Geological Study of Active Cold Seeps in the Syn-collision Accretionary Prism Kaoping Slope off SW Taiwan

Abstract

Pogonophoran tube worms, elongated pyrite tubes and authigenic carbonate nodules are used to evaluate the occurrence of potential cold seeps in the syn-collision accretionary prism Kaoping Slope off SW Taiwan. At least two species of pogonophoran tubeworms were found in surface and core sediments. Pyrites occur in three different forms: fillings inside foraminiferal chambers, cements between calcareous microfossils, and elongated tubes. The bottom water off SW Taiwan is aerobic, but authigenic pyrites are found in the surface sediments at several sites, suggesting the existence of local reducing environments enabling the formation of pyrites. These environments are most likely caused by the occurrence of active cold seeps where methane expulses. Authigenic carbonates with highly depleted carbon isotope values (-54 to -43‰) were found at more than 5 locations, in agreement with a methane-derived source for the carbon.

Integrating these geological and biological results, this study demonstrates that there are active cold seeps with methane expulsions in several anticline hinge zones east of the associated thrust faults in water depths of 800 - 1500 m of the frontal syn-collision accretionary prism Kaoping Slope. The existence of these high potential active cold seep sites is also supported by the occurrences of distinct bottom simulating reflectors (BSRs), shallow sulfate-methane interface and high concentrations of dissolved methane in bottom water.

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