Seismological estimates of means of isostatic support of the Sierra Nevada

Abstract

The modern topography of the Sierra Nevada (California, UNITED STATES) has been attributed to rapid uplift following foundering of negatively buoyant lithosphere into the asthenosphere since ca. 10 Ma. Uplift now manifests as ~2 km mean topographic relief between the crest of the southern Sierra Nevada and the western foothills and 1-2 km between the Sierran crest and adjacent Basin and Range. In this study, we use seismic P-wave velocity structures derived from teleseismic tomography to estimate the lithospheric density structure in the region and thus infer the current sources of topographic support. We exploit the different derivatives of crustal density with temperature and wave speed to attempt to identify a single solution for crustal density and temperature that satisfi es fl exural isostasy and the P-wave tomography. This solution yields both temperature variations compatible with observed heat fl ow and Bouguer gravity anomalies concordant with observations. We fi nd that the topographic gradient between the crest and the eastern Great Valley is due to both crustal and mantle sources. Despite a greater thickness, the foothills crust is less buoyant than that beneath the range crest, accounting for ~1 km of the topographic difference. High densities are due principally to composition. High-velocity upper mantle (~50-100 km depth) is also observed beneath the foothills but not the range crest, and this contrast explains an additional 1 km of topographic difference. Miocene or more recent removal of such upper mantle material from the Sierran crest, as inferred from xenoliths, would have triggered rapid uplift of ~1 km. Our fi ndings are consistent with the removal of negatively buoyant material from beneath the Sierra Nevada since the Miocene.