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California |
Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
The late Mesozoic Great Valley forearc basin of northern and central California evolved from a residual forearc basin formed on top of young oceanic crust to a composite forearc basin resting on both oceanic and continental crust. Depositional environments preserved in outcrop along the west side of the Sacramento Valley evolved from deep oceanic floor and slope in the Late Jurassic to basin plain in the Early Cretaceous to submarine fan complexes in the Late Cretaceous to slope and shelf in the Palaeogene. The forearc basin widened and enlarged, and was supplied with voluminous sediment primarily derived from the coeval magmatic arc to the east. However, significant quantities of ophiolitic detritus (chert, and sedimentary, mafic volcanic and mafic metavolcanic lithic fragments) occur in the lower Great Valley strata, thus indicating erosion of ‘tectonic highlands’ formed during arc-arc collision immediately preceeding initiation of the late Mesozoic subduction regime.
The Great Valley forearc owes its geometry and history to the peculiarities of subduction initiation and to the shapes of the continental margin and magmatic arcs extant before initiation of the Great Valley subduction phase. The residual forearc developed on oceanic crust that had formed previously by spreading behind an east-facing intraoceanic arc. After this east-facing arc collided with the west-facing continental-margin arc, a new trench formed west of the suture belt. The shape of the new continental margin was irregular, so that a wide residual forearc basin formed in the Great Valley area, whereas no residual forearc basin formed to the north (Klamath area) or possibly, to the south (southern California borderland area that has been dislocated by Neogene strike-slip motions). The northern coastal promontory (Klamath area) provided much of the ophiolitic detritus within the lower part of the Great Valley strata. Sediment derived from the Klamaths was transported southward directly into the forearc basin, as well as westwards and then southwards into the trench west of the forearc basin. The Great Valley forearc basin is preserved and displayed so well today due to its mode of initiation (residual forearc formed in a previously backarc area) and its mode of termination (northward migration of a triple junction that converted a convergent margin into a transform margin).
