FOREARC STRIKE-SLIP DISPLACEMENT AS AN ALTERNATIVE TO SUBDUCTION EROSION, WITH A CALIFORNIA EXAMPLE (NACIMIENTO FAULT)

The enigmatic Nacimiento fault of the central Coast Ranges has been variably interpreted as a dextral, sinistral or reverse fault. The presently dominant intrepretation is that the Nacimiento fault is a reverse fault that accommodated subduction erosion, by which the central to eastern part of the Cretaceous California batholith (e.g., Sierra Nevada – Peninsular Ranges) was thrust over the western part of the batholith and the entire forearc basin, resulting in juxtaposition of batholithic Salinia against the accretionary Franciscan Complex. This juxtaposition occurred concurrently with Laramide flat-slab subduction and underplating of the Pelona-Orocopia-Rand schist (POR). Reviews divide global subduction zones into two types: accretionary and erosional. None of these plate margins contains an example of overthrusting within the forearc area of the magnitude suggested by the Nacimiento reverse-fault model. The closest modern analog to the likely configuration of the Salinian continental margin near the end of Laramide deformation might be southern Mexico, where arc plutons are exposed at the forearc coast (e.g., Acapulco). Although this margin is interpreted as experiencing subduction erosion, the primary reason that parts of the plutonic arc and forearc are “missing” is because they have been displaced to the southeast (as the Chortis block) by sinistral slip along the Polochic-Motagua fault system. Thus, the southern Mexico forearc might be an excellent modern analog for the Nacimiento forearc following sinistral slip. This model predicts that, as a trench-trench-transform triple junction migrated southeastward along the continental margin, a new forearc margin was exposed sequentially from northwest to southeast during flat-slab subduction induced by intersection of the continental margin with an aseismic oceanic ridge. This scenario is consistent with northwest-to-southeast younging of POR and initial forearc sediments deposited on the newly exposed plutonic crust.