Abstract: Paleomagnetism, Paleogeographic Origins, and Remagnetization of the Coast Range Ophiolite and Great Valley Sequence, Alta and Baja California

HAGSTRUM, JONATHAN T.

Reliable paleomagnetic data for the Coast Range ophiolite (CRO) and Great Valley sequence (GVS) of North America have been difficult to obtain. In Alta and Baja California, characteristic remanent magnetizations for these rocks and their equivalents often post-date folding, show only normal polarity where both polarities are expected, and tend to have directions similar to younger geomagnetic field directions. These magnetizations are clearly secondary and may have been acquired during deep subaerial weathering, burial and uplift, or low-temperature chemical alteration. Similarly, Mesozoic oceanic rocks of the Franciscan Complex have been extensively overprinted, but high-blocking temperature components of magnetization showing polarity stratigraphies and equatorial paleolatitudes have been found in hematite-bearing chert sequences from northern California to Mexico. Upper Jurassic red tuffaceous chert above the CRO at Stanley Mountain in southern California also has a high-temperature component with dual polarities that is interpreted as a primary magnetization. Separation of this primary component from an overprint component is best shown by samples with negative-inclination primary directions, and a corrected mean direction using only these samples indicates an initial paleolatitude of 32 degrees plus or minus 8 degrees N. At Mount Diablo in northern California, Middle to Upper Jurassic pillow lavas and diabase sills of the CRO have two components of remanent magnetization. The characteristic component shows two polarities as well, and its corrected direction indicates an initial paleolatitude of 20 degrees plus or minus 12 degrees N. The general agreement of these results suggests that the CRO in California has not undergone large-scale northward transport with respect to cratonic North America since its formation. Reliable estimates of paleolatitude for the lower GVS remain unavailable.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California