Bouguer gravity values at about 11,000 stations in east-central California range from -14 mgal near Merced to -274 mgal in Long Valley. Gravity lows in the west and south parts of the San Joaquin Valley and over local basins south and east of the Sierra Nevada are produced by large thicknesses of Upper Cretaceous and Cenozoic deposits. A large regional gravity low over the Sierra Nevada can be explained by isostatic compensation of the range together with the effect of the relatively low-density rocks of the Sierra Nevada batholith. A broad gravity ridge along the east side of the San Joaquin Valley shows excellent correlation with a similar magnetic ridge where the two sets of data are available, suggesting that both anomalies are caused by a dense, magnetic mass buried at an estimated depth of 5-10 miles. Seismic refraction measurements further indicate that the thickness of the earth's crust under the valley is less than 12 miles. Thus, the anomalous mass is in the lower part of the earth's crust and is conceivably related to the more mafic rocks of the earth's upper mantle.
