A Gravity Model for the Coso Geothermal Area, California

Abstract

Two and three dimensional gravity modeling was done using gridded Bouguer gravity data covering a 45 x 45 km region over the Coso geothermal area in an effort to identify features related to the heat source and to seek possible evidence for an underlying magma chamber. Isostatic and terrain corrected Bouguer gravity data for about 1300 gravity stations were obtained from the U.S. Geological Survey. After the data were checked, the gravity values were gridded at 1 km centers for the area of interest centered on the Coso volcanic field. Most of the gravity variations can be explained by two lithologic units: (1) low density wedges of Quaternary alluvium with interbedded thin basalts filling the Rose Valley and Coso Basin/Indian Wells Valley, and (2) low density over of Tertiary volcanic rocks and intercalated Coso Formation. A 3D iterative approach was used to find the thickness of both units. The gravity anomaly remaining after effects from Units 1 and 2 are removed is a broad north south trending low whose major peak lies 5 km north of Sugarloaf Mountain, the largest of the less than 0.3 m.y. old rhyolite domes in the Coso Range. Most of this residual anomaly can be accounted for by a deep, low density prismatic body extending from 8 to about 30 km below the surface. While some of this anomaly might be associated with fractured Sierran granitic rocks, its close correlation to a low velocity zone with comparable geometry suggests that the residual anomaly is probably caused by a large zone of partial melt underlying the rhyolite domes of the Coso Range.