LATE CENOZOIC 40AR/39AR AGES OF FAULT ROCKS FORMED ALONG THE WEST SALTON DETACHMENT SYSTEM, SOUTHERN CALIFORNIA
East-down low-angle normal faults of the west Salton detachment system (WSDS) overprint more ductile structures of the Late Cretaceous eastern Peninsular Ranges mylonite zone (EPRMZ). Faults of the WSDS have been considered to be Cretaceous, middle Miocene, or late Miocene to Quaternary in age, but some clearly cut late Miocene-Quaternary sediments. Dating of pseudotachylyte (PST) offers one potential means of establishing the timing of slip along faults for which the displacement history is unclear (e.g., those cutting only Cretaceous rocks), but PST is a complex material and 40Ar/39Ar ages are difficult to interpret in the absence of Ar diffusion data and well constrained thermal histories of the fault surroundings. PST veins of the EPRMZ have yielded Late Cretaceous-early Tertiary 40Ar/39Ar ages whereas a PST from the WSDS yielded a 10 Ma K/Ar age.
In the Whale Peak and Yaqui Ridge area, field, petrographic, and SEM data show that WSDS fault rocks range from ultracataclasite to PST(?). In both, SEM analysis indicates that the only K-bearing phase is feldspar. Glass has not been identified but hydrous mafic minerals are generally absent (in contrast to the tonalitic protolith), suggesting possible partial melting. 40Ar/39Ar step-heating analyses of PST(?) samples yield total gas 40Ar/39Ar ages ranging from 6 to 25 Ma. Notably, the youngest gas fractions in all samples are <10 Ma, suggesting late Miocene fault activity. K-feldspar 40Ar/39Ar and apatite thermal history data from footwall samples beneath the fault damage zone show that the Yaqui Ridge footwall had already cooled to <125°C by ~50 Ma, indicating that the PST(?) formed at low ambient temperatures. Hump-shaped PST(?) age spectra, characteristic of low-temperature grain size reduction in K-feldspar, indicate that our samples experienced fault-related grain-scale deformation and Ar loss. However, model bulk closure temperatures calculated from PST(?) 39Ar diffusion properties are < 100°C, so the 40Ar/39Ar ages could reflect denudation-related footwall cooling and/or the time of PST/ultracataclasite formation, but support late Miocene fault activity in either case.