ABSTRACT: Paleomagnetism of the Mesozoic Coast Range Ophiolite, California

HAGSTRUM, JONATHAN T., U.S. Geological Survey, Menlo Park, CA

Paleomagnetic data bearing on the paleogeographic origins of the Coast Range Ophiolite of western California have been difficult to obtain. In northern California, characteristic remnant magnetizations for samples from the Del Puerto Canyon, Stonyford, and Paskenta regions postdate folding, show only normal polarity where both polarities are expected, and tend to have directions similar to younger geomagnetic field directions. These magnetizations clearly are secondary and have been acquired during either deep subaerial weathering, burial and uplift, or low-temperature chemical alteration of these rocks. Limited data from two other localities (Mt. Diablo, Potter Valley) show paleolatitudes concordant with Jurassic North America, but fold and reversal tests are lacking. In southern Cal fornia, characteristic magnetizations for remnants of the Coast Range Ophiolite (Stanley Mountain, Point Sal, Cuesta Ridge, Llanada) consistently indicate magnetization acquisition at low paleolatitudes, but field stability tests are also lacking, and again only normal polarity is indicated at each locality. These results also are consistent with remagnetization, which probably occurred during accretion of these rocks to the continental margin at southerly paleolatitudes. Similarly, oceanic rocks of the Franciscan Complex appear to have been extensively remagnetized during accretion, but primary high-blocking temperature components of magnetization showing polarity stratigraphies and deposition at equatorial paleolatitudes have been found in

radiolarian chert sequences at several localities. Primary magnetizations may be intermittently preserved within the Coast Range Ophiolite as well, and investigations are presently underway at promising localities (Mt. Diablo, Stanley Mountain) in an attempt to document them and determine reliable paleolatitudes of origin for these rocks.

AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)