Temporal variability of gas seeps offshore New Zealand: multi-frequency geoacoustic imaging of the Wairarapa area, Hikurangi margin

Cold seeps on Opouawe Bank, situated in around 1000 m water depth on the Hikurangi Margin offshore North Island. New Zealand, were investigated using multibeam bathymetry, 75 and 410 kHz sidescan sonar imagery, and 2–8 kHz Chirp sediment echosounder data. Towed video camera observations allowed ground-truthing the various geoacoustic data. At least eleven different seep locations displaying a range of seep activity were identified in the study area. The study area consists of an elongated, northward-widening ridge that is part of the accretionary Hikurangi Margin and is well separated from direct terrigenous input by margin channels surrounding the ridge. The geoacoustic signature of individual cold-seep sites ranged from smooth areas with slightly elevated backscatter intensity resulting from high gas content or the presence of near-surface gas hydrates, to rough areas with widespread patches of carbonates at the seafloor. Five cold seeps also show indications for active gas emissions in the form of acoustic plumes in the water column. Repeated sidescan sonar imagery of the plumes indicates they are highly variable in intensity and direction in the water column, probably reflecting the control of gas emission by tides and currents. Although gas emission appears strongly focused in the Wairarapa area, the actual extents of the cold seep structures are much wider in the subsurface as is shown by sediment echosounder profiles, where large gas fronts were observed

La Ride méditerranéenne (Méditerranée orientale): apports de la cartographie multifaisceaux à l'analyse morphologique d'un prisme en accrétion-collision

Géomorphologie : Relief, Processus, Environnement, v. 2, p. 91-104, 2005International audienc

Geology of mud volcanos in the Eastern Mediterranean from combined sidescan and submersible surveys

Deep Sea Research Part I: Oceanographic Research Papers, v. 52, n. 3, p. 457-475, 2005. http://dx.doi.org/10.1016/j.dsr.2004.10.005International audienc

The Rhodes Basin, a Post-Miocene Tectonic Trough

New light is shed on the enigmatic Rhodes deep marine basin by a set of underway geophysical data across it, including swath bathymetry, seismic reflection profiles, gravity and magnetic data. This deep trough (as much as 4485 m) to the east of the Island of Rhodes and close to southwestern Turkey, contains no Messinian evaporites and only a thin Pliocene-Quaternary sedimentary section (< 1000 m) overlying an acoustic basement. This basement is likely composed of pre-Miocene tectonized rocks tentatively related to neighbouring Hellenide-Tauride alpine orogens. Reverse faulting, strike-slip faults, sedimentary nappes and mass sliding are presently occurring in the Rhodes Basin mainly around its edges. This tectonic activity, together with a relatively important seismicity (particularly along its northwestern margin), indicates that the basin is still evolving. We suggest that the Rhodes Basin results from a general collapse of what is now its brittle basement, in connection with the progressive development of transform motion along the eastern branch of the Hellenic Arc. If our hypotheses are correct, the Rhodes Basin is a relatively recent (post-Miocene) foundered trough, somewhat similar to pull-apart basins which have also developed along the transpressive branch of the Hellenic subduction zone, particularly along the Pliny Trench. (C) 2000 Elsevier Science B.V