Structure and 40Ar/39Ar K-Feldspar Thermal History of the Gold Butte Block: Reevaluation of the Tilted Crustal Section Model
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spe463-15 page 331 The Geological Society of America Special Paper 463 2010 Structure and 40Ar/39Ar K-feldspar thermal history of the Gold Butte block: Reevaluation of the tilted crustal section model Karl E. Karlstrom* Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Matt Heizler New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA Mark C. Quigley† Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA ABSTRACT This paper reevaluates the geometry and processes of extension in the bound- ary zone between the western Colorado Plateau and the Basin and Range Province. Based on new mapping of extensional detachment faults, restored cross sections, and 40Ar/39Ar K-feldspar thermochronology, we present an alternative to the previ- ously published model that the Gold Butte block is a tilted 15–18-km-thick intact basement crustal section. Mapping of windows of crystalline basement at 1:12,000 scale delineates a bedding-parallel detachment fault system that parallels the Great Unconformity in the Tramp Ridge block, just north of the Gold Butte block. Above this detachment fault, extensional allochthons containing Upper Paleozoic through Tertiary (>18 Ma) rocks exhibit tilting due to westward translation and tilting. We project this geometry above the Gold Butte block itself based on restoration of slip across the Gold Butte fault. This reconstruction suggests that the detachment system extended over lateral distances of >1000 km2, helping defi ne a region of relatively modest extension (~25% for cover; 10% for basement) within the Nevada transition zone between the Colorado Plateau and Basin and Range. In agreement with previously published mapping and structural cross sections, our restored cross sections suggest that extensional deformation initiated with forma- tion of hanging-wall anticlines above a listric Grand Wash fault system and evolved via a combination of both listric faulting and domino-block translation and tilting. New data presented in this paper document that extension was also facilitated by slip on bedding-subparallel detachment zones in the Bright Angel Shale, along the basement unconformity, and along other zones of weakness, such that the extended *[email protected] †Current address: Department of Geological Sciences, University of Canterbury, Christchurch 8140, New Zealand. Karlstrom, K.E., Heizler, M., and Quigley, M.C., 2010, Structure and 40Ar/39Ar K-feldspar thermal history of the Gold Butte block: Reevaluation of the tilted crustal section model, in Umhoefer, P.J., Beard, L.S., and Lamb, M.A., eds., Miocene Tectonics of the Lake Mead Region, Central Basin and Range: Geological Society of America Special Paper 463, p. 331–352, doi: 10.1130/2010.2463(15). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved. 331 spe463-15 page 332 332 Karlstrom et al. Paleozoic cover was partly decoupled from less-extended basement. This detach- ment system ramps down into basement to merge with the South Virgin–White Hills detachment at the west end of Gold Butte, the principal extensional detachment of the region. Our mapping and structural model suggest that movement on these detach- ment faults initiated at low angle. Further, using the geometry from restored cross sections, we infer that the deepest rocks now exposed in the western Gold Butte block resided at depths of ~4 km below the Great Unconformity (~8 km below the surface) rather than the previously published 15 km below the unconformity (~19 km below the surface). New 40Ar/39Ar K-feldspar thermochronology from the Gold Butte block, added to a compilation of published thermochronologic data, is used to help evaluate alternative models. K-feldspar multiple diffusion domain (MDD) modeling suggests that rocks throughout all but the westernmost part the block had cooled through 150–200 °C before the Phanerozoic and resided at temperatures <200 °C prior to onset of rapid Miocene extension at 17 Ma. Pre-extensional (pre–17 Ma) 100 °C and 200 °C isotherms were located near the east and west ends of the basement block, respectively. Muscovite, biotite, and K-feldspar from a 70 Ma Laramide pluton deep in the block give 40Ar/39Ar ages of 70, 50, and 30 Ma, respectively. MDD modeling of K-feldspar from this sample is compatible with cooling the westernmost part of the block from 225 °C to 150 °C between 17 and 10 Ma. Available thermochronology can be explained by either structural model: our model requires pre-extensional geo- thermal gradients of ~25 °C/km, rather than 15–20 °C/km as previously published. INTRODUCTION AND GEOLOGIC BACKGROUND the North and South Virgin Mountains of Nevada (Fig. 1) are seg- mented into basement-cored crustal blocks by low-angle normal The Gold Butte region of southern Nevada forms part of faults and