Abstract

The Ainsa basin is part of the inner south-Pyrenean foreland basin and is thrusted in its northern part by the Cotiella nappe. The interpretation of the geochemical composition of calcite cements in shear veins from the autochthonous Eocene basin fill at the thrust front reveals the fluid flow history during the emplacement of the Cotiella nappe. From rocks below the thrust zones, isotopic and elemental composition and fluid inclusion data show that cements were generated during early burial diagenesis from a formation fluid derived from Eocene seawater at temperatures between 160 degrees C and 185 degrees C. Within the thrust zones, strontium isotopic compositions from calcite cements in shear veins indicate circulation of fluids with different composition. Microstructural analysis and petrographical data reveal that shear veins formed in soft sediment during thrust emplacement. A broad calculation of the amount of fluid necessary to precipitate the amount of calcite veins indicates a considerable participation of external water. To evaluate fluid flow from consolidation of the autochthonous rocks, we apply a finite-element model that calculates consolidational fluid flow during the nappe emplacement. We test different cases for nappe permeability and permeability conditions in the thrust zone. Results show that the permeability of the nappe is the principal factor affecting the resulting flow system and that channeled flow through permeable thrust zones alters the flow system. Calculated flow velocities for all cases remain below the values required to generate sufficient fluid mass transport to explain the observed geochemical patterns. Thus, other processes than sediment consolidation such as seismogenic pumping in the thrust zone and interaction with topography driven flow regimes during nappe emplacement may have played an important role during thrusting.

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