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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 Monterey Formation (deposited 20 to 9 million years ago--within the Miocene Epoch of the Tertiary Period). Being in the rain shadow of the central Coastal and 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. The San Andreas Fault extends from the Salton Sea in southern California northward for nearly 800 miles were it extends out to sea at Cape Mendicino. The fault system began to develop nearly 30 million years ago when orientation and relative motion of boundaries between the North American and Pacific crustal plates formed a great strike-slip fault system. Geologic evidence suggests that rocks have moved hundreds of miles northward along the west side of the San Andreas Fault relative to the eastern side. The San Andreas Fault in the Carrizo Plain has the largest accumulated post-early Miocene offset and is the oldest reach of the entire active fault system--196 miles (315 km) in roughly the past 15 million years (Page, 1990). |
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