THE EARLY PALEOGENE TRANSITION FROM THIN-SKINNED TO THICK-SKINNED SHORTENING IN THE POTOSÍ UPLIFT, SIERRA MADRE ORIENTAL, NORTHEASTERN MEXICO

The Jurassic Minas Viejas Formation is host to a Late Cretaceous to early Paleogene-aged décollement in the Sierra Madre Oriental, Mexico. The Minas Viejas Formation consists primarily of weak evaporite that accommodated thin-skinned shortening with minimal deformation to underlying Triassic-Jurassic red beds. Thin-skinned shortening transitioned to thick-skinned, resulting in the development of the Potosí uplift – one of the largest and well exposed thick-skinned uplifts in the orogen. New mapping and structural analysis provide insight into the geometry and kinematics of the uplift, and zircon (U-Th)/He thermochronometry records exhumation associated with the uplift.

Thick-skinned deformation involved folding of sub-décollement strata into a NNW-trending anticlinorium, development of cleavage, thrust and conjugate strike-slip faulting, and the formation of extension fractures associated with barite mineralization. These structures consistently record subhorizontal shortening, directed ~057° in the southern uplift, ~071° in the northern uplift. Paleocene 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 Potosí uplift range from ~66–54 Ma, whereas dates in the northern part of the uplift range from ~49–44 Ma. We attribute the transition to thick-skinned shortening to the elimination of a planar weak zone at the base of the décollement as 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 décollement 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 Potosí 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.