Structural evolution of the Potosí uplift, Sierra Madre Oriental, northeastern Mexico
Date
2019
Authors
Williams, Stewart Alexander, author
Singleton, John, advisor
Ridley, John, committee member
Laituri, Melinda, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
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 is dominated by rheologically-weak evaporite that accommodated thin-skinned deformation, forming the Sierra Madre Oriental fold-thrust belt with minimal deformation to the underlying Triassic-Jurassic red beds. Thin-skinned shortening above the décollement temporally transitioned to thick-skinned shortening, resulting in exhumation of the décollement and development of the Potosí uplift, one of the largest and well exposed thick-skinned uplifts in the orogen. Detailed geologic mapping and structural analysis provide insight into the geometry and kinematics of the Potosí uplift, and (U-Th)/He thermochronometry and vitrinite reflectance record the burial history and timing of exhumation associated with the uplift. Thick-skinned deformation involved folding of sub-décollement strata into a NNW-SSE-striking anticlinorium, development of cleavage, ENE-WSW directed thrust faulting, conjugate strike-slip faulting, and formation of ENE-WSW-striking extension fractures associated with barite mineralization. These structures consistently record ENE-directed subhorizontal shortening. Shortening is directed ~062° in the southern part of the uplift and ~077° in the northern part of the uplift. Thick-skinned deformation modified pre-existing geometries of thin-skinned folds involving Cretaceous overburden strata above the Jurassic evaporite décollement. Zircon (U-Th)/He cooling dates (ZHe) and vitrinite reflectance data indicate that the entire evaporite décollement was buried to ≥185°C, consistent with development of phyllitic fabrics in the basal part of the Minas Viejas Formation. Paleocene to mid-Eocene zircon (U-Th)/He cooling dates in the Triassic-Jurassic red beds below the evaporite décollement directly record the timing of exhumation associated with thick-skinned deformation, and suggest that the thick-skinned uplift was a continuation of earlier thin-skinned shortening as opposed to a distinct tectonic event. A zircon (U-Th)/He date from the southern Potosí uplift is ~66 Ma, whereas ZHe dates in the northern part of the uplift range from ~49–43 Ma. Two samples from the nearby Aramberri uplift to the south of the Potosí uplift have mean ZHe dates of ~63–54 Ma. The transition from thin- to thick-skinned shortening may be attributed to the evolution of mechanical stratigraphy in the décollement. Thin-skinned detachment folding resulted in significant migration of evaporite away from synclinal keels, effectively eliminating a planar weak zone at the base of the décollement and creating salt welds between carbonates or shales within the décollement with underlying red beds. The along-strike differences in timing of thick-skinned exhumation and the shortening directions may also be attributed to differences in mechanical stratigraphy. Thicker intervals of evaporite in the northern part of the uplift allowed thin-skinned shortening to continue while the southern part of the uplift transitioned to thick-skinned shortening as the weak evaporite décollement was exhausted. As a result, stress-strain trajectories in the northern part of the uplift refracted clockwise towards the area with more propagation of deformation. Our findings provide a new insight into the geometry, kinematics, and timing of deformation associated with the Potosí uplift, and may provide a framework for studying other thick-skinned uplifts in the Sierra Madre Oriental, and more generally orogenic belts that record a transition in deformation styles.
Description
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