EQUATORIAL ORIGIN FOR LOWER JURASSIC RADIOLARIAN CHERT IN THE FRANCISCAN COMPLEX, SAN-RAFAEL-MOUNTAINS, SOUTHERN CALIFORNIA

Lower Jurassic radiolarian chert sampled at two localities in the San Rafael Mountains of southern California (similar to 20 km north of San ta Barbara) contains four components of remanent magnetization. Compon ents A, B', and B are inferred to represent uplift, Miocene volcanism, and subduction/accretion overprint magnetizations, respectively. The fourth component (C), isolated between 580 degrees and 680 degrees C, shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during, or soon after, deposition . Both sequences are late Pliensbachian to middle Toarcian in age, and an average paleolatitude calculated from all tilt-corrected C compone nts is 1 degrees +/- 3 degrees north or south. This result is consiste nt with deposition of the cherts beneath the equatorial zone of high b iologic productivity and is similar to initial paleolatitudes determin ed for chert blocks in northern California and Mexico. This result sup ports our model in which deep-water Franciscan-type cherts were deposi ted on the Farallon plate as it moved eastward beneath the equatorial productivity high, were accreted to the continental margin at low pale olatitudes, and were subsequently distributed northward by strike-slip faulting associated with movements of the Kula, Farallon, and Pacific plates. Upper Cretaceous turbidites of the Cachuma Formation were sam pled at Agua Caliente Canyon to determine a constraining paleolatitude for accretion of the Jurassic chert sequences. These apparently unalt ered rocks, however, were found to be completely overprinted by the A component of magnetization. Similar in situ directions and demagnetiza tion behaviors observed in samples of other Upper Cretaceous turbidite sequences in southern and Baja California imply that these rocks migh t also give unreliable results.