MICROPLATE CAPTURE, ROTATION OF THE WESTERN TRANSVERSE RANGES, AND INITIATION OF THE SAN-ANDREAS TRANSFORM AS A LOW-ANGLE FAULT SYSTEM

Tectonic rotation of the western Transverse Ranges block is explained by capture of the partially subducted Monterey microplate by the Pacif ic plate at about anomaly 6 time (ca. 20 Ma). As Pacific-Monterey spre ading slowed and eventually ceased, the slip vector along the gently n ortheast dipping subduction interface beneath the California margin ch anged from slightly oblique subduction to transtensional dextral trans form motion. This change in slip vector and a shift of Pacific plate m otion eastward along the already subducted Monterey plate interface im ply that the San Andreas transform began as a system of low-angle faul ts that locally subjected the overriding continental margin to distrib uted basal shear and crustal extension. This basal shear produced the rotated western Transverse Ranges. This model helps explain the tintin g of initial rotation and basin formation, the sudden appearance of wi dely distributed transform motion well inland of the margin in early M iocene time, why the western Transverse Ranges uniquely rotated as a l arge coherent crustal block, and several fundamental structural charac teristics of central and southern California. The model also provides major constraints on the amount of Pacific-North America strike-slip m otion, the position through time of offshore oceanic plates relative t o onshore geology, and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves in to a transform system.