Structural, stratigraphic, and thermochronologic studies provide insight into the formation of basement-cored uplifts within the Colorado Plateau–Basin and Range transition zone in the Lake Mead region. Basement lithologic contacts, foliations, and ductile shear zones preserved in the core of the Virgin Mountain anticline parallel the trend of the anticline and are commonly reactivated by brittle fault zones, implying that basement anisotropy exerted a strong influence on the uplift geometry of the anticline. Potassium feldspar 40 Ar/ 39 Ar thermochronology indicates that basement rocks cooled from ≥250–325 °C to ≤150 °C in the Mesoproterozoic and remained at shallow crustal levels (<5–7 km) until they were exhumed to the surface. Apatite fission-track ages and track length measurements reveal a transition from slow cooling beginning at 30–26 Ma to rapid cooling at ca. 17 Ma, which we interpret to mark the change from regional post-Laramide denudational cooling to rapid extension-driven exhumational cooling by ca. 17 Ma. Middle Miocene conglomerates (ca. 16–11 Ma) flanking the anticline contain locally derived basement clasts with ca. 20 Ma apatite fission-track ages, implying rapid exhumation rates of ≥500 m m.y. −1 . The apparently complex geometry of the anticline resulted from the superposition of first-order processes, including isostatic footwall uplift and extension-perpendicular shortening, on a previously tectonized and strongly anisotropic crust. A low-relief basement-cored uplift may have formed during the Late Cretaceous–early Tertiary Laramide orogeny; however, the bulk of uplift, exhumation, and deformation of the Virgin Mountain anticline occurred during middle Miocene crustal extension.

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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 geothermal gradients of ~25 °C/km, rather than 15–20 °C/km as previously published.

Abstract The central Colorado landscape bears a strong imprint of post-Laramide (late Eocene to Quaternary) tectonics, volcanism, climate change, and drainage rearrangement. This field trip will examine the post-Laramide evolution of central Colorado, traversing the Front Range, from the Colorado Piedmont on the east to the upper Arkansas valley segment of the Rio Grande Rift on the west (Fig. 1 ). The first day of the trip will involve a transect from the Denver-Colorado Springs section of the Piedmont across the southern Front Range, South Park, and Mosquito Range to the upper Arkansas valley. On this day we will focus on questions concerning the roles of tectonics and climate in driving post-Laramide landscape changes, examining structural, sedimentological, paleontological, geomorphic, and fission track evidence that has been used to reconstruct post-Laramide history. We will end the day with an initial overview of rift-related structures, sediments, and geomorphology as we enter the upper Arkansas valley. We will spend the second day in the southern portion of the upper Arkansas valley and the adjacent Poncha Pass transfer zone, examining structural and sedimentological evidence for the nature and timing of Neogene and Quaternary faulting and graben formation, and the character of the transfer zone. On our final day we will traverse back to the Piedmont, this time traveling down the canyons of the Arkansas River. We will examine rift-related structures and sediments in the Pleasant Valley graben and at the northern end of the Wet Mountain Valley, and will discuss the history of Cenozoic and earlier faulting in the area, the evolution of the Arkansas River drainage, and its recent downcutting history. We will end the trip with a discussion of the Neogene and Quaternary erosional history of the High Plains and Piedmont, and possible implications of this history for the driving mechanisms of landscape change.