Large-magnitude continental extension: An example from the central Mojave metamorphic core complex

John M. Fletcher* Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 John M. Bartley } Mark W. Martin* Isotope Geochemistry Laboratory and Department of Geology, University of Kansas, Lawrence, Kansas 66045 Allen F. Glazner Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27599 J. Douglas Walker Isotope Geochemistry Laboratory and Department of Geology, University of Kansas, Lawrence, Kansas 66045

ABSTRACT of ductile deformation and either the proximity or relative volume of extension-related igneous rocks. This suggests that models that The central Mojave metamorphic core complex is defined by a invoke a single upper-crustal genetic relationship, such as magma- belt of Miocene brittle-ductile extension and coeval magmatism. tism triggering extension or vice versa, do not apply to the central The brittle-ductile fault zone defines a basin-and-dome geometry Mojave metamorphic core complex. that results from the interference of two orthogonal fold sets that Systematic variation in the relative timing of dike emplace- we infer to have formed by mechanically independent processes. ment and mylonitization suggests that, at the time of dike emplace- One fold set contains axes that lie parallel to the extension direction ment, rocks in the Mitchel Range were below the brittle-ductile of the shear zone and has a maximum characteristic wavelength of transition while those in the Hinkley Hills were above it. The Hink- about 10 km. The axial surfaces of these folds can be traced from ley Hills and Mitchel Range are separated by ϳ2 km in the dip the footwall mylonites, through the brittle detachment, and into direction of the fault zone, which suggests that the vertical thick- hanging-wall strata. However, folds of mylonitic layering have ness of the brittle-ductile transition probably was between 100 and smaller interlimb angles than those of the brittle detachment, sug- 950 m. gesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime, possibly after movement INTRODUCTION across the fault zone ceased. Mesoscopic fabrics related to the transport-parallel fold set indicate that the folds record true crustal Brittle-ductile faulting as recorded in Cordilleran metamorphic shortening perpendicular to the extension direction. We interpret core complexes are now recognized throughout the world as an these folds to form in response to elevated horizontal compressive important mechanism of extension in previously overthickened con- stress perpendicular to the extension direction and suggest that this tinental crust. Examples include late- to post-Caledonian rifting on stress regime may be a natural consequence of large-magnitude the western margin of Norway (Norton, 1986; Andersen and extension. Jamtveit, 1990), extension in the Andean magmatic arc (Mpodozis The other fold set has axes perpendicular to the extension and Allmendinger, 1993), syncontractional spreading on the Ti- direction and a characteristic maximum wavelength of about 50 km. betan plateau (Burg et al., 1984; Burchfiel and Royden, 1985), col- Mesoscopic fabrics related to these folds include northwest-striking lapse of the Cycladic blueschist belt in the central Aegean Sea (Lee joints, kink bands, and en echelon tension-gash arrays. These fab- and Lister, 1992), and extension of continental crust at the tip of a rics formed after mylonitization and record both layer-parallel ex- propagating oceanic rift in the Solomon Sea (Davies and Warren, tension and northeast-side-up subvertical shear. The postmylonitic 1988; Hill et al., 1992; Baldwin et al., 1993). Additionally, spreading fabrics are kinematically compatible with rolling-hinge-style iso- at some mid-ocean ridges seems to involve such fault zones, based static rebound of the footwall following tectonic denudation. on the results from ocean drilling (e.g., Cannat, 1987; Cannat et al., The relative timing of extension-related magma intrusion and 1987; Dick et al., 1987a) and the study of ophiolites (e.g., Varga and ductile deformation varies through the central Mojave metamor- Moores, 1985; Harper, 1985). phic core complex. On the scale of the small mountain ranges that Much of the work on metamorphic core complexes in the past make up the central Mojave metamorphic core complex, no corre- decade has centered on three main areas: (1) understanding rela- lation was observed between either shear zone thickness or intensity tionships between magmatism and extensional deformation, (2) un- derstanding the characteristically nonplanar geometry of the low- angle fault zones, and (3) understanding mechanics of low-angle *Present address: Fletcher: Departamento de Geologia Centro de In- normal faulting. This paper addresses issues concerning each of vestigacio´n Cientı´fica y Educacio´n Superior de Ensenada, B.C. Ensenada, B.C., Mexico; Martin: Servicio Nacional de Geologia y Mineralia-Chile, these areas. Grupo de Geologia Regional, Avda. Santa Maria 0104, Casilla 1347, San- In many metamorphic core complexes, crustal extension oc- tiago, Chile. curred during igneous activity and genetic links therefore have been

GSA Bulletin; December 1995; v. 107; no. 12; p. 1468–1483; 13 figures.

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inferred between the two processes. Elevated mantle heat flow formation of the undulations. Rock fabrics related to these two fold and/or the emplacement of mafic magma at the base of the crust has sets have important implications for the origin of the dome-and- been proposed to cause thermal softening, triggering extension (e.g., basin fault geometry, as well as for the kinematics and dynamics of Rehrig and Reynolds, 1980; Sonder et al., 1987; Wernicke et al., large-magnitude extension. 1987; Gans et al., 1989). Lister and Baldwin (1993) recently pro- This paper defines the areal extent and characterizes the rock posed a more intimate genetic relationship, suggesting that mid- types and fabrics of a large-displacement extensional shear zone crustal magmatism triggers extension and controls the location of exposed in the central Mojave Desert. Field relations and structural the brittle-ductile transition in the crust. They suggest that mid- data in this paper complement geochronologic data from the central crustal magmatism is responsible for many characteristic features of Mojave metamorphic core complex published separately by Walker core complexes, including ductile deformation of footwall rocks, et al. (in press). Extension-related intrusive rocks are used to limit spatial patterns of cooling ages, and characteristic geometric fea- the timing of deformation and to recognize and correlate Miocene tures of the fault zones. Abundant extension-related intrusions and rock fabrics throughout the multiply deformed terrane. Field rela- excellent exposures in the central Mojave metamorphic core com- tions in the central Mojave metamorphic core complex also help to plex permit tests of such proposed genetic relationships. characterize the brittle-ductile transition during crustal extension. Brittle-ductile detachment faults in core complexes character- istically define dome-and-basin structural topography that results Geographic and Tectonic Setting from interference of two orthogonal sets of upright folds (Spencer, 1982, 1984; Davis and Lister, 1988; Yin, 1991; Yin and Dunn, 1992). Miocene crustal extension and basin development are recog- Several models have been proposed to explain the origin of these nized in the central Mojave Desert from the Rodman Mountains to folds, and each makes specific predictions for (1) macroscopic ge- the Buttes (ϳ90 km along strike; Fig. 1). A single master detach- ometry of layered rocks in the upper and lower plates, (2) meso- ment fault may underlie the entire belt. However, differences in scopic finite strain recorded in the upper and lower plates, and (3) structural style and extension magnitude permit the belt to be di- relative timing of mylonitization, deposition of rift sediments, and vided into two main segments. The southeastern segment in the

Figure 1. Geologic map of the Miocene extensional belt in the central Mojave Desert, California (The Buttes to Rodman Mountains) showing the mylonitic and nonmylonitic segments of the detachment fault system. Modified in part after Dokka (1989) and Walker et al (in press); see text.

