MAGNETOTELLURIC EVIDENCE FOR A BRITTLE-DUCTILE TRANSITION, PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIA

A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60-90 lan. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11-12 km is sparse, others have concluded that the brittle-ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle-ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton Trough.