S-wave tomographic model of the Sierra Nevada, California: Constraining thermal and compositional effects through Vp/Vs, anisotropy, and attenuation

Abstract:

The lithospheric seismic structure of the Sierra Nevada, California has long been recognized as an important tool for determining the uplift mechanism(s) of this range and its effect on the tectonic evolution of the western United States. Past studies have generally observed that at upper mantle depths, the Sierran crest is characterized by slower wavespeeds, suggestive of buoyant material while the western foothills are characterized as regions of faster, more dense material. Although there exists many different tomographic models of the Sierra Nevada, its exact geometry and structure are difficult to constrain and vary. For instance, the high wavespeed Isabella anomaly has been imaged in many different studies and has been interpreted as delaminated lithosphere, convectively downwelling lithosphere, and a remnant slab. Challenges in estimating a tomographic model include discrepancies in data coverage, resolution, and accounting for the effects of anisotropy and attenuation. It is still not well understood whether seismic velocity variations in the Sierra Nevada are thermal or compositional in origin. To address this question, we will use a three-dimensional P- and S-wave tomographic model of the Sierra Nevada and vicinity to characterize its lithospheric thermal and compositional structure. Travel times for direct S_fast and S_slow using regional-to-teleseismic S- and SKS- phases will be measured to invert for Vp, Vp/Vs, and percent anisotropy. Teleseismic P-wave travel times used to calculate Vp and Vp/Vs were measured from a previous study. Vp/Vs and percent anisotropy will be used to evaluate regions where compositional effects are prevalent. Regions with higher anisotropy could be indicative of olivine-rich, strained mantle. However, lower anisotropy could be indicative of isotropic minerals such as eclogite. High Vp/Vs could suggest more garnet and pyroxene or a decrease in Mg. We will also calculate S_fast and S_slow wave differential attenuation, dt*, to evaluate regions where thermal or scattering effects dominate.