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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 1. (January)

First Page: 29

Last Page: 40

Title: Franciscan-Knoxville Problem: Relationship Between an Accretionary Prism and Adjacent Fore-Arc Basin

Author(s): R. J. Korsch (2)


The relationship between the Franciscan complex and Knoxville shales in the Californian Coast Ranges has been the subject of debate for a considerable period of time. Initially, gradational and unconformable relationships for the contact were proposed. This was followed by the recognition, at some localities, of a thrust fault contact. Plate tectonics has provided a framework for interpretation of the Franciscan complex as an accretionary prism related to subduction of oceanic crust and the Great Valley sequence, including the Knoxville shales, as fore-arc basin deposits. Thus, the contact between the two units was the initial site of the Benioff zone, which then migrated westward as the accretionary prism developed. In the Franciscan complex and Great Valley sequence man thrusts have been recognized recently. At different localities, the Franciscan complex can be observed juxtaposed against various units of the Great Valley sequence, suggesting a complicated thrusting history subsequent to initiation of the Benioff zone. Some of the thrusts had movement during development of the accretionary prism, but movement on others may be post-subduction in time. The term "Coast Range thrust" has been used to refer to the contact, but in recent years its meaning has become blurred by liberalization and excessive use. It is proposed that the term be used only as originally proposed, that is, as the thrust that juxtaposes the Franciscan complex and the ophiolitic base of the Great Valley sequence.

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