FOLD AND SHEAR ZONE DEVELOPMENT WITHIN THE EVAPORITE DéCOLLEMENT OF THE SIERRA MADRE ORIENTAL FOLD BELT, NORTHEASTERN MEXICO

Upper Jurassic evaporites exposed in the Galeana area of northeastern Mexico preserve a diverse suite of well-exposed structures. Here, the décollement of the Laramide-age Sierra Madre Oriental fold belt has been exhumed by the thick-skinned, late- or post-Laramide Potosí uplift. We have mapped (at 1:10,000 scale) an 8 km long outcrop strip that exposes the full décollement stratigraphy. The ~850 m thick décollement interval consists of calcareous gypsum-anhydrite with 5 regionally-persistent carbonate members (up to 120 m thick) and numerous thinner (<5 m) carbonate interbeds. These carbonate units delineate macroscopic structural patterns within the décollement, and define two broad structural domains. The larger western domain is characterized by north, north-northeast and northeast-trending macroscopic folds that affect the middle and upper parts of the décollement section. Most folds are tight to isoclinal and overturned toward the east or southeast. In places, these folds are refolded by later east-trending folds. The smaller eastern domain exposes the lower part of the décollement, and is characterized by open folds, thrust repetition of carbonate members and a regionally persistent shear zone developed within the lowermost evaporite interval. This shear zone (~55 m thick) is marked by spectacular banded mylonites, porphyroclasts, and intrafolial isoclinal folds. Primary sedimentary structures in this interval have been completely obliterated. Such shear zone textures are only locally developed in western domain rocks. Thinner carbonate interbeds in both domains are commonly boudinaged, and where boudinage is extreme a mélange texture is developed. The thick carbonate members locally show map-scale boudinage. Décollement structures represent two distinct kinematic processes. Folding of western domain rocks likely reflects redistribution of décollement material in response to folding of the overburden. Western domain thrusts and shear zones likely reflect heterogeneous bulk simple shear associated with the bedding-parallel displacement of the rocks above the décollement relative to the sub-décollement basement. Stratigraphic compartmentalization of the structures associated with each process reflects strain partitioning during décollement deformation.