Abstract

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. The block occupied the forearc region of a subduction zone, collecting continental shelf sediments equivalent to those in the Great Valley belt farther north. In the mid-Cenozoic, a spreading center approached the trench in this region, contributing to the uplifts recorded by the Sespe Formation terrestrial rocks. Between 27 and 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 the early stage of plate boundary evolution (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, eruption of Conejo-age volcanics, and deposition of 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 ongoing folding, uplift, tilting, and faulting of the modern Channel Islands.