Chapter 2: Tectonic History of the Transverse Ranges: Rotation and Deformation on the Plate Boundary

Eleanor S. Bartolomeo & Nicole Longinotti

The Santa Barbara Channel Islands are located on the western edge of the Transverse Ranges block, within the California Continental Borderland. These islands have been uplifted by faulting and folding associated with the collision of the North American and Pacific plates and the rotation of the Transverse Ranges block. Paleomagnetic data indicates that the Transverse Ranges block has rotated 80-110° from its original orientation (Kamerling and Luyendyk, 1985). This rotation occurred during the evolution of the current plate boundary, which took place in three stages: subduction, transtension and transpression. In the Mesozoic, the western edge of North America was a subduction zone with the Farallon plate subducting eastwards under the North America plate. As the spreading ridge between the Farallon and the Pacific plates approached the continental margin, spreading along the Pacific-Farallon ridge slowed as the remaining fragments of the Farallon plate were captured by the Pacific plate and began to move with Pacific plate motion - predominately northwest. This changed the continental margin from a subduction zone to a transtensional boundary. Since portions of the captured Farallon underlay North America, this change in the direction of plate motion caused several blocks of continental crust to break off, including the Transverse Ranges block and the Outer Continental Borderland. The other blocks were captured by the Pacific plate, but the Transverse Ranges block was trapped at its northern end by a step in the continental margin and so began to rotate. This rotation and extension caused extensive normal faulting and the opening of a number of basins, similar to the Basin & Range province. This rotation continued until the Pacific plate captured Baja California and began transporting it northwest, pressing its northern end against southern California. This changed the transtensional regime to a transpressional regime in the California Continental Borderland, causing extensive folding and the inversion of the existing normal faults to reverse. These processes of folding and uplift exposed and shaped the Santa Barbara Channel Islands.