Popis: |
Cold seeps have been observed with manned submersibles at many sites on the Tokai margin. Most sites are found on the hanging wall of major thrust faults, within 1–2 km of their outcrop. Few sites relate to purely erosional features. This relationship between thrusting and venting may be explained either by fluid flow along fault zones, by fluid flow along permeable layers outcropping at the fault scarp, or by diffuse upward flow of fluids released during footwall compaction. We use deep-towed side-scan sonar and sediment sounder profiles acquired at the toe of the Nankai accretionary wedge to identify the context of seepage at a smaller scale. Modes of fluid expulsion inferred from observations in this zone include flow channelling along strata, along thrusts, along gravity faults, and along joint fractures as well as diffuse flow. We then combine visual observation and monitoring of electrochemical potential (Eh) to assess the relative importance of seepage sites. We show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites, cemented by authigenic carbonates. These were sampled and have permeability high enough to channel fluid flow (1–4×10−14 m2). We suggest they are the main conduits near the seafloor. However, seepage sites are also observed directly on the outcrop of major thrusts, indicating that they do play a role as fluid conduits. We observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance. |