Interpretive geophysical cross section for central California. (a) Published magnetic and seismic interpretations along section location indicated in Figure 1 (San Francisco Bay) based on the interpretations of Fuis and Mooney (1990) and Ponce et al. (2003a,[b], 2008). The region labeled ‘ophiolite’ represents accreted oceanic lithosphere (by our interpretation, partially serpentinized mantle with or without altered crust) that is, according to this paper, now dehydrating. The MTJ passed through this area at approximately 10 Ma BP. HF, Hayward Fault; MD, Mount Diablo; SAF, San Andreas Fault; CRO, Coast Range ophiolite; Kjf, Franciscan Complex; GVS, Great Valley sequence; Tsd, Tertiary sedimentary rocks. The top of a possible stalled Farallon slab is shown as a dashed line, as suggested by the seismic evidence in Figure 5. (b) Magnetic profile along cross section in Figure 2. Sharp magnetic peaks generally correspond to outcrops of largely serpentinite-bearing bodies and shallow subsurface distributions of such rocks, often along faults in the San Andreas Fault System. The main trend of the magnetic field has been modeled according to the structure shown in (a). The distribution of serpentinites in the shallow crust may be associated with vertical mobilization of serpentine from serpentinized former forearc mantle by release of water by a mechanism suggested in this paper.