GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 167-7
Presentation Time: 10:15 AM

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


WILLIAMS, Stewart A.1, SINGLETON, John S.1, PRIOR, Michael G.1, MAVOR, Skyler P.1 and CROSS, Gareth E.2, (1)Department of Geosciences, Colorado State University, 1482 Campus Delivery, Fort Collins, CO 80523, (2)Plymouth, PL1, United Kingdom

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.