Paleomagnetism of Middle Miocene volcanic rocks in the Mojave‐Sonora Desert region of western Arizona and southeastern California

Abstract

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave‐Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low‐angle normal faults but predate high‐angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time‐distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave‐Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post‐middle Miocene high‐angle normal faulting in this region; (2) there has been no detectable post‐middle Miocene latitudinal transport of the region; and (3) long‐term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations.