Active fluid and gas transport were measured and observed along more than 200 km of the convergent margin of Costa Rica during cruise SO144-2 aboard RV Sonne. Ten profiles were run with the TV-sled OFOS, eight of which detected the dense occurrence of cold vent sites. This discovery shows that seafloor fluid expulsion is widely spread along the Pacific margin of Costa Rica. Surficial evidence of fluid expulsion is indicated by the appearance of chemosynthetic vent organisms such as bacterial mats, vesicomyid, solemyid and mytilid bivalves and tubeworms. Numerous active vents were indicated by elevated methane concentrations (£ 200 nmol L -1 ) in the bottom water. Although fluid-venting activity was known previously from a small area south of Nicoya Peninsula, the present study documents active seepage at landslides, headwall scarps related to seamount subduction, morphological intersections of faults and mid-slope mud volcanoes.
The Kurile Basin in the Okhotsk Sea, northwestern Pacific, is a back‐arc basin located behind the Kurile Island Arc. It is underlain by oceanic crust and its origin has been attributed to back‐arc spreading. Two models for the opening of the Kurile Basin exist, for which the spreading axis is oriented northeast–southwest and northwest–southeast, respectively. New data are presented here on the morphostructure of the slope of the northern Kurile Basin and of the central Kurile Basin which support a strike of the spreading axis in the latter direction. Bathymetric as well as single‐channel and multichannel seismic reflection data demonstrate the existence of dominant northwest‐striking normal faults on the northern slope of the Kurile Basin. In the central Kurile Basin a basement rise striking north‐northwest–south‐southeast (here named the Sakura Rise) was mapped. The rise morphology has the distinct imprint of a rift structure with symmetrical volcanic edifices on the rise axis and faulted blocks that tilt in opposite directions on the flanks. These data suggest that the Kurile Basin opened in a northeast–southwest direction. In the generally accepted plate tectonic reconstructions, northwest–southeast spreading associated with dextral strike–slip along the north–south‐striking shear zone of Sakhalin and Hokkaido islands has been assumed. In the present model, spreading in the Kurile Basin was presumably connected with dextral displacement along a northeast‐striking shear zone on the southern segment of the Okhotsk Sea.
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