High temperatures and inverted metamorphism in the schist of Sierra de Salinas, California

New field and thermobarometric work in the Californian Salinian block clarifies current and pre-Tertiary relationships between the schist of Sierra de Salinas and Cretaceous arc-related granitic rocks. The contact is variably preserved as a brittle fault and high-temperature mylonite zone, the Salinas shear zone, which represents the contact between North America and sediments accreted above the Farallon slab between similar to 76 Ma and similar to 70 Ma. Near granulite facies, prograde replacement of homblende with clinopyroxene is associated with deformation of plutonic rocks at the base of the upper plate. In the lower plate, the schist of Sierra de Salinas, garnet-biotite thermometry indicates decreasing temperatures down-section from at least 714 degrees C to -575 degrees C over an exposed thickness of similar to 2.5 km, consistent with petrologic evidence of an inverted metamorphic gradient. The measured temperatures are significantly higher than observed at shallow levels above subducting slabs or predicted by 2D computational models assuming low shear stresses. Previous workers have called upon shear heating to explain similar observations in the correlative Pelona schist, an unlikely scenario given the results of recent rock deformation experiments which predict that feldsparquartz-mica aggregates are far too weak to withstand stresses of similar to 70 MPa required by the shear heating hypothesis. As an alternative, we propose that high temperatures resulted from conductive heating while the leading edge of the schist traveled similar to 150 km beneath the recently active Salinian continental arc during the initiation of shallow subduction. Weakening of the schist due to high temperatures helped facilitate the collapse of the Salinian arc as the schist was emplaced. Schist emplacement coincided with loss of lower, mafic portions of the arc, and therefore evolution of the Southem California crust towards a more felsic composition. (c) 2005 Elsevier B.V. All rights reserved.