TECTONIC AND MAGMATIC EVOLUTION OF THE NORTHWESTERN BASIN AND RANGE TRANSITION ZONE: MAPPING AND GEOCHRONOLOGY FROM THE BLACK ROCK RANGE, NV

The northern Black Rock Range on the northwestern margin of the Basin and Range province preserves a remarkably complete record of Cenozoic magmatism. Geologic mapping and ⁴⁰Ar/³⁹Ar geochronology from the northern Black Rock Range document three significant episodes of Eocene to middle Miocene volcanism. Eocene (35 Ma) basalts directly overlie Mesozoic granites and arc-related volcanic and sedimentary rocks. These Eocene flows are overlain by a ~1 km sequence of locally derived Oligocene to early Miocene (27-21 Ma) bimodal volcanic rocks, comprising the bulk of the Cenozoic section. These bimodal units include rhyolitic lavas, variably welded rhyolitic air-fall and ash-flow deposits, and basalt flows. In the neighboring Pine Forest Range ~20 km to the north, similar Oligocene to early Miocene units are overlain by more than 500 m of ~16.4 Ma Steens basalt flows capped by ~16 Ma rhyolitic ash-flows. In the northern Black Rock Range, the ~16.4 Ma middle Miocene basalts are absent from the section, and ~16 Ma rhyolitic ash flows directly overlie the early Miocene flows. Despite the complexities of locally derived lava flows, the Cenozoic section is broadly conformable and dips gently (~5^o-10^o) to the northwest. The region experienced no significant tilting between 35 and 15 Ma, with moderate uplift (~1 km) and tilting (~5^o-10^o) occurring after 15 Ma. This tectonic history is consistent with that of the nearby Pine Forest and Santa Rosa Ranges, where low-temperature thermochronology documents footwall exhumation along the range-bounding normal faults beginning less than 12 Ma. Basaltic and rhyolitic volcanism in northwestern Nevada spanned more than 20 my, with the majority of these units derived from local vents and dikes. Together with recently completed work from neighboring ranges, our results suggest that the evolution of the northwestern margin of the Basin and Range is characterized by long-lived and voluminous volcanism followed by low magnitude (5-15%) extension along high-angle normal faults.