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Reconstruction of a giant submarine slope-failure on the northern edge of New Zealand

April 26, 2001

The continental margins, submarine areas at the boundary between the continent and the oceanic abyssal floor are unstable. This instability is manifested by submarine slides or collapse events. These are sometimes catastrophic, taking away portions of coast down to the deep ocean floor or locally causing tsunami. Such slope-failures are rare in the human time-scale, but frequent in geological terms. They can occur on all continental margins and are particularly spectacular and violent on the flanks of oceanic volcanoes (Hawaii, Canary Islands, Vanuatu, for example) and on the so-called "active" submarine margins (2). Beyond their effects on coastlines, these submarine geological processes could have a more wide-ranging impact, on the climate, in that they liberate a large volume of greenhouse gases, mainly methane formerly trapped in sediments. There are many different causes of the submarine slope failures including: earthquakes, tectonic deformation, sea-level changes and rapid thickening of layers of margin sediments.
Between 1993 and 1999, IRD scientists, in collaboration with the CNRS and the University of Nice (Research unit UMR Géosciences Azur) (1) and New Zealand researchers from NIWA conducted a geophysical study of an active convergent continental margin, situated close to the northern coast of New Zealand, southern end of the Kermadec-Hikurangi subduction margin. The campaign, performed using a multibeam swath bathymetry probe installed on the IFREMER research vessel Atalante discovered an exceptionally large slope failure on that margin which had caused an enormous avalanche. Following geophysical and geological investigations conducted later, the research team were able to reconstruct the detailed morphology of the slide, analyse the avalanche debris, date the events and identify the causes.
The slope-failure, exceptional in scale, was the impulse for a giant avalanche. The debris cover an area of 3400 km_ with a volume of 3000 km3 and are up to 2 km thick. A three-dimensional map of the slip gives a definite idea of the size of the event. Under the effect of gravity and tectonic stresses, an enormous slide-block (abvout 40 x 35 km) of the submarine continental slope detached itself at 1200 m depth, leaving a scar, an indentation about 1500 m high indicative of the violence of the break. The portion of coast broke up into 100 or so huge blocks of sediment (of over 1 km long). The largest of these was 18 km long and over 2 km high, whereas about 20 were more than 5 km long. The fact that they were displaced over nearly 50 km on the abyssal plateau (3500 m below sea level) demonstrated the high amount of energy released when the avalanche occurred.
Seismic reflection profiles, obtained from acoustic waves emitted by an air gun and recorded by a multi-channel array of hydrophones pulled by the research ship, determined the thickness and the geological structure of the avalanche debris. The images obtained show that, although they were displaced and shattered, some blocks have maintained their original sedimentary fine structure. The blocks are now part of a complex sedimentary matrix and the whole reaches a thickness of 1.7 to 2 km. Other seismic profiles indicate that the sediments initially present on the abyssal plain were severely disturbed by the avalanche and were covered up by a dense mud flow, 100 m thick and spreading up to about 100 km on to the open oceanic plateau.
On analysis of cores taken from the fine cover beds (80-100 m) which mask the avalanche debris and the mud flow, the geologists were able to date this major event to about 170 000 years B. P. Several indicators suggest that this slope failure results partly from the oblique subduction of one or more submarine volcanoes. Moved along as if on a conveyor belt by the Pacific plate at 6 cm per year, they would have collided with the New Zealand continental margin, the edge of the Australian plate, about 1.5 to 2 million years ago, lifting it up and deforming it, before it collapsed in their wake 170 000 years B. P.
The Géosciences Azur research unit is continuing the study of the Ruatoria slip. Work is in progress to decipher other submarine slip events and their relation to seismic activity on the active margins of the South West Pacific and also of the Andes in the context of research programmes on instability of sedimentary structures.

(1) CNRS, Université de Nice, Université Paris VI and IRD
(2) Regions of intense deformation, where the lithospheric plate pushes progressively down, by subduction under an adjacent plate.

Institut de Recherche pour le Développement, Paris (IRD)