Late Cretaceous deformation at the western Acatlán Complex – zircon (U-Th)/He thermochronometry

The Mexican orogen represents the southern continuation of the North American Cordilleran orogenic system and plays a critical role in shaping the tectonic, structural, and thermal evolution of western Mexico. Although the age of deformation in central and northern Mexico is continuously being constrained, deformation in southern Mexico remains poorly understood due to limited geochronological data. Thick-skinned structures exposed along the western margin of the Acatlán Complex are interpreted as part of the Mexican orogen and, therefore, offer an important opportunity to refine the timing and tectonic evolution of deformation in southern Mexico. This study presents new zircon (U-Th)/He thermochronology data revealing Late Cretaceous deformation and exhumation along the western margin of the Acatlán Complex. Field observations along the western margin of the Acatlán Complex show that the southeast-dipping S1 foliation of metamorphic rocks is structurally parallel to the La Encinera Shear Zone main fabric, a Carboniferous ductile structure with complex deformation features. In contrast, the similarly oriented Papalutla Fault represents a Late Cretaceous brittle structure juxtaposing the Acatlán Complex and the Guerrero-Morelos Platform, cross-cutting the thin-skinned structures of the platform. Zircon He data reveal two thermal events: (1) a post-Jurassic to pre-Cenomanian heating phase that partially reset the zircon He system, likely related to ∼1.6 km of burial and regional increase of geothermal gradient, and (2) a cooling phase marking the onset of exhumation that started during the Cenomanian, predating initial deformation of the Mexican orogen and deposition of the Mezcala Formation west of the Acatlán Complex.