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Rodman Mountains and Newberry Mountains exposes no footwall Mitchel Range and provides the thickest exposures of the brittle- mylonites nor extension-related plutonic rocks. In contrast, the ductile fault zone, as well as the most intense mylonitic fabrics. The northwestern segment of the fault system, exposed from the Mitchel Hinkley domain, which includes the Hinkley Hills and Lynx Cat Range to the Buttes, is characterized by well-developed footwall Mountain, contains exposures of the fault zone that are structurally mylonites and widespread extension-related igneous intrusions. up-dip from the Mitchel domain. The Buttes domain, which in- Based on the offset of regional pre-Tertiary lithologic markers in the cludes the Buttes and surrounding hills, makes up the westernmost central Mojave Desert, ϳ40–50 km of northeast-directed displace- exposures of the brittle-ductile detachment in the central Mojave ment occurred across the northwestern brittle-ductile fault segment metamorphic core complex and lies along strike from the other (Glazner et al., 1989; Martin et al., 1993). In contrast, field relations domains. in the Rodman Mountains suggest that the southeastern fault seg- ment accomplished significantly less displacement, which is consist- LITHOLOGIC AND STRUCTURAL CORRELATIONS ent with the lack of mylonites (Bartley and Glazner, 1991, and un- pub. field data). Martin et al. (1993) suggested that the boundary Rock Types between these two segments is defined by an unexposed right-lateral fault system that transfers extension from the Colorado River Pre-Tertiary Rocks. The ductilely deformed footwall of the Trough to the central Mojave. central Mojave metamorphic core complex is predominantly com- The two segments of the extensional terrane approximately posedofpre-Tertiarymetaigneousandmetasedimentaryrocks.Meta- correspond to the Daggett and Waterman terranes of Dokka (1989). igneous rocks are dominated by a plutonic complex made up of However, Dokka (1989) also placed a terrane boundary (Waterman/ quartz diorite, hornblende-biotite diorite, hornblende gabbro, and Edwards) within our northwestern fault segment, separating the subordinate two-mica granite. Metasedimentary wall rocks of this Buttes–Harper Lake area from the rest of the core complex. Much igneous complex are composed of varying rock suites across the of the Edwards Terrane also includes unextended or very weakly central Mojave metamorphic core complex. In the Mitchel Range extended crust in the western Mojave Desert (Bartley et al., 1990b). and Hinkley Hills, feldspathic micaceous quartzite is the most abun- We found no evidence that either the style or magnitude of crustal dant metasedimentary rock, with subordinate pelitic schist, ortho- extension changes between the Buttes–Harper Lake area and the quartzite, bluish-gray graphitic calcite marble with siliceous string- rest of the brittle-ductile fault segment. ers, white calcite marble, and tan dolomitic marble. Kiser (1981) The northwestern segment of the Mojave extensional belt has correlated this sequence with Vendian-Cambrian strata of the Cor- been termed the central Mojave metamorphic core complex (Glazner dilleran miogeocline. In the Buttes domain, marbles are virtually et al., 1989; revised by Fletcher and Bartley, 1994). This paper syn- identical to those in the Hinkley Hills and Mitchel Range, but the thesizes and interprets extensional deformation and coeval igneous metaclastic rocks are more calcareous and less aluminous. Amphi- intrusion in the footwall of the central Mojave metamorphic core bolitic carbonaceous quartzite is the most abundant metasedimen- complex, which is exposed in three areas that we refer to as domains tary rock type in this domain. Also, a significant amount of para- (Fig. 2). The Mitchel domain includes the Waterman Hills and amphibolite is present here but is virtually absent in the other

Figure 2. Geologic map of the central Mojave metamorphic core complex showing the three domains of this study: WH and MR make up the Mitchel domain; HH and area to the immediate northwest make up the Hinkley domain; B is the Buttes domain. Area ornamented with wavy lines is alluvi- um-covered but inferred to be underlain by Miocene mylonite. Abbreviations: B, Buttes; CM, Calico Mountains; FP, Fremont Peak; GF, Garlock Fault; GH, Gravel Hills; HH, Hinkley Hills; IM, Iron Mountain; L, Lead Mountain; LCM, Lynx Cat Mountain; LM, Lane Mountain; MH, Mud Hills; MR, Mitchel Range; SAF, San Andreas fault; WH, Waterman Hills.

