J. Douglas Walker

Variations across and along a major continental rift: An interdisciplinary study of the Basin and Range Province, western USA

Abstract Geological, geochemical, and geophysical data gathered within the central part of the Basin and Range and adjacent areas of the western USA suggests that considerable heterogeneity characterizes Cenozoic extension in this region. Good exposure and an abundance of pre-rifting markers indicate 250 km of extension of the upper crust over the past 16 m.y. Extension of several hundred percent has occurred in two distinct deformational domains, Death Valley and Lake Mead, separated by a relatively unextended block, the Spring Mountains. The limited topographic differences between extended and unextended regions imply that material with a crustal density has been added to the extended regions. Although igneous activity can provide some of this added material, kinematics of extension within the Death Valley region suggest that lateral flow of the middle and lower crust into the extended areas accounts for much of the needed material. Such flow is consistent with geochemical analysis of intermediate to silicic volcanic rocks in the Death Valley area. These volcanic rocks contain isotopic and geochemical trends similar to Mesozoic plutonic rocks from the western part of the Sierra Nevada, about 150–200 km to the west, thus suggesting that the upper crust has moved by that amount relative to deeper crustal levels. Geochemical analyses of basaltic magmas in the region indicate that two mantle reservoirs are present: an OIB-type asthenosphere, and an old, Precambrian continental lithosphere. The ancient lithospheric mantle is preserved beneath the Central Basin and Range, but to the west and north the basaltic rocks have a signature compatible with an asthenospheric origin. These differences indicate that the degree of thinning and removal of the mantle lithosphere varies considerably across the Central Basin and Range. These differences are compatible with the inference from geological and geophysical arguments that thinning of the mantle lithosphere at the latitude of the Central Basin and Range is localized beneath the Sierra Nevada. Geophysical measurements have shown that the thickness of the crust varies little from a mean of about 30 km over the entire Basin and Range; the crust under the high Sierra Nevada to the west might have about the same thickness. Estimates of the buoyancy of the crust and mantle based on P-wave crustal structures suggest that the most buoyant, and thus probably the warmest, mantle lies under the Sierra Nevada and not under areas of strongly thinned upper crust of the Death Valley and Lake Mead regions to the east. Similar analyses indicate that the extended upper crust of the Northern Basin and Range overlies an upper mantle more buoyant than that of the Southern and Central Basin and Range; this is in accord with geochemical and seismological inferences. Thus, the style of lithospheric extension varies considerably both along and across the strike of the Basin and Range.

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

The central Mojave metamorphic core complex is defined by a belt of Miocene brittle-ductile extension and coeval magmatism. The brittle-ductile fault zone defines a basin-and-dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes. One fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km. The axial surfaces of these folds can be traced from the footwall mylonites, through the brittle detachment, and into hanging-wall strata. However, folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment, suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime, possibly after movement across the fault zone ceased. Mesoscopic fabrics related to the transport-parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction. We interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large-magnitude extension. The other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km. Mesoscopic fabrics related to these folds include northwest-striking joints, kink bands, and en echelon tension-gash arrays. These fabrics formed after mylonitization and record both layer-parallel extension and northeast-side-up subvertical shear. The postmylonitic fabrics are kinematically compatible with rolling-hinge-style isostatic rebound of the footwall following tectonic denudation. The relative timing of extension-related magma intrusion and ductile deformation varies through the central Mojave metamorphic core complex. On the scale of the small mountain ranges that make up the central Mojave metamorphic core complex, no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension-related igneous rocks. This suggests that models that invoke a single upper-crustal genetic relationship, such as magmatism triggering extension or vice versa, do not apply to the central Mojave metamorphic core complex. Systematic variation in the relative timing of dike emplacement and mylonitization suggests that, at the time of dike emplacement, rocks in the Mitchel Range were below the brittle-ductile transition while those in the Hinkley Hills were above it. The Hinkley Hills and Mitchel Range are separated by ∼2 km in the dip direction of the fault zone, which suggests that the vertical thickness of the brittle-ductile transition probably was between 100 and 950 m.

Late Silurian plutons in Yucatan

This is the published version. Copyright 1996 American Geophysical Union. All Rights Reserved.

