Description
Many recent paleomagnetic studies in the western United States and Mexico have found magnetic inclinations that appear to be too shallow for the rocks to have originated at their present latitude. This suggests a northward translation of the rock units. The objective of this study was to extend or confine the area of this anomaly in the region of south-central Arizona. The focus of this study was the Ajo Range, a north-northwest trending block faulted mountain range composed of latite, tuff, rhyolite, and basalt flows. The Childs latite and lower portions of the Rhyolite of Montezumas Head were sampled. The Childs latite has been dated at 18 ± .5 my, and the Rhyolite of Montezumas Head has an age of 16 ± 1 my. A total of 259 cores were drilled at 31 sampling sites. Only the 23 sites with the most reliable data were included in the final calculations. The paleomagnetic data indicate a northward translation of 8 ± 6 degrees (about 900 km), and no significant amount of rotation. This suggests that the Ajo Range may be included with the other studies which show abnormally shallow inclinations. A regional average of 4 degrees (about 450 km) of northward translation was calculated for 15 paleomagnetic studies done east of the San Andreas fault system and west of the craton. A translation gradient was established showing progressively less translation from west to east. Several possibilities exist that may explain this apparent northward motion. First, the North American plate may have had a small northward component of absolute motion during the last 20 million years. Second, that a magnetic field anomaly has existed in western North America for at least 20 my, producing shallow inclinations relative to geographic latitudes. The third explanation is that dextral shear has occurred east of the San Andreas fault system in the process of moving the Pacific plate pq.st the North American plate. This study concludes that a part of the regional average of 450 km of apparent northward translation may be attributed to dextral shear; however, the remainder may be due to either of the previously described possibilities or a combination of the two.
