VELOCITIES OF SOUTHERN BASIN AND RANGE XENOLITHS - INSIGHTS ON THE NATURE OF LOWER CRUSTAL REFLECTIVITY AND COMPOSITION

To reconcile differences between the assessments of crustal compositio n in the southern Basin and Range province on the basis of seismic ref raction and reflection data and lower-crustal xenoliths, we measured v elocities of xenoliths from the Cima volcanic field in southern Califo rnia. Lower-crustal samples studied included gabbro, microgabbro, and pyroxenite. We find that the mafic xenolith velocities are compatible with regional in situ measurements from seismic refraction studies, pr ovided that a mixture of gabbro and pyroxenite is present in the lower crust. Supporting this model are observations that many of the lower- crustal xenoliths from the Cima volcanic field are composites of these rock types, with igneous contacts. Vertical incidence synthetic seism ograms show that a gabbroic lower crust with occasional pyroxenite lay ering can produce a reflective lower crust that is similar in texture to that shown by seismic reflection data recorded nearby.