Model for sinistral offset of the Cretaceous magmatic arc of southwestern North America along the Nacimiento fault system

Finite-element paleogeologic restoration of southwestern North America at 60 Ma shows the western edge of Cretaceous granodiorite plutons, formed as a continuous magmatic arc above the Franciscan subduction zone, to be offset ~400 km in southern California, USA. This offset is successfully restored by reversing sinistral slip along a proposed Nacimiento–Caborca–Durango–Zacatecas (NCDZ) fault trace active approximately from 75 Ma to 60 Ma. The Franciscan trench west of the finite-element model can be restored by extending the NCDZ fault northward through coastal Franciscan terrane, where it formed a trench– trench–fault triple junction. As the trench–trench–fault triple junction migrated southeastward along the continental margin, a new forearc margin was exposed sequentially from NW to SE, beneath which the Pelona–Orocopia–Rand Schist was underplated. NCDZ fault displacement occurred concurrently with Laramide flat-slab subduction and intersection of the continental margin with an aseismic oceanic ridge. The trench–trench–fault triple junction would have tracked the intersection point of the NE–SW ridge with the NW–SE continental margin. Tests of this model (following palinspastic restoration) include: (1) The oldest ages of Pelona–Orocopia–Rand Schist should young NW–SE, (2) the initiation of forearc deep-marine sedimentation directly on Mesozoic magmatic-arc basement of Salinia should young NW–SE, (3) the detrital-zircon age spectra and other provenance indicators should correlate with potential source areas, (4) the metamorphic and plutonic belts of the batholithic belt should realign, (5) the Great Valley forearc belts should realign, and (6) the Franciscan belts should realign. Finite-element modeling provides rigorous constraints on paleogeologic and paleogeographic reconstructions and suggests additional avenues of investigation.