REGIONAL EXTENT OF GREAT-VALLEY BASEMENT WEST OF THE GREAT-VALLEY, CALIFORNIA - IMPLICATIONS FOR EXTENSIVE TECTONIC WEDGING IN THE CALIFORNIA COAST RANGES

Interpretation and modeling of the magnetic field of central Californi a indicate that the magnetic basement of the forearc deposits of the G reat Valley sequence extends westward beneath the coeval subduction-re lated rocks of the Franciscan Complex. The basement surface slopes gen tly to the west, reaching midcrustal depths (15-19 km) at distances of 50-100 km west of the Great Valley. This magnetic basement is disrupt ed by the Hayward-Rodgers Creek Fault system and is cut by the San And reas Fault at the south end of the Great Valley and possibly throughou t much of central California. The widespread presence of the Great Val ley basement beneath rocks of the Franciscan Complex implies that the basement is more extensive than proposed in earlier interpretations ba sed on seismic studies near the Franciscan Complex-Great Valley sequen ce contact. This result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones. The easter n boundary fault of the Franciscan Complex (Coast Range Fault) is not (and never was) a subduction zone thrust fault but rather was original ly a roof thrust (wedge-roof fault) formed above the eastward wedging mass of Franciscan Complex intruded along the top of the basement bene ath the Great Valley deposits. This tectonic interpretation offers a s olution for the question of how high-pressure metamorphic rocks of the Franciscan Complex were juxtaposed at the Coast Range Fault against l ow-pressure metamorphic rocks of the Great Valley sequence. This inter pretation also implies an older flat-lying thrust fault (wedge-floor f ault) that forms the top of magnetic basement between the active San A ndreas and Hayward Faults at depths of 15-17 km. This older thrust fau lt may today transfer strain between the two young strike-slip faults, possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults. The former east dipping subduction z one along which the rocks of the Franciscan Complex accumulated must l ie west of the western limit of the Great Valley magnetic basement.