Abstract
Submarine landslides are documented on active margins globally, where they tend to be concentrated along the frontal thrust of an accretionary prism. Historical evidence has suggested a variety of causal mechanisms that could generate submarine landslides on active margins. These include preconditioning mechanisms such as oversteepening, long-term tectonics, free gas and gas hydrates and climate change. Documented triggering mechanisms can include seismic ground shaking, excess pore pressure within the sediment, tidal-loading, and erosion. This study focuses on determining which mechanisms are most likely to have caused submarine landslides in the past by investigating mass transport deposits (MTDs) buried in the subsurface of anticlinal thrust ridges located on the accretionary wedge of southern Hikurangi margin. Repeat surveying at the margin and around the globe has failed to detect large landslide events and their cause, suggesting a need to investigate critical processes and factors contributing to these events other than earthquakes.
Geophysical investigations of these complex events are fundamental to improving our understanding of historical landslide frequency and their potential to generate tsunamigenic hazards. This study uses 2-D seismic data collected aboard the TAN1808 voyage along the Hikurangi Margin to map preserved MTDs that have been imaged off the southern Wairarapa coast. These data contribute to New Zealand’s first margin-wide MTD database. For this study, three sites have been used in the characterisation of MTDs that occur on anticlinal thrust ridges located close to the deformation front; these sites have contrasting subsurface characteristics and bathymetric expressions on the present-day seafloor. Analysis of the seismic data at these three locations has been undertaken using the state-of-the-art IHS Markit Kingdom software package which has facilitated a constrained geological interpretation of the seismic reflection data.
The results of this study provide a detailed characterisation of submarine landslides occurring along the targeted active margin. In total, 16 MTDs have been mapped on Mungaroa Ridge (oldest), four on Pahaua Ridge, and seven on Glendhu and Honeycomb ridges (youngest). An in-depth analysis of seismic data reveals that most mapped MTDs were emplaced within the syn-tectonic phase of thrust ridge growth, revealing that the growth of thrust ridges likely contributes to seafloor instabilities and gives rise to casual mechanisms that can be activated by tectonic deformation. In order to determine whether the MTDs were deposited before or post ridge growth, kinematic indicators and broad-scale geomorphic features associated with slope instability were identified. The unique features observed and identification techniques applied can be applied to seismic data examples from other accretionary margins around the world.