SEEP FAUNAS AND OTHER INDICATORS OF METHANE-RICH DEWATERING ON NEW-ZEALAND CONVERGENT MARGINS

Fluid seeps, many marked by rich, distinctive, chemosynthetic faunas l iving independently of the Earth's photosynthesis-based food chain, ar e indicators of sedimentological and tectonic processes at convergent margins. Thirteen seep sites, seven marked by distinctive faunas and o thers marked by carbonate chimneys, carbonate crusts, or plumes of low -density fluids on sounder records, are reported from the vicinity of the convergent plate boundary through New Zealand. Sites extend from E ast Cape in the northeast to Otago and Puysegur Ridge in the southwest . Some reports are based on data from fishing and research cruises and some are from archived material. Seeps with different characteristics occur in three geological environments: (1) Calyptogena-based communi tes and active plumes occur on upper slope ridges of subduction margin s; (2) Maorithyas sp., chimneys, and carbonate crusts occur at shelf e dges and canyon heads, often away from subduction margins; (3) Calypto gena sp. and diapirs also occur at the edges of large slumps that have collapsed into sedimentary basins. Seeps generally indicate methane-r ich dewatering from overpressured sediments, with the nature and locat ion of faunas and associated deposits indicating the nature and origin of expelled fluids, their vigour of flow, and their pathways to the s eabed. The subduction margin seeps are characterised by many of the ta xa identified at methane-rich seeps elsewhere. There are three new spe cies of Calyptogena, some live Bathymodiolus-like mussels, vestimentif eran tube worms, and a variety of limpets and other gastropods. The fa unas and associated crusts, chimneys, and fluid plumes occur on mid to upper slope ridges of pre-subduction margin sediments that occur to l andward of any frontally accreted prism. They also occur landward of t he stability field for the methane clathrates, which may form a barrie r to fluid flow on the lower slope. They are associated with carbonate cements with depleted C-13 concentrations indicative of hydrocarbons having a thermogenic origin. It is inferred that fluids are derived by compressional dewatering, mineral dewatering, and chemical alteration of organic matter from slices of water-saturated trench and oceanic s ediments underthrust to 4-6 km beneath the continental slope. Producti on is high because of the conveyor-belt input of thick sediments and t heir rapid underthrusting and burial. Percolation of the overpressured fluids to the upper slope is facilitated by steeper and more dilation al faults than on the lower slope, by permeable lenses in the pre-subd uction sediments, and by lack of a clathrate barrier. Seeps occur wher e permeable layers or fault zones crop out on slope ridges. Shelf edge and canyon head seeps characterised by Maorithyas sp. and carbonate c himneys or crusts are generally inferred to be relict from the seepage of organic-rich groundwater at the height of the last glacial age. Se eps from the toes of slides indicate rapid overloading of basin sedime nts after failure of nearby slopes.