordant with emplacement of some Late Cretaceous granites et al., 2002). Within the roof zone a solid-state the overlying wall-rock screens and solid-state from the Iron, Old Woman, Granite, and New deformation fabric comprises a >1.3-km struc- foliation. Ion microprobe U-Pb analyses of zir- York Mountains. tural thickness of mylonitic rocks. Foliation in con of two phases of the batholith yield similar

Geological Society of America Bulletin, May/June 2008 519 Wells and Hoisch ages of 75.4 ± 2.1 and 75.6 ± 1.7 Ma (Wells et Granite Mountains and the Raft River, Albion, and Grouse Creek al., 2002), within 2σ analytical error of the age Jurassic and Cretaceous plutonic rocks (How- Mountains of northwestern Utah and southern (73.9 ± 1.9 Ma) reported by Barth et al. (2004). ard et al., 1987b) in the Granite Mountains of the Idaho (Fig. 1). Although Late Cretaceous gran- Biotite 40Ar/39Ar analyses from 3-km intervals Mojave National Preserve (Fig. 2) were emplaced ites of the Great Basin defi ne the northward con- along a 9-km lineation-parallel transect show a at midcrustal depths; barometry of Cretaceous tinuation of the Cordilleran-type peraluminous consistent progression of isochron ages that are and Jurassic granitoids indicates 4–6 kbar pres- granite belt (Barton, 1990; Miller and Barton, younger toward the east—from 66.6 ± 0.3 Ma sures (Anderson et al., 1989; Kula, 2002; Kula 1990), in comparison with the Mojave there (west) to 59.1 ± 0.7 Ma (east) (Fig. 3A). These et al., 2002). The presence of screens of Paleo- are fewer recognized surface exposures of the data, together with (U-Th)/He apatite and zircon zoic rock in these midcrustal intrusions sug- granites and Cretaceous extensional structures. ages, which are consistent with westward tilt of gests intrusion through tectonically buried rocks. There is also a less extensive modern U-Pb geo- the range in Miocene time (Brichau et al., 2006), Barometry, geochronology, and thermochronol- chronology, resulting in an unclear relationship support an initial component of east dip to the ogy of 72–76 Ma granites indicate intrusion at between Cretaceous intrusion and extension in shear zone. Downdip top-to-the-east kinematics ~4.5 kbar, followed by rapid cooling through the Great Basin. and bracketing U-Pb zircon and 40Ar/ 39Ar biotite hornblende 40Ar/39Ar closure interpreted as con- ages indicate extensional shearing between 75 ductive cooling to country rock temperatures Funeral Mountains and 67 Ma. (Fig. 3C) (Kula et al., 2002). Continued rapid The Funeral Mountains, located in Death cooling from 350 to 180 °C from 73 to 69 Ma, as Valley National Park (Fig. 1), preserve a Bar- Old Woman–Piute Mountains shown by K-feldspar multiple diffusion domain rovian metamorphic fi eld gradient in Protero- The Old Woman Mountains, Kilbeck Hills, (MDD; Lovera et al., 1991, 1997) is interpreted zoic metasedimentary rocks that resulted from and Piute Mountains (Fig. 2) are underlain by as having resulted from extensional exhumation differential Mesozoic thrust burial; metamor- greenschist- to amphibolite-facies Cambrian to (Fig. 3C) (Kula, 2002; Kula et al., 2002). phism increases from subgreenschist facies in Triassic cratonal metasedimentary rocks and the SE to upper amphibolite facies and 7.5–9 Proterozoic crystalline basement intruded by New York Mountains kbar pressures to the NW across a distance of Jurassic and Late Cretaceous granitoids. The The Pinto shear zone records Late Cretaceous ~40 km (Labotka, 1980; Hodges and Walker, Jurassic and older rocks exhibit extensive Meso- extensional unroofi ng of the Teutonia batholith 1990; Hoisch and Simpson, 1993; Applegate zoic ductile deformation, most prominently in the New York Mountains (Fig. 2) (Miller et and Hodges, 1995). Although the metamorphic within the NE-striking Scanlon thrust and its al., 1996; Beyene, 2000; Wells et al., 2005). The rocks were exhumed during Cenozoic motion associated basement-cored nappes (Miller et al., shear zone deforms the middle Cretaceous (90 along the Boundary Canyon detachment fault 1982; Howard et al., 1987a). Thermobarometry Ma ± 