Timing and mechanisms of basement uplift and exhumation in the Colorado Plateau-Basin and Range transition zone, Virgin Mountain anticline, Nevada-Arizona

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 infl uence 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 fi ssion-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) fl anking the anticline contain locally derived basement clasts with ca. 20 Ma apatite fi ssiontrack ages, implying rapid exhumation rates of ≥500 m m.y. –1 . The apparently complex geometry of the anticline resulted from the superposition of fiprocesses , 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.