Accretionary history of an oceanic crustal assemblage: The Permanente terrane near Pacifica, California

Previous studies of the Calera limestone (Permanente terrane, Franciscan complex) of California have shown that deposition occurred at approximately 25°N paleolatitude atop an oceanic plateau or rise, and that it was subsequently transported northeast and accreted to the North American continental margin in the early Tertiary. The Permanente terrane in the study area along the Pacific coast south of San Francisco contains a suite of rock types present in north dipping imbricate thrust slices possibly related to the subduction and accretion of an oceanic crustal sequence. Geochemical studies of greenstones, thought to represent fragments of the dismembered oceanic crust, indicate affinities with mid-ocean ridge basalts and within-plate basalt. These greenstones may represent fragments of the rise upon which the Calera limestone was deposited. Metamorphic minerals in the greenstones indicate both sea-floor metamorphism and locally, high-pressure, low-temperature metamorphism. It is concluded tentatively that the entire Permanente terrane in the study area underwent high-pressure, low-temperature metamorphism but that temporal or geologic conditions prevented widespread recrystallization or development of blueschist facies minerals. Structures in the deformed rocks can be related to underthrusting beneath the continental margin and attachment to the overriding plate. Studies of veins indicate several stages of dewatering. It is likely that dewatering was a major process in the attachment event, as shown by veins associated with this phase of deformation. Structurally interleaved mélanges and broken formation contain sedimentary rock clearly derived from a continental margin; these rocks probably represent trench and trench slope facies that were underthrust with the accreting oceanic crustal sequence.