2, U-Pb zircon) Mid Hills monzogranite (Wright and Troxel, 1993; Hoisch and Simpson, of Cretaceous granodiorite and pelitic schist in and late (ca. 75 Ma) porphyry dikes and records 1993), an earlier history of partial exhumation its contact aureole indicates intrusion pressures downdip, top-to-the-southwest shear during in the Late Cretaceous has also been suggested of 4–5 kbar, following crustal thickening asso- decreasing temperatures from uppermost green- (Applegate et al., 1992). Shear zones record ciated with initial nappe emplacement (Foster schist–lowermost amphibolite facies to catacla- top-to-NW shear down the fi eld metamorphic et al., 1992; Rothstein and Hoisch, 1994). The stic conditions. The geometry and kinematics, gradient (Applegate and Hodges, 1995). The most pervasive penetrative deformation in the hanging-wall deformation style, progressive age of the early extensional deformation is con- Old Woman Mountains apparently records a changes in deformation temperature, and dif- strained by U-Pb dating of synkinematic peg- younger extensional overprint. Synmagmatic ferences in hanging wall and footwall thermal matite (72 ± 1 Ma, zircon) and postkinematic deformation within the Old Woman pluton histories allow a clear interpretation of the zone pegmatite (70 ± 1 Ma, zircon) (Applegate et (74 ± 3 Ma; Foster et al., 1989) and solid-state as due to extension, not shortening (Wells et al., al., 1992). In addition, the Harrisburg fault in mylonitic deformation within the pluton and 2005). Deformation is well constrained between the Panamint Range has been interpreted as the wall rock screens along its tabular margins have ca. 74 and 68 Ma by 40Ar/39Ar thermochronol- southern continuation of Late Cretaceous shear been interpreted as extensional, recording ~E-W ogy of the exhumed footwall, including MDD zones of the Funeral Mountains (Hodges et al., extension and vertical shortening (McCaffrey modeling of K-feldspar, which shows cooling 1990; Andrew, 2000). et al., 1999). A top-to-the-southwest fabric that rates of 62–76 °C/m.y. (Fig. 3D). The magnitude overprints the nappes within the Scanlon thrust of exhumation along the shear zone remains East Humboldt Range, Wood Hills, Pequop zone (Nicholson, 1990; Owens, 1995), and a 1- unclear owing to uncertainties in the emplace- Mountains km-thick top-to-the-southwest mylonitic shear ment depth of the Mid Hills monzogranite and Cretaceous exhumation of metamorphic rocks zone that deforms the Cretaceous plutons and in the paleogeothermal gradient. in the East Humboldt Range core complex and wall rocks along the western margin of the Old the adjacent Wood Hills and Pequop Mountains Woman Mountains (western Old Woman Moun- Great Basin (Fig. 1) is indicated by dated decompressional tains shear zone; Carl et al., 1991), have also been P-T paths (Hodges et al., 1992b; McGrew and interpreted as extensional. Thermochronometric The record of Late Cretaceous extension Snee, 1994; McGrew et al., 2000) and the con- studies from the footwall of the shear zone indi- of the Sevier belt is more cryptic in the Great temporaneous large-displacement Pequop nor- cate rapid cooling from intrusion temperatures Basin region than in the Mojave Desert, but mal fault (Camilleri and Chamberlain, 1997). (~750 °C) at 74 Ma to fi ssion track closure in it may have been more widespread than cur- Similar to the Funeral Mountains, these ranges apatite (~100 °C) at 66 Ma (Fig. 3B). In light of rently recognized. This event in the Great Basin collectively preserve a Barrovian metamorphic the recognition of the synextensional nature of is currently best understood in three areas: the fi eld gradient, with meta morphism increasing the Old Woman pluton (McCaffrey et al., 1999), Funeral Mountains of southeastern California; toward the northwest owing to increased tectonic extension was active from intrusion at 74 Ma to the East Humboldt Range, Wood Hills, and burial, with local metamorphic grade increases <68 Ma (Carl et al., 1991; Foster et al., 1992). Pequop Mountains of northeastern Nevada; near Jurassic, Cretaceous, and Tertiary plutons

