Gravity anomalies in the central Sierra Nevada, California

Abstract

More than 1000 measurements of gravity in the central Sierra Nevada and adjacent regions to the east show a decrease in regional Bouguer gravity values from −75 mgal at the western edge of the mountains to a minimum of about −235 mgal just west of the Sierra crest, a distance of only 100 km. Farther east, a positive regional gradient of about 0.6 mgal/km continues into the Inyo-White Mountains for a distance of at least 60 km. Local gravity lows are associated with Cenozoic sediments in Owens Valley, Long Valley, and Mono Basin to the east of the Sierra Nevada and with a thick, narrow deposit of Pleistocene sediments in Yosemite Valley. Local gravity highs are associated with roof pendants of meta volcanic rocks which extend down into the Sierra Nevada batholith to depths of more than 2 km. The variation in the regional Bouguer gravity values of about 160 mgal can be explained by a thickening of the earth's crust from 35 to 52 km beneath the central Sierra Nevada, combined with an eastward decrease in the density of the Sierra Nevada batholith from 2.76 to 2.64 g/cm3. Gravity data support Romney's conclusion, based on seismic evidence, that a local mountain root exists under the central and southern parts of the Sierra Nevada and not under the lower northern part, in general accordance with regional isostasy.