HIGH-RESOLUTION SEISMIC IMAGES OF THE FORMOSA RIDGE OFF SOUTHWESTERN TAIWAN WHERE “HYDROTHERMAL” CHEMOSYNTHETIC COMMUNITY IS PRESENT AT A COLD SEEP SITE Liu, Char-Shine; Morita, Sumito; Liao, Yi-Hsiang; Ku, Chia-Ken; Machiyama, Hideake; Lin, Saulwood; Soh, Wonn
A high-resolution seismic reflection survey was conducted during the NT07-05 cruise over the Formosa Ridge offshore southwestern Taiwan where strong and continuous bottom simulating reflections (BSR) have been observed. Previous seafloor pictures taken from a deep-towed camera indicate that there are some chemosynthetic colonies. During the NT07-05 cruise, not only large and dense chemosynthetic communities were confirmed at the plume site, ROV Hyper-Dolphin has also discovered that both deep-sea mussel Bathymodiolus platifrons, and galatheid crab Shinkaia crosnieri are vigorously populated at this site. By integrating swatch bathymetry, multichannel seismic and high-resolution seismic reflection data, we now have a better understanding on the structural characters of the cold seep site. The cold seep is situated at the summit of the Formosa Ridge southern peak. Submarine canyons that incised continental slope on both sides of the ridge are the controlling factors of the ridge formation. The sedimentary strata are generally flat lying but have been deformed by mass wasting processes. Strong BSR is observed 400 to 500 ms below the seafloor of the ridge, with many bright reflections beneath it. There is a narrow vertical blanking zone raising from BSR to the crest of the ridge. This narrow zone is interpreted to be the fluid conduit of the seep site. BSR may form a good cap to trap gas below, and this “gas reservoir” is shallower than the canyon floors on either side of the ridge. We suggest that this “ridge type” gas reservoir configuration enables the cold sea water to get into the fluid system, and forms a special kind of “hydrothermal” circulation that feeds the unusual chemosynthetic communities observed at the Formosa Ridge cold seep site.
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