Deep-Seated Bedrock Landslides and Submarine Canyon Evolution in an Active Tectonic Margin: Cook Strait, New Zealand

Abstract

The Cook Strait sector of the Hikurangi subduction margin, off south-east central New Zealand, is dominated by a multi-branched canyon system where landslides are widespread. The objective of this study is to determine the character, origin, and influence of these landslides on the evolution of the canyon system. Multibeam bathymetry covering seven submarine canyons is utilised to characterise landslides’ spatial distribution, morphological attributes and area-frequency characteristics. We demonstrate that mass movements within the Cook Strait canyons consist of spatially dense, predominantly retrogressive, small, deep-seated, translational bedrock landslides occurring in Late Cenozoic sequences. These landslides affect up to a quarter of the canyoned area. Concentration of landslides in the shallow canyon reaches (down to 800 m) is attributed to the influence of oceanographic processes originating on the continental shelf such as tide-generated currents, dense shelf water cascading and internal waves. Canyon incision and wall undercutting, locally favoured by underlying lithological control, are proposed as major landslide drivers in Cook Strait. Ground motion during regional earthquakes is considered a secondary cause. Retrogressive landslides are responsible for canyon widening and wall retreat, cross-sectional asymmetry, preconditioning for additional failure, destabilisation of adjacent slopes and delivery of sediment into canyon floors.