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Abstract: This study uses fluid inclusions in quartz and calcite veins to characterize the fracture-controlled paleohydrology of a map-scale, evaporite-cored detachment fold in the Sierra Madre Oriental of northeastern Mexico. Field observations indicate that the veins are tectonic in origin, and that they formed in a general sequence that corresponds to four broad stages of progressive folding. We collected samples from each vein stage in various structural and stratigraphic positions across the fold. After determining the mineral paragenesis and origin of inclusions in each sample, we used standard microthermometric techniques to measure the homogenization temperatures (Th ), salinities and eutectic temperatures of the available two- and three-phase aqueous inclusions. Neither Th , salinity nor eutectic temperature varied systematically with inclusion origin or mineral type. The data are also not significantly correlated with structural position or vein type, but are strongly partitioned by stratigraphy, suggesting that the area was a vertically stratified hydrologic system consisting of three regional paleohydrostratigraphic units. An upper unit comprised the Indidura Formation through Difunta Group, and is characterized by Th near 150°C and salinity <5wt% NaCl equivalent. A middle unit comprised the Taraises and Cupido Formations, and is characterized by Th near 150°C and salinities near 12 or 22wt% NaCl equivalent. The lower unit comprised the Zuloaga and La Casita Formations, and is characterized by an average Th near 225°C and salinities in three groups near 12, 22 or 36wt% NaCl equivalent. The preservation of multiple, distinct fluid types in many veins suggests that fracture development during folding and uplift created a second paleohydrologic system that overprinted the vertically stratified system. Fracture-controlled fluid migration in this second system occurred in periodic, repeated, and spatially variable pulses. [Copyright &y& Elsevier]

Abstract: Geologists now recognize the presence and abundance of detachment folds in various contractional settings. Several two-dimensional geometric and kinematic models exist to describe the development of such structures. These models typically are area-balanced, are interpreted to develop by hinge migration, limb rotation, or a combination of the two processes, and possess a constant regional level outside the fold itself. We present two new two-dimensional geometric and kinematic models for detachment folds that incorporate hinge migration and limb rotation as their deformation mechanisms. These area-balanced models differ from previous models, however, in that they allow ‘hinterland inflation’ to occur, thereby creating a higher local regional level in the hinterland relative to the foreland. This local regional difference across the detachment fold may reflect hinterland uplift due to thickening of the incompetent unit, foreland deflation of the incompetent unit as material migrates into the fold core during fold growth, and/or migration of material out of the fold core as the fold tightens. We illustrate the utility of these models by constructing pseudo-three-dimensional representations of the western termination of the Nuncios Fold Complex in the Monterrey Salient, northeastern Mexico. Our results suggest that the limb rotation model more accurately portrays the overall three-dimensional fold geometry for the Nuncios Fold Complex, matches the observed hinterland inflation, and predicts more reasonable detachment depths for the structure along its length. These results agree with interpreted kinematics based on field observations and illustrate how this modeling approach may help constrain interpretations of detachment folds with hinterland inflation in areas lacking sufficient data. [Copyright &y& Elsevier]