Evaporite deformation in the Sierra Madre Oriental, northeastern Mexico : décollement kinematics in an evaporite-detached thin-skinned fold belt
Décollements are important tectonic elements in thin-skinned fold-thrust belts. However, few studies have addressed the internal structure of décollements because most are deeply buried and internal features typically cannot be resolved in seismic reflection images. Upper Jurassic evaporite exposures in the Potosí uplift of northeastern Mexico provide a unique tectonic window into the décollement of the Laramide-age Sierra Madre Oriental fold belt. In order to constrain the three-dimensional geometry of décollement structures, I mapped a ~20 km2 portion of the décollement at a scale of 1:10,000. I created a new stratigraphy for the décollement interval during mapping, and made detailed structural observations at targeted sites. The ~900 m thick décollement interval consists of gypsum with five carbonate members (up to 120 m thick) and numerous thin (<5 m) carbonate interbeds. These carbonate units delineate map-scale structural patterns and define two structural domains. The middle and upper parts of the décollement in the western domain contain map-scale folds with local map-scale boudinage and thrust faults. The eastern domain exposes the lower part of the décollement, and contains thrust repetitions of carbonate members and a regionally-persistent basal shear zone. These map relationships indicate a stratigraphic variation in structural style. Western domain folds and eastern domain thrust sheets both appear to be related kinematically to overburden folding. In contrast, the basal shear zone accommodated décollement-parallel shear strain in response to overburden translation. Folding and faulting of carbonate members and intervening gypsum units drove localization of simple shear into the basal shear zone, because only the lowermost gypsum interval maintained a favorable orientation sub-parallel to the regional transport direction throughout deformation. This investigation demonstrates that décollements have complex internal structural patterns that are below typical seismic resolution and lateral variations in structural style that cannot be reconstructed from single well cores or small outcrops. Décollement stratigraphy controls variations in strain magnitude within the décollement interval, so that previous models that invoke homogeneous strain within the décollement are incorrect. Complex, laterally-variable structural style and stratigraphic control of strain distribution could be general characteristics of décollements where the décollement interval contains significant contrasts in bed rheology.