The Coso Range lies in the southwest corner of the Basin and Range province, east of the Sierra Nevada and north of the Garlock Fault. The range, circular in form, is covered by a thin sequence of late-cenozoic volcanics which overlie highly fractured crystalline basement, similar in composition to the nearby Sierra Nevada (Duffield et al., 1980). Thirty-eight rhyolite domes of Pleistocene age were emplaced near the center of the range and are aligned approximately parallel to the direction of Basin and Range faulting (Figure 1). Bacon et al. (1980), using both age-dates and chemical analyses, inferred that the domes originated from a magma source beneath the central and largest of the rhyolite domes, Sugarloaf Mountain (Figure 1). A shallow source was inferred because active fumeroles, hydrothermally altered zones, and high heat flow values (Combs, 1980) are presently observed in this vicinity.
In an effort to collect seismic data which could be used to identify a magma body beneath Sugarloaf Mountain, a 16 station seismograph array was operated in the Coso Range from September 1975 through September 1977. During the two years of network operation, over 4000 local earthquakes were located in the vicinity of the Coso Range (Walter and Weaver, 1980a, 1980b). In October of 1976, a seismic refraction experiment was conducted in the Coso Range in order to develop a crustal velocity model for use in interpreting the earthquake data.
