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

Abstract

Analysis of seismic-refraction/wide-angle reflection travel-time and seismic amplitude data collected during the 1993 Southern Sierra Nevada Continental Dynamics (SSCD) project 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 Pg and Pn 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 Pn velocities (7.6–7.9 km s−1) and a laminated transitional Moho imaged beneath the Sierran Batholith also differ from the higher Pn 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 south-ernmost 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 Pn, 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.