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domains. We interpret these lithologic variations to reflect differ- spar is cataclastically deformed and hydrothermally altered; most of ences in either age or facies of the sedimentary protolith. the strain in feldspar is accomplished by the cryptocrystalline alter- Pre-Tertiary rocks variably contain the Tertiary mylonitic fabric ation product, perhaps by grain boundary sliding or alteration-in- and a high-grade gneissic fabric probably related to Mesozoic con- duced dynamic recrystallization. Dolomite is typically cracked and traction. Pelitic rocks preserve relict upper amphibolite-facies as- contains abundant glide twins. 40Ar/39Ar release spectra in a mylo- semblages including garnet ϩ biotite ϩ muscovite ϩ quartz Ϯ kya- nitic pre-Tertiary quartzite limit peak temperatures of mylonitiza- nite Ϯ sillimanite Ϯ staurolite. High-grade fabrics in the central tion to 300–400 ЊC because phengitic white mica yields a disturbed Mojave metamorphic core complex and vicinity appear to be of two spectrum at about 52 Ma and biotite is totally reset at 20–22 Ma different ages, Late Cretaceous in the Buttes (Glazner et al., 1994; (Wanda Taylor and John Bartley, unpub. data). M. W. Martin, unpub. data) and Late Jurassic in the Iron Mountains Mylonitic foliation is folded at all scales about an axis oriented (Boettcher and Walker, 1993). In each area the gneissic fabric is subparallel to the stretching lineation of the shear zone. In all three steep and predominantly strikes northeast. domains, poles to the mylonitic foliation define a great circle girdle Miocene Igneous Rocks. The central Mojave metamorphic core with a best-fit fold axis oriented subparallel to the local stretching complex is intruded by several extension-related igneous phases. lineation (Figs. 3B, 3F, and 3J). Fletcher and Bartley (1994) con- U/Pb zircon data from all samples of these rocks define a single cluded that the mylonitic fabric records horizontal shortening per- discord with a lower intercept of 21.9 Ϯ 0.8 Ma, and some of the pendicular to the stretching lineation of the shear zone through the calculated error is interpreted to reflect real variation in ages of the formation of synmylonitic folds and L-tectonites. We demonstrate suite of intrusive rocks (Walker et al., in press). Miocene granite below that these folds are related to northeast-trending upright crops out in all three domains and may form a continuous batholith folds of the detachment fault. Variations in the character of my- at depth. Primary minerals in the medium-grained equigranular lonitization and synmylonitic folding across the central Mojave meta- granite include oligoclase, orthoclase, quartz, biotite, and minor morphic core complex are documented below. hornblende. Texturally, oligoclase is euhedral and cyclically zoned, Postmylonitic Fabrics. Rocks in the footwall of the central Mo- whereas orthoclase poikilitically overgrows all other primary min- jave metamorphic core complex contain two main classes of post- erals and is partially replaced by myrmekite. mylonitic fabrics: joints and a composite fabric of kink bands and en Dikes in the central Mojave metamorphic core complex range echelon tension-gash arrays (Figs. 3C, 3G, 3K, 3D, 3H, and 3L). in composition from basalt to rhyolite, but the majority (60%–80%) Both fabrics strike northwest, dip steeply, and are predominantly are dacitic to rhyodacitic. Basaltic dikes were not analyzed by found in rocks that were previously mylonitized. Joints are abundant Walker et al. (1995), but these dikes never contain a mylonitic fabric in all rock types. They occur as single and conjugate sets that are and may be much younger, possibly related to Pliocene basalt flows oriented subperpendicular to the stretching lineation and commonly at Black Mountain (Fig. 1). Rhyolite dikes are generally aphanitic, accommodate millimeter-scale tensile and normal-sense offsets but some are porphyritic with 10% plagioclase, quartz, and rare (Fig. 4). Joints in the Mitchel Range are divisible into two groups garnet phenocrysts. Dacite dikes are commonly porphyritic with depending on the plunge of the stretching lineation. Where the 30%–40% phenocrysts of plagioclase ϩ quartz Ϯ biotite Ϯ stretching lineation plunges to the southwest, joints mainly dip to hornblende. the northeast; where the stretching lineation plunges to the north- A dacite dike in the Mitchel Range yielded a zircon U/Pb age east, the joints dip to the southwest (Fig. 3C). This pattern suggests of 23 Ϯ 0.9 Ma (Walker et al., 1990), but dikes in the Hinkley Hills that the joints formed before or during folding of the shear zone have complex U/Pb systematics that are presently uninterpretable about a northwest-rending axis and that the maximum extension (Walker et al., in press). However, the dikes in both areas are litho- direction in the brittle regime crudely coincided with that in the logically identical, and available geochronologic data at least permit earlier ductile regime. the possibility that they were intruded synchronously. If this is true, The composite kink-band and en echelon tension-gash fabric is field relations in the central Mojave metamorphic core complex can best developed in strongly layered rocks. The composite fabric is be used to characterize the nature of the brittle-ductile transition, as common in ultramylonitic rocks of the mafic igneous complex and discussed below, because the relative timing of ductile deformation in Miocene dikes but is rare in quartzite and marble. Tension-gash and magma emplacement varies systematically between domains. arrays commonly occur within kink bands and may accommodate dilation during kinking. However, tension-gash arrays and kink Deformational Fabrics bands each occur independently of each other. Planar tension gashes are typically oriented 45Њ–20Њ from the array boundary, which Synmylonitic Fabrics and Folds. In all domains, the mylonitic suggests that, in some cases, they accommodated boundary- stretching lineation plunges to northeast or southwest (Figs. 3A, 3E, perpendicular extension as well as boundary-parallel simple shear and 3I) and shear sense across the foliation is consistently northeast (cf. Ramsay and Huber, 1987). directed (Dokka, 1989; Bartley et al., 1990a). The mylonitic fabric In the Mitchel Range, the composite fabric is subvertical and forms L- and LS-tectonites and varies in intensity from protomylo- consistently shows northeast-side-up shear sense (Bartley et al., nitic to ultramylonitic. Protomylonitic rocks commonly are pure L- 1990a). The fabric commonly is oriented subperpendicular to my- tectonites and rarely contain a measurable foliation. Mylonitization lonitic foliation such that it accomplished neither layer-parallel involved transitional brittle-plastic deformation mechanisms (Bart- shortening nor extension, although some contractional kink bands ley et al., 1990a; Fletcher and Bartley, 1994). Quartz and calcite were observed in the Mitchel Range (Bartley et al., 1990a). The occur as mosaics of subgrains and neoblasts, which suggests that cleavage typically is better developed in the hinge and southwest- dislocation creep was the dominant deformation mechanism. Feld- dipping limb of the antiformal arch. In the Buttes and Hinkley Hills,

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/12/1468/3382205/i0016-7606-107-12-1468.pdf by guest on 28 September 2021 Figure 3. Equal-area stereoplots of Miocene deformational fabrics in ductilely sheared rocks from each domain. (A, E, I) Mylonitic stretching lineation. Shaded boxes are maximum eigen values of the data sets. (B, F, J) Poles to mylonitic foliation. Shaded boxes are minimum eigen values that define the best-fit macroscopic fold axis. In each subdomain the mean stretching lineation orientation is nearly parallel to the best-fit macroscopic fold axis. (C, G, K) Poles to postmylonitic joints. Shaded boxes are maximum eigen values. In the Mitchel Range, joints dip to the northeast in area where the stretching lineation plunges to the southwest and vice versa. (D, H, L) Poles .number of samples ؍ to the postmylonitic composite kink-band and tension-gash cleavage. Shaded boxes are minimum eigen values; n

