Interpretation and modeling of the magnetic field of central California indicate that the magnetic basement of the forearc deposits of the Great Valley sequence extends westward beneath the coeval subduction-related rocks of the Franciscan Complex. The basement surface slopes gently to the west, reaching midcrustal depths (15--19 km) at distances of 50--100 km west of the Great Valley. This magnetic basement is disrupted by the Hayward-Rodgers Creek Fault system and is cut by the San Andreas Fault at the south end of the Great Valley and possibly throughout much of central California. The widespread presence of the Great Valley basement beneath rocks of the Franciscan Complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the Franciscan Complex-Great Valley sequence contact. This result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones. The eastern boundary fault of the Franciscan Complex (Coast Range Fault) is not (and never was) a subduction zone thrust fault but rather was originally a roof thrust (wedge-roof fault) formed above the eastward wedging mass of Franciscan Complex intruded along the top of the basement beneath the Great Valley deposits. This tectonic interpretation offers a solution for the question of how high-pressure metamorphic rocks of the Franciscan Complex were juxtaposed at the Coast Range Fault against low-pressure metamorphic rocks of the Great Valley sequence. This interpretation also implies an older flat-lying thrust fault (wedge-floor fault) that forms the top of magnetic basement between the active San Andreas and Hayward Faults at depths of 15--17 km. This older thrust fault 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 zone along which the rocks of the Franciscan Complex accumulated must lie west of the western limit of the Great Valley magnetic basement.