K-Ar ages of approximately 155 m.y. were measured on minerals from ophiolites at two localities in the California Coast Ranges. Upper Jurassic rocks of the Great Valley sequence are n i depositional contact with underlying ophiolites, which are interpreted as Mesozoic oceanic crust that formed during differentiation of mantle material at a spreading ridge some 15 m.y. before deposition of the Great Valley sequence began.

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