MULTIPROXY PROVENANCE STUDY OF THE CHIVILLAS FORMATION: A RECORD OF THE MIDDLE JURASSIC TO EARLY CRETACEOUS GEODYNAMIC EVOLUTION OF SOUTHEASTERN MEXICO

The Middle Jurassic to Early Cretaceous tectonic configuration of southeastern Mexico was controlled by a transtensional regime associated with the final stage of the opening of the Gulf of Mexico. During this period, sedimentary basins were established within central and southern Mexico, which include detrital successions derived mostly from different igneous and metamorphic basement units, and also from Jurassic or older sedimentary successions. One of these basins is the Cuicateco, whose Early Cretaceous sedimentation is represented by the Chivillas Formation. The Chivillas Formation is made up of siliciclastic turbidites, volcanoclastic deposits, and volcanic rocks. Both the paleontological record and detrital zircon geochronology constrain the maximum age of deposition to the Early Cretaceous.

A provenance analysis was performed using U-Pb data from detrital zircon and apatite grains, as well as apatite geochemistry, and fission track analysis. The new detrital zircon and apatite U-Pb geochronology data of the Chivillas Formation allows identifying three main source areas: Early Neoproterozoic, Permian-Carboniferous, and Early Cretaceous. Trace element concentrations in the detrital apatite reveal that the Grenvillian grains have Sr content correlated with negative Eu anomalies. Also, the REE concentrations in the Grenvillian grains is higher than in the Permian-Carboniferous grains.

The relative mechanical instability of apatite suggests nearby sources, such as the metamorphic rocks of the Grenvillian Oaxacan Complex, the igneous rocks belonging to the Permian-Carboniferous magmatic arc, and those belonging to Early Cretaceous magmatic arc, which was developed along the Pacific margin of Mexico. Apatite fission track analyses indicate that the Chivillas Formation was exhumed quickly during the middle Eocene, and thermal modeling indicates that exhumation lasted 7 Ma. This exhumation post-dated the contractional Laramide event and is probably associated with the reactivation of the Oaxaca fault, one of the most prominent discontinuities in southern Mexico, which acted as an extensional structure during the Cenozoic.