Fluid migration during Eocene thrust emplacement in the south Pyrenean foreland basin (Spain): an integrated structural, mineralogical and geochemical approach

Abstract

Abstract In the frontal part of the south Pyrenean Eocene thrust-fault system, syn-kinematic fluid flow during the early compressional deformation of the foreland basin marls is evidenced macroscopically by the abundance of calcite shear veins within the thrust-fault zones and folds. The geometry and distribution of the veins are indicative of the mechanisms and kinematics of fluid-deformation relationships, and give assessment of the fluid migration paths. The crack-seal mechanism of formation of the shear veins attests to the episodic nature of fault-slip and associated fluid flow in fractures. The distribution of the veins suggests that the main source of fluid was the dewatering of the overpressured, poorly permeable marls from the thrust footwalls, probably related to both (i) vertical compaction due to burial under thrust sheets and (ii) tectonic horizontal shortening. These fluids drained upwards towards the thrust-fault zones, in which they migrated laterally towards the thrust front due to the anisotropy of the fracture permeability in these zones. The geochemistry of the vein-filling minerals and their comparison with the geochemistry and mineralogy of the host marls are indicative of the fluid types, fluid origins, fluid-sediment interactions, and fluid migration paths. The δ 34 S and 87 Sr/ 86 Sr ratio of the host marl calcite and of the calcite and celestite in the veins away from the thrust-fault zones indicate that the original water trapped interstitially in the marls was Eocene seawater. The elemental composition (Ca, Sr, Mg, Mn, and Fe), δ 18 O, and δ 13 C of the same samples reveal a change of the pore-water composition from marine to formation water during the early burial stage. Fluid-inclusion analyses of the celestite in the veins reveal the presence of a hot, saline ascending fluid restricted to these discontinuities, where it was mixed with the local formation water. These two types of fluids drained towards the thrust-fault zones where they acquired a higher 87 Sr/ 86 Sr ratio, probably related to local fluid-sediment reactions. Indeed, dickite precipitated during cleavage formation in the most intensely strained part of the fault zones, and its formation was probably mainly controlled by stress. δ 18 O depletion in the calcite from the structurally highest/innermost thrust-fault zones suggests also the influence of meteoric water derived from the emerged part of the belt in these structures. The earlier fluid regime in the Ainsa basin was an intergranular (porous) flow regime (compactional flow) allowing for a pervasive isotopic, and elemental exchange of the marls prior to vein formation. With the onset of compressional deformation, channelized flow along tectonic slip surfaces became dominant.