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Geology of National Parks, 3D and Photographic Tours

Wallace Creek

The Carrizo Plain is perhaps best known for its spectacular preservation of landforms associated with the San Andreas Fault. This view is looking east along Wallace Creek toward the Temblor Range. The location of the photograph is situated nearly directly on the active trace of the San Andreas Fault. Bends in the stream uphill from this location are probably the location of additional faults associated with the San Andreas Fault zone. Outcrops in the distance are part of the Miocene Monterey Formation (Dibblee, 1972 and 1973).

Being in the rain shadow of the central Coastal and Western Transverse Ranges, the overall low amount of precipitation results in slow erosion rates relative to the amount of tectonic motion along the San Andreas Fault. As a result, the surficial features associated with the fault are relatively "fresh" in appearance. Features include offset, beheaded, and abandoned stream channels, fault scarps, sag ponds, linear ridges, and shutter ridges that display recent strike-slip motion (click here for an illustration of these features). Major earthquakes produced much of this offset motion as demonstrated by the last major earthquake in the region, the magnitude 8.2 Fort Tejón earthquake of 1857. As much as 22 feet (7 meters) of right lateral- displacement occurred in the Carrizo Plain area during that great earthquake (Grant and Sieh, 1993).

The San Andreas Fault extends from the Salton Sea in southern California northward for nearly 800 miles where it extends out to sea at Cape Mendocino. The fault system began to develop nearly 30 million years ago along the developing boundary between the North American and Pacific crustal plates. These two plates have been moving approximately parallel to one another, in opposite directions. Geologic evidence suggests that rocks have moved hundreds of miles northward along the west side of the San Andreas Fault system relative to the eastern side. For example, motion along the proto-San Andreas split apart an old volcano that formed about 24 million years ago (Early Miocene). The remains of the western half of the volcano, Pinnacles Volcanics (at Pinnacles National Monument), moved 196 miles (315 km) northward; the eastern half of the volcano, the Neenach Volcanics, remained behind on the eastern side of the proto-San Andreas Fault (Sims, 1993). The Carrizo Plain lies about halfway between the two.

 

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