Description
Interactions between the Farallon and North American plates and, later, the Pacific and North American plates subjected the western United States to dextral shear through most of the Cenozoic. One result of this shear has been the clockwise rotation of crustal blocks, particularly in southern California. Concordant declination results obtained from 17 volcanic flows in the southeasternmost Chocolate Mountains and the southern Trigo Mountains in southeastern California and southwestern Arizona indicate that the area has been rotated 40 ± 15 degrees clockwise since 13 mya. Declination data obtained from 12 cooling units in the Palo Verde Mountains and the northern Trigo Mountains indicate that this northern region has not been rotated. Several paleomagnetic studies in southern California have shown substantial local declination anomalies within mountain ranges which, as a whole, have not been rotated. Two such local declination anomalies have been observed in the lower Colorado River region. These declination anomalies are observed much too frequently to be attributed solely to paleoexcursions of the earth's magnetic field. Instead, they are attributed to local block rotations. Previous studies suggest that the Pacific plate has moved 1000 km northward with respect to the North American plate since 29 mya. This motion has been taken up on borderland faults, the San Andreas fault system, basin and range extension, and the opening of the Gulf of California. However, the sum of all these displacements may not account for all of the predicted motion. The San Andreas fault system is generally regarded as the present boundary between these two plates. Previous workers have suggested that the boundary extends inland east of the San Andreas fault. If this is true, then areas where the motion has taken place should be recognizable. An inclination anomaly of 5 ± 9 degrees has been obtained from a total of 31 Oligo-Miocene units. While this value is not statistically significant, it is consistent with other paleomagnetic studies in the region that also suggest a substantial northward motion has occurred east of the San Andreas fault.
