BASIN AND RANGE CRUSTAL AND UPPER MANTLE STRUCTURE, NORTHWEST TO CENTRAL NEVADA

We present a interpretation of the crustal and uppermost mantle structure of the Basin and Range of northwestern Nevada based on seismic refraction/wide-angle reflection, near-vertical reflection, and gravity data. In comparison to most previous estimates, we find that the crust is somewhat thicker (32-36 km versus 22-30 km), and the uppermost mantle velocity is somewhat higher (8.0 km/s versus 7.3-7.9 km/s). Along our transects, the crust is thinnest (32 km) in the Carson Sink-Buena Vista Valley region and increases by 2-4 km to the west and east, respectively. There is considerable complexity throughout the crust where velocities range from of 2.5 km/s at the surface to 7.4 km/s in the lowermost crust. Variations in velocity and structure of the upper crustal layers reveal apparent basement velocity depressions (areas of lower velocities extending up to 10 km in depth) that underlie some surface ranges as well as the basins. The middle crust rises from about 20 km beneath central Nevada to within 12 km of the surface beneath the area of thinnest crust and is characterized by a modest (approximately 0.1 km/s) change in velocity and low-velocity gradients. These midcrustal layers mark the onset of high crustal reflectivity and the apparent limiting depth to which Basin and Range faults can be traced in near-vertical reflection profiles, suggesting that these midcrustal layers represent the transition between the brittle and ductile zones of the crust. The lower crust is more structurally complex, with layers thickening and thinning in a systematic manner with the upper crustal layers; generally, where there are velocity depressions in the upper crust, the lower crust is thickest and shallowest. The geometry of these lower crustal layers (derived from refraction modeling) coincides with changes in the crustal reflectivity, determined from the Consortium of Continental Reflection Profiling reflection data. The lower crustal layer is unusually high in velocity (7.4 km/s) and is likely the layer identified as mantle in some previous studies. We do not identify the 7.4 km/s layer as mantle because (1) there is an underlying layer with a velocity (8.0 km/s) that is more consistent with the worldwide average velocity for the upper mantle, and (2) the 7.4 km/s layer does not correspond to the "reflection" Moho. Gravity modeling and comparison to existing seismic models show a general consensus in many aspects with respect to crustal structure. This new model forms the basis for speculation on some of the processes associated with rifting of the Basin and Range Province. One such process, lithospheric magmatism, is inferred from the strong attenuation of transmitted seismic waves, which occurs at the same interface at which high-amplitude, bright spot reflections originate. Unlike previous models, the overall structure and velocity of the crust and uppermost mantle of our new model are similar to other regions worldwide which have undergone high degrees of extension.