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derlain by the Miocene Waterman Hills Granite, whereas the Mitchel Range is dominated by the heterogeneous pre-Tertiary basement complex (Fig. 5A). The two assemblages are separated by a klippe of Miocene volcanic and sedimentary rocks. Despite the structural differences summarized below, both assemblages record noncoaxial constrictional strain (Fletcher and Bartley, 1994). In the Waterman Hills, finite strain increases structurally up- ward. Most of the shear zone is an outcrop belt, several hundred meters wide, of protomylonitic L-tectonite that is transitional be- tween undeformed granite in the structurally lowest exposures and 10–20 m of ultramylonite beneath the detachment (Bartley et al., 1990a; Fletcher and Bartley, 1994). LS-tectonites are rare and found mainly in the ultramylonite. Finite strain in the protomylonitic L- tectonite is strongly constrictional, from which Fletcher and Bartley (1994) inferred that the rocks record a significant amount (ϳ75%) of horizontal shortening perpendicular to the inferred transport di- A rection of the shear zone. The shear zone is thicker in the pre-Tertiary rocks of the Mitchel Range than in the Waterman Hills granite and includes the most intense mylonitization found in the central Mojave metamor- phic core complex. Ultramylonite is exposed down to the lowest exposed structural level, Ͼ1 km beneath the detachment. Compo- sitional layering is fully transposed into the ultramylonite fabric. Quartzite and marble units generally do not exceed5minthickness, but individual horizons commonly can be traced for 1–4 km. The shear zone contains weakly deformed lozenges up to 1 km2 in out- crop area, bounded by steep strain gradients that can be mapped as contacts (Fig. 5A). Mylonite bounds the lozenges structurally both above and below (Fletcher, 1994); therefore, the strain gradients do not represent a ‘‘mylonite front’’ similar to that described in the Whipple Mountains (e.g., Davis and Lister, 1988; Reynolds and Lister, 1990). LS-tectonites predominate in the Mitchel Range but L-tecto- B nites also are common. The LS-tectonites record large-magnitude plane strain at the grain scale (Fletcher and Bartley, 1994), but the Figure 4. Postmylonitic deformational fabrics. (A) Outcrop photo fabric is strongly folded about an axis oriented subparallel to the of conjugate joints showing small normal-sense shear offsets. (B) extension direction of the fault zone. These nearly coaxial folds Monoclinal kink bands and tension-gash arrays on saw-cut surface range from upright and open to recumbent and isoclinal, but most of Miocene dacite from The Buttes. The bands and arrays are sub- are close and asymmetric. Fletcher and Bartley (1994) argued that perpendicular to the mylonitic foliation and accommodate north- this variation of fold styles represents an evolutionary sequence with east-side-up shear, but the two are not always spatially coincident. progressive mylonitization, from open folds initiated with hinges nearly parallel to the transport direction toward recumbent isoclinal many exposures of the composite fabric include conjugate sets, but folds. The initial upright folds are inferred to record the same sub- the northeast-side-up set is predominant. Displacement across con- horizontal shortening of the shear zone perpendicular to the exten- jugate sets of kink bands and tension gash arrays accomplishes layer- sion direction that is recorded by constrictional strain in the Mio- parallel extension. cene granite. However, relationships in the other domains suggest In summary, both the joints and the composite cleavage mainly that other folding processes operated as well. accommodate layer-parallel extension or northeast-side-up subver- The detachment fault in this domain defines a macroscopic tical shear. We discuss below the implications of this pattern for upright antiform-synform pair with an axis subparallel to the exten- mechanics of the extension-perpendicular fold set. sion direction and a wavelength of ϳ10 km (Fig. 5B). The synform in the detachment fault coincides with a tight syncline in hanging- DOMAINAL VARIATIONS IN MYLONITIZATION wall strata. The antiform in the detachment coincides with the AND MAGMATISM range-scale antiformal culmination in mylonites of the Mitchel Range. The mylonites also contain shorter wavelength (ϳ2 km) Mitchel Domain upright folds and, in the northwestern Mitchel Range, synformal klippen are preserved near the hinge-surface trace of one of these The extensional ductile shear zone in the Mitchel domain af- shorter-wavelength synforms (Fig. 5B). Antiformal and synfor- fected two rock assemblages. The Waterman Hills mainly are un- mal folds of the detachment therefore broadly correspond to

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A

B Figure 5. Geologic map and cross section of the Mitchel Range and Waterman Hills. (A) Geologic map of Mitchel Range and Waterman Hills showing the brittle detachment, mylonitic fabrics in the ductile shear zone, macroscopic folds, and cross-section line. (B) Simplified cross section of the fault zone in the Mitchel Range and Waterman Hills showing relationship of folds in the footwall mylonites and hanging-wall strata to undulations of the brittle detachment. Cross-section line is perpendicular to the transport direction.

folds of more than one wavelength in both footwall mylonites and adjacent wall rocks suggests that the granite was emplaced syn- hanging-wall strata. However, folds of the mylonites are tighter kinematically and only recorded the latter stages of mylonitization than those of the brittle detachment (Fig. 5B). These relation- (Fletcher and Bartley, 1994). Mylonitic fabrics are equally well de- ships indicate that much of the upright folding occurred when veloped in dacite dikes and their wall rocks, and, within the ultra- footwall rocks lay in the ductile regime. We interpret undulations mylonites, the dikes are transposed into parallel with the mylonitic of the detachment to reflect continued fold growth in the brittle fabric. Relations of rhyolite dikes commonly are similar, but some regime. dikes are less mylonitized and are oriented at high angles to the Magmatism. Field relations suggest that extension-related ig- mylonitic fabric. Some of the rhyolite dikes clearly were emplaced neous rocks in this domain were emplaced synkinematically. The synkinematically, but the transposed and highly deformed dikes may difference in strain magnitude between the Miocene granite and have been emplaced either pre- or synkinematically. Based on tim-

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A

B Figure 6. Geologic map and cross section of the Hinkley Hills. (A) Geologic map of Hinkley Hills. (B) Cross section perpendicular to the transport direction of the fault zone in the Hinkley Hills, showing truncation and transposition of older high-grade fabric by the folded ultramylonite zone.

ing relationships in the Hinkley Hills discussed below, we suspect brings the three matching rock suites in the two domains into con- that even the highly deformed dikes in the Mitchel Range were tiguity (Fig. 2). Therefore, we interpret the Hinkley domain to be emplaced synkinematically. the up-dip continuation of the fault zone in the Mitchel domain. Although the rock assemblages are virtually identical in the two Hinkley Domain domains, the character of deformation and relative timing of dike emplacement differ markedly. The overall map pattern of the Hinkley domain matches that of Miocene shear strain is heterogeneous in the Hinkley Hills such the Mitchel domain (Fig. 6A), with pre-Tertiary rocks on the south that an older, probably Mesozoic, sillimanite-grade fabric is well and Miocene granite on the north, separated by a klippe of Tertiary preserved in much of the area. The older fabric generally strikes strata preserved in a synform in the detachment surface (Fig. 2). northeast, dips steeply, and is coarser grained than the Miocene Removing roughly 5 km of right slip across the Harper Lake Fault mylonitic fabric; however, it can be difficult to distinguish the two

