Abstract
RV Sonne cruise SO139 discovered active fluid venting at an anticline structure in the fore-arc basin of the Sunda Arc south of Java. Fluid venting is indicated by methane anomalies in the water column, elevated heat flow, vent-typical macrofauna, authigenic carbonate precipitation at the sea floor and methane-rich pore fluids of the sediments. The vent site ‘Snail Hill’ is located at an up to 90 m elevated topographic high which formed immediately atop the folded basin sequence. Gas escaping from the site is the source for methane anomalies in the water column with values up to 5000 nl/l; depth positions of maximum concentrations correspond to the peak elevation of the topographic high. Heat-flow values close to the site are elevated three to five times relative to regional background values. The main venting area at 2910–2920 m water depth is restricted to an elongated area near the summit. It is characterised by clusters of giant white bivalves (probably Vesicomyidae), black sulphidic sediment patches and authigenic carbonate slabs. In addition, we observed low seep activity over a relatively large area as suggested by the widespread distribution of burrowing bivalves ( Acharax sp.) and of pogonophoran tube worms ( Lamellisabella sp. and probably Oligobrachia sp.). Pore water of reduced salinity at the vent site suggests destabilisation of gas hydrates. Bottom-simulating reflectors (BSRs) rising steeply towards the vent location and potential pathways for rising fluids observed on seismic records support this interpretation; the lack of a (visible) BSR below the vent suggests a perforation of the hydrate-stability zone in the crestal part of the fold. The position of the vent site on top of a tectonic structure which is linked to oblique subduction suggests that the Snail Hill site is not a singular vent phenomenon. We speculate that venting is a common process along compressional/transpressional zones (Ujung Kulon fault zone, Mentawai fault zone) in the northwestern fore arc of the Sunda convergent margin.
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