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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.
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