TEMPORAL-SPATIAL FRAMEWORK OF THE FRANCISCAN COMPLEX, CALIFORNIA BEARING ON CONTINUITY OF SUBDUCTION, AND ACCOMMODATION OF INTERNAL STRIKE-SLIP FAULTING

The Franciscan subduction complex of California records sporadic accretion along the subduction interface during continuous eastward subduction of the Farallon plate from ca. 176 to 12 Ma, from subduction initiation to subduction termination by ridge subduction and subduction-transform transition. Subduction continues north of the Mendocino triple junction. The continuity of subduction, including during periods of non-accretion/subduction erosion is attested by the lack of structural-thermal overprinting of subduction zone metamorphism. This contrasts with >15 km depths of the present transform regime wherein subduction-transform transition and slab window formation generated high-temperature (low to medium pressure) metamorphism, magmatism (including many intrusive bodies), and strike-parallel stretching lineations associated with the metamorphic overprint. Franciscan structural architecture is characterized by folded low-angle sheets of rocks accreted at different times. Accretion ages young progressively structurally downward, consistent with progressive but not continuous, accretion inconsistent with syn-subduction strike-slip faulting within the subduction complex. Nearly all of the strike-slip faulting recorded by Franciscan rocks is associated with post-subduction dextral faults of the San Andreas fault system. About 200 km of pre-transform dextral slip may be accommodated along the eastern contact of the granitic Salinian Block and the Franciscan, but this has also been proposed as a west-directed detachment emplaced as a result of extension of the southern Sierra Nevada/northern Mojave region. Shortening of Franciscan units accounts for only about 2000 km of the 13000 km of subduction expected from plate motions suggesting that most subduction was accommodated in a non-accreting mode and/or that subduction erosion has removed part of the accreted record. Far-traveled coherent oceanic crustal material is of mid-ocean ridge (MORB) and ocean island basalt (OIB, ie off axis) affinity. There are no arc or continental slices within the Franciscan accreted units. Geochemical character and detrital zircon age populations for Franciscan clastic rocks are the same as for coeval forearc basin deposits of the Great Valley Group; inboard North American sources are indicated.