A Paleomagnetic Investigation of the Mojave-Sonora Megashear Hypothesis in North-Central and Northeastern Mexico

The Mojave-Sonora megashear (MSM) hypothesis attempts to explain the relationship between northern Mexico and North America. According to the MSM hypothesis, terranes in northern Mexico, including the Tampico terrane, 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 Triassic and Jurassic rocks at sites north and south of the MSM in north-central and northeastern Mexico, including: volcanics from the Nazas formation; turbidite sandstones from the Zacatecas formation; red beds from the La Boca and La Joya formations; and carbonates and dolostones from the Zuloaga formation. Virtual geomagnetic poles (VGPs) and rotation and flattening values computed from site mean directions indicate lateral continuity of the northern part of the Nazas volcanic arc with respect to North America since the Late Triassic. Up to 90° of counterclockwise vertical axis rotation of the La Boca and La Joya formations with respect to Triassic and Jurassic North American reference poles indicate that northeastern Mexico and North America had distinct Mesozoic tectonic histories. Our paleomagnetic results suggest that the Tampico terrane was separate from North America during the Triassic and Jurassic and was accreted to North America by the Oxfordian. Zuloaga formation sites that were not remagnetized in the Late Cretaceous and Tertiary have minor amounts of vertical axis rotation that reflect the effects of folding during the formation of the Sierra Madre Oriental fold belt. Overall, our results do not support the MSM hypothesis, but instead lend support to models associated with the opening of the Gulf of Mexico.