contemporaneous high-angle strike-slip faults (Beard, the transitional boundary zone between the relatively unde- 1996). The Precambrian-cored blocks have strikingly different formed Colorado Plateau to the east and the highly extended geometries. The Gold Butte block exposes a 20-km-wide expo- regions of the Great Basin (Fig. 1). This has been a type area sure of Proterozoic basement west of and beneath ~50°E-tilted for developing and testing models of extensional processes in Paleozoic strata. The dipping strata correlate with nearby fl at- continental settings. For example, this region was the centerpiece lying Grand Canyon successions, and simple restoration of their for models of isostatic uplift of footwall blocks during extension steep dip has led to the tipped crustal section model (Fig. 2B; (Wernicke and Axen, 1988) and related models for isostatically Fryxell et al., 1992). On the west edge of the Gold Butte block, driven tilting and folding of initially high-angle faults to become and extending to the south and north, there is a low-angle ex- low-angle normal faults (Duebendorfer and Sharp, 1998; Brady tensional detachment fault system called the South Virgin–White et al., 2000). This area has also provided an important fi eld labo- Hills detachment fault (Fig. 1; Duebendorfer and Sharp, 1998), ratory to evaluate the question of whether low-angle faults move which dies out southward against the Black Mountain accommo- at low angle, or are rotated to low angle due to isostatic footwall dation zone (Faulds and Varga, 1998) and correlates northward response (rolling hinge model; Buck, 1988; Wernicke and Axen, with the Beaverdam and Snake Range detachments. The trace of 1988) and/or due to progressive domino faulting (Proffett, 1977; this detachment system is just east of the trace of frontal thrusts Chamberlain, 1983; Axen, 2007). The region has been a test bed of the Sevier thrust system (Fig. 1). for understanding the extent to which thrusts may be reactivated North of the Gold Butte basement block, across the Gold as normal faults (Wernicke et al., 1985; Axen, 1993), and it has Butte strike-slip fault (Fig. 1), the Tramp Ridge and the Lime spawned diverse models to explain the extensional geometries, Ridge blocks are characterized by small basement windows ex- for example, N-S shortening synchronous with E-W extension posed beneath generally E-tilted sections of Paleozoic rocks. (Anderson and Barnhard, 1993; Anderson et al., 1994) and upper- Farther north, the Virgin Mountain anticline (VMA of Fig. 1) crustal response to lower-crustal fl ow (Langenheim et al., 2001). is a large NE-trending basement-cored uplift (Quigley, 2002; The central topic of this paper, the Gold Butte block, is one Quigley et al., this volume), and the Beaverdam Mountains key to resolving these different models. It has been proposed to be expose a small basement block beneath the Beaverdam detach- a 15–18-km-deep intact crustal section that was initially bounded ment zone (Fig. 1). by steeply dipping normal faults that were tilted to shallow dips The Gold Butte block has been interpreted to have been due to isostatic uplift resulting from regional detachment faulting uplifted via isostatic uplift of the footwall of the South Virgin– (Fig. 2A; Wernicke and Axen, 1988; Fryxell et al., 1992; Reiners White Hills detachment fault accompanying extensional break- et al., 2000; expanded upon herein). This and adjacent parts of away of Basin and Range blocks from the Colorado Plateau spe463-15 page 333 Structure and K-feldspar history of the Gold Butte block: Reevaluating the tilted crustal section model 333 Index Map North Virgin BEAVERDAM Colorado MOUNTAINS NV Mountains Plateau Gold Butte UT CO Block R E IV Cerbat LGG MORMON R Area of Fig 4 Mountains MTNS IN Transition NV G lt thrust R VI Basin andzone Range VMA n Mtns Mormo Piedmont fau AZ NM Mesquite BasinMesquite TERTIARY/QUATERNARY ROCK kerville Ridge Bun Figure 1. Location map of the Gold MIDDLE MIOCENE RED 40 Butte block and North Virgin Mountains SANDSTONE CONGLOMERATE (adapted from Beard, 1996) showing fault EARLY MIOCENE RAINBOW blocks cored by Precambrian basement GARDENS MEMBER idge (black). Inset shows location of Gold PALEOZOIC/MESOZOIC ack R Whitney Butte and Lower Granite Gorge (LGG) ROCKS Bl LIME Pocket in the context of the transitional bound- PROTEROZOIC ROCKS ent RIDGE fault m BLOCK Grand ary zone between the Colorado Plateau Wash and Basin and Range Province. Locations SEVIER THRUST FAULT MUDDY Trough Muddy Mts. thrust MTNS of Figures 3, 4, and 5 are also shown. NORMAL FAULT s Detach TRAMP VMA—Virgin Mountain anticline. e Hill RIDGE STRIKE-SLIP FAULT “Pig’s hit BLOCK Ear” Wash DETACHMENT FAULT W SYSTEM n- Virgi Area of Fig 3 50 La s Vegas GOLD rand shear G zone South BUTTE ead em BLOCK ke M La fault syst LAKE Area of Fig 5 Las Vegas MEAD COLORADO PLATEAU 36°N 0 10 km WHITE HILLS 114°W across the Grand Wash fault system (Fig.