Seismic refraction and potential field modeling of the northwestern Basin and Range transition zone, Nevada and California

Seismic-velocity and potential-field modeling along an E-W profile provides new constraints on the crustal structure of the northwestern Basin and Range transition zone. Our data span the change in tectonic setting from low-magnitude (<15%) Basin and Range extension in northwestern Nevada to relatively unextended volcanic plateaus in northeastern California, effectively filling the gap in geophysical data between the 1986 PASSCAL survey from northwestern and central Nevada and the 1981 USGS survey from northeastern California. Velocity modeling of the 300 km wide-angle refraction dataset collected in summer 2005 by Stanford University documents significant westward crustal thickening and upper crustal velocity heterogeneity. The degree of this crustal thickening can be accounted for by simply restoring the minor extension recorded in the upper crust in the eastern portion of the study area, suggesting crustal response to extension was homogeneous over the entire crustal column. The USGS collected 84 gravity stations and about 250 line-kilometers of truck-towed magnetometer data along the 2005 seismic refraction line for potential field modeling. A gravity profile pulled from a compilation of new and existing regional gravity data, reveal a sharply delineated ~25mGal isostatic anomaly low that coincides with a broad topographic plateau situated between Surprise Valley to the west and the Black Rock Desert to the east. Although little is known of subsurface lithologies in this area --no basement rocks are exposed because the region is blanketed by Cenozoic volcanic cover -- the gravity low is probably caused by a thick sequence of low-density Cenozoic deposits and/or anomalously low-density basement rocks. The margins of this low are characterized by pronounced gravity gradients and correspond with two prominent crustal discontinuities. The western margin of the gravity low marks the western extent of the Great Basin Province, whereas the eastern margin is aligned with a major tectonostratigraphic terrane boundary. This terrane boundary separates a thick sequence of Cenozoic volcanic (dominantly basaltic) rocks that overlie oceanic lower-crustal basement to the northwest from crust dominated by Mesozoic and Paleozoic strata and Mesozoic intrusives to the southeast. The velocity models show low velocities in the shallow crust beneath the plateau coinciding with the gravity low, consistent with the presence of low-density Cenozoic deposits. However, not all of the anomaly can easily be accounted for in the shallow crust, as the required densities are too low for any reasonable bulk lithology, and far lower than the densities of rocks exposed at the surface. Furthermore, the velocity models show low velocities that extend into the deeper crust (to 20 km), suggesting that the anomalous nature of the crust beneath this region involves both shallow deposits and basement rock.