Paleomagnetism and neogene tectonics of the Northern Channel Islands, California

Abstract

Geologic observations and previous paleomagnetic studies have suggested that the western Transverse Ranges arrived at their anomalous east‐west orientation by clockwise tectonic rotation. A paleomagnetic study of the northern Channel Islands was undertaken, in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern California Borderland. The islands of Anacapa, Santa Cruz, Santa Rosa, and San Miguel form an east‐west trending chain and are considered part of the Transverse Range physiographic province. Oligocene through Miocene volcanic and intrusive rocks occur on these islands, and were sampled for this paleomagnetic study. Eocene sandstones were sampled on San Miguel Island, and Eocene sandstones and Miocene siltstones were sampled on Santa Cruz Island. Paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69° to 81° from expected directions and inclinations which are too shallow by 10° to 25°. Normal and reversely magnetized units yield antipodal mean directions. The mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests. These results suggest that the northern Channel Islands have been tectonically rotated into place since early(?) Miocene time as the outer borderland area translated northwestward within a large shear zone between the Pacific and North American Plates. The data from the northern Channel Islands when averaged with the data from the Santa Monica Mountains yield a result of I = 36.1° ± 5.1°, D = 72.6° ± 6.3° for the southern part of the western Transverse Ranges (52 units, 405 samples). The shallow paleomagnetic inclinations suggest a northward latitude translation of 14.0°+3.7° or −3.9° for the western Transverse Ranges. However, most plate tectonic reconstructions suggest only 4°±3° of northward translation. The discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal. Other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components, or motions of small tectonic plates between the North American and Pacific plates as the Farallan plate broke up and was subducted.