SEISMIC EVIDENCE FOR A LOWER-CRUSTAL DETACHMENT BENEATH SAN-FRANCISCOBAY, CALIFORNIA

Results from the San Francisco Bay area seismic imaging experiment (BA SIX) reveal the presence of a prominent lower crustal reflector at a d epth of similar to 15 kilometers beneath San Francisco and San Pablo b ays. Velocity analyses indicate that this reflector marks the base of Franciscan assemblage rocks and the top of a mafic lower crust. Becaus e this compositional contrast would imply a strong theological contras t, this interface may correspond to a lower crustal detachment surface . If so, it may represent a subhorizontal segment of the North America and Pacific plate boundary proposed by earlier thermomechanical and g eological models.