Franciscan Complex

Late Mesozoic terrane of heterogeneous rocks in the California Coast Ranges

The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew Lawson, who also named the San Andreas fault that defines the western extent of the assemblage. The Franciscan Complex is dominated by greywacke sandstones, shales and conglomerates which have experienced low grade metamorphism. Other important lithologies include chert, basalt, limestone, serpentinite, and high pressure, low temperature metabasites (blueschists and eclogites) and meta limestones. Fossils like radiolaria are found in chert beds of the Franciscan Complex. These fossils have been used to provide age constraints on the different terranes that constitute the Franciscan. The mining opportunities within the Franciscan are restricted to deposits of cinnabar and limestone. The outcrops of the formation have a very large range, extending from Douglas County, Oregon to Santa Barbara County, California. Franciscan like formations may be as far south as Santa Catalina Island. The formation lends its name to the term describing high pressure regional metamorphic facies, the Franciscan facies series.

The Franciscan Complex is an assemblage of metamorphosed and deformed rocks, associated with east dipping subduction zone at the western coast of North America. Although most of the Franciscan is Early/Late Jurassic through Cretaceous in age (150 66 Ma), some Franciscan rocks are as old as early Jurassic (180 190 Ma) age and as young as Miocene (15 Ma). The different age distribution represents the temporal and spatial variation of mechanisms that operated within the subduction zone. Franciscan rocks are thought to have formed prior to the creation of the San Andreas Fault when an ancient deep sea trench existed along the California continental margin. This trench, the remnants of which are still active in the Cascadia and Cocos subduction zone, resulted from subduction of oceanic crust of the Farallon tectonic plate beneath continental crust of the North American Plate. As oceanic crust descended beneath the continent, ocean floor basalt and sediments were subducted and then tectonically underplated to the upper plate. This resulted in widespread deformation with the generation of thrust faults and folding, and caused high pressure low temperature regional metamorphism.