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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B9, PAGES 19,271–19,297, 2001

The giant Ruatoria debris avalanche on the northern Hikurangi margin, New Zealand: Result of oblique seamount subduction

Jean-Yves Collot

UMR Géosciences Azur, Institut de Recherche pour le Développement, Villefranche sur mer, France

Keith Lewis

National Institute of Water and Atmospheric Research, Wellington, New-Zealand

Geoffroy Lamarche

National Institute of Water and Atmospheric Research, Wellington, New-Zealand

Serge Lallemand

Laboratoire de Géophysique, Tectonique et Sédimentologie, Université de Montpellier II, Montpellier, France

Abstract

Despite convergent margins being unstable systems, most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins. Swath bathymetry and seismic profiles of the northern Hikurangi subduction system, New Zealand, show a tapering 65–30 km wide by 65 km deep margin indentation, with a giant, 3150±630 km³, blocky, debris avalanche deposit projecting 40 km out across horizontal trench fill, and a debris flow deposit projecting over 100 km. Slide blocks are well-bedded, up to 18 km across and 1.2 km high, the largest being at the avalanche deposit's leading edge. Samples dredged from them are mainly Miocene shelf calc-mudstones similar to those outcropping around the indentation. Cores from cover beds suggest that failure occurred ∼170±40 ka, possibly synchronously with a major extension collapse in the upper indentation. However, the northern part of the indentation is much older. The steep, straight northern wall is close to the direction of plate convergence and probably formed around 2.0–0.16 Ma as a large seamount subducted, leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60° angle between groove and oversteepened margin front. The triangle collapsed as a blocky avalanche, leaving a scalloped southern wall and probably causing a large tsunami. Tentative calculations of compacted volumes suggest that the indentation is over 600 km³ larger than the avalanche, supporting a two-stage origin that includes subduction erosion. Since failure, convergence has carried the deposits ∼9 km back toward the margin, causing internal compression. The eventual subduction/accretion of the Ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts.

Received 13 April 2000; accepted 22 December 2000.

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Citation: Collot, J.-Y., K. Lewis, G. Lamarche, and S. Lallemand (2001), The giant Ruatoria debris avalanche on the northern Hikurangi margin, New Zealand: Result of oblique seamount subduction, J. Geophys. Res., 106(B9), 19,271–19,297.

Copyright 2001 by the American Geophysical Union.