520 Geological Society of America Bulletin, May/June 2008 Cretaceous extension and delamination in the Cordilleran orogen

11 10 A 6 B Rims 7 9 85 Ma 8 Upper 7 Zone Exhumation 6 50 Ma 5 30–40 Ma Rims 4 3 2 22–30 Ma Cores Lower Zone 1 Cores 20 Ma 0 3 0 200 400 600 800 1000 400 450 500 550 600 650

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, from the Wood Hills (Camilleri and Chamber- amphibolite facies (475–510 °C) (Wells et al., 1997). Thermobarometric and geochronologic lain, 1997). Muscovite 40Ar/39Ar ages of 75 Ma 1998). In contrast, mineral assemblages and studies of the deepest level rocks in the East from uppermost greenschist facies rocks in the geothermometry of the Neoproterozoic pelitic Humboldt Range defi ne a decompressional Pequop Mountains may also record such cool- schist of Mahogany Peaks in the footwall indi- pressure-temperature-time (P-T-t) path from >9 ing; alternatively they may record thermal reset- cates pressures and temperatures >6.5 kbar and kbar and 800 °C to ~5 kbar and 630 °C meta- ting (Thorman and Snee, 1988). The signifi cant 590 °C, respectively. The metamorphic grade morphic conditions (Fig. 4A; McGrew et al., imprint of Eocene and Oligocene high-grade discordance of ~80–110 °C across the fault is 2000), bracketed between a Late Cretaceous metamorphism and plutons, dikes, and sills, consistent with the postmetamorphic omission intrusion of leucogranite (84.8 ± 2.8 Ma, Pb- and Oligocene mylonitic deformation has made of 3–4 km of rock. Because the fault places Pb zircon) at peak metamorphic conditions and it diffi cult to unravel potential Late Cretaceous younger strata over older, and shallower (colder) Oligocene extensional shearing. Of the total exhumational structures from the deep levels of over deeper (hotter) rocks, the fault is interpreted decompression, >2.5 kbar is interpreted to pre- the core complex (Snoke et al., 1997). as extensional in origin. date 40Ar/39Ar hornblende ages of ca. 50–63 Ma Cretaceous exhumation is also evident in the and intrusion of 40 Ma (±3 Ma, U-Pb zircon) Raft River, Albion, and Grouse Creek P-T paths recorded in the pelitic schist of Ste- diorite sills with Al-in-hornblende pressure Mountains vens Spring, which lies in the footwall of the estimates of 4.5–5.5 kbar (Wright and Snoke, In the Raft River, Albion, and Grouse Creek Mahogany Peaks fault. Garnets in two zones of 1993; McGrew and Snee, 1994). In the Peqoup Mountains of northwestern Utah and south- the schist of Stevens Springs from Basin Creek Mountains the west-rooted Pequop low-angle ern Idaho (Fig. 1), two distinct but supporting in the northern Grouse Creek Mountains grew by normal fault juxtaposes a nonmetamorphosed observations record Late Cretaceous exten- different reactions at different times and record over a metamorphosed miogeoclinal section sion: (1) normal faults, including the Mahogany different segments of a composite P-T path (Fig. and caused an estimated 10 km of vertical thin- Peaks fault (Wells, 1997; Wells et al., 1998); and 4B). The P-T path provides a record of two peri- ning, cutting out an inferred older thrust (Thor- (2) P-T paths that require marked exhumation in ods of thrusting separated by marked decom- man, 1970; Camilleri and Chamberlain, 1997). between two periods of thrusting (Hoisch et al., pression and heating (Hoisch et al., 2002), the The Pequop fault cuts a prograde metamorphic 2002; Harris et al., 2007). latter interpreted to record tectonic exhumation. fabric dated at 84.1 Ma (±0.2 Ma, U-Pb) on The Mahogany Peaks fault separates the The youngest thrust burial event has been dated metamorphic titanite from uppermost green- Neoproterozoic schist of Mahogany Peaks by Th-Pb dating of monazite inclusions in garnet schist facies rocks and is overlain by 41 Ma and quartzite of Clarks Basin from Ordovi- (Hoisch and Wells, 2004). Garnet growth and volcanic rocks (Brooks et al., 1995; Camilleri cian carbonate rocks and crops out discontinu- inferred renewed thrust burial initiated at 69.5 and Chamberlain, 1997). Slip on the Pequop ously throughout the Raft River, Grouse Creek, ± 5.2 Ma (95% confi dence interval) and thus fault is interpreted to have caused cooling at and Albion Mountains (Wells et al., 1998). provide a lower bound on the earlier period of 75 Ma, as evident from U-Pb dating of titanite The Ordovician rocks of the hanging wall are exhumation and heating interpreted from decom- (75 ± 1 Ma) in diopside zone metacarbonate metamorphosed at upper greenschist–lower pression P-T paths and geologic structures.

