Klaucke, Ingo , Sarkar, Sudipta , Bialas, Jörg , Berndt, Christian , Dannowski, Anke , Dumke, Ines, Hillman, Jess, Koch, Stephanie, Nodder, Scott D., Papenberg, Cord and Schneider von Deimling, Jens (2018) Giant depressions on the Chatham Rise offshore New Zealand – Morphology, structure and possible relation to fluid expulsion and bottom currents. Marine Geology, 399 . pp. 158-169. DOI 10.1016/j.margeo.2018.02.011.

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Abstract

Highlights

• Large seafloor depressions with diameters of up 10 km across have been mapped on the southern Chatham Rise, New Zealand.
• Seismic reflection data show scarce indications for vertical fluid flow but no clear link between fluid flow and depressions.
• Methane gas or methane hydrates appear to be absent on the southern Chatham Rise.
• Seismic evidence suggests that vertical fluid flow was likely fuelled by polygonal faulting and silica diagenesis
• The depressions are the results of erosion and sediment drift deposition of bottom currents associated with the Subtropical Front.

Abstract

Several giant seafloor depressions were investigated on the Chatham Rise offshore New Zealand using mainly bathymetric and seismic data, supplemented by sediment cores and reported porewater geochemistry data. The depressions have diameters of up to 11 km and occur on the southern flank of the Chatham Rise in water depths between 600 and 900 m, i.e. roughly underneath the location of the strongest thermal gradients of the Subtropical Front (STF) and characterized by eastward flowing currents. With up to 150 m of relief the depressions cut into post-Miocene deposits. Some of the depressions are partially filled with drift deposits that have similar seismic characteristics as the surrounding sediments and consist of alternations of silty muds and silts. Seismic profiles also show completely filled depressions that no longer have a bathymetric expression. Despite several pipe structures indicating vertical fluid flow, neither active fluid seepage nor indications for past fluid seepage are present at the seafloor of the Chatham Rise. Also, both pore water geochemistry and geophysical data do not show indications for an existing or past gas hydrate system in the area. Instead, seismic data suggest widespread polygonal faulting and the presence of silica diagenetic fronts. The release of mineral-bound water during silica diagenesis or fluid expulsion during sediment compaction can explain the presence of vertical fluid flow features but not the giant depressions themselves. Instead, the depressions are interpreted as the result of scouring by strong bottom currents for which fluid venting may have created the nucleation points.

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