26. HYDROGEOCHEMISTRY, ODP LEG 110: AN OVERVIEW1

Drilling of the northern Barbados accretionary complex transect during Leg 110 of the Ocean Drilling Program pro­ vided a unique opportunity to study the effects of sedimentary thickening, overthrusting, and faulting on the evolution of the chemical composition of interstitial waters. The production of low-chloride fluids farther in the complex has en­ abled evaluation of the main pathways of fluid expulsion from the complex. These are (1) along faults in the accretion­ ary prism itself and (2) along the decollement and through underlying sandstones. In the latter case, methane anoma­ lies, presumably caused by methane production as a result of thermal decomposition of organic matter farther into the complex, accompany the chloride anomalies. Major cation distributions (Ca2+, Mg2+, K+, Na+) are primarily af­ fected by alteration of volcanic ash and exchange with formation waters in underlying basement rocks. However, as a result of tectonic activity in the complex, exchange with the overlying ocean is diminished and concentration-decreases in Mg2+, S042_, and K+ become larger in an arcward direction. Major cation distributions rule out the importance of pervasive advection through the sediments, thus confirming that the main pathways for fluid expulsion are along faults, the decollement, and underlying sandstones.