Tectonic rotation of the western Transverse Ranges block is explained by capture of the partially subducted Monterey microplate by the Pacific plate at about anomaly 6 time (ca. 20 Ma). As Pacific-Monterey spreading slowed and eventually ceased, the slip vector along the gently northeast dipping subduction interface beneath the California margin changed from slightly oblique subduction to transtensional dextral transform motion. This change in slip vector and a shift of Pacific plate motion eastward along the already subducted Monterey plate interface imply that the San Andreas transform began as a system of low-angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension. This basal shear produced the rotated western Transverse Ranges. This model helps explain the timing of initial rotation and basin formation, the sudden appearance of widely distributed transform motion well inland of the margin in early Miocene time, why the western Transverse Ranges uniquely rotated as a large coherent crustal block, and several fundamental structural characteristics of central and southern California. The model also provides major constraints on the amount of Pacific-North America strike-slip motion, the position through time of offshore oceanic plates relative to onshore geology, and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system.