Detrital-zircon geochronology of Triassic fluvial and submarine fan deposits from northeastern Mexico: Stratigraphy, paleogeography and tectonic implications

Upper Triassic strata are exposed in several localities in northeastern México. Marine sequences outcropping in San Luis Potosí and Zacatecas are considered parts of a large submarine fan system (“Potosí fan”) related to the paleo-Pacific margin of Pangea. Upper Triassic fluvial deposits of “El Alamar River” (“El Alamar formation”) outcropping in Nuevo León and Tamaulipas, are linked to the Potosí fan, accord to paleocurrent, geochemical, and detrital zircon data. Marine and continental sequences show continental block and recycled orogenic provenances. Detrital zircon geochronology (U-Pb) by LA-MC-ICPMS shows three main populations of Grenvillian (900-1250 Ma), Pan-African (450-700 Ma) and Permo-Triassic zircons (245-280 Ma). The presence of these zircons indicates deposition close to the Oaxaquia block, the Pan-African terrains such as Yucatan and eastern Texas, and the Permo-Triassic magmatic arc, and does not support large left-lateral displacement as proposed for the Mojave Sonora megashear (MSM) hypothesis. Triassic strata in northeastern Mexico lie south of the inferred trace of the MSM. Had the fault existed, these units should be reconstructed 800 km to the northwest in Triassic time. Besides the absence of detrital zircons from Precambrian provinces such as Yavapai and Mazatzal, and mid-Proterozoic zircons, Pan-African zircons cannot be easily found had these rocks been in a position suggested by reconstruction of displacement by the MSM. Furthermore, Grenvillean zircon sources in northwest Mexico are restricted to ages ca. 1.1-1.2 Ga whilst zircons in the Zacatecas and El Alamar formation are more typical of Oaxaquia (0.9-1.25 Ga Ma). The only model that permits left-lateral displacement in the region is a transform along the southern continental margin of North America with an oceanic plate to the south, subducting eastward under a static Grenvillean-Pan-African block.