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ABSTRACT: Tectonic Erosion, Accretion, Back-Arc Extension and Slab Length along the Kermadec Subduction Zone

Bernard Pelletier

The Kermadec trench, elongated N20°E from 26°S to 38°S, marks the convergence (7-9 cm per year) between the Pacific plate and a microplate that comprises the Kermadec arc and is separated from the Indo-Australian plate by the extension of the Havre back-arc troughs (1-2 cm per year). Morphology and structure of the Kermadec trench drastically changes near 32°S. Bathymetric and seismic data suggest that the northern part of the Kermadec subduction zone is characterized by tectonic erosion and the southern part by accretion, the transition between the two styles occurring at 32°S where the trench rapidly shallows and is displaced eastward. The thickness of the sedimentary cover of the plunging plate and the convergence rate are not here major factor controlling the tectonic style at the trench because they do not change significantly. In other respects, the slab length rapidly varies at 32°S, the slab tip reaching the depth of 250-300 km in the south and 550 km in the north. It is proposed that the tectonic processes at the trench and thus the morphology and structure of the inner and outer slopes are directly related to the slab length, accretion being associated with a short slab and, in contrast, tectonic erosion with a long slab. Also the morphology of the Havre back-arc domain changes near 32°S, from a relatively shallow trough in the north (2500 m) to an abnormally deep trough in the south (more than 4000 m). Possibly, structure of back-arc domain also depends on the slab length and the tectonic processes at the tre ch. Clearly the Kermadec trench/arc/back-arc system is an exceptional area to study the possible correlations between the slab length and the tectonic style of a convergent margin. Several cruises using swathmapping and multichannel seismic will be proposed in the future to address these problems.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990