The
Franciscan Formation (the type section of which is located in the
city of San Francisco) is a unique complex of diverse igneous, sedimentary,
and metamorphic rock types that formed between 200-80 million years
ago. (Click to zoom in on figure at left.) The unit is widely distributed
throughout the California Coast Ranges, both north and south of San
Francisco, and is generally found east of the San Andreas fault. Its
history pre-dates the evolution of the San Andreas transform fault
system and indicates that a subduction zone existed along the coast
of California during the Mesozoic Era.
An
understanding of Franciscan Complex rock types can be gained by considering
both the rock and plate tectonic cycles where processes such as melting,
crystallization, weathering, erosion, and burial lithify rocks. (Click
to zoom in on the rock cycle at right.) These rock-forming processes
produce materials with distinctive compositions and textures that
provide clues to the environments and conditions of formation that
and are often most dynamically illustrated at plate boundaries.
