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Regional Geology of North America

West Coast Borderlands

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The West Coast (Washington, Oregon, and California) is an active continental margin with all the associated geologic features: mountains, rugged coastlines, volcanoes, and earthquakes. Collectively the mountains in the region are called part of the greater Western Cordilleran, but the region is subdivided into several provinces based on both geography and geology. The regional geology is directly related to the plate tectonics of the region (Figure 228). In the north, the Juan de Fuca Plate is actively being subducted beneath northern California, Oregon, Washington, and British Columbia. This subduction is resulting in the formation of the series of volcanoes that make up of the Cascade Ranges that run roughly parallel to the Pacific coast, but about 70 to 120 miles inland from the coastline. Between the Cascades and the Pacific shoreline are the Pacific Borderlands and Coast Ranges.

The San Andreas Fault System is the major feature controlling the geology and geography of California. North of where the San Andreas Fault runs out to sea at Cape Mendicino, subduction is still actively consuming the Juan de Fuca Plate along the Cascadia Subduction Zone. South of Cape Mendicino part of the North American Plate has split away and is now attached to the Pacific Plate and as being carried northward on the west side of the San Andreas Fault. The rate and amount of movement movement can be seen how the Baja Peninsula has been rifted away from Mexico. The rifting open the Gulf of California basin starting about 23 million years ago as the North American Plate gradually moved westward over a spreading center between the ancient Farallon Plate and the Pacific Plate. The change of the relative plate motions of the 3 plates resulted in the formation of the San Andreas Fault system as the Farallon Plate vanished beneath North America. The Juan de Fuca and Cocos Plates are remnants of the once large Farallon Plate.
Major plate tectonic features of the West Coast of the Western United States and Mexico.
Figure 228. Major plate tectonic features of the West Coast of the Western United States and Mexico.     1/20/2017
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