Abstract

Two types of serpentinites are known in the California Coast Ranges. The first, “intrusive” (herein renamed non-detrital) is the most widespread, particularly along the thrust contact separating the Franciscan terrane from the coeval, and apparently overlying the Great Valley sequence. The second type, detrital serpentinite, is interfingered with sediments in the lower portion of the Great Valley sequence, probably as a result of submarine slumping during Tithonian and Early Cretaceous times.

Detailed mapping of both types of serpentinite bodies shows they exhibit extrusive features indicative of recent remobilization by superficial readjustments. Because of this remobilization, many bodies of detrital serpentinite may have been overlooked.

The combined study of detrital and non-detrital serpentinites has important implications for the interpretation of Coast Range geology in the light of global tectonic concepts. Detrital serpentinites were probably deposited at the base of a middle Mesozoic continental slope, perhaps in an oceanic trench. The source material was not derived from adjacent landmasses and probably was extruded onto the sea floor by tectonic activity. Non-detrital serpentinites and overlying mafic rocks represent the basement upon which the Great Valley sequence was laid. The ultramafic-mafic assemblage may represent a remnant of oceanic crust or an ophiolite extrusion.

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