Seep faunas and other indicators of methane‐rich dewatering on New Zealand convergent margins

Abstract

Fluid seeps, many marked by rich, distinctive, chemosynthetic faunas living independently of the Earth's photosynthesis‐based food chain, are indicators of sedimentological and tectonic processes at convergent margins. Thirteen seep sites, seven marked by distinctive faunas and others 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 East 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 communites and active plumes occur on upper slope ridges of subduction margins; (2) Maorithyas sp., chimneys, and carbonate crusts occur at shelf edges and canyon heads, often away from subduction margins; (3) Calyptogena sp. and diapirs also occur at the edges of large slumps that have collapsed into sedimentary basins. Seeps generally indicate methane‐rich dewatering from overpressured sediments, with the nature and location of faunas and associated deposits indicating the nature and origin of expelled fluids, their vigour of flow, and their pathways to the seabed.

The subduction margin seeps are characterised by many of the taxa identified at methane‐rich seeps elsewhere. There ire three new species of Calyptogena, some live Bathy‐modiolus‐like mussels, vestimentiferan tube worms, and a variety of limpets and other gastropods. The faunas and associated crusts, chimneys, and fluid plumes occur on mid :o upper slope ridges of pre‐subduction margin sediments that occur to landward of any frontally accreted prism. They also occur landward of the stability field for the methane clathrates, which may form a barrier to fluid flow on the lower slope. They are associated with carbonate cements with depleted ¹³C 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 sediments underthrust to 4–6 km beneath the continental slope. Production is high because of the conveyor‐belt input of thick sediments and their rapid underthrusting and burial. Percolation of the overpressured fluids to the upper slope is facilitated by steeper and more dilational faults than on the lower slope, by permeable lenses in the pre‐subduction sediments, and by lack of a clathrate barrier. Seeps occur where permeable layers or fault zones crop out on slope ridges.

Shelf edge and canyon head seeps characterised by Maorithyas sp. and carbonate chimneys or crusts are generally inferred to be relict from the seepage of organic‐rich groundwater at the height of the last glacial age. Seeps from the toes of slides indicate rapid overloading of basin sediments after failure of nearby slopes.

Keywords:

• seep
• seep fauna
• Calyptogena
• methane
• pore fluid
• subduction
• convergent margin