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Late Mesozoic Franciscan rocks of the California Coast Ranges, chaotically deformed and consisting of graywacke, micrograywacke, shale, chert, and mafic pillow lava, are considered to have been deposited in and adjacent to a northwest‐trending oceanic trench. Principally on the east, contemporaneous, well‐bedded conglomerate, lithic sandstone, siltstone, and shale of the Great Valley sequence evidently were laid down on a continental shelf, slope, and sea floor environment. The junction between these two sequences, one the ensimatic eugeosynclinal mélange of Franciscan rocks, the other the more orderly Great Valley miogeosynclinal strata overlying chiefly sialic‐type basement, is marked by the South Fork Mountain‐Stoney Creek‐Ortigalita (–Sur‐Nacimiento) fault system. This structural break is interpreted as the crustal expression of a Late Mesozoic Benioff zone. Episodic or relatively continuous underthrusting of the trench mélange beneath this former seismic shear zone is held responsible for the contrasting characteristics of the Franciscan rocks and the simultaneously deposited Great Valley sequence, as well as for their ubiquitous tectonic juxaposition. A speculative four‐state tectonic model is proposed that involves: (1) rapid Late Jurassic relative northeastward or eastward spreading of the Pacific Ocean floor, coupled with westward or southwestward encroachment by the continental lithospheric plate; (2) mid‐Cretaceous buoyant uplift of portions of the eugeosynclinal prism during a period of less intense spreading; (3) accelerated post‐Cretaceous convergence between oceanic and continental lithospheric plates; and (4) Miocene‐Pleistocene diapiric uplift of the Franciscan mélange related to northwestward seafloor spreading.

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