Fracture-controlled paleohydrology in a map-scale detachment fold: Insights from the analysis of fluid inclusions in calcite and quartz veins

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 <5 wt% NaCl equivalent. A middle unit comprised the Taraises and Cupido Formations, and is characterized by Th near 150 °C and salinities near 12 or 22 wt% 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 36 wt% 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.