FLUID-FLOW IN FORELAND BASINS DURING EMPLACEMENT OF THRUST SHEETS - MODELING THE SOUTH-PYRENEAN AINSA BASIN

The Ainsa basin is part of the inner south-Pyrenean foreland basin and is thrusted in its northern part by the Cotiella nappe. The interpret ation 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. F rom rocks below the thrust zones, isotopic and elemental composition a nd 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 th rust zones, strontium isotopic compositions from calcite cements in sh ear veins indicate circulation of fluids with different composition. M icrostructural analysis and petrographical data reveal that shear vein s formed in soft sediment during thrust emplacement. A broad calculati on of the amount of fluid necessary to precipitate the amount of calci te 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 flo w during the nappe emplacement. We test different cases for nappe perm eability and permeability conditions in the thrust zone. Results show that the permeability of the nappe is the principal factor affecting t he resulting flow system and that channeled flow through permeable thr ust zones alters the flow system. Calculated flow velocities for all c ases remain below the values required to generate sufficient fluid mas s transport to explain the observed geochemical patterns. Thus, other processes than sediment consolidation such as seismogenic pumping in t he thrust zone and interaction with topography driven flow regimes dur ing nappe emplacement may have played an important role during thrusti ng.