Magnitude and significance of Miocene crustal extension in the central Mojave Desert, California

The newly recognized Waterman Hills detachment fault (WHDF) of the central Mojave Desert, California, is significant because it provides the first unambiguous evidence for large-scale core complex-style crustal extension in the central Mojave Desert, and because it has significantly rearranged the pre-Miocene paleogeography of the Mojave Desert. The WHDF places steeply dipping to overturned Miocene volcanic and sedimentary rocks upon mylonitic pre-Tertiary basement. The mylonites, which apparently formed during extension, are predominantly L-tectonites which manifest top-to-northeast shear. The WHDF dips to the northeast beneath dominofaulted ranges of the central Mojave Desert and detachment faults of the Colorado River trough, forming an imbricated early Miocene system of detachment faults. Extension continued in the Colorado River trough after extension had ceased in the central Mojave Desert. Tentative correlations of Mesozoic intrusions suggest about 40 km of slip across the WHDF, which carries eugeoclinal Paleozoic rocks in its hanging wall and cratonal/miogeoclinal Paleozoic rocks in its footwall. Restoration of 40 km of slip (1) removes a prominent kink in the boundary between eugeoclinal and cratonal/miogeoclinal facies, (2) aligns cratonal/miogeoclinal strata near Victorville more closely with the late Paleozoic continental margin farther north, (3) places cratonal/miogeoclinal rocks structurally beneath eugeoclinal rocks, implying that the facies were stacked by thrusting, and (4) straightens the western margin of the Late Jurassic Independence dike swarm.

Geology of the Inyo Mountains Volcanic Complex: Implications for Jurassic paleogeography of the Sierran magmatic arc in eastern California

An ∼3.1-km-thick volcanic complex exposed in the southern Inyo Mountains, east-central California, records Jurassic subaerial depositional environments along the east flank of the Sierran arc. This complex, which we name the Inyo Mountains Volcanic Complex, is subdivided into lower, middle, and upper stratigraphic intervals. The 200–580-m-thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast. Mafic lava flows and rare reworked tuff in this interval record the onset of Jurassic(?) volcanism in this part of the arc. The 300–700-m-thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The >2260-m-thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes. Three new U-Pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca. 169 Ma to 150 Ma. The uppermost part of the complex remains undated but probably accumulated prior to 140 Ma. The Inyo Mountains Volcanic Complex is part of a belt of volcanic complexes that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that they represent a distinctive arc-flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to exclusively subaerial settings, substantial (>30%) portions of epiclastic strata, and existence at times of north- to northeast-inclined paleoslopes. We infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc-flank province were rarely if ever proximal to major eruptive centers, complexes in two areas (White Mountains and eastern Mojave Desert) were at times located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eastward-projecting volcanic salients separated by arc recesses—typified by the Inyo Mountains area—in which epiclastic deposition was dominant.

Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the Mojave Desert region, California

The geologic history of the outer continental margin (eugeoclinal) rocks in the El Paso Mountains and northern Mojave Desert has long been important in models for the development of the active continental margin in the western Cordillera. Current interpretations call for either strike-slip or thrust juxtaposition of eugeoclinal rocks against miogeoclinal/cratonal (platformal) rocks, or some combination of both strike-slip faulting and thrusting. Two broad and interrelated aspects of the history of the eugeoclinal rocks are at issue: (1) How much primary displacement is necessary to account for the present position of the eugeoclinal rocks? and (2) When were the eugeoclinal rocks thrust against platformal rocks? This study primarily addresses the second issue. Lithologic correlation indicates that the outcrop belt of eugeoclinal rocks is bounded by platformal rocks to the east and south. Platformal rocks are also present to the west, but many of these exposures restore to a position structurally beneath the eugeoclinal rocks when Tertiary extension is restored, implying stacking by a thrust. New U-Pb zircon geochronology and whole rock geochemistry and Sr, Nd, and Pb isotopic data for plutons in the El Paso Mountains and northern Mojave Desert lend insight into the timing of this thrusting. The outcrop belt of eugeoclinal rocks coincides with the only known Permian and Triassic plutons in the El Paso Mountains and northern Mojave Desert, which are dated at 260 to 240 Ma by U-Pb zircon in this report. Initial Sr and ϵ Nd(t) isotopic values from these plutons are distinct ≤0.704 and ≥+2, respectively) from continental lithosphere isotopic signatures ≥0.705 and ≤−2) of both Middle Jurassic plutons in the same area and Triassic plutons in the southern and eastern Mojave Desert. Feldspar common lead data for the Permian and Triassic plutons within the eugeoclinal outcrop belt also indicate limited crustal involvement and do not overlap previously reported values for common lead data from Mesozoic plutons in the eastern Mojave Desert region where Proterozoic basement is widespread. The observations and data reported here indicate that Late Permian–Early Triassic plutons in the northern Mojave Desert and El Paso Mountains were generated within or passed primarily through oceanic lithosphere, but later Jurassic plutons were derived from and/or interacted extensively with continental lithosphere. We hypothesize that the eugeoclinal rocks were deposited on oceanic crust that was thrust eastward over Precambrian cratonal basement and overlying strata between approximately 240 Ma and 175 Ma, a time of little documented tectonic activity in the Mojave Desert region. Postulated Permian or late Middle Jurassic east-directed thrusting is incompatible with our data and observations. The data also fill an important gap in palinspastic reconstructions of the early Mesozoic arc and indicate that this arc was northwest-trending and oblique to Paleozoic facies trends in the Mojave Desert.

