SEDIMENT DEWATERING AND PORE FLUID MIGRATION ALONG THRUST FAULTS IN AFORELAND BASIN INFERRED FROM ISOTOPIC AND ELEMENTAL GEOCHEMICAL ANALYSES (EOCENE SOUTHERN PYRENEES, SPAIN)

The lower Eocene Ainsa basin was formed during the first stages of the south-Pyrenean foreland basin evolution due to southwestward migratio n of imbricated thrust-folds. Isotopic and elemental geochemistry of s yn-kinematic veins (calcite and celestite) and their marry host-rock, sampled in three thrust-fault zones and one footwall syncline, allows us to characterize the origin of pore fluids and the early stages of t heir evolution and circulation during the early deformation of the bas in-fill. The isotopic composition of sulfur and the Sr-87/Sr-86 ratios of calcites and celestites from the veins in the footwall syncline sh ow that the original fluid had the isotopic composition of Eocene seaw ater. The different Sr-87/Sr-86 ratio in veins from the thrust-fault z ones compared with the same ratio in the marry host-rock of the footwa ll syncline indicates that the thrust-fault zones acted as conduits fo r advective fluids. The relatively high Sr-87/Sr-86 ratio in the veins related to the thrust-fault zones indicates that the fluid originated from the interaction of seawater with an external fluid coming from d eeper sources or from the meteoric weathering of the emerged part of t he belt. delta(18)O and delta(13)C values of calcites show that the is otopic composition of the calcite cements in veins was controlled by t he isotopic composition of the marry host sediment. Depletion of both delta(18)O and delta(13)C with respect to Eocene seawater composition, together with elemental geochemistry of calcite cements in the veins, points to burial transformations of a seawater-derived fluid to a for mation water composition. The distribution of delta(18)O and delta(13) C values of the marry host-rock and calcite cements in veins of the fo ur outcrops probably resulted from differences in the meteoric water i nfluences. The hydrogeological regime at the toe of the submarine thru st system was dominated by tectonically-induced dewatering of the fore land basin sediments. The thrust-fault zones were the channelizing pat hs for migration of fluids expelled from the surrounding sediments, as well as fluids derived from more internal parts of the belt.