Geological Society of America Bulletin, May/June 2008 521 Wells and Hoisch

Magmatic Arc (Wood and Saleeby, 1998), interpreted as the based on preserved cation diffusion profi les in isostatic response to removal of lower crust orthopyroxene, suggest removal of mantle more The Mesozoic magmatic arc of the south- and mantle lithosphere and to mid- to upper- recently than ca. 150 Ma, and prior to late Mio- west Cordillera underwent cooling and exhu- crustal extension (Saleeby, 2003). Numerous cene eruption of the host basalt. Arc-like mantle mation associated with removal of lower crust extensional allochthons are distributed about the wedge compositions of the accreted mantle sug- and mantle lithosphere in the Late Cretaceous western Mojave, southern Sierra–Tehachapi, gest removal during or shortly following arc at the onset of Laramide tectonism and fl at-slab and the Salinian block (Wood and Saleeby, production (Lee et al., 2000, 2001a). Mesozoic subduction (George and Dokka, 1994; Grove et 1998; Saleeby, 2003). K-Ar and 40Ar/39Ar cool- removal of mantle lithosphere is consistent with al., 2003a; Saleeby, 2003). Interpretations for ing ages in other areas of the western Mojave support of high elevations inferred for the Sierra cooling vary between erosional and extensional Desert (Evernden and Kistler, 1970; Armstrong Nevada from low temperature thermochronom- denudation, with or without subduction refrig- and Suppe, 1973; Kistler and Peterman, 1978; etry and paleoelevation studies (House et al., eration, depending on location (Dumitru et al., Miller and Morton, 1980; Jacobson, 1990) sug- 1997; Mulch et al., 2006; Cecil et al., 2006). 1991; George and Dokka, 1994; Grove et al., gest the possibility of widespread exhumation 2003a; Saleeby, 2003). of the western Mojave Desert at 75–67 Ma, DISCUSSION although the relative contributions of extension Northern Peninsular Range Batholith and erosion remain unclear. Based on our current understanding, any In the northeastern Peninsular Range batho- Similar relations are evident in the Salinian model to explain Late Cretaceous (75–67 Ma) lith, indistinguishable apatite fi ssion track ages block. The Sierra de Salinas schist is in low- extension of the Sevier hinterland must account and track length distributions over a 2.3 km angle fault contact beneath Cretaceous arc and for (1) the localization of extension along the crustal section indicate rapid cooling between older metamorphic framework rocks (Barth et axis of prior crustal thickening; (2) signifi cant 80 and 74 Ma (George and Dokka, 1994). Late al., 2003; Kidder and Ducea, 2006), similar to Late Cretaceous exhumation of up to 14 km; Cretaceous rapid cooling (≤80 °C/m.y., 76–72 the relations between the Pelona-Orocopia- (3) the common association of extension with Ma) is further indicated by K-feldspar MDD Rand schists and the southern Sierran–western plutonism, usually of peraluminous composi- analysis (Grove et al., 2003a) (Fig. 3E). This Mojave arc and framework rocks (Saleeby, tion; (4) extension being synmagmatic to post- rapid cooling has been interpreted as having 2003). Following attainment of metamorphic magmatic, but apparently not premagmatic; resulted from erosional denudation induced conditions of ~7.5 kbar and 800 °C between 81 (5) a deep crustal source for Late Cretaceous by the isostatic response to tectonic removal (±3) and 76 (±1) Ma, the framework rocks in Cordilleran-type peraluminous granites, but with of mantle lithosphere–lower crust, with a pos- the Coast Ridge belt cooled through garnet Sm- a juvenile component; (6) the local development sible contribution from subduction refrigera- Nd (76 ± 1 Ma), biotite K-Ar (75 ± 4 Ma), and of elevated temperatures (~600 °C), where Cre- tion (George and Dokka, 1994; Grove et al., apatite fi ssion track (71 Ma), suggesting cool- taceous plutons are absent, at middle crustal 2003a). Thermochronology of detrital minerals ing rates during exhumation between 120 and depths (~4 kbar) following signifi cant exhuma- of the forearc sediments supports a component 75 °C/m.y. (Kidder et al., 2005) (Fig. 3F). These tion; (7) extension and magmatism occurring at of basement cooling owing to erosion (Lovera results are consistent with 40Ar/39Ar cooling the onset of slab fl attening during an increase et al., 1999). However, the high rates of cooling ages of biotite and hornblende from Cretaceous in plate convergence rate; (8) extension and may indicate a contribution from yet unrecog- granitoids in the Sierra de Salinas (Kistler and anatexis north and south of the inferred segment nized extensional structures. Champion, 2001; Barth et al., 2003). Metamor- boundary of the Farallon slab; and (9) extension phic framework rocks are overlain by Maastrich- and magmatism synchronous with continued Sierra Nevada, Western Mojave, and Salinia tian marine and terrestrial deposits, interpreted shortening in both the Laramide foreland prov- The southernmost Sierran, western Mojave, by Grove (1993) as deposited in grabens. ince and the Sevier fold-thrust belt northeast of and Salinian segments of the Mesozoic Cordil- Mantle xenolith studies from the central the Mojave Desert. leran arc, segments adjacent to the Mojave sec- Sierra Nevada (locality 11, Fig. 1) provide evi- tor of the Sevier orogen in pre-late Tertiary res- dence for removal of all but a thin remnant of Rejection of Alternative Mechanisms for torations (Grove et al., 2003b; Saleeby, 2003), ancient mantle lithosphere and for its replace- Synconvergent Late Cretaceous Extension show evidence for both removal of lower crust ment by asthenosphere (Lee et al., 2000, 2001a; and mantle lithosphere and widespread exten- cf. Saleeby et al., 2003). Petrologic and Re-Os Mechanisms that may be applicable to the sion in the Late Cretaceous (Saleeby, 2003). isotopic studies of xenoliths in late Miocene backarc Sevier-Laramide orogen, analogous in The Sierra Nevada batholith exhibits a south- basalt show the mantle lithosphere as vertically tectonic setting to the modern subandean thrust ward-increasing depth of exposure, from ~2 stratifi ed with shallow (<45–60 km) and cold belt and its internal plateau (Jordan et al., 1983; kbar in the south-central Sierra to ~9 kbar meta- Proterozoic mantle showing evidence for heat- Allmendinger et al., 1997), are considered morphic conditions in the Tehachapi Range to ing, whereas deeper (~45–100 km), younger below. As currently understood, Late Cretaceous the south (Ague and Brimhall, 1988; Pickett mantle lithosphere shows evidence for cooling extension in the Sevier orogen was dominantly and Saleeby, 1993; Saleeby, 2003). The deep- (Lee et al., 2000, 2001a). These observations perpendicular to the trend of the orogen, and est exhumed rocks are in low-angle fault con- are interpreted to record removal of mantle therefore we do not consider explicitly mecha- tact with underlying schist associated with the lithosphere, followed by upwelling and accre- nisms that result in orogen-parallel extension Pelona-Orocopia-Rand schist belt, consistent tion of conductively cooled asthenosphere to the (e.g., Ave Lallemant and Guth, 1990; McCaf- with delamination of mantle lithosphere and base of the thinned lithosphere, in turn causing frey and Nabelek, 1998; Murphy et al., 2002). lower crust and detachment along the base of heating of the relict mantle lithosphere (Lee et Late Mesozoic to early Cenozoic plate the felsic arc (Saleeby, 2003). Thermochronol- al., 2000, 2001a). Although the timing of the velocities and subducting plate geometry along ogy indicates a Late Cretaceous age for denuda- implied events is diffi cult to date precisely, the western margin of North America preclude tion and northward tilt of the southern Sierras thermal arguments of Lee et al. (2000, 2001a), a reduction in convergent velocity and slab

