MIDDLE MIOCENE HIGH-MAGNESIUM ANDESITE AND HIGH-SILICA ADAKITE IN THE JACUMBA VOLCANICS AND ALVERSON FORMATION OF SAN DIEGO AND IMPERIAL COUNTIES, CALIFORNIA

The central part of the Baja California Peninsula is a classic locality where distinctive post-subduction volcanic geochemistry was recognized nearly 30 years ago in the form of Late Miocene to Quaternary high magnesium andesite (bajaite) volcanic fields, thought to derive from partial melting of subducted oceanic crust. The distribution of central Baja California high-magnesium andesite correlates tightly with the lateral extent of fossil Farallon plate slab, as imaged by seismic tomography, along a segment of the margin where subduction ceased before the Pacific-Farallon spreading center reached the trench. Here we report discovery of Middle Miocene high-magnesium andesite and high-silica adakite within the Jacumba Volcanics and Alverson Formation of southern California, 350 km to the north of the nearest Baja California examples, within a presumed slab-free segment of the margin. The Jacumba Volcanics and Alverson Formation volcanic fields both record an evolution from initial primitive high-magnesium andesite eruptions to subsequent high-silica adakite. Recognition of these volcanic rock types in southern California adds important new insight into interpretation of post-subduction magmatism and mantle dynamics along the margin.