Fault Trends as a High Resolution Measure of Pacific-North America Plate Evolution
In the rifted inner southern California Borderland there are faults that have classic characteristics of transform motion. Spreading ridges and volcanic chains are terminated and offset by these faults which form sets in distinct orientations. Many faults from central to Baja California also have these distinct orientations. Some of these trends are different than expected relative plate motions from plate circuit reconstructions. Since transform faults define the direction of relative plate motion, these fault trends reflect changes in relative plate motion through time and provide a more detailed record than can be obtained with plate circuit techniques. A trend of 330° is shown by the Patton Escarpment, San Diego Trough fault, Santa Lucia Escarpment, and the Coronado Bank fault. This trend is related to subduction. As the Pacific plate came into contact with the North America plate this pre-existing trend was close to the relative plate motion oriented at 300° and resulted in a transtensional environment. Within this environment the crust was extended and right-stepping, northwest-trending transform faults formed and bounded areas of extension, incipient rifting, and volcanism. Several faults have this 300° trend. As the new Pacific-North America Plate boundary evolved, the relative plate motion shifted to an azimuth of 323° at the latitude of the Borderland. This trend of 323° is strongly represented by many faults in the Borderland, and southern and Baja California. However, we observe an intermediate fault trend defined by volcanoes, spreading centers, and extensional faulting bounded by transform faults oriented at 310°-313°. The San Jacinto, Elsinore, and San Clemente Bend Region faults are also in this orientation. We believe this fault trend and perhaps additional minor fault sets represent intermediate stages in the evolution of relative motion between the Pacific and North American Plates.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009