STRUCTURE AND MAGMATIC EVOLUTION OF THE NORTHERN BLACK ROCK RANGE, NEVADA: PREPARATION FOR A WIDE-ANGLE REFRACTION/REFLECTION SURVEY

Preliminary geochronology, field mapping, and geochemistry from the northern Black Rock Range constrain the Cenozoic structural and magmatic history of this region in preparation for a 260km wide-angle refraction/reflection survey. Low-magnitude (~10%) extension along high-angle faults postdates voluminous late Oligocene to Miocene rhyolitic and basaltic magmatism. New ⁴⁰Ar/³⁹Ar dates on alkali olivine basalts, and coeval rhyolitic ignimbrites and lavas indicate ca. 35Ma age for the inception of volcanism, dramatically increasing the length of known volcanism in this part of the Basin and Range. This pattern of late Miocene extension postdating latest Eocene to Miocene volcanism is consistent with previous studies from the Pine Forest and Santa Rosa Ranges, where ~15% Tertiary extension is restricted to the last 10-12Ma, with peak extension probably occurring around 8Ma. A large region of northwestern Nevada thus remained relatively unextended during latest Eocene to Miocene volcanism, and subsequent extension was minor compared to the amount (50-100%) documented in parts of the Basin and Range to the south and east. Despite relatively low supracrustal extension, the crust in northwestern Nevada is today among the thinnest in the Basin and Range, raising the possibility that the region underwent flow of a ductile middle or lower crust away from the area to more extended regions to the south. To investigate this hypothesis, a 260km seismic refraction-reflection experiment is planned for September 2004 that should constrain the Moho, velocity structure, and reflective character of the crust in this poorly-known region, and tie the 1986 PASSCAL survey in northwestern Nevada with the 1981 USGS survey across northeastern California.