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TitreGeophysical signatures of cold vents on the northern Cascadia margin
AuteurRiedel, M; Paull, C K; Spence, G; Hyndman, R D; Caress, D W; Thomas, H; Lundsten, E; Ussler, W; Schwalenberg, K
Source 2009, 1 pages
LiensOnline - En ligne
Séries alt.Secteur des sciences de la Terre, Contribution externe 20090380
RéunionAmerican Geophysical Union, Fall Meeting; San Francisco; US; décembre 14-18, 2009
Mediaen ligne; numérique
ProvinceRégion extracotière de l'ouest
Sujetstopographie du fond océanique; topographie du fond océanique; caractéristiques structurales; failles; hydrocarbures; gaz; hydrate; méthane; géologie marine; combustibles fossiles
ProgrammeCaractérisation des Hydrates de gaz, Hydrates de gaz
Résumé(disponible en anglais seulement)
The accretionary prism of the northern Cascadia margin is a classic gas hydrate research area. Ocean Drilling Program Leg 146 and Integrated Ocean Drilling Program (IODP) Expedition 311 documented that gas hydrate is widely distributed across the margin. In recent years an increased research focus has been on cold vents, where methane gas is actively released. Two recent expeditions funded by the Monterey Bay Aquarium Research Institute (MBARI) were conducted in the area of IODP Sites U1327 and U1328. An autonomous underwater vehicle (AUV) was used to map the seafloor bathymetry followed by dives with the ROV Doc Ricketts for ground truth information of various seafloor morphological features identified. The two cruises revealed many new seafloor features indicative of methane venting that were previously unknown. Bullseye Vent (BV) has been extensively studied using seismic imaging, piston coring, heat-flow, controlled-source EM, and deep drilling. BV is seismically defined by a circular wipe-out zone but the new AUV data show that BV is rather an elongated depression. BV is associated with a shoaling in the BSR, but lacks evidence for the existence of an underlying fault in the previous data. Although a massive gas-hydrate plug was encountered within the top 40 mbsf in the IODP holes, the ROV observations only revealed some platy methane derived carbonate outcrops at the outer-most rim of the depressions, a few beds of Vesicomya clams, and no observed gas vents, which together do not indicate that BV is especially active now. Further northeast of BV, but along the same trend, active gas venting was found associated with seafloor blistering and bacterial mats suggesting that there is an underlying fault system providing a fluid flow conduit. The newly discovered vent area has few seismic line crossings; however the available seismic data surprisingly are not associated with wipe-out zones. Another prominent fault-related gas vent also was investigated during the two MBARI expeditions in 2009 (Spinnaker Vent, SV). Seismic profiles over SV show blanking and a slight uplift of the BSR that underlies the vent-area. The seafloor morphological expressions (trending over ~400 m) are similar to the elongated series of depressions seen at BV, but SV overall appears more active and younger due to the presence of widespread chemosynthetic communities, methane bubbling, massive outcrops of methane-derived carbonate as well as seafloor gas-hydrate bearing mounds. The seafloor features at SV all follow a fault trend that is clearly seen on the AUV bathymetry map, as also suggested by the earlier seismic data. Together the new MBARI expeditions and previous studies show that the area investigated on the N. Cascadia margin is dominated by fluid escape features. At least 12 cold vents (7 with bubble-plumes) are now identified within an area of ~10 km2 making a re-evaluation of the methane hydrate and associated underlying fluid-flow regimes an important focus of future studies.