POSSIBLE TRIPLE JUNCTION AND PLATE PULL-APART ORIGIN FOR MONTEREY BAY AND THE MONTEREY SUBMARINE CANYON, CALIFORNIA

The Monterey canyon is one of the deepest, most developed submarine canyons cut into the western North American continental shelf, and the surrounding Monterey bay is one of the largest coastal indentations. Theories advanced to explain this dramatic topography generally start with the assumption that the submarine canyon was cut by turbidite and debris flows originating near the mouth of an unidentified major river. In this report, the location of the canyon and bay are attributed, rather, to a short-lived triple junction at the southern end of the nascent San Andreas plate boundary. Between about 22 and 19 Ma, the Pacific plate was in contact with the rim of the North American plate from Point Mendocino to the Monterey area, but not farther south. As this first section of the continental rim began to attach itself to the Pacific plate, it would have been carried up the coast, creating a pull-apart zone at its southern end with up to 100 km of extension. About 19 Ma, the Monterey microplate stalled offshore, causing the triple junction to shift to the south. The Monterey region then became simply a weak, fractured zone between two migrating pieces of the continental margin, a likely spot to concentrate continental erosion and seaward removal of sediments.