Franciscan Complex

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Street incision with heavily folded cherts rich in radiolaria . Glen Canyon Park , San Francisco

The Franciscan Complex (also Franciscan Formation , Franciscan Group , Franciscan Assemblage , Franciscan Series ) is a rock unit built up of very different types of rock , more than a thousand kilometers long on the west coast of North America from Oregon in the north to southern California , which also includes large parts the San Francisco Peninsula is building. The Terrane owes its name to the geologist Andrew Cowper Lawson , who also named the San Andreas Fault that runs through the complex.

Rock content and tectonics

The Franciscan Complex is made up of very diverse rocks. Represented are mafic volcanic rocks ("greenstones"), deep-sea chert , gray-wacky sandstones , limestones , serpentinites , clay slate and metamorphic rocks , all of which are disturbed , flaky and mixed in a chaotic manner ( tectonic mélange ).

The complex is the main component of the California Coast Mountains . The remarkable tectonic folding and scaling of the stacked layers are a reflection of the plate tectonic forces that created the coastal mountains of the American West. The association of the complex with the ophiolites of the Coast Range and the rocks of the Great Valley emerged as an accretion wedge through the subduction of the Pacific, predominantly oceanic crust under North America and its involvement in the lateral displacement along the San Andreas Fault and the accompanying faults.

Prehistoric tools made from Franciscan flint

Archaeological finds of prehistoric Indian stone tools made from Franciscan flint in the western USA prove that the material was traded over long distances and that a trading network between the various tribes already existed at this time. An example of this are corresponding finds in central California, which prove a trade between the tribes in the San Francisco Bay Area with the Chumash .

Individual evidence

  1. ^ Gerhard H. Eisbacher: North America . In: Geology of the Earth . 1st edition. tape 2 . Ferdinand Enke Verlag, Stuttgart 1986, ISBN 3-432-96901-5 .
  2. ^ W. Porter Irwin: Understanding the "Franciscan Complex". ( September 23, 2006 memento on the Internet Archive ) United States Geological Survey
  3. ↑ CM Hogan: Morro Creek edited by A. Burnham, 2008.

literature

  • CM Wentworth, MC Jr. Blake, DL Jones, AW Walter, MD Zoback : Tectonic wedging associated with emplacement of the Franciscan assemblage, California Coast Ranges. In: MC Blake (Ed.): Franciscan geology of northern California. Pacific Section, Society of Economic Paleontologists and Mineralogists, Field Trip Guidebook 43, 1984, pp. 163-173.

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