Miocene unroofing of the Canyon Range during extension along the Sevier Desert Detachment, west central Utah

This is the published version. Copyright 2001 American Geophysical Union. All Rights Reserved.

Relations between hinterland and foreland shortening: Sevier orogeny, central North American Cordillera

The tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold-and-thrust belt in the United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data critical to understanding the developmere of internal and external parts of orogens. The Garden Valley thrust system, part of the central Nevada thnkst belt, crops out in south-central Nevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrust system consists of at least four principal thrust plates composed of strata as young as Pennsylvanian in age that are unconformably overlain by rocks as old as Oligocene, suggesting that contraction occurred between those times. New U/Pb dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to intrusion, suggest that regionally these thrusts were active before -85-100 Ma. The thrust faults are characterized by long, relatively steeply dipping ramps and associated folds that are broad and open to close, upright and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate-Mount Irish thrust, southward with the Gass Peak thrust of southern Nevada. This correlation carries the following regional implications. At least some of the slip across Jurassic to mid-Cretaceous foreland thrusts in southern Nevada continues northward along the central Nevada thrust belt rather than noaheastward into Utah. This continuation is consistent with age relations, which indicate that thrusts in the type Sevier belt in central Utah are synchronous with or younger than the youngest thrusts in southern Nevada. This in turn implies that geometrically similar Sevier belt thrusts in Utah must die out southward before they reach Nevada, that slip along the southem Nevada thrusts is partitioned

Timing and kinematics of deformation in the Cronese Hills, California, and implications for Mesozoic structure of the southwestern Cordillera

A southeast-vergent ductile shear zone is exposed in the Cronese Hills in the central Mojave Desert. This shear zone formed under greenschist facies conditions and placed mylonitic metaplutonic and metavolcanic rocks upon folded Triassic( ) metasedimentary rocks. U-Pb zircon ages of prekinematic and postkinematic plutons constrain the age of deformation to between 169 and 154 Ma (late Middle or early Late Jurassic). The authors believe that the Cronese shear zone forms part of the southern continuation of the east Sierran thrust system. Mesozoic thrust faults may have controlled preservation of Jurassic arc-related rocks and may be responsible for structural stacking of Paleozoic eugeoclinal and miogeoclinal strata in the central Mojave Desert.

Cenozoic evolution of the Mojave block of southern California

The recorded Cenozoic history of the Mojave Desert region of southern California began in the latest Oligocene, when intense volcanism and tectonism interrupted a long early Tertiary silence. Volcanism commenced across the region in an east-west band ca. 24–22 Ma. Northwest of Barstow, volcanism was accompanied by intense crustal extension and development of a metamorphic core complex. Outside of this relatively restricted area, extension was minor or absent. After extension ceased ca. 18 Ma, volcanism shifted to small-volume eruptions of basalt. Post-extensional deformation has largely been by strike-slip faulting along northwest-striking dextral faults and west-striking sinistral faults, and total dextral slip across the Mojave Desert region since the early Miocene is 45–60 km. Strike-slip deformation has been overprinted locally by intense north-south contraction that is the dominant style of deformation in the western Mojave block. Paleomagnetic data indicate that parts of the Mojave block were rotated clockwise, although the magnitude and timing of this rotation are poorly determined. The best evidence for large (45) rotation comes from the area east of Barstow, where large clockwise declination anomalies and Mesozoic and Cenozoic dikes with anomalous strikes may reflect early Miocene clockwise deflection along the Mojave River fault. Volcanism and tectonism in the Mojave block resulted from interactions among the North American, Pacific, and various oceanic plates. Patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent, but do correlate with the position of the subducted Mendocino fracture zone. Plate-circuit reconstructions suggest that the driving force for extension was divergence between the Pacific and North American plates along the transform margin that separated the two. This hypothesis accounts for the direction, magnitude, and rate of extension in the Mojave block.