Abstract: Microplate Capture, Rotation of the Western Transverse Ranges, and Initiation of the San Andreas Transform as a Low-Angle Fault System

Craig Nicholson, Christopher Sorlien, Tanya Atwater, John Crowell, Bruce Luyendyk

The western Transverse Ranges (WTR) have tectonically rotated substantially since the early Miocene and are continuing to rotate today. This rotation has been closely linked to the evolving Pacific-North American transform boundary and, recently, to large-scale extension and rifting of the inner California borderland. However, it has never been explained why the WTR should accommodate such plate boundary deformation by tectonic rotation, or exactly why they should have developed when and where they did. We propose that rotation of the WTR block is the direct result of capture of the partially subducted Monterey microplate by the Pacific plate previously documented to have occurred at about anomaly 6 time (~20 Ma). As Pacific-Monterey spreading slowed and eventually ceased, the slip ve tor along the gently dipping subduction interface beneath the California margin changed from slightly oblique subduction to transtensional dextral transform motion. This implies that the San Andreas transform initiated as a system of low-angle faults, which locally subjected the overriding continental margin to distributed basal shear and crustal extension. This model helps explain the timing of initial WTR rotation and associated basin formation, the sudden appearance of widely distributed transform motion well inland of the margin in the early Miocene, why the WTR uniquely rotated as a large coherent crustal block, and some of the fundamental structural characteristics of central and southern California. The model also provides major constraints on the amount of Pacific-North America s rike-slip motion, the relative position through time of offshore oceanic plates with respect to onshore geology, and a general explanation of what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994