Thermochronology of fault rocks from the mid-Miocene South Virgin-White Hills detachment, Arizona and Nevada

One of the most prominent low-angle normal fault systems in the Basin and Range province of the western U.S. is the ~100 km-long South Virgin-White Hills detachment. Multiple low-temperature thermochronometers have been used to document rapid Miocene exhumation of the footwall of this detachment, most notably within the Gold Butte block of southern Nevada. Less attention has been paid to the actual faults that comprise the detachment system, one of which (the Salt Spring fault) is very well-exposed over an along-strike distance of about 5 km just south of Lake Mead. In this location, the fault dips ~25° and separates Proterozoic crystalline rocks in the footwall from Tertiary gravels and volcanic rocks in the hanging wall. At one exceptional outcrop, the fault consists of a 9 m-thick stratified zone of brecciated granite in a clay matrix (fault breccia), overlain by a relatively thin (<1 m) layer of finer-grained, foliated, clay-rich fault gouge that is in direct contact with overlying Tertiary gravels. Clay minerals are a minor constituent of the crystalline footwall rocks but are abundant in the fault zone. We utilized apatite fission track and illite/muscovite 40Ar/39Ar thermochronology to investigate the thermal evolution of the fault gouge. Apatite grains entrained in the gouge have fission track ages of ~15 Ma, comparable to previously published apatite fission track ages from structurally deep parts of the Gold Butte block and the immediate footwall of the Salt Spring fault, near the fault rock sampling site. Encapsulated 40Ar/39Ar data from micron-scale size fractions of the gouge yield staircase-shaped step-heating spectra that reach maximum ages of 80 to 110 Ma. The primary K-bearing phases in these size fractions are illite and muscovite. In structurally deep parts of the Gold Butte block, previously published muscovite 40Ar/39Ar ages are ~90 Ma. We interpret the staircase-shaped 40Ar/39Ar spectra of the fault gouge 1) to indicate the presence of retentive muscovite Ar domains with Late Cretaceous apparent ages, and 2) to result from the degassing of smaller, less retentive muscovite domains and/or mixing with younger, possibly less retentive illite formed authigenically in the fault zone. These 40Ar/39Ar and apatite fission track results suggest that along this particular segment of a major extensional system, mineral grains in fault gouge preserve thermochronological data that reflect the exhumation history of the footwall.