DEPOSITION OF FRANCISCAN COMPLEX CHERTS ALONG THE PALEOEQUATOR AND ACCRETION TO THE AMERICAN MARGIN AT TROPICAL PALEOLATITUDES

Red radiolarian cherts from three localities within the Franciscan sub duction complex of northern California contain three components of rem anent magnetization which are best isolated by progressive thermal dem agnetization. The first component, usually removed by 300-degrees-C, h as an in situ direction similar to the present axial-dipole field and is probably a recently acquired thermoviscous overprint. A second comp onent, generally removed between 300 and 630-degrees-C, has constant ( normal) polarity and direction within each section and is interpreted to have been acquired by low-temperature chemical alteration during su bduction and accretion at the continental margin. The third component, isolated between approximately 560 and 680-degrees-C, has both normal and reversed polarities, passes a fold test, and is inferred to have been acquired during or soon after deposition. The available paleomagn etic, biostratigraphic, and geochemical data indicate deposition of th ese cherts along the paleoequator (0-degrees-2-degrees-N or S paleolat itude) between Pliensbachian and Oxfordian time as the oceanic plate m oved eastward, relative to North America, beneath the equatorial zone of high biologic productivity. Between Bathonian and Cenomanian time, the chert sequences apparently moved progressively away from the paleo equator (2-degrees-15-degrees-N or S), and were soon after accreted to the American continental margin. Plate reconstruction models for the Farallon plate corroborate low-paleolatitude trajectories from ridge c rest to subduction zone (for example, from 3-degrees-S to 11-degrees-N ), and they imply subsequent northward translation of the Franciscan C omplex (>4,000 km) by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates.