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 composition in the southern Basin and Range province on the basis of seismic refraction and reflection data and lower-crustal xenoliths, we measured velocities of xenoliths from the Cima volcanic field in southern California. 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, provided 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 seismograms show that a gabbroic lower crust with occasional pyroxenite layering can produce a reflective lower crust that is similar in texture to that shown by seismic reflection data recorded nearby.