Description
Relative and absolute motion paleomagnetic studies suggest more northward translation has occurred in the continental borderland of western North America than the documented maximum value of approximately 500 km along the San Andreas transform since the Neogene. Sea floor magnetic anomalies suggest there has been as much as 700 km of northwest oriented relative dextral slip motion between the Pacific and North American plates in the past 30 my, while paleomagnetic studies west of the San Andreas suggest an average northward translation of approximately 10 degrees of latitude (representing northward translation of the Pacific plate), and studies east of the San Andreas suggest an average value of approximately 2 degrees of latitude (representing northward translation of the North American plate). Given the magnitude of motion between the North American and Pacific plates, and the discrepancy between the amount of observable offset and the amount predicted, the objective of the study was to define a west-to-east zone in which less northward translation could be demonstrated by progressively steepening paleomagnetic inclinations, and thereby suggest a possible zone(s) where yet to be discovered dextral-shear faults oriented parallel to the San Andreas might exist. Paleomagnetic sampling was conducted in Miocene and Oligo-Miocene volcanic units from the Colorado River region in southeastern California and in western Arizona, and from a region along the western coast of mainland Mexico. The resultant regional average of 7 degrees of latitude for the Colorado River region agrees with an average value for other paleomagnetic studies west of the San Andreas (representing the Pacific plate), while the resultant mainland Mexico regional average of 4 degrees agrees with studies performed east of the San Andreas (representing the North American plate). Beck statistics calculated for the regional averages are semi- quantitative at best (no locations had a sufficient number of sites to average secular variation), but probably have some significance based on the total number of points in time (20 sites in the Colorado River region, and 12 in the mainland Mexico region); they indicate the value for northward translation in the Colorado River region is significant (7 degrees of latitude). Given the observed offset along the San Andreas transform, and the magnitude of motion between the Pacific and North American plates predicted by seafloor magnetic anomalies and inferred by absolute motion paleomagnetic studies, one of three possibilities is suggested: first, the whole of the North American continent has had an absolute northward component of motion in the last 30 mybp; second, there has been more northward displacement on yet undiscovered northwest trending dextral shear faults east of the San Andreas, and east of the present study area within the Basin and Range province; or third, possible magnetic field effects resulted in shallow magnetic inclinations relative to geographic latitudes for many rock units within the continental borderland of western North America.
