POST-9.5 MA TILTING OF THE DUBLIN HILLS: MECHANISMS OF CRUSTAL EXTENSION IN THE DEATH VALLEY REGION, EASTERN CALIFORNIA

New ⁴⁰Ar/³⁹Ar ages for biotite from two samples of rhyolite and a volcanic breccia obtained from the Shoshone Volcanics in the Dublin Hills, California are statistically indistinguishable, suggesting that much of the volcanism at that site occurred close to 9.5 Ma. The volcanic rocks dip to the southeast at 10°-20°, and unconformably overlie east-dipping sedimentary rocks as old as the Neoproterozoic-Cambrian Wood Canyon Formation with a discordance of 20°-30°. In the absence of evidence for growth in the volcanic succession, we infer that much or all of the extension-related tilting occurred after 9.5 Ma.

The picture that is now emerging across a series of tilted fault blocks to the east of central Death Valley is one of a few faults becoming active more or less coeval with magmatism between ~13-9.5 Ma (east of the Resting Spring Range, Greenwater Range, the Sheephead fault at Sheephead Pass, Black Mountains), with more widespread faulting after much of the magmatism had ended (Resting Spring Range, Dublin Hills, Sheephead Mountain, Black Mountains). Volcanism in the Greenwater Range continued until ~3.5 Ma. Faulting persisted longest in the vicinity of the Black Mountains, with exhumation locally in excess of 10-12 km between 11.6 Ma and the present, and particularly from 8-6 Ma. However, Neoproterozoic to Cambrian sedimentary rocks were exhumed widely by erosion prior to the onset of extension in an area that is now bounded by the right-lateral Furnace Creek and Sheephead faults (Resting Spring Range, Dublin Hills, Eagle Mountain, Sheephead Mountain, Furnace Creek Wash, southern Black Mountains).

These data are inconsistent with regional tectonic denudation and the hypothesized northwestward migration of a rolling hinge (e.g., Holm and Dokka, 1993, EPSL, v. 116, p. 63-80). Instead, they corroborate an earlier interpretation of COCORP seismic reflection data (Serpa et al., 1988, GSAB, v. 100, p. 1437-1450): that extension was accommodated by a series of discrete normal faults rooted in the mid-crust. A stretch (or beta factor) of 2.53 between the northern Panamint Range and northern Nopah Range (Renik and Christie-Blick, 2013, Tectonics, v. 32, doi:10.1029/2012TC003170) is for the most part explicable with a simple domino model.