Late Triassic–Middle Jurassic Regional Syn-Rift Extension of Overthickened Late Paleozoic Collisional Orogenic Crust in Mexico and Its Influence on Rift-Basin Development: A Paradigm Shift for Models of Western Pangean Breakup?

The Colombia–Mexico overlap problem complicating Pangean reconstructions is often circumvented by allowing major sinistral slip along a “Mojave–Sonora Megashear,” but projecting such a transform across NE Mexico is problematic. Fieldwork fails to recognise it, and there is no marginal offset along eastern Mexico nor displacement of the Tamaulipas arch. An alternative model stems from realizing 1) pre-Mesozoic basement underlies only the eastern half of central and southern Mexico, reducing the scale of overlap; 2) little-deformed Triassic-Jurassic synrift deposits overlie mid-crustal granitoids and metamorphic rocks, the exhumation of which requires explanation; 3) a regional 4–7 km Mesozoic synrift/postrift sedimentary section implies that rifting thinned basement to 25–28 km from a greater thickness. Using the southward azimuth of the stretching lineation in the Sierra Juarez mylonites (Graham et al. 2020) as a representative example, these observations collectively suggest major southward continental extension of overthickened Late Paleozoic crust from an orogenic core or belt in northern Mexico, locally exposing deeply exhumed crustal levels from under low-angle detachments, potentially with enough cumulative extension to circumvent the overlap problem (Pindell et al. 2020). We highlight cases across eastern and southern Mexico of anatectites, metamorphic rocks, and synrift mylonites exposed prior to and during synrift deposition, and show how the detrital zircon record of Permian to Jurassic sedimentary basins supports the hypothesis of widespread extension and multiphase rift basin formation. Our model provides a potential paradigm shift for understanding the Permian–Middle Jurassic evolution of Mexico.