SALINIA: A CRUSTAL CROSS SECTION THROUGH A SHALLOW SUBDUCTION ZONE EXPOSING THE SUBDUCTION MEGATHRUST

Shallow subduction and subduction erosion/tectonic underplating represent important processes at convergent margins, yet their products are rarely documented in geologic record. The Salinian block in central California is a prime example of an exposed cross section in an environment characterized by shallow subduction; moreover, the paleo-subduction megathrust is exposed as a shear zone. The Salinas shear zone (SSZ) is a Latest Cretaceous ductile thrust fault in central California – the upper plate consists of continental arc rocks and the lower plate represents a sequence of tectonically underplated accretionary wedge meta-sediments (the schist of the Sierra de Salinas). The juxtaposition of the two rock types that were originally some 150 km apart took place in the latest Cretaceous, in the early stage of the Laramide orogeny. Extensional collapse of the upper plate resulted in >30 km of exhumation in the latest Cretaceous, bringing the deep arc rocks of the SSZ and the schist to the surface. Much of the upper plate of the shear zone is exposed at paleo-depths of 30-35 km and has been metamorphosed in the amphibolite facies during arc magmatism. Shear heating had an important role and led to the development of localized granulite facies metamorphism in the SSZ. Granulite facies rocks require heating, dewatering and partial melting of pre-existing rocks. The shear stress along the shear zone that can be determined from excess heat is about 80-100 MPa, consistent with the shear stress drops required by mega-earthquakes along subduction planes. If the fault surface or shear zone would act as a sealant for fluids such as water, granulite facies metamorphism would not develop at 30-35 km deep.