Abstract

Tectonic wedging is currently deforming rocks along the western margin of the Central Valley of California. Structural relations in the strata of the Mesozoic Great Valley Group fore-arc basin indicate that earlier phases of wedging were coeval with subduction and accretion of the Franciscan Complex to the west. The present-day geometry of tectonic wedging is incompatible with blueschist metamorphism in the Franciscan complex because the detachment for the wedge system would have prevented the transport of accretionary-wedge materials to great depth. We propose a sequence of tectonic events that reconciles these tectonic processes. Franciscan subduction and metamorphism began at ∼165 Ma, and Great Valley Group deposition began in the Tithonian. Shortening affected Great Valley Group rocks in the Hauterivian-Barremian and may have coincided with a gap in blueschist metamorphism in the Franciscan. Major uplift of Franciscan blueschist relative to the unmetamorphosed Great Valley Group rocks began in the Albian-Cenomanian. This uplift was accommodated along an east-dipping extensional fault system between the Franciscan Complex and the structurally overlying Coast Range ophiolite and Great Valley Group. Movement on this fault system caused the western side of the fore-arc basin to subside, resulting in uninterrupted deposition of Great Valley Group sediments from the Cenomanian to the Campanian-Maastrichtian. The youngest exposed blueschists in the Franciscan were metamorphosed at 80–90 Ma. Uplift of blueschist relative to Great Valley Group rocks must have been complete prior to development of the present style of east-vergent tectonic wedging. This style of deformation probably began between the Maastrichtian and Paleocene, resulting in exhumation and deformation of both Franciscan and Great Valley Group rocks. The development of wedging was coincident in part with the shallowing of subduction dip and voluminous frontal accretion of the coastal belt of the Franciscan.

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