Abstract Title: Cenozoic volcanic and structural evolution of the northwestern Basin and Range Province: Implications for large-scale flow of the lower crust. Abstract Author(s): Colgan, Joseph P (Stanford) - Dumitru, Trevor A (Stanford) - Lerch, Derek (Stanford) - Miller, Elizabeth L (Stanford) Abstract: In northwestern Nevada, the total amount of Basin and Range extension is low (about 15%) compared to the amount documented to the south in central Nevada (55-100%). Detailed mapping and 40Ar/39Ar dating of 38-16 Ma volcanic rocks in the Pine Forest and Black Rock Ranges demonstrates that Tertiary rocks in northwestern Nevada are conformable and not disrupted by angular unconformities formed during multiple episodes of faulting and tilting. Rather, Tertiary volcanic rocks were tilted during a single episode of slip along the faults that bound the modern ranges. Detailed apatite fission-track studies of the Santa Rosa and Pine Forest ranges, coupled with reconnaissance fission-track sampling of surrounding ranges, indicate that major fault slip on modern range-bounding normal faults was ongoing ca. 6-8 Ma and began as early as 10-12 Ma. Extension is thus of low magnitude and postdates major Oligocene and middle Miocene volcanism. Existing but limited geophysical data from northern Nevada suggest that the crust in northwestern Nevada is as thin or thinner (~30km) than the crust in central Nevada, which has been extended 50-100% or more. This combination of thin crust and little supracrustal extension raises the possibility that northwestern Nevada was significantly thinned by flow of the middle or lower crust into more extended regions to the south. Our upcoming seismic experiment is designed to evaluate this hypothesis by collecting data on the thickness, velocity structure, and reflectivity of the crust. These data will be integrated with the results of our ongoing geologic and thermochronologic studies to assess when and to what extent this part of Nevada may have been affected by lower-crustal flow.