MID-MIOCENE SYNEXTENSIONAL DEPOSTION DURING NORTH-SOUTH EXTENSION IN A WEST-NORTHWEST TRENDING HALF GRABEN IN THE WESTERN GREAT BASIN

The Palmetto Mountains in the northern part of the southern Walker Lane in west-central Nevada expose three sequences of Miocene volcanic and volcaniclastic rocks ranging from 250 to 1990 m thick. The Cenozoic rocks were deposited within and outside of a WNW–trending half-graben and record synextensional and post-extensional deposition. The half-graben is 11 km long and 4 km wide and contains (1) a basal unit of lower to middle Miocene andesite lahar, flows, tuff and volcaniclastic sediments, (2) a mid-Miocene ignimbrite, and (3) an upper Miocene rhyolite tuff. Within the half-graben, andesite flows, lahar, and sediments are 1.5 km thick but are only 120 to 210 m thick outside of the basin. The andesite is lithologically correlated to units dated at 17.4 ± 0.6 to 15.7 ± 0.5 Ma elsewhere in the region. WNW–striking faults forming the northern and southern boundaries of the half-graben separate the basin-fill from Paleozoic metasediments and a Mesozoic pluton. The basin is segmented by N-S–striking transfer faults that accommodate along-strike changes in thickness and dip direction of the basin-fill. Inside of the basin, the andesite succession forms of a stratal-wedge with dips of up to 55° that shallow upsection. Within the wedge, individual units thicken into the WNW–striking basin-bounding faults. In contrast, andesite deposited outside the basin is relatively flat-lying and infills minor erosional depressions in underlying rocks. The half-graben is overlapped by 90 m of mid-Miocene ignimbrite that shows no sign of synextensional deposition. Half-graben faults are locally buried by rhyolite tuff dated at 6.9 ± 0.9 to 6.02 ± 0.03 Ma. Fault-slip inversion indicates multiple extension directions (N-S, ENE, and WNW) during the late Cenozoic. Lineated slickensides indicating N-S extension are restricted to the andesite succession and middle Miocene ignimbrite and are consistent with the orientation E-W–striking andesite dikes cutting andesite within the half-graben. Although N-S extension persisted during deposition of the ignimbrite, the half-graben activity was suppressed and no longer creating accommodation space by the mid-Miocene. The upper Miocene rhyolite tuff is not involved in N-S extension, which ceased after deposition of the andesite and overlying ignimbrite and prior to 6.9 to 6.0 Ma.