ContentsPutah and Cache: Clear Lake or Lypoyomi

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Geology of Putah-Cache: The Franciscan Complex

Eldridge M. Moores and Judith E. Moores

The Franciscan Complex is especially visible along the northern shore of Clear Lake. A diverse collection of igneous, sedimentary and metamorphic rocks, the Franciscan Complex was produced in a subduction zone. Sediments, submarine volcanoes, and oceanic crust were scraped from the down-going plate and attached to the North American plate. Some of this material was thrust beneath the continent far enough so that it began to melt to produce the magmas that gave rise to the granitic rocks of the Sierra Nevada. Much of it, however, was left along the margin of the continent and now forms the basement rocks beneath San Francisco, the Berkeley-Oakland Hills, Marin, Sonoma, and Napa County, and the western margin of the Putah-Cache bioregion. These tectonic blocks are large rock fragments, often several meters to hundreds of meters long, that have been mixed together by movements within the Earth's crust along a subduction plate boundary. Most of these rocks were highly squeezed or smeared out along the plate boundary, so that they appear complex and messy in outcrops or road cuts. Many of these rocks are highly folded; others show many irregular fractures, along which the rocks break apart. Some rocks show "chunks" or "knobs": hard rock mixed in a softer surrounding matrix of sheared shale or serpentine. Such a diverse mixture of rocks, called a "mélange" by geologists (from the French for "mixture"), is characteristic of rocks that have accumulated along convergent plate margin boundaries throughout the world. Some of the Franciscan tectonic blocks, as well as some of the matrix rocks, exhibit the results of metamorphism (recrystallization) at about 20-30 km depth and 200-300 degrees C. Geologists call these rocks high pressure/low temperature metamorphic rocks because they reflect colder-than-average temperatures at a given depth in the Earth. These rocks are often called "blueschists" because their minerals are often roughly aligned and some of the minerals are blue. Blueschists are present in the Franciscan along the western part of the Putah-Cache bioregion.

Most Franciscan rocks formed from 65-160 Ma, during the Jurassic and Cretaceous period of the Mesozoic era. Younger (Cenozoic) Franciscan rocks are present along the northern California coast, and Franciscan-like rocks are currently forming offshore or beneath the continent north of Cape Mendocino, where the oceanic Juan de Fuca plate is still subducting beneath North America.

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