522 Geological Society of America Bulletin, May/June 2008 Cretaceous extension and delamination in the Cordilleran orogen

rollback as viable mechanisms for Late Cre- this hypothesis include uncertainties in the abso- contributing factor to the north (Great Basin), taceous synconvergent extension of the Sevier lute convergence rate between the Farallon and followed replacement of lithosphere by asthe- orogen. The orthogonal component of the plate North American plates (Engebretson et al., 1985; nosphere. Fluid fl ux into the lower crust from convergent velocities increased over this time Stock and Molnar, 1988), the timing of termina- a dehydrating Farallon slab may have further interval (Engebretson et al., 1985), and this was tion of arc magmatism and its causes (e.g., 76– promoted rheological weakening and crustal a period of slab shallowing rather than steep- 82 Ma; Mattinson, 1990; Kistler and Champion, melting, leading to magmatism and extension. ening. An increase in horizontal compressive 2001; Ducea, 2001), and in the causes of slab We suggest that delamination may have aided stress is predicted from both an increased sur- fl attening (Cross and Pilger, 1982; Henderson in the shallowing of the slab by removal of a face area of plate contact and increased conver- et al., 1984). A suffi cient slab rollback effect is physical impediment. Otherwise, attainment gence rate (Dickinson and Snyder, 1978; Bird, required at the trailing edge of the aseismic ridge of the shallow-angle geometry would require 1988; Livaccari and Perry, 1993). to generate extension in the overriding plate “mechanical removal” or “subduction erosion” Orogenic wedge mechanics (Platt, 1986; Wil- without invoking slab breakoff. Slab breakoff by traction (e.g., Bird, 1984, 1988; Hamilton, lett, 1999), including the response to localized is precluded because shortening deformation of 1988; Spencer, 1996). Removal of lithospheric crustal shortening via underplating and duplex the Rocky Mountain foreland and underplating mantle would further facilitate the underplating faulting, may have been a contributing factor in of the Pelona-Orocopia-Rand schist continued of weak, water-rich metasedimentary protoliths causing Late Cretaceous extension in the hin- into the early Tertiary (Dickinson et al., 1988; for the Pelona-Orocopia-Rand schist. We note terland of the Idaho-Utah-Wyoming sector of Grove et al., 2003b). The thermal, chemical, and that the complete (Bird, 1984, 1988) or partial the Sevier fold-thrust belt, but it is diffi cult to xenolith evidence for Late Cretaceous invasion (Spencer, 1996) removal of mantle lithosphere apply to the Mojave sector. Duplex faulting to of basalt into the lower crust (see below) and by lateral displacement, and its replacement produce culminations at the craton-miogeocline the observation of initial extension as synmag- by subducted oceanic lithosphere, may result transition and farther east in the hinterland of the matic are more supportive of asthenosphere, in a similar response of increased buoyancy of Idaho-Utah-Wyoming sector thickened the rear rather than the Farallon plate, directly underly- the overriding plate, leading to uplift, but will of the foreland wedge (Yonkee, 1992; DeCelles ing North American lithosphere prior to and per- lack the thermal and magmatic effects resulting and Mitra, 1995; DeCelles et al., 1995; Mitra haps during the initial stages of extension. Fur- from replacement by asthenosphere (Dumitru and Sussman, 1997). Farther west, culminations thermore, if we accept the notion of a segmented et al., 1991; English et al., 2003). Expulsion of marking the location of the core complexes of slab at face value, the model cannot be applied asthenosphere during subsequent shallowing of the Great Basin may have served a similar role to localities that overlie the inferred steeper slab the slab, perhaps not complete northeast of the in accomplishing continued internal thicken- segment in the northern Great Basin. exposures of the Pelona-Orocopia-Rand schist, ing during evolution of the orogenic wedge. In occurred as the position of the shallowly dipping contrast, the contractile belt in eastern Califor- Proposed Mechanisms: Delamination Farallon plate propagated northeastward beneath nia lacked an initial wedge-shaped sedimentary Followed by Slab Dehydration North America. The removal of lithosphere may prism, resulting in discontinuous thrusts involv- have been piecemeal or wholesale and variable ing crystalline rocks commonly not of décolle- Based on the observations from the Mojave along orogenic strike (Fig. 5), but the similar- ment style (Burchfi el and Davis, 1971; Reyn- Desert and Great Basin summarized above, ity in timing of magmatism and extension (e.g., olds et al., 1986). We conclude that the dynamic we propose the following two-stage evolution the eastern Mojave Desert region) suggests that and geometric models of orogenic wedges (e.g., during the inception of the Laramide orogeny removal occurred over a short time interval for Platt, 1986; Willett, 1999), including underplat- in the southwestern Cordillera (Fig. 5). Firstly, >400 km along strike. Similarly, lithospheric ing as a framework for driving extension, are the removal of mantle lithosphere and eclog- delamination during progressive slab fl attening probably not viable for the Mojave sector of the itic lower crust (Saleeby et al., 2003) owing to has been invoked for the Andes (Kay and Kay, Sevier fold-thrust belt. delamination and/or subduction erosion beneath 1993; Whitman et al., 1996; Kay and Abbruzzi, Saleeby (2003) proposed that synconvergent the arc (southern Sierra, Salinia, northern Pen- 1996), where the geophysical signature of a extension and westward “breachment” of the insular Ranges), and removal of mantle litho- thinned mantle lid is preserved. southern Sierran, Mojave, and Salinian arc sphere owing to delamination in the backarc Below we explore why the geologic con- segments developed in the passing wake of (Sevier hinterland and eastern Mojave Desert), sequences of delamination of mantle litho- an underthrust aseismic ridge (Henderson et resulted in isostatic uplift, upwelling of asthenosphere—an increase in Moho temperature, al., 1984; Barth and Schneiderman, 1996) in sphere, incursion of decompression basaltic par- decompression partial melting of astheno- response to the progressive increase in density tial melts and their associated heat into the lower sphere, surface uplift, and extension—are most of trailing abyssal oceanic lithosphere relative crust (most profound in the eastern Mojave), compatible with observations (1) through (9) to the subducted aseismic ridge. Although the surface erosion, and extension. Heating of the from the selected areas as above, as well as observations summarized here bolster the evi- lower crust led to anatexis and the reduction of observations from mantle xenoliths and petrol- dence presented by Saleeby (2003) for Laramide lower crustal viscosity, promoting channel fl ow ogy of crustal metamorphic and igneous rocks extensional fragmentation of the forearc, arc, of the lower crust and decoupling between the from elsewhere in the southwest Cordillera. and eastern arc fringe, the proposed mechanism upper-middle crust and the remaining subcon- for extension faces challenges in application to tinental mantle lithosphere and allowing the Heating and Deep Production of the eastern arc fringe in the eastern Mojave Des- upper-middle crust to respond to lateral gradi- Cordilleran-Type Peraluminous Granites ert region, and to regions north of the inferred ents in potential energy and to extend. slab segment boundary. Initiation of extension Secondly, fl attening and dehydration of The Cordilleran-type peraluminous gran- as shown here is well constrained at 73–75 Ma, the Farallon slab, thought to be most relevant ites are widely viewed as representing crustal requiring the aseismic ridge to be east of Cali- to areas south of the inferred slab infl ection melts with only a minor mantle contribution; fornia by this time. Diffi culties in corroborating (Mojave Desert), but perhaps also a lesser however, the mechanism of production of the

