The history of poleward motion of the Baja California peninsula is a subject of continuing geologic investigations, including paleomagnetic studies during the past decade. Geologic correlation seem not to necessitate northward movement, while paleomagnetic data suggest movement of 1000 km or more. We have made paleomagnetic measurements on Cretaceous and Eocene sedimentary rocks of San Miguel Island, California, which are western lithofacies of rocks cropping out on the Baja peninsula. A total of 97 drill cores taken from Upper Cretaceous turbidites, though weakly magnetized, were found to clean best in response to thermal demagnetization, revealing a characteristic remanence. The characteristic magnetization has an inclination 43.9¿, declination of 24.0¿, and an α95 of 3.9¿. This direction is 19¿ shallower and rotated 50¿ clockwise from the expected direction for the present site position and corresponds to a paleolatitude of 26¿N.

A total of 117 drill cores of middle Eocene rocks revealed characteristic magnetizations in response to either alternating field or thermal demagnetizations. Some samples suffer from a secondary thermal magnetization from early Miocene volcanic activity, requiring a thermal cleaning that incompletely removes the overprint. A mean direction of magnetization can be derived from the intersection of planes fitted to the paths of demagnetization. The characteristic magnetization for the Eocene rocks has an inclination of 31.9¿, a declination of 71.0¿, and an α95 of 3.8¿. This direction is 19¿ shallower and rotated 79¿ clockwise from the expected Eocene direction for this site and corresponds to a latitude of 17¿N. These paleomagnetic data, in combination with recently published data from the Baja peninsula and Peninsular Ranges of southern California, indicate motion trajectories that mimic the North American craton for the period Cretaceous to late Eocene time. During the Oligocene the Baja peninsula and adjoining regions to the west moved northward relative to the craton, arriving at a pre--Gulf of California position not later than earliest Miocene time. A comparison of the northward motion of the Baja peninsula with that of the Santa Lucia--Orocopia allochthon shows that both areas underwent rapid poleward translations, but these motions occurred at different times.