Terrestrial-style slow-moving earthflow kinematics in a submarine landslide complex

Morphometric analysis of Simrad EM300 multