Azimuthal Anisotropy From Surface Waves in the Great Basin

The Great Basin, Nevada, is a region of widespread crustal extension and magmatism. It is also characterized by a thin lithosphere (less than 100 km) and a high average heat flow, except in central Nevada where the heat flow is significantly reduced. In this study, we modeled seismic velocities and anisotropy in this region in order to understand how mantle dynamics relate to these geophysical and geological observations. We employed a two-station method to measure fundamental mode Rayleigh wave dispersion between periods of 16 and 170s using USArray Transportable Array (TA) data. We employed 22 events with high quality dispersion curves measured between stations aligned approximately along a common great-circle path (within 3 degrees). The excellent lateral and azimuthal coverage provided by TA seismic stations enabled us to produce azimuthally anisotropic phase velocity maps between periods of 16 and 100s, which gives constraints on shear-wave velocity structure down to depths of 150 to 200 km. Measurement uncertainties were large at periods greater than 100s, and no significant deviation from our reference model (a slightly modified Tectonic North America (mTNA) velocity model) were found. The isotropic part of the maps displays lateral changes in phase velocities between periods of 16 and 28s, but they become generally lower than predicted by mTNA at larger periods over the entire study region. Given that the sensitivity of 28s Rayleigh waves to shear wave velocities peaks at about 40 km depth, these results suggest a reduction in shear wave velocities compared to mTNA located below a thin lithospheric lid of about 60 km. In addition, we found that the data favor no or very little azimuthal anisotropy in central Nevada between 16 and 28s. For periods between 33 and 100s azimuthal anisotropy is required to significantly improve the data fit. This change appears thus to coincide with a lithosphere-asthenosphere transition. Other constraints on azimuthal anisotropy in the region include shear wave splitting, which exhibits a marked reduction in azimuthal anisotropy in central Nevada, comparable to our results at short periods. This indicates that the origin of the low shear-wave splitting signal lies at least partially in the lithosphere. In addition, this reduction in azimuthal anisotropy is coincident with a columnar zone of increased seismic velocities imaged by relative delay time tomography, which, combined with other geological and geophysical constraints, was interpreted as a lithospheric downwelling. The requirement by surface waves of upper asthenospheric anisotropy suggests that this may be a region of lateral flow feeding the lithospheric downwelling. In addition, the absence of azimuthal anisotropy found in the lithosphere could be the signature of past lithospheric delamination and subsequent downwelling, in which case it is very likely associated with the presence of radial anisotropy, such as the one found by others in the crust using ambient noise tomography.