Geological Society of America Bulletin, May/June 2008 523 Wells and Hoisch

Mojave Great Basin 95 Ma 95 Ma Shortening LB CN SFTB A Shortening ES SFTB D Shortening

Moho Moho

End load

Isotherm Isotherm Topography 2X vertical

0 100 200 km

75 Ma 75 Ma LFP SFTB LFP Extension B Extension Shortening E Shortening Shortening bp Melting bp Melting Uplift Uplift dm dm

Delamination Delamination 70 Ma 70 Ma LFP SFTB LFP Extension Shortening C F Shortening Shortening

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 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 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 counterfl ow 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 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.

granites remains controversial. Crustal thicken- as best exemplifi ed in the Mojave Desert source, such as the Neoproterozoic siliciclastic ing, fl uid infi ltration from a shallow Laramide region, are pre- or synextensional rather than rocks of the miogeocline of the Great Basin, slab, increased mantle heat fl ux, magmatic postextensional, and intersection of P-T paths and these rocks are absent in the eastern Mojave underplating, decompression, and delamination with dehydration melting curves requires rapid Desert region. Furthermore, the low melt frac- have all been invoked to explain melting of the decompression (Fig. 6). Muscovite dehydration tion produced by muscovite dehydration melt- lower crust (Foster and Hyndman, 1990; Hoisch melting resulting from crustal thickening has ing may also have been insuffi cient for separa- and Hamilton, 1990; Patiño Douce et al., 1990; been locally invoked for the Ruby Mountains tion, migration, and accumulation (Clemens and Miller and Barton, 1990; Hodges and Walker, and East Humboldt Range of the Great Basin Vielzeuf, 1987; Patiño Douce et al., 1990). It is 1992). Dehydration partial melting owing to (Lee et al., 2003). However, the production of debated whether in situ heat production follow- rapid decompression is unlikely to have been of large quantities of melt requires the presence of ing crustal thickening was suffi cient to induce widespread importance because most granites, large volumes of a muscovite-rich metapelite biotite dehydration partial melting, which could

