Late Oligocene to Early Miocene North-South Extension in the Western Great Basin
In the central and southern Walker Lane, a network of six east-northeast east-west, and westnorthwest trending half-grabens, spanning an area of 15,000 km2, controlled the spatial distribution and thickness of late Oligocene to early Miocene volcanic and volcaniclastic rocks. The basins range from 10 to 25 km long, are 4 to 7 km wide, and from 0.5 to 1.5 km deep. Internally, the basins are segmented by north-northeast and north-northwest striking transfer faults that accommodated along-axis changes in basin geometry and across-axis dog-leg steps of up to 15 km. The basin-fill consists of synextensional rhyolite tuff, andesite, and volcaniclastic rocks that form asymmetric stratal wedges that thicken to the north and south into basinbounding extensional faults. In several locations, the bounding faults are overlapped by postextensional deposits of tuff and andesite that maintain uniform stratigraphic thicknesses throughout the region. The half-grabens controlled deposition of rhyolite tuff ranging in age from 27.4 to 23.75 Ma and andesite dated at 22 to 16.6 Ma. Individual faults were variously sealed by 23.75 Ma rhyolite tuff and 16.1 to 15.7 Ma andesite. Half graben development ceased by the mid-to-late Miocene and the basins were subsequently dissected by Late Miocene to Quaternary faults. Analysis of 1335 fault-slip measurements collected within late Cenozoic and pre-Cenozoic rocks along major basin-controlling faults indicates the half-grabens formed in a period of regional north-south extension and predated west-northwest extension associated development of Late Miocene and Pliocene to contemporary basins and topography. Locally, east-west and east-northeast striking andesitic dikes intrude the basin-fill sequences and are consistent with emplacement during north-south extension.