Structure and 40Ar/39Ar K-feldspar thermal history of the Gold Butte block: Reevaluation of the tilted crustal section model

This paper reevaluates the geometry and processes of extension in the boundary 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 previously 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 km 2 , helping define 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 formation 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 Paleozoic cover was partly decoupled from less-extended basement. This detachment 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 detachment 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