Why are there no faults in the Great Valley of central California?

The Great Valley is a basin, initially forming some ~100 million years ago as a low area between the subducting ocean plate on the west (diving down under the North American plate) and the volcanoes to the east (now the Sierra Nevada mountains). Since its formation, the Great Valley has continued to be low in elevation. Starting about 15 million years ago the tectonics changed in California and instead of the ocean plate diving down under the North American plate, it began to slide along it, with the ocean plate moving northward. This movement occurs along the San Andreas fault and the many other faults that are roughly parallel to it.

The faults on the east side of the Great Valley, mostly in Nevada, are the result of the North American plate pulling apart there, in a different tectonic setting that results in the linear mountain ranges and long valleys you can see there. The faults just to the east of the Great Valley are mostly old faults and may or may not still be active today. So there is movement of faults in two separate regimes: sideways motion along the San Andreas system to the west-southwest, and pull apart motion along the faults mostly in Nevada to the east-northeast of Sacramento.

(contributed by Heidi Stenner)

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