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Reconnaissance Geology of Cedros Island, Baja California, Mexico





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Cedros Island is composed of late Jurassic melange of Franciscan aspect (herein designated Cedros Formation) which nearly everywhere is in tectonic contact with ophiolite and a superjacent sedimentary sequence of middle Jurassic to late Cenozoic age. Analogy with structural relationships in California suggests that the Cedros Formation was subducted beneath the ophiolite as oceanic and continental plates converged. Subduction was initiated during latest Jurassic time and concluded during the early Cretaceous. The San Carlos fault, presumably the subduction surface, may be a southern extension of the Coast Range thrust. During late Cenozoic time, three bodies of Cedros Formation were uplifted along faults which truncated ophiolite, superjacent rocks through Pliocene age, the San Carlos fault, and the San Agustin shear zone.