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

Jean-Yves Collot (1) , Keith Lewis (2) , Geoffroy Lamarche (3) , Serge Lallemand (4)

1 Service de la Géologie de Nouvelle Calédonie
2 UAG UFR SM - Université des Antilles et de la Guyane - UFR des sciences médicales
3 NIWA - National Institute of Water and Atmospheric Research [Wellington]
4 Laboratoire de Géophysique, Tectonique et Sédimentologie

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 km 3, 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 ø 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 km 3 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.

Mots clés: Hikurangi margin; New-Zealand; seamount subduction

Domaines: Tectonique

Jean-Yves Collot, Keith Lewis, Geoffroy Lamarche, Serge Lallemand. The giant Ruatoria debris avalanche on the northern Hikurangi margin, New Zealand: Result of oblique seamount subduction. Journal of Geophysical Research, 2001, 106 (B9), pp.19271-19279. ⟨10.1029/2001JB900004⟩. ⟨hal-01261406⟩