FLUID CIRCULATIONS IN MODERN AND ANCIENT ACCRETIONARY WEDGES

Investigations on modern accretionary zones linked to subduction have shown that fluid circulations play an important role in the formation and evolution of accretionary wedges in subduction processes. Studies have recently been developped in ancient complexes where fluid charact eristics are fossilized throughout the evolution within geochemical ma rkers from syntectonically deformed veins. These studies provide addit ional data to those conducted by exploration and seismic on modern acc retionary wedges. The fluids are expelled from the sediments mainly al ong the decollement zones and to a lesser extent diffused in microfrac tures. They originate at depth from 8 to 14 km causing high temperatur es and overpressures in the sediments. Thermal anomalies are the rule along the deformation. However, there is a relative disequilibrium wit h the wall rocks which favour transient fluid-flow regimes instead of a permanent one. This is also attested by the successive generations o f currently veining, showing differences in compositions in hydraulic breccias. Low salinity waters are involved in veins and in expelled fl uids implying a dilution mechanism of original seawater. Organic matte r such as oil and methane is an important constituent of the fluid pha ses entrapped in veins. Methane may increase the fluid molar volume an d then the fluid pressure. An anomalous concentrations have been measu red present-day within expelled fluids. It may result parity from the destabilization of gas clathrates, mainly methane, as indicated by sei smic data (Bottom Simulating Reflectors). Methane is mainly biogenic, but advections of thermogenic gas are sometimes observed. Characteriza tions of palaeofluids along the whole deformation in ancient prisms ma y develop rapidly through the improvement of microanalytical technique s and particularly of those applied to fluid inclusion characterizatio n. Such studies are still rare and will afford very useful data additi onal to those obtained in active wedges.