The early Palaeogene transition from thin-skinned to thick-skinned shortening in the Potosi uplift, Sierra Madre Oriental, northeastern Mexico

The central Sierra Madre Oriental in Mexico developed primarily in the Late Cretaceous to early Palaeogene as a thin-skinned fold belt above a decollement hosted in Jurassic evaporite. In some regions shortening transitioned to sub-decollement structural levels, resulting in thick-skinned uplifts such as the Potosi uplift in Nuevo Leon. Thick-skinned deformation in the Potosi uplift involved folding of sub-decollement strata into an NNW-trending anticlinorium, cleavage development, thrust and conjugate strike-slip faulting, and extension fractures associated with barite mineralization. These structures consistently record subhorizontal shortening, directed similar to 52-65 degrees in the southern uplift, similar to 69-72 degrees in the northern uplift. Palaeocene to mid-Eocene zircon (U-Th)/He cooling dates record the timing of exhumation associated with thick-skinned uplift and suggest a continuation of shortening rather than a separate tectonic event. Zircon (U-Th)/He dates across the southern Potosi uplift and the Aramberri uplift, similar to 50 km to the south, range from similar to 66-53 Ma, whereas dates in the northern part of the Potosi uplift range from similar to 49-44 Ma. We attribute the transition to thick-skinned shortening to the mechanical strengthening of a planar decollement as rheologically weak evaporite was evacuated beneath synclinal keels of detachment folds. Along-strike differences in timing of exhumation and shortening directions may relate to differences in mechanical stratigraphy. Thicker intervals of evaporite in the northern uplift allowed thin-skinned shortening to continue while the southern uplift transitioned to thick-skinned shortening as the evaporite decollement was exhausted. As a result, stress-strain trajectories in the northern uplift refracted clockwise during continued deformation. Our findings provide new insight into the structural evolution of the Potosi uplift and may provide a framework for studying other thick-skinned uplifts in the orogen, and more generally orogenic belts that record a transition in deformation styles during progressive shortening.

Automated summary

This study reveals the structural evolution of the Potosi uplift in the central Sierra Madre Oriental in Mexico, involving thick-skinned uplift and shortening. The timing of exhumation associated with thick-skinned uplift suggests a continuation of shortening rather than a separate tectonic event, with along-strike differences in timing of exhumation and deformation styles.