Polyphase emplacement of a 30 km3 blocky debris avalanche and its role in slope-gully development

Abstract

Failure of the upper slope of the Hikurangi subduction margin resulted in emplacement of ∼30 km3 of debris across ∼250 km2 area of a mid-slope basin. The landslide deposit is well preserved in slope morphology. In this study we use morphometric analysis of Simrad EM300 multibeam data and stratigraphic analysis of multichannel seismic reflection data to assess a possible role for deep-seated bedrock failure in the erosional development of the upper continental slope. We interpret the blocky debris avalanche deposit as resulting from retrogressive polyphase failure. Late stage events are inferred to have impacted earlier debris and caused localised secondary remobilization failure. Morphometric analysis of upper-slope gully development indicates that gully systems become less mature to the north. Among several possible explanations for this pattern of gully development we infer a causal link between large-scale mass failure retrogression and the initiation of slope gully erosion systems. © Springer Science+Business Media B.V. 2012.