CRUSTAL STRUCTURE AND COMPOSITION OF THE SOUTHERN FOOTHILLS METAMORPHIC BELT, SIERRA-NEVADA, CALIFORNIA, FROM SEISMIC DATA

The Foothills Metamorphic Belt is an accreted terrane consisting of Pa leozoic and Mesozoic metamorphic rocks that separates the Great Valley from the Sierra Nevada batholith in northern and central California. Until recently, the only available geophysical data for this area were reconnaissance refraction surveys, and gravity and magnetic data. New insights into the structure of the deep crust are provided by the int erpretation of a seismic reflection profile (CC-2), acquired in 1984 b y the U. S. Geological Survey at the southern end of the Foothills Met amorphic Belt. Our interpretation is constrained by a new seismic velo city model derived from coincident microearthquake data. Earthquake hy pocenters that occur at unusually great depths of 12 to 30 km make the data set particularly useful for obtaining deep crustal velocity info rmation. The velocity model shows velocities of 5.2 to 6.3 km s-1 for the upper 12 km of the crust, and 6.7 to 6.8 km s-1 from 12 km to an e stimated Moho at 32 km. The upper crustal velocities correspond to met amorphic rocks and serpentinites of the Foothills Metamorphic Belt as well as to diorites and granodiorites of the Sierra Nevada batholith, while the lower crustal velocities are interpreted to represent interm ediate to mafic granulites. The majority of the earthquake hypocenters as well as a 6.7 km s-1 layer in the velocity model corresponds in de pth to thick zones of west dipping midcrustal reflections that may rep resent major shear zones formed during the late Jurassic Nevadan oroge ny or synbatholithic ductile shear zones that accommodated crustal ext ension associated with batholith intrusion. These reflections are trun cated updip by an inferred subvertical contact that coincides with the western edge of the Sierra Nevada batholith and the southward trace o f the Bear Mountains fault zone. The updip truncation of midcrustal sh ear zones and high lower crustal velocities indicate that strike-slip faulting and magmatic underplating can be important processes during t he docking and welding of an accreted terrane.