Thin crust and active upper mantle beneath the Southern Sierra Nevada in the western United States

Analysis of seismic-refraction/wide-angle reflection travel-time and seismic amplitude data collected during the 1993 Southern Sierra Nevada Continental Dynamics (SSCD) protect resolves a small crustal root (40-42 km thick) centered 80 km west of the Sierran topographic crest. The SSCD experiment consisted of a west-east profile across the Sierra Nevada at approximately 36.5°N and a north-south profile extending the length of Owens Valley, located eastward of the Sierran topographic crest. Two-dimensional finite-difference travel-time inversion of P_g, and P_n arrivals resolves upper-crustal velocities of 6.0 to 6.4 km s^-1 within the Sierran Batholith and Basin and Range, an upper-crustal west-dipping wedge of higher velocities (6.8-7.2 km s^-1) consistent with ophiolitic material underlying the Great Valley sedimentary sequence, and higher velocities in the lower crust beneath the Basin and Range (6.8-7.0 km s^-1) than those beneath the Sierran Batholith (6.6 km s^-1). Low average P_n velocities (7.6-7.9 km s^-1) and a laminated transitional Moho imaged beneath the Sierran Batholith also differ from the higher P_n, velocities (7.9-8.0 km s^-1) and sharp first-order Moho observed beneath the Basin and Range. The crust decreases in thickness both westward of the root to 28-34 km beneath the Great Valley and eastward to 35 km beneath the highest Sierran topography and decreases further to 27-30 km beneath the Basin and Range. Crustal thickness also appears to increase southeast to northwest from 29-30 km beneath the Garlock Fault in the south to 38-40 km beneath the north end of Chalfant Valley. Juxtaposition of the crustal model with previous P-wave tomography models of the southernmost Sierra Nevada upper-mantle reveal that the thickest Sierran crust on the west-east profile overlies a pronounced upper-mantle high-velocity anomaly (+5%), whereas the region of laminated Moho overlies a flanking upper-mantle low-velocity region (-3%). The upper-mantle velocity anomalies, relatively low P_n, and relatively flat Moho, observed beneath the Sierran crest suggest that the recent uplift of the Sierra Nevada is due to asthenospheric flow and/or lithospheric thinning beneath the southeastern Sierra Nevada and Basin and Range.