The Fault and the Sea

Back in the Mesozoic Era, the oceanic Farallon Plate bordered the western edge of the Americas. Since then, as the Atlantic Ocean opened, the American Plates have been pushed westward and most of the Farallon has subducted beneath them. Today, remnants of the Farallon Plate include the Juan de Fuca Plate of the Pacific Northwest, the Cocos Plate off Mexico and Central America, and the Nazca Plate, off the west coast of South America. All three of these remnant plates continue to subduct, as evidenced by volcanism along and near the western edge of the Continents.

Between the Juan de Fuca and Cocos remnants, the Farallon has completely subducted and the edge of the Pacific and North American Plates meet; for millions of years, this was a rather passive margin. Then, about 5 million years ago, the southern end of this border began to rift; known as the East Pacific Rise, this rifting tore the Baja Peninsula from the edge of northern Mexico, opening the Sea of Cortez. Further north, the margin persists as the famous San Andreas Fault, a transform fault between the North American and Pacific Plates; in concert with the opening of the Sea of Cortez, the Pacific Plate has been sliding northwestward along the edge of North America.

This motion is gradually widening the Sea of Cortez and, intermit-tently, forcing Southern California, which sits on the Pacific Plate, to grind its way along the San Andreas Fault; pressure that builds between the Plates is released by earthquakes, common events throughout the region. Compression along the San Andreas has also forced up the Transverse Ranges and, eventually, the combined mass of Southern California and the Baja will separate from North America and become a long Pacific island.