Much of western North America is composed of land segments that were welded to the primary craton as the Atlantic opened and the Farallon oceanic plate was forced to subduct beneath the Continent as it advanced westward; these segments arrived as volcanic island arcs, created by the subduction itself, or as exotic terranes that were carried in by the oceanic plate. Remnants of the Farallon Plate are still subducting in the Pacific Northwest and along the west coast of Central and South America but the North American and Pacific Plates are now in contact across Southern California; in fact, this segment of the State, from San Francisco to San Diego arrived as an exotic terrane atop the Pacific Plate late in the Tertiary Period. Other than Hawaii, this is the only segment of the U.S. that does not lie on the North American Plate.
The east boundary of this Salinas Terrane (and thus of the Pacific Plate) is the famous San Andreas Fault. About five million years ago, as the East Pacific Rise developed, rifting the Baja Peninsula from Mexico, the Pacific Plate was forced to inch northwestward along the west margin of the North American Plate. Where the fault curves westward, north of the Los Angeles Basin, increasing pressure within the Salinas Terrane forced up the Transverse Ranges and this area of resistance remains a potent source of major earthquakes. Nevertheless, the Plate motion continues, averaging 35 mm per year but occurring as intermittent, quake inducing slips.
Over time, the Salinas Terrane will break free of its contact with North America and ride to the northwest atop the Pacific Plate. Destined to be an island for 20 million years or more, it will eventually merge with the Aleutian Chain of Alaska as its transporting plate dives into the Aleutian Trench. Enjoy the sunny, warm climate while you can!
