Chemistry, source, and behavior of fluids involved in alpine thrusting of the Central Pyrenees

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Abstract
Quartz veins occupying extensional fractures that formed relatively late in the Alpine thrusting history of the Pyrenees contain abundant inclusions of hypersaline brines. Small amounts of bitumen also occur. A range of brine compositions is apparent from the freezing behavior of fluid inclusions from different samples, with large variations in Na/Ca ratio, but many individual samples contain only one brine composition. All brine types display a bimodal distribution of homogenization temperature, which is interpreted as the result of fluctuation in fluid pressure from a maximum value equal to lithostatic at 5–7 km depth and around 270°C, to a value up to 700 bars lower. This is ascribed to dilatancy pumping. Chemical analyses of brines leached from two samples yields balanced analyses for 14 elements. Both analyses appear to represent sedimentary formation waters and are closely analogous to oil field brines. A basement origin is precluded by the presence of bitumens and the low K/Ca ratios. Differences between the analyses probably reflect different source bed mineralogy, including the presence or absence of dolomite, but Triassic red beds are likely to be a major source. The occurrence of these brines in the fault planes provides constraints on both the sources and movement of the fluids involved in thrusting. Significant infiltration of either surface waters or metamorphic water of dehydration can be ruled out because of the high salinities (typically, 20–25 wt % dissolved salts). Hence although permeability was locally high due to fracturing around the faults, fluids were pumped into these fractures from sedimentary formations adjacent to the fault. Individual fluid pathways were short‐lived relative to the duration of the deformation event, so that different fluids were pumped into the faults at different times in response to local fault propagation.