Abstract

The Coast Ranges are considered in the light of the “new global tectonics.” It is proposed that at the end of the Nevadan orogeny, a large prism of sediments, ranging in age from Late Jurassic to Late Cretaceous, was deposited across the boundary of the oceanic and the steep-sided sialic crusts. Westward movement of North America, away from the mid-Atlantic rise, caused the oceanic block to underthrust the continental block, in the Coast Range orogeny. The thrust fault, here named the Coast Ranges thrust, passed through the post-Nevadan sediment prism at a gentle angle and separated it into two blocks. The hanging-wall block is relatively undisturbed, and its sediments are identified as the Great Valley sequence. The footwall block is greatly disturbed into a tectonic jumble of sediment and ocean crustal igneous rocks, collectively identified as the Franciscan. Rocks in the footwall block are subjected to various facies of high-pressure and low-temperature metamorphism.

On approaching the steep-sided sialic crust, the footwall block folded the crust downward and backward and continued under it at a steepened angle. Peridotite from the oceanic crust or from the mantle, mobilized by serpentinization, worked up the thrust plane and was dispersed in the footwall mélange. No batholithic instrusions and no mountain making accompanied the Coast Range orogeny.

The tectonic event of the Coast Range orogeny terminated with the propagation of the right-lateral transcurrent San Andreas fault. This change is estimated to have taken place in Eocene time. The San Andreas fault has offset the Coast Ranges thrust fault approximately 310 mi since its inception. The relative pressure of the ocean basin toward the continent was partly alleviated by strike-slip of the ocean basin, relatively northwesterly with respect to the continent. The orientation of the San Andreas is not ideal for the relief of this strain and compressional folding has also occurred. No evidence of the formation of deep-ocean trenches or island arcs has been found.

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