PARTIAL MELTING, COUNTERCLOCKWISE P-T PATH, AND RAPID EXHUMATION ABOVE AN ANCIENT FLAT SLAB: INSIGHTS FROM THE SAN EMIGDIO SCHIST, SOUTHERN CALIFORNIA

The San Emigdio and related Pelona, Orocopia, Rand, and Sierra de Salinas schists of Southern California were underplated beneath the southern Sierra Nevada batholith and adjacent southern California batholith along a shallow segment of the subducting Farallon plate in Late Cretaceous – early Tertiary time. These subduction accretion assemblages represent a regional, deeply exhumed, shallowly dipping domain from an ancient slab segmentation system and record the complete life cycle of the segmentation process from initial flattening and compression to final extensional collapse. An important unresolved question regarding shallow subduction zones concerns how the thermal structure evolves during the slab flattening process. We present new field relations, thermobarometry, thermodynamic modelling, and garnet diffusion modelling that speak to this issue and elucidate the tectonics of underplating and exhumation of the San Emigdio Schist. We document an upsection increase in peak temperature (i.e., inverted metamorphism), from 590 to 700 ˚C, peak pressures ranging from 8 to 11 kb, limited partial melting, microstructural evidence for large seismic events, rapid cooling (825 to 380 ˚C / Myr) from peak conditions, and a counterclockwise to “out and back” P-T path. While inverted metamorphism is a characteristic feature of southern California schists, the presence of partial melt and high temperatures (> 650 ˚C) are restricted to older exposures. Progressive cooling and tectonic underplating beneath an initially hot upper plate following the onset of shallow subduction provides a working hypothesis explaining high temperatures and partial melting in San Emigdio and Sierra de Salinas schists, lower temperatures in comparatively young Pelona, Orocopia, and Rand schists, inverted metamorphism in the schist as a whole, and the observed P-T trajectory calculated from the San Emigdio body. These results are consistent with an inferred tectonic evolution from shallow subduction beneath the then recently active Late Cretaceous arc to exhumation by rapid trench-directed channelized extrusion in the subducted schist.