524 Geological Society of America Bulletin, May/June 2008 Cretaceous extension and delamination in the Cordilleran orogen have produced the larger melt fractions required for migration and accumulation of melts in plu- 1 2 tons (Patiño Douce et al., 1990; Barton, 1990). Alternatively, the gradual heating of the lower crust as a consequence of asthenospheric coun- f 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- 3 taceous. Delamination, and the resulting incursion of basaltic asthenospheric melts produced

by decompression partial melting, could have (kb) P 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 4 felsic and mafi c magmas (e.g., Robinson et al., 1986; Kapp et al., 2002). Furthermore, mid-oceanic-ridge basalt (MORB)–source lower crustal T (°C) xenoliths of Late Cretaceous age from the Cima Figure 6. P-T diagram illustrating possible causes for melting of lower crust. Wet volcanic fi eld (Fig. 2) suggest asthenospheric melting of muscovite granite in the system K2O-Na2O-Al2O3-SiO2-H2O is shown by upwelling beneath the eastern Mojave Desert the dotted-dashed line (labeled 1, from Thompson and Tracy, 1979). Melting reac- (Leventhal et al., 1995). All lower-crustal xeno- tions in the system Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O (NaKFMASH)are shown liths at Cima are consistent with partial melts of as dashed lines (Spear et al., 1999). Al2SiO5 polymorphic transformations are shown the asthenosphere, suggesting that invasion of in solid lines (Pattison, 1992). Light gray polygon shows the region of muscovite basalt into the lower crust was widespread. dehydration partial melting as determined from experiments on natural materi- Late Cretaceous heating of the crust is evi- als (Patiño Douce, 1999). Dark gray polygon shows the region of biotite dehydra- dent from the P-T path from the Grouse Creek tion partial melting as determined from experiments on natural materials (Patiño Mountains in northwest Utah, which shows con- Douce, 1999). NaKFMASH reactions: muscovite dehydration partial melting in the tinuous heating during a sequence of compres- presence of quartz and albite (labeled 2); two biotite dehydration partial melting sion, decompression, and renewed compression reactions (labeled 3); and wet melting of pelitic schist (labeled 4). Arrow shows the in the Late Cretaceous (Hoisch et al., 2002; Har- adiabatic decompression path of Harris and Massey (1994). As—Al2SiO5; Ab— ris et al., 2007). The initial heating during thick- albite; And—andalusite; Bt—biotite; Cd—cordierite; Gt—garnet; Ilm—ilmenite; ening recorded by the path (marked upper zone, Kf—K-feldspar; Ky—kyanite; L—liquid; Mus—muscovite; Opx—orthopyroxene; Fig. 4B) is typical and expected in thrust settings Pl—plagioclase; Qz—quartz; Sil—sillimanite. (e.g., England and Thompson, 1984) owing to thermal relaxation and radiogenic heating of the thickened pile; however, continued heating following exhumation would be diffi cult to explain uration temperatures from many Mojave Late the crystallization of hydrous melts beneath the without invoking an additional heat source. Cretaceous Cordilleran-type peraluminous gran- level of exposure. The infl ux of fl uids expelled Because the additional heating took place in the ites with abundant inherited zircon indicate low from the Farallon plate and hydration of the absence of exposed age-correlative plutons in magma temperatures (725–825 °C) requiring overlying North American lithosphere have also the nearby region, delamination accompanied signifi cant water at the melting site, more than been suggested to have occurred over a broad by asthenospheric upwelling seems necessary can be ascribed to in situ dehydration reactions region of the western United States to account to explain the continued heating in this area. in the lower crust (Miller et al., 2003). Evidence for anomalous elevation, low seismic velocities, for Late Cretaceous fl uid fl ux is also provided by and melt production as far east as the Colorado Fluid Flux From Laramide Slab metamorphism of calcite-cemented sandstone Mineral Belt (Humphreys et al., 2003). zones in the Supai Formation to massive (>90%) In the Mojave region, melting of the lower wollastonite in the Big Maria Mountains (Fig. Weakening of Lower Crust and Decoupling crust may have been promoted further by the 2) (Hoisch, 1987). At the upper greenschist to infi ltration of fl uids derived from the shallowing lower amphibolite facies conditions of metamor- Widespread melting and attendant rheo- Farallon slab, perhaps from water-rich under- phism, fl uid-to-rock ratios of >17:1 are required logical weakening of the lower crust, which plated metasediments (Pelona-Orocopia-Rand to produce the observed quantity of wollastonite are required in the production of Cordilleran- schist) in the subduction channel (Hoisch and (Hoisch, 1987). The minimum quantity of fl uid type peraluminous granites, may have played Hamilton, 1990; Malin et al., 1995). Zircon sat- required is much too large to be derived from a role in facilitating synconvergent extension.

