TECTONIC UNDERPININGS OF THE SIERRA NEVADA AND SURROUNDINGS: RESOLVING THE PHYSICAL STATE OF THE UPPER MANTLE USING TELESEISMIC SHEAR WAVES

We refine our understanding of the upper mantle processes driving tectonism in the Sierra Nevada and volcanism to its east by incorporating teleseismic shear waves in a tomographic inversion of the Sierra Nevada and vicinity. Because variations in composition and melt content are expected to have shear-wave signatures differing from a purely thermal interpretation possible with just P-waves, we can explore for garnet-rich lithologies and high-melt regions in the upper mantle. We also investigate the interplay between lithology, melt, and mantle deformation by accounting for anisotropy in our measurements. S-waveforms were rotated into the Sierran S_Fast and S_Slow directions as observed from SKS-splitting measurements. Teleseismic P-, S_Fast-, and S_Slow- arrival times were then inverted for 3-D perturbations in Vp, Vp/Vs_Mean, and percent anisotropy using three surface wave starting models. We observe the highest Vp/Vs anomalies in regions marked by young volcanism. These include the Long Valley-Mono Lake magmatic system and the Clear Lake and Coso volcanic fields. The largest of these anomalies is the Long Valley-Mono Lake magmatic system extending to depths greater than 100 km. Peak Vp/Vs perturbations of +4% are found at 40 km depth. High velocities related to the Isabella and Gorda anomalies are characterized by low Vp/Vs values with peak perturbations of -2% and low anisotropy. For the Isabella anomaly, there is a profound difference between the velocity and the Vp/Vs geometries. The velocity results image the Isabella anomaly as an eastward plunging singular body. However, Vp/Vs results image the Isabella anomaly as two separate bodies whose grouped geometry is also plunging to the east. This dichotomy might indicate that the material making up the P-wave Isabella anomaly is actually compositionally heterogeneous. Within the Sierra Nevada, the highest anisotropy anomalies are largely contained within the central portion of the range and the adjacent section of the Great Valley. Such features suggest that the tectonic and deformational regime beneath the Sierra Nevada is diverse and segmented.