STOKING THE LOWER CRUST OF CONTINENTAL ARCS WITH SUBDUCTION ACCRETION ASSEMBLAGES: AN EXAMPLE FROM SOUTHERN CALIFORNIA

Continental arc magmatism is widely regarded to reflect melting of mantle wedge material by addition of fluids produced through metamorphism of trench sediments and oceanic crust in “normally” dipping subduction zones. A corollary to this concept is that shallow subduction and associated mantle wedge removal provides an explanation for the termination and/or migration of arc magmatism. However, during the slab flattening process, subduction accretion assemblages may be transported into the magmatic source region of an active arc. Subsequent partial melting of fertile underplated material may trigger a brief episode of high-flux magmatism prior to arc shutoff.

The southern California schist (locally referred to as the Pelona-Orocopia-Rand and related San Emigdio and Sierra de Salinas schists) tectonically underplated the lower crust of the Sierra Nevada magmatic arc during a Late Cretaceous episode of shallow subduction. Here I summarize recent isotopic and geochronologic work on the southern California schist and overlying plutonic rocks and suggest that as the Cretaceous slab flattened, the schist was emplaced into the root zone of the Sierran arc, where ensuing partial melting led to an order of magnitude increase in upper plate magmatic flux. I hypothesize, based on this southern California example, that subduction accretion assemblages may be conveyed into continental arc source regions during slab shallowing, leading to brief (<10 m.y.) episodes of high-flux magmatism.

Further evaluation of this hypothesis can come from geochronologic, geochemical, and isotopic analysis of upper-plate and lower-plate rocks in exhumed shallow subduction systems (e.g., the Condrey Mountain schist, northern California; the Coolwater culmination, Idaho; the North Cascades crystalline core, Washington; the Central Gneiss Complex, British Columbia; and the Qiangtang terrane, Tibet) or from geochemical and/or geophysical investigation of modern analogs (e.g., shallow subduction and associated magmatism in central Mexico).