THE TIMING AND EVOLUTION OF CENOZOIC EXTENSIONAL NORMAL FAULTING IN THE SOUTHERN TOBIN RANGE, NORTHWEST NEVADA

The Tobin Range lies near the transition between more extended terrane (~100%) to the south and east, and generally less extended terrane (possibly ~15%) to the north and west. Despite these different estimates of extension, geophysical data suggests that the crust is relatively thin on both sides of the transition.

Geologic mapping has been employed to establish the Cenozoic stratigraphy, geometry and timing of normal faults, and magnitude of extension in the vicinity of Golconda Canyon in the southern Tobin Range. The Golconda Canyon area lies near the westernmost extent of the Caetano paleo-trough, an east-west channel that likely predates basin and range extension there. Infilling the trough is a series of Oligocene to Miocene volcanic rocks. From the base to the top the volcanic stratigraphy in the trough consists of a series of thin basaltic lava flows; the Caetano rhyolite ignimbrite (~34 Ma); a >1000m-thick sequence of andesite lavas, lahars, and ignimbrites (~32 Ma); and a unit of syn-tectonic landslide mega-breccias consisting principally of rhyolite ignimbrite clasts. These units are overlain by units not confined by the paleo-trough that include a sequence of fluvial and lacustrine tuffaceous sedimentary rocks and a 50m-thick series of basaltic lavas (inferred ~10 to 20 Ma).

Gently to moderately west-dipping NNE-striking normal faults cut the base of the andesitic sequence and older units, which dip 30°E. Several of these faults do not cut and are overlain by younger andesitic lavas of the sequence, which suggests that andesitic magmatism coincides with normal faulting initiation. The landslide mega-breccias are localized in the hanging wall (south side) of a large normal fault system that strikes northwest across the range. The Miocene basaltic lavas dip gently (~15°) and are cut by north-striking faults dipping to the west. The western limit of the Tobin Range is a large normal fault that ruptured during the 1915 Pleasant Valley earthquake.

Preliminary estimates suggest the southern Tobin Range has undergone ~50% east-west crustal extension. The initiation and majority of extension is early Oligocene, consistent with the regional pattern of westward younging normal fault inception in northwest Nevada. Ongoing geologic mapping and 40Ar/39Ar dating are being used to refine estimates.