Geological Society of America Bulletin, May/June 2008 525 Wells and Hoisch

A reduction in strength is predicted by thermal Molnar, 1997). Because backarc shortening dur- extensional exhumation. Furthermore, we argue softening through the temperature sensitivity of ing the Sevier-Laramide orogeny was protracted that the exhumation of mid-crustal rocks record- plastic fl ow laws (Kohlstedt et al., 1995), and by for up to 110 m.y., delamination during rather ing 6–9 kbar Mesozoic pressures (e.g., Hoisch melt-induced weakening through anatexis and than after mountain building and plate conver- and Simpson, 1993; Lewis et al., 1999; McGrew production of migmatites (Jamieson et al., 1998; gence is permitted. The recognition of a thin et al., 2000; Hoisch et al., 2002; Harris et al., Handy et al., 2001). Experimental petrology and and fertile Archean mantle lithosphere beneath 2007) requires multiple exhumation events, as petrochemical modeling of the Cordilleran-type the eastern Mojave from Re-Os isotopic stud- Cenozoic detachment fault systems typically peraluminous granites indicate that the crustal ies of mantle xenoliths in Pliocene basalts of the exhumed pre-Cenozoic crustal levels no deeper source was deep (e.g., Patiño Douce, 1999; Kapp Cima volcanic fi eld (Lee et al., 2001b) shows a than 10–15 km (e.g., Richard et al., 1990; Hur- et al., 2002), consistent with weakening of lower dense mantle lithosphere capable of foundering low et al., 1991; Wells et al., 2000). crust. The site of melting was probably diffuse and a thickness compatible with prior removal. Mechanisms to trigger extension during plate and sheetlike, because it was controlled by the Should delamination in the presence of astheno- convergence require either a reduction in hori- thermal and lithologic structure of the lithosphere sphere have been responsible for the removal of zontal compressive stress or an increase in forces (e.g., Sawyer, 1998), facilitating mechanical mantle lithosphere and lower crust, rather than resulting from lateral contrasts in gravitational decoupling between the upper mantle and overly- subduction erosion by traction, then the forma- potential energy. The initiation of extension in ing crust. If extension initiated and/or continued tion of eclogites in the deep levels of the arc the Late Cretaceous of the western United States after the low-angle slab made contact with east- (Ducea, 2001; Saleeby et al., 2003) may have was associated with anatexis of the lower crust ern Mojave continental lithosphere, fl uid infi ltra- been signifi cant in the arc region. and pluton emplacement at mid-crustal levels, tion from a devolatilizing Farallon slab would suggesting a common root cause. The metamor- have further weakened the lower crust (Hoisch CONCLUSIONS phic, magmatic, thermal, and kinematic histories et al., 1988; Malin et al., 1995; Humphreys et of the arc and backarc lead us to propose that al., 2003). This weakened lower crust may have The Mesozoic arc and backarc belt of short- delamination of mantle lithosphere, aided by undergone channel fl ow in response to pressure ening of the southwest Cordilleran orogen decoupling of the crust from the mantle through gradients between the topographically elevated underwent widespread and large-magnitude a reduction in the viscosity of the lower crust, orogen and the lower elevations to the east, and exhumation in the Late Cretaceous at the onset provides the most plausible explanation (Fig. 5). fl ow may have been dynamically related to uplift of the Laramide orogeny. Although the relative of the Colorado Plateau and shortening in the contribution of extension and erosion to the ACKNOWLEDGMENTS Laramide belt (Livaccari, 1991; McQuarrie and total exhumation probably varied signifi cantly Financial support for our research in the Mojave Chase, 2000), similar to the linkage between between localities, extension was nonetheless Desert was provided by U.S. National Science Foun- crustal fl ow under the Tibetan Plateau and the prevalent north and south of the inferred seg- dation grant EAR 96-28540 (M.L.W.), and in the marginal belts of crustal shortening (e.g., Roy- ment boundary (Saleeby, 2003) in the Laramide Great Basin by NSF grants EAR-9805076 (T.D.H.) den, 1996; Royden et al., 1997). slab. The record of extensional exhumation is and EAR-9805007 (M.L.W.). We have benefi ted from most clear in the eastern Mojave Desert region, discussions of Mojave geology and Laramide tectonics with D. Foster, M. Grove, K. Howard, E. Humphreys, Precondition for Delamination where, in comparison with the Great Basin, C. Jacobson, J. Kula, C. Miller, D. Miller, T. Spell, abundant Late Cretaceous plutons provide age and P. Gans. This manuscript benefi ted from informal Mass balance considerations permit the litho- constraints, and subsequent Late Cretaceous to reviews by M. Nicholl and A. Simon. We would like spheric mantle beneath the Sevier hinterland early Tertiary shortening was partitioned east to thank C. Jacobson and an anonymous reviewer for their detailed reviews and thoughtful comments that and the Mesozoic arc to have been substantially of the inactive fold-thrust belt. Additionally, the helped us to improve the paper. thickened and subsequently removed. 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