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the shallow mylonitic fabric on the steep northeast-striking gneissic fabric. However, such transposition will not produce the upright folds or asymmetric folds with interlimb angles Ն90Њ, which we infer to have formed in response to horizontal shortening perpendicular to the extension direction. Magmatism. Miocene granite crops out poorly in the low hills and pediment north of Mount General (Fig. 2), where the granite contains a mylonitic fabric and thus was emplaced pre- or synkine- matically. Miocene dikes make up about 20% of exposures in the Hinkley Hills. The dikes generally are more altered than those in the Mitchel Range but are otherwise petrographically identical to dikes elsewhere in the central Mojave metamorphic core complex. The dikes typically strike west-northwest and dip moderately to steeply, but gently dipping sills injected along the mylonitic layering also are observed. Nearly all dikes crosscut the mylonitic fabric, although a few dikes on the eastern edge of the Hinkley Hills contain the my- lonitic fabric. These relations indicating postmylonitic dike intrusion contrast strongly with relations in the Mitchel Range. However, all of the dikes are truncated by the brittle detachment on the flanks of Mount General (Fig. 6A), and therefore these dikes must have been emplaced during extension.

The Buttes Domain

The Buttes domain forms the westernmost known exposure in the central Mojave Desert of rocks that record Miocene brittle- ductile extension. The dominant rock fabric in the region is a Mesozoic gneissic fabric that we interpret to have developed during migmatization of pelitic rocks. A leucosome layer from the migma- tite yielded a concordant U/Pb monazite age of 94.8 Ϯ 1.0 Ma (M. W. Martin, unpub. data). This fabric is variably overprinted by Figure 7. Stereoplots of mesoscopic fold-related fabrics in the Miocene mylonitization along discrete shear zones that generally Mitchel Range and Hinkley Hills. Fold hinges plot as a cluster that are 5–20 m thick. However, in the central and southern Buttes, the is subparallel to the stretching lineation of the shear zone. Poles to main mylonitic shear zone is at least 70 m and 200 m thick, respec- hinge surfaces define a great circle girdle suggesting that they are tively (Fig. 8). In neither area is upper shear zone boundary exposed. also folded about macroscopic axes that are nearly parallel to the Thinner shear zones located structurally below the thick shear-zone number of samples. exposures are interpreted to be branches off of the main extensional ؍ stretching lineation. Abbreviation: n shear zone. It is uncertain whether the brittle detachment is exposed in the fabrics in the field. A prominent ultramylonitic shear zone is ap- Buttes. The best candidate for such a structure occurs in the western proximately 30–100 m thick and contains several upright antiforms Buttes (Dokka et al., 1988, 1991; Dokka, 1989; Fig. 8). The fault dips and synforms with an axis oriented subparallel to the extension di- shallowly and contains southwest-plunging slickensides which are rection (Fig. 6B). Complex drag folds with inclined hinge surfaces consistent with the orientation of the mylonitic stretching lineation. define the intersection between the steep high-grade fabric and the However, the footwall contains only a 5- to 20-m-thick cataclastic gently dipping ultramylonite zone. In general, dolomitic marble and zone and no mylonite, and thus it differs from any other exposure quartzofeldspathic rocks are sharply truncated, but quartzite and of the detachment in the central Mojave metamorphic core com- calcite marble are transposed into parallelism with the shear zone. plex. The fault separates Mesozoic mafic plutonic rocks in the foot- Quartzite and calcite marble layers are attenuated, typically by wall from medium-grained granite in the hanging wall. Neither the 50%–90%, and we infer that these are the least competent rock sense nor amount of offset across the fault is clear, but the hanging- types in the area. LS-tectonites predominate in the ultramylonitic wall granite has not been recognized elsewhere in the footwall of the shear zone. Brittle faults at a low angle to the ductile fabric also are central Mojave metamorphic core complex and could represent far- common, however. Therefore, even on the macroscopic scale, the traveled basement. However, the granite resembles similar granite shear fabric in the footwall shows transitional brittle-ductile that is abundant west of the Buttes domain; if correct, this corre- characteristics. lation does not delimit well the slip magnitude across the fault. Mesoscopic folds in the ultramylonite zone are coaxial with the Another fault exposed in the Buttes domain strikes northwest stretching lineation (Fig. 7) and display the same spectrum of ge- and dips about 30Њ to the northeast. Based on offset of a macroscopic ometries as that in the Mitchel Range. The large-scale drag folds in fold hinge in the northern Buttes (piercing point in Fig. 8), this fault the area imply that some recumbent folds formed by overprinting of accommodated ϳ600 m of right-normal oblique slip.

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Figure 8. Geologic map of the Buttes.

In the Buttes domain, it is difficult to distinguish mesoscopic zone, whereas the northern limb is defined by the high-grade fabric. folds related to mylonitization from those related to the earlier In the northern Buttes, the mylonitic fabric is best developed in the high-grade deformation. However, the two macroscopic folds north-striking limb. Therefore, we interpret these folds to have re- mapped in the area may have formed during Miocene mylonitiza- sulted from localized shear and transposition of the high-grade fab- tion. In both folds, the mean orientation of the mylonitic foliation ric into parallelism with mylonitic shear zones. lies between the orientations of the fold limbs and approximately Magmatism. Similar to the Mitchel Domain, Miocene granite contains the fold axis. Therefore, the mylonitic fabric could repre- in the Buttes Domain contains a pronounced strain gradient with sent the axial-planar cleavage of the folds (Fig. 9). In the synform in structural level. Granite at the bases of most of the tors (ϳ75mof the southern Buttes, the entire southern limb is a mylonitic shear relief) in the central Buttes is either undeformed or protomylonitic

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with an L-tectonite fabric. This fabric progressively grades into SC- mylonite on most of the summits. The pluton-wall rock contacts are not well exposed, and detailed relative timing relations could not be determined. Dacite and felsite dikes show mutually crosscutting relations with the shear zones, suggesting that emplacement oc- curred both before and after mylonitization. In some cases, dacite dikes in the Buttes contain a wall-parallel mylonitic fabric that affects the country rock only within 0.1–1.0 m of the contact. Such strain localization could have occurred as a result of thermal weakening of wall rocks during dike emplacement or hydrolytic weakening caused by wall-rock hydration along the dikes. The relations suggest that deformation may have overlapped in time with emplacement.

