Serdar Özalaybey, Martha K. Savage*, Anne F. Sheehan**, John N. Louie, and James N. Brune
Seismological Laboratory, University of Nevada Reno, Reno, NV
* also at Institute of Geophysics, Victoria University of Wellington, New Zealand
**CIRES, University of Colorado, Boulder, CO
We have derived one-dimensional shear-velocity structures at nine broadband seismic stations situated in the northern Basin and Range Province from the combined analysis of receiver function and surface-wave phase velocity data. A new, modified method based on the joint inversion of receiver functions and surface-wave phase velocities resulted in well-determined shear-velocity structures that are consistent with the compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. This new inversion method takes advantage of average velocity information present in the surface-wave method and differential velocity information contained in the receiver function method, thus minimizing the non-uniqueness problem resulting from the velocity-depth trade-off.
The crustal thickness, average crustal and upper mantle velocities are the most robust parameters determined from the joint inversions. These parameters show excellent correlations with the 1986 PASSCAL, wide angle reflection and refraction experiment results in northwestern and central Nevada, and other refraction and earthquake tomographic experiment results that have been used to infer the crustal and upper mantle structure of the province. An unusually thick (38 km), and faster crust and upper mantle are found in central and eastern Nevada compared to the thin (28 to 34 km), and slower crust and upper mantle of the western Basin and Range. We interpret the regions of thicker and faster crust and, upper mantle as zones that have undergone less Cenozoic extension relative to the surrounding regions to the west and north. The thick crust and consequently greater depth to the dense mantle material is consistent with the Butterfly gravity low pattern described by Eaton (1978) in central and eastern Nevada. A simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface, indicating a classical Airy type of compensation in the province.
We have made a detailed analysis of the shear-wave (S-wave) velocity model derived from the receiver functions at station BMN and compressional-wave (P-wave) velocity models derived from the PASSCAL experiment in northwestern Nevada. The most interesting feature of these models is the presence of negative-velocity gradients in the S-wave model with no corresponding velocity decrease in the P-wave models between 10 and 24 km depth range. This combined velocity model may be explained by high pore fluid pressures at these depths. This model is in favor of the layered pore fluid pressure models proposed to explain the origin of extensive middle to lower crust continental seismic reflections and high electrical conductivity. An upper mantle, gradational low-velocity zone (LVZ) is present between 32 and 38 km in the S-wave model. This upper mantle shear-wave LVZ is consistent with partial melt, which may be considered as the source material for magmatic underplating in this region.