Abstract: Plate Tectonic History of the Transverse Ranges and Channel Islands, Southern California.

ATWATER, TANYA, Dept. Geological Sciences, University of California, Santa Barbara, CA 93106-9630

The Channel Islands occupy the southern edge of the Transverse Ranges block and thus the islands share and illuminate the history of this block. In the Mesozoic and early Cenozoic, the Transverse Ranges block was oriented north-south, so that the Channel Islands probably lay near San Diego (as shown by Poway conglomerate equivalents). The block occupied the arc-trench gap region of a subduction zone, collecting continental shelf sediments (Great Valley equivalents). In the mid-Cenozoic, a spreading center approached the trench in this region (causing the Sespe formation regressions?).

Starting about 25-18 Ma, the Pacific plate made contact with North America and continental pieces began to break off and join the Pacific plate, gradually establishing the modern San Andreas plate boundary.

In an early stage, Miocene (18-12 [-5?] Ma), the northern plate boundary lay within and inboard of the Salinian block, but then bent south-westward through the Southern California Borderland in a transtensional geometry. This transtensional phase is responsible for the extension and reconfiguration of the continental rim, including the onset of rotation and left lateral shearing of the Transverse Ranges Block and deposition of San Onofre-type breccias,. Conejo-age volcanics, and marine basin sediments including the Monterey and Sisquoc formations.

In a late stage, Plio-Pleistocene (5-0 Ma), Baja California joined the Pacific plate and began to obliquely ram into southern California. This transpressional phase is responsible for the ongoing folding, faulting, and vertical displacements, including the uplift, tilting and faulting of the modern Channel Islands.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California