3-DIMENSIONAL CRUSTAL STRUCTURE OF THE SOUTHERN SIERRA-NEVADA FROM SEISMIC FAN PROFILES AND GRAVITY MODELING

Traveltime data from the 1993 Southern Sierra Nevada Continental Dynam ics seismic refraction experiment reveal low crustal velocities in the southern Sierra Nevada and Basin and Range province of California (6. 0 to 6.6 km/s), as well as low upper mantle velocities (7.6 to 7.8 km/ s),The crust thickens from southeast to northwest along the axis of th e Sierra Nevada from 27 km in the Mojave Desert to 43 km near Fresno, California. A crustal welt is present beneath the Sierra Nevada, but t he deepest Moho is found under the western slopes, not beneath the hig hest topography. A density model directly derived from the crustal vel ocity model but with constant mantle density satisfies the pronounced negative Bouguer anomaly associated with the Sierra Nevada, but shows large discrepancies of >50 mgal in the Great Valley and in the Basin a nd Range province. Matching the observed gravity with anomalies in the crust alone is not possible with geologically reasonable densities; w e require a contribution from the upper mantle, either by lateral dens ity variations or by a thinning of the lithosphere under the Sierra Ne vada and the Basin and Range province. Such a model is consistent with the interpretation that the uplift of the present Sierra Nevada is ca used and dynamically supported by asthenospheric upwelling or litho sp heric thinning under the Basin and Range province and eastern Sierra N evada.