Gas and fluid venting at the Makran accretionary wedge off Pakistan

The Makran accretionary complex shows a distinct bottom-simulating reflecto r, indicating a thick gas-hydrate-bearing horizon between the deformational front and about 1350 m water depth which seals off the upward flow of gas- charged fluids. A field of presently inactive mud diapirs with elevations u p to 65 m was discovered in the abyssal plain seawards of the deformation f ront, suggesting that in the past conditions were favorable for periodic bu t localized vigorous mud diapirism. Regional destabilization of the gas hyd rate leading to focused flow was observed when deep-penetrating, active fau lts reach the base of the gas-hydrate layer, as in a deeply incised submari ne canyon (2100-2500 m water depth). At this location we discovered seeps o f methane and H2S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the lan dward part of the gas-hydrate layer below the Makran margin is being progre ssively uplifted. Due to reduced hydrostatic pressure and rising ocean bott om-water temperatures, gas hydrates are progressively destabilized and diss ociated into hydrate water, methane and H2S. Sediment temperatures lie outs ide the methane stability field wherever water depth is less than 800 m. Ab ove this depth, upward migration of fluids to the seafloor is unimpeded, th us explaining the abundance of randomly distributed gas seeps observed at w ater depths of 350 to 800 m.