Abstract

The Sur fault zone is a complex series of high-angle and low-angle faults that juxtapose a northeast block of competent crystalline rocks against a southwest block of generally sheared Franciscan rocks. The Franciscan rocks are an assemblage of sandstone, siltstone, greenstone, and chert. This assemblage can be divided into a northwestern terrane characterized by unmetamorphosed sheared and unsheared units containing detrital K-feldspar and a southeastern terrane with a pervasive dip-slip tectonite fabric throughout, metamorphic mineral assemblages indicative of high-pressure, low-temperature metamorphism, and no K-feldspar. A lawsonite isograd in Franciscan sandstone crosses the southeastern terrane. K-Ar dates indicate a minimum age of Late Cretaceous for high-pressure metamorphism of the Franciscan.

During mid- to Late Cretaceous time, accumulation of Franciscan rocks was closely followed by subduction and high-pressure, low-temperature metamorphism. Low-pressure, high-temperature metamorphism and plutonism occurred concurrently in the Salinian block. In Late Cenozoic time, the San Andreas fault probably offset the Sur-Nacimiento fault from the Great Valley subduction zone.

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