THIN CRUST AND ACTIVE UPPER-MANTLE BENEATH THE SOUTHERN SIERRA-NEVADAIN THE WESTERN UNITED-STATES

A nalysis of seismic-refraction/wide-angle reflection travel-time and seismic amplitude data collected during the 1993 Southern Sierra Nevad a Continental Dynamics (SSCD) project resolves a small crustal root (4 0-42 km thick) centered 80 km west of the Sierran topographic crest. T he SSCD experiment consisted of a west-east profile across the Sierra Nevada at approximately 36.5 degrees N and a north-south profile exten ding the length of Owens Valley, located eastward of the Sierran topog raphic 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)) con sistent with ophiolitic material underlying the Great Valley sedimenta ry sequence, and higher velocities in the lower crust beneath the Basi n and Range (6.8-7.0 km s(-1)) than those beneath the Sierran Batholit h (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 fir st-order Moho observed beneath the Basin and Range. The crust decrease s 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 a nd decreases further to 27-30 km beneath the Basin and Range. Crustal thickness also appears to increase southeast to northwest from 29-30 k m beneath the Garlock Fault in the south to 38-40 km beneath the north end of Chalfant Valley. Juxtaposition of the crustal model with previ ous P-wave tomography models of the southernmost Sierra Nevada upper-m antle reveal that the thickest Sierran crust on the west-east profile overlies a pronounced upper-mantle high-velocity anomaly (+5%), wherea s the region of laminated Moho overlies a flanking upper-mantle low-ve locity region (-3%). The upper-mantle velocity anomalies, relatively l ow P-n, and relatively flat Moho, observed beneath the Sierran crest s uggest that the recent uplift of the Sierra Nevada is due to asthenosp heric flow and/or lithospheric thinning beneath the southeastern Sierr a Nevada and Basin and Range. (C) 1998 Elsevier Science B.V. All right s reserved.