Description
Miocene paleomagnetic directions obtained from twenty basalt flows and one andesite dome along the western margin of the Salton Trough display both anomalously shallow inclinations and clockwise deflections from the expected Tertiary direction. These units appear to represent at least ten separate points in time, spanning a period of about 7 million years. When the inclinations from these separate points in time are averaged, the data suggests a post upper Miocene northward translation for the western Salton Trough of about 7 degrees of latitude (770 kilometers). This data describes a greater amount of northward movement than can be accounted for by the currently accepted history of the North American and Pacific plate margin. Possible solutions to this discrepancy include: A component of northward motion for the North American plate between 21 and 10 million years ago, the existence of an axial dipole shifted north of the equator during most of the Miocene, or a greater rate of displacement between the Pacific and North American plates than described by Atwater and Molnar (1973). The apparent paleomagnetic stability of most of North American during the Miocene argues against the first two possible solutions. Therefore, the data suggests a greater displacement between the Pacific and North American plates and within the proto-Gulf of California than previously deduced. Finally, three of the four sampling sites in the Southern Coyote Mountains and the one unit sampled in the Impact Area describe apparent clockwise rotations that vary from 111 degrees to 40 degrees. The stratigraphically higher units generally show the least amount of rotation. This rotation is probably the result of right lateral shearing acting on extensionally loosened crustal fragments within the proto-Gulf of California.