DISCUSSION

Basin-and-Dome Geometry of Detachment Faults

The master low-angle normal faults of metamorphic core com- plexes typically are affected by two orthogonal sets of upright folds that have different characteristic wavelengths and accomplish dif- ferent types of finite strain. Extension-parallel folds have a maxi- mum characteristic wavelength of about 10–20 km. Extension-per- pendicular folds typically include a single antiform-synform pair with a maximum characteristic wavelength of about 50–100 km. In the central Mojave metamorphic core complex, both fold sets also include shorter-wavelength folds. We infer these two fold sets to result from independent mechanical processes. Extension-Parallel Folds. In the central Mojave metamorphic core complex, several lines of evidence suggest that the extension- parallel folds formed during active displacement across the fault system and record horizontal shortening perpendicular to the ex- tension direction. The folds are observed in upper-plate strata, in the brittle detachment, and in lower-plate mylonites. The hinge- Figure 9. Stereoplots of poles to mylonitic foliation and com- surface traces of the upright folds spatially coincide in all of the positional layering in the Buttes showing relationships between ori- folded layers. However, in the Mitchel Range and elsewhere (e.g., entation of macroscopic folds and local mylonitic fabric. In both Davis and Lister, 1988; Mancktelow and Pavlis, 1994), macroscopic areas of the Buttes, the mean orientation of the mylonitic foliation folds in the mylonites have smaller interlimb angles than the un- contains the macroscopic fold hinge, which suggests that the my- dulations in the detachment. We interpret this relation to indicate lonitic foliation is the axial planar fabric. that the folds began forming in the ductile regime and continued to amplify in the brittle regime. In the central Mojave metamorphic core complex, ductile shearing followed two distinct constrictional strain paths that accomplished horizontal shortening perpendicular displacement. Yin (1991) showed that buckling of a horizontal elas- to the extension direction (Fletcher and Bartley, 1994): either di- tic sheet only can occur if the horizontal stress is greater than the rectly at the grain scale to form L-tectonites or by a combination of vertical normal stress. Because the maximum normal stress ordi- plane strain at the grain scale and synmylonitic folding. In other narily is vertical in an extensional regime, buckling of horizontal Cordilleran core complexes, the youngest rift-related sediments are layering should be suppressed. If extension-parallel folds form by also folded (e.g., Compton, 1975; Yin and Dunn, 1992; Dueben- buckling, some process must decrease the ratio of the normal stress dorfer and Simpson, 1994), which suggests that shortening perpen- acting across the layering to the horizontal compressive stress per- dicular to the extension direction may have outlasted displacement pendicular to the extension direction. Because fold orientations con- across the fault zone. sistently parallel the extension direction, as it varies along the Cor- Previously proposed genetic models for the upright extension- dillera (e.g., Wernicke, 1992), we infer that the stress-modifying parallel folds can be divided into three types: (1) buckling in re- process must be inherent in the dynamics of large-magnitude sponse to increased horizontal compression normal to the extension extension. direction, (2) bending in a heterogeneous vertical stress field, or (3) Fletcher and Bartley (1994) suggested that the normal stress on the folds reflect original topography on the fault surface rather than fault zone mylonites (or any other layered medium) may be less than deformation of a previously more planar surface (Fig. 10). Only the the lithostatic load if the layers are not horizontal. Buckling insta- first type of model can cause true crustal shortening during active bility thus could arise if the horizontal compressive stress were equal

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and produce shortening within the denuded region and extension along the margins or transfer zones (Fig. 11C). Alternatively, the margins of the extended terrane could move laterally inward and the associated extension in this lateral dimension could occur in crust that is far removed from the region of extension. Variations in vertical stress in the plane perpendicular to the extension direction could arise from (1) differential tectonic unroof- ing between the extended terrane and its lateral margins (Spencer, 1982, 1984) or (2) buoyant forces associated with uncompensated undulatory crustal roots or lower-density plutons (Yin, 1991). Al- though differential unroofing between the extended terrane and its unextended margins is an important source for vertical stress het- erogeneities, this unroofing should produce folds with wavelengths of the order of the lateral dimension of the extended terrane, that is, greater than the generally observed 10–20 km. Yin (1991) showed that it is mechanically feasible to create folds with the observed maximum characteristic wavelength, given a grid-like network of uncompensated crustal roots or plutons with the proper size and spacing. However, whether such a configuration of uncompensated

Figure 10. Schematic cross sections of core-complex fault zones, drawn perpendicular to the extension direction, showing the three main classes of proposed genetic models for extension- parallel folds/undulations. See text for explanation.

to or even less than the lithostatic load, depending on the inclination and material properties of the layered medium. Mechanical effects of tectonic denudation may also encourage extension-perpendicular buckling. Tectonic denudation should not only reduce the vertical normal stress on the remaining rocks, but also may increase horizontal stress. In laterally homogeneous and confined lithosphere, horizontal stress increases as some function of the vertical stress (␳gh); in time-relaxed viscous crust, horizontal stress equals vertical stress (Fig. 11A). Prior to denudation, the integrated horizontal force from this stress profile is supported by the full crustal thickness. However, if the lower crust is weak, such that the crust is decoupled from the upper mantle, then, after tec- tonic denudation, the horizontal stress in the denuded region must increase because the same horizontal force acts over a smaller cross- Figure 11. Dynamic model for the formation of extension- sectional area (Fig. 11B). parallel folds as expressed in cross-section views perpendicular to Isostatic rebound of the footwall will reduce the gravitational the extension direction. (A) Before denudation the horizontal potential gradient along the margins of the extended terrane. How- stresses on the boundary and within the block are equal because the ever, if isostatic compensation occurs by lateral flow of midcrustal integrated horizontal force operates over the same crustal thick-

material with a low effective viscosity, then the upper crust may be ness: ⌬hint and ⌬hext. (B) After denudation, force balance requires mechanically decoupled from the lower crust and mantle (Block and horizontal stress to increase in the thinned portion of the crust: the Royden, 1990; Wernicke, 1990), and the horizontal force, generated integrated horizontal force remains constant but is distributed over

by the topographic gradient, will be distributed only in the extended a shorter distance in the denuded portion of the crust (⌬hint < upper crust. Sediment-transport and facies patterns in the main rift ⌬hext). Additionally, vertical stress decreases at a given horizon in basin of the central Mojave metamorphic core complex suggest that the footwall in response to denudation. Lower boundary of extended significant lateral highlands existed west of Fremont Peak and east upper crust could be low-viscosity compensating material like that of Lead Mountain (Fillmore and Walker, in press; Fig. 2). proposed by Block and Royden (1990). (C) The crust may respond If the elevated horizontal stress exceeds the yield strength of by shortening in the denuded region and extending along the lateral the rocks, the margins of the extended terrane could collapse inward margins.

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crust exists in the wide variety of extended continental and oceanic it follows the trailing edge of the displaced hanging wall (Buck, 1988; crust remains to be demonstrated. Additionally, these mechanisms Wernicke and Axen, 1988). These models predict deflections of the should produce extension above the antiformally deflected crust crust that vary in time and space and thus at least permit the pos- that is not observed in the central Mojave metamorphic core sibility for folds with steeper back-dipping limbs to occur. Differ- complex. ential tectonic unroofing might also explain the fold asymmetry Some workers have interpreted extension-parallel folds/undu- (Spencer, 1984). lations to represent original corrugations in the fault (Davis and Determining the nature of crustal deflections that occur in re- Hardy, 1981; Spencer, 1985; John, 1987; Davis and Lister, 1988) sponse to the migrating center of uplift or ‘‘rolling hinge’’ has been which may form by linking of separate straight fault segments during the focus of many recent studies (e.g., Bartley et al., 1990a; Axen and the early stages of fault propagation. Concordance between the brit- Wernicke, 1991; Selverstone et al., 1993; Manning and Bartley, tle fault and the lower-plate mylonites would require either that the 1994). Based upon the preliminary results from the Mitchel domain, mylonites were ductilely molded to the base of the rigid hanging- Bartley et al. (1990a) interpreted the postmylonitic deformational wall block (Spencer and Reynolds, 1991) or that corrugations of the fabrics to reflect isostatic rebound by flexural failure, but Axen and brittle fault project downward into similar corrugations in the duc- Wernicke (1991) pointed out that the data were equally consistent tile regime. However, neither of these mechanisms is likely to pro- with rebound by subvertical noncoaxial shear. The data presented in duce widespread constrictional strain; molding the footwall mylo- this paper shed more light on the controversy. nite to a nonplanar hanging-wall block should actually result in The predominance of the monoclinal composite fabric on the flattening strains. back-dipping limb suggests that the rotation of this portion of the Extension-Perpendicular Folds. The broad antiformal arch of shear zone through horizontal included a significant component of fault zones in nearly all core complexes consists of a limb that dips northeast-side-up shear (Fig. 12). Although the composite fabric is in the same direction as the active shear zone and one tilted through more penetrative, we liken it to the antithetic shear zones that Reyn- horizontal to show apparent reverse-sense displacement. Following olds and Lister (1990) interpreted to have caused back-rotation of Reynolds and Lister (1990), we refer to these as the fore- and back- the shear zone in the Harcuvar Mountains, South Mountains, and dipping limbs, respectively. In the central Mojave metamorphic core Santa Catalina Mountains in southern Arizona. The origin of joints complex, the antiformal arch is best preserved in the Mitchel Range and mesoscopic normal faults is more uncertain. These could have where it is asymmetric: the fore-dipping limb dips 10Њ–20Њ and the formed in response to extension around the outer arc of the anti- back-dipping limb dips 40Њ–50Њ. Although we cannot rule out local formal flexure at the top of a fault ramp (Fig. 12), as similar struc- contraction across the Harper Lake fault (e.g., Bartley et al., 1990b) tures were interpreted in the Raft River Mountains (Manning and as the cause of this asymmetry, other Cordilleran core complexes Bartley, 1994). Alternatively, they also could have accommodated show a similar geometry (e.g., Reynolds and Lister, 1990; Lister and layer-parallel extension at the synformal hinge at the bottom of a Baldwin, 1993). fault ramp (cf. Wernicke and Axen, 1988, Fig. 4c). Each interpre- The only mesoscopic fabrics that could be related to this long- tation is consistent with the fanning of joint orientations around the wavelength northwest-trending fold are joints and the composite fold. Other important aspects of rolling-hinge models, such as kink-band and tension-gash cleavage. Both fabrics formed postmy- whether the antiform migrated through the footwall or if the fault lonitically, predominantly accomplish northeast-side-up subvertical had a steep orientation while active, cannot be answered with this shear and/or layer-parallel extension, and are primarily found in data set. previously mylonitized rocks. North-east-side-up shear is mainly ac- complished by monoclinal forms of the composite cleavage (Fig. 4B) that is preferentially developed on the back-dipping limb. Joints are common on both the fore- and back-dipping limbs of the antiform. The most popular models for extension-perpendicular folds in- voke heterogeneous vertical stress that arises from buoyant loads generated either by tectonic denudation or by magmatic inflation. Uplift above a symmetric low-density pluton should produce an asymmetrically warped fault zone with a steeper fore-dipping limb, because the fault initially dipped in this direction. This is the op- posite of the observed fault-zone geometry in the central Mojave other metamorphic core complexes (e.g., Reynolds and Lister, 1990). Also, tectonic removal of crust should generate much larger vertical loads than density differences between granitic plutons and country rock because the density contrast between rock and air is greater than the density contrast between any two crustal rock types. Therefore, although low-density plutons may induce significant ver- tical loads in core complexes, we consider tectonic denudation to be the dominant source of vertical loads in the central Mojave meta- morphic core complex. Figure 12. Schematic cross sections showing kinematics of the Recent models of isostatic rebound of tectonically denuded rolling hinge: subvertical shear on back-dipping limb and flexural crust predict a center of uplift that migrates through the footwall as failure around hinge of antiform.

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Role of Magmatism in Extensional Deformation migration of magma into the middle and upper crust rather than the reverse. The common temporal and spatial coincidence of igneous ac- In summary, the general coincidence of ductile extension and tivity and deformation in many core complexes has led workers to plutonism in the central Mojave Desert are broadly consistent with infer genetic relationships between the two phenomena. On the potential genetic relationships on the lithospheric scale. However, lithospheric scale, the most commonly inferred link involves thermal on the scale of the domains in this study, field relations do not weakening of gravitationally unstable crust either through thermal support any single cause-and-effect relationship between ductile ex- relaxation following contraction (Glazner and Bartley, 1985), ele- tension and magmatic intrusions. vated mantle heat flow (Sonder et al., 1987; Wernicke et al., 1987; Axen, 1993), or the emplacement of mafic magma in the lower crust Thickness of the Brittle-Ductile Transition (Gans et al., 1989). The initial thermal pulse triggers extensional collapse, which leads to smaller-scale sympathetic relationships be- The contrast in relative timing of dike emplacement in Hinkley tween magmatism and extensional deformation. Some of the ways Hills and Mitchel Range may provide a snapshot of variations in that magmatism can enhance extensional deformation on a smaller deformational style in the down-dip direction of the central Mojave scale include (1) localized thermal weakening on the margins of metamorphic core complex fault zone. In the Hinkley Hills, dikes plutons and dikes, (2) reorientation of stress trajectories and de- crosscut semiductile mesoscopic fabrics in the footwall but are velopment of locally elevated deviatoric stress around margins of themselves crosscut by the brittle detachment. However, structurally magma bodies (e.g., Pollard and Seagall, 1987; Parsons and Thomp- down-dip in the Mitchel Range, dikes are crosscut by and, in most son, 1993), and (3) magmatic hydrofracture or brittle failure in re- cases, fully transposed into parallelism with the footwall mylonitic sponse to elevated pore pressure associated with magma. Some of fabric. These relations suggest dikes in the Hinkley Hills were em- the ways that extensional deformation can induce the generation placed after the footwall rocks had passed through the brittle-ductile and migration of magma include (1) rapid isothermal decompres- transition but well before ductile deformation had ceased in the sion by tectonic denudation and (2) creation of dilatant openings footwall rocks of the Mitchel Range. We infer that postemplace- formed as a result of rock failure and strain incompatibilities. ment offset of the dikes occurred almost entirely across the brittle Lister and Baldwin (1993) proposed that local thermal anom- detachment in the Hinkley Hills, whereas, in the Mitchel Range, this alies associated with midcrustal intrusions (plutons, dikes, and dike offset predominantly occurred across the ductile shear zone swarms) trigger core-complex-style extension. They cited examples (Fig. 13). in the southern Basin and Range of ductile strain gradients around After displacement across the younger Harper Lake Fault is intrusions and suggested that cooling-age patterns record transient restored, ϳ2 km in the down-dip dimension separates brittlely de- geotherms associated with midcrustal igneous intrusion. These au- formed dikes in the Hinkley Hills from ductilely deformed dikes in thors specifically predicted the presence of syn- or prekinematic the Mitchel Range. If the dikes were emplaced synchronously in igneous rocks in the immediate vicinity of ductilely deformed foot- both areas, we would interpret the intervening 2 km of unexposed wall rocks and suggested that drilling or thermochronology could be ground to contain the former brittle-ductile transition. The maxi- used to identify them and test their hypothesis. However, we suggest mum thickness of the transition depends on the active dip of the that other field criteria may also shed light on this issue. fault zone and the amount of ductile stretching that occurred be- On the scale of the Mojave Desert as a whole, outcrops of tween the Hinkley Hills and Mitchel Range before rocks exposed in Miocene granite coincide with areas of brittle-ductile extensional the Mitchel Range were transported passively to the surface in the detachment. However, variations in the intensity of ductile defor- footwall of the brittle detachment (Fig. 13). Assuming active fault mation do not correlate with the extent of granite exposures and, in inclinations of 15Њ–45Њ and ductile stretches of 1.5–5.0, the maxi- the Mitchel domain where pluton/wall-rock relationships are best mum vertical thickness of the brittle-ductile transition is 100–950 m exposed, most of the ductile deformation preceded emplacement of (Fletcher, 1994). This estimation is almost an order of magnitude the pluton. thinner than thicknesses predicted by monomineralic deformation The central Mojave metamorphic core complex contains rare experiments (e.g., Hirth and Tullis, 1994) and flow laws of phases examples of strain localization around the margins of dikes (e.g., in that make up the strongly heterogeneous continental crust like that the Buttes). However, more typically igneous rocks display no pref- in the central Mojave (e.g., Kirby and Kronenberg, 1987). Future erential development of ductile strain. The vast majority of dikes studies in the central Mojave metamorphic core complex will more were intruded after mylonitization (e.g., Hinkley Hills) or into wall completely characterize the age of the suite of dikes in both areas rock that records no ductile strain (e.g., at Fremont Peak and Lead and explain the discrepancy between measured and predicted tran- Mountain) (Fig. 1). The undeformed dikes generally are oriented sition thicknesses. parallel to the joint fabric, which may have aided and controlled their emplacement. We suggest below that postmylonitic dikes in CONCLUSIONS the Hinkley domain may have been emplaced in rocks located near the brittle-ductile transition at that time. However, most of the duc- (1) Synmylonitic folds exhibit a wide range of styles and gen- tile shearing occurred in the structurally lower Mitchel domain, de- erally have axes parallel to the extension direction of the shear zone. spite the fact that it contains significantly fewer dikes. Finally, be- We infer that they formed mainly by two main processes: (a) am- cause most of the dikes in the central Mojave metamorphic core plification of upright open folds that initially contained axes nearly complex were emplaced after or during mylonitization, one could parallel to the transport direction and (b) transposition of a preex- perhaps argue more successfully that ductile extension triggered the isting steep northeast-striking gneissic fabric into parallelism with

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after mylonitization and records layer-parallel extension and north- east-side-up subvertical shear that may reflect isostatic rebound of the footwall. (3) The regional distribution of magmatism and large-magni- tude brittle-ductile extension broadly coincide. However, on the scale of individual mountain ranges, the relative timing of magma- tism and ductile deformation varies throughout the central Mojave metamorphic core complex. Additionally, shear zone thickness and degree of mylonitization show no correlation with proximity to or volume of extension-related intrusions. Therefore we are skeptical of models that suggest that midcrustal magmatism triggers extension or vice versa. Instead, we prefer the hypothesis that large-magnitude extension and magmatism both are manifestations of common larg- er-scale processes in the lithosphere. (4) A preserved crustal section containing the brittle-ductile transition may be present in the central Mojave metamorphic core complex. Approximately 2 km of dip-parallel distance separates a portion of the footwall where dikes are strongly overprinted by my- lonitization from another portion where lithologically identical dikes postdate mylonitization but are crosscut by the brittle detach- ment. If the dikes were emplaced over the same time interval, those in the Mitchel Range would have been emplaced below the brittle- ductile transition and those in the Hinkley Hills would have been emplaced above the transition.

ACKNOWLEDGMENTS

This study was partially funded by National Science Foundation grants EAR8816944 and EAR8916838. John Bendixen and Rob Fillmore assisted in various aspects of the field work. Stereograms were generated using Stereonet 4.5a by Richard Allmendinger. Thoughtful reviews by Keith Howard, Steve Reynolds, and Jon Spencer added considerably to this paper.

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