Thermochronology of the salt spring fault: Constraints on the evolution of the South Virgin-White Hills detachment system, Nevada and Arizona, USA

Abstract

We present new clay mineralogy and muscovite and illite 40Ar/39Ar data from fault gouge and immediately adjacent wall rocks from the Salt Spring fault, the central portion of the Miocene South Virgin-White Hills detachment system in southern Nevada and northern Arizona. In combination with a wealth of published regional thermochronology data, we find that useful age information can be obtained from 40Ar/39Ar step-heating spectra of fine-grained clays in both sedimentary rocks and fault gouge derived from them. This information can be used to investigate the provenance of detrital clays in lowgrade sediments, the source of clays in fault gouge, and potentially to constrain thermal histories. A new muscovite 40Ar/39Ar age from the footwall near the Salt Spring fault is ca. 900 Ma, in contrast with previously reported ages of ca. 90 Ma from near the South Virgin-White Hills detachment system at Gold Butte, Nevada, located 20 km along strike to the north. This discrepancy in muscovite cooling ages supports prior interpretations of significant along-strike variations in the magnitude of footwall exhumation. 40Ar/39Ar data from clay-size sediment in a supradetachment basin at the Salt Spring Wash area show no influence from detrital ca. 900 Ma or older muscovite, but do show an influence from detrital muscovite with Cretaceous apparent ages, suggesting that these sediments were derived from Gold Butte to the north, and not from directly updip areas to the east. These observations suggest a Miocene paleotopographic configuration wherein Gold Butte formed an upland area that shed detritus southward into the supradetachment basin during exhumation of the Salt Spring footwall. Fault gouge from the Salt Spring detachment is similar to fine-grained sediment from the adjacent supradetachment basin both in terms of clay mineralogy and 40Ar/39Ar results. The gouge is rich in illite and smectite, clays that have low frictional coefficients, are relatively impermeable, and would have reduced the shear strength of the detachment at shallow depths. Apatite grains entrained in the gouge have a mean fission-track age of 15 Ma and mean track lengths that are statistically indistinguishable from previously published data from footwall samples. Collectively, these observations suggest that clay-rich gouge along this segment of the fault formed principally by scraping and incorporating clays and other minerals from the base of the supradetachment basin and that the fault zone did not experience temperatures measurably greater than the footwall. © 2011 Geological Society of America.