A paleomagnetic investigation of the Mojave-Sonora megashear hypothesis in north-central and northeastern Mexico.

摘要

The Mojave-Sonora megashear (MSM) hypothesis attempts to explain the spatial relationship of geology in northern Mexico with respect to North America. According to the MSM hypothesis, terranes in northern Mexico were displaced 800-1000 km along a left-lateral strike slip system during the Oxfordian. In this study, the MSM hypothesis is evaluated with new paleomagnetic data from north-central and northeastern Mexico. More than 175 paleomagnetic samples were collected from seventeen sites at ten localities situated north and south of the postulated trace of the MSM. The rocks sampled include: volcanics from the Upper Triassic to Middle Jurassic Nazas formation; turbidite sandstones from the Upper Triassic Zacatecas formation; red beds from the Upper Triassic La Boca and Lower to Middle Jurassic La Joya formations; and carbonates and dolostones from the Oxfordian Zuloaga formation. Step-wise alternating field (AF) and thermal demagnetization were used to isolate characteristic remnant magnetization (ChRM) vectors, and thermoremnant induced remnant magnetism (TIRM) techniques were applied to samples for rock magnetic studies. Virtual geomagnetic poles (VGPs) and rotation and flattening values computed from the site mean directions indicate Late Cretaceous and Early Tertiary remagnetization of the Zacatecas formation in western San Luis Potosi and the Zuloaga limestone in Nuevo Leon, Coahuila, and San Luis Potosi. Minor vertical axis rotation and near zero flattening values calculated from site mean directions for the Nazas formation in northern Durango indicate lateral continuity of the northern part of the Nazas volcanic arc with respect to North America since the Late Triassic and do not support the lateral displacement predicted by the MSM hypothesis. Up to 90° of counterclockwise vertical axis rotation of the La Boca and La Joya formations and comparison of their VGPs with respect to Triassic and Jurassic North American reference poles indicate that northeastern Mexico and North America had distinct Mesozoic tectonic histories. The post-Late Triassic rotation and flattening values for northeastern Mexico are not adequately explained by the MSM hypothesis alone. The Zuloaga formation sites both north and south of the MSM that were not remagnetized in the Late Cretaceous and Tertiary have minor amounts of vertical axis rotation. These vertical axis rotations are thought to reflect the effects of folding during the formation of the Sierra Madre Oriental fold belt and are inconclusive with respect to the Mojave-Sonora megashear hypothesis. The MSM hypothesis calls for a through-going fault system from Death Valley, CA to the Gulf of Mexico, so if northeastern Mexico was affected by the MSM, north-central Mexico should have also been affected. Since the Nazas formation in north-central Mexico does not appear to be offset by the MSM, the results of this study do not provide conclusive evidence for the MSM in north-central and northeastern Mexico.