San Andreas Fault in Tejon Pass

November 21, 2005

Tejon_pass_fault copy

Provided and copyright by: David Lynch
Summary authors & editors: David Lynch

The San Andreas Fault marks the boundary between the North American and Pacific Plates in California. These two tectonic plates are sliding horizontally past each other along the transform fault. Rock on the Pacific Plate is being carried northwest and juxtaposed against different rock at the edge of the North American Plate. Normally, the interface is buried by Quaternary sediments or overgrown by vegetation. In Tejon Pass near the town of Gorman, however, the contact between the two plates is clearly evident. The fault runs up a hillside and separates rocks of different composition and color. To the southwest is grey, metamorphic quartz monzonite and to the northeast across the fault is rich brown sedimentary sandstone and siltstone. These two rocks could not have formed together and therefore must have been brought together by fault motion. Between the gray and tan is a black layer of fault gouge, rock that has been pulverized and cooked by frictional heating generated by earthquakes. The entire hillside is granulated and appears gouged due to the constant grinding of the plates. A diagonal color discontinuity defines the surface rupture of the 1857 earthquake, which moved the rocks on one side of the fault about 30 feet (9 m) relative to other. This magnitude 8.0 temblor was the largest earthquake in California’s recorded history.

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