MESOZOIC-CENOZOIC ACCRETIONARY EXPANSION OF CALIFORNIA

Following Permian-Triassic truncation of the continental margin by strike slip along the sinistral California-Coahuila transform, accretion of multiple tectonic belts between mid-Triassic and mid-Cenozoic time expanded the California continental margin by 125-250 km. Ongoing accretion was arrested by Miocene development of the dextral San Andreas transform, which has offset the accretionary belts, but the structural underpinnings of the western Klamath Mountains, Sierra Nevada foothills, Great Valley and the Coast Ranges exclusive of the Salinian block were put in place by tectonic accretion associated with continuing subduction along the flank of the Cordilleran magmatic arc. Accreted tectonic elements include disrupted terranes composed of mélange and broken formation which did not exist as discrete geologic entities until assembled along the continental margin at subduction zones, but also intraoceanic island arcs and interarc basins separated from more interior pre-mid-Triassic tectonic elements by mélange belts that are continuous longitudinally for the length of California. Unlike the Franciscan assemblage on the west, older accretionary belts in the Klamath Mountains and the Sierra Nevada have been intruded by post-accretion arc plutons, but this later history should not blind us to their fundamental character as mélange belts and island arcs of oceanic origin. The sediment fill of evolving forearc basins and younger successor basins mask the structural architecture of basement over large areas, but enough segments of the underlying substratum emerge from beneath the sedimentary blanket to allow reconstruction of successive accretionary belts. The sources at depth for Mesozoic batholithic magmas remain uncertain in detail, but significant mantle components represent an additional dimension of crustal accretion in California. Accretionary expansion of oceanward flanks of arc orogens and mantle contributions to arc magmatism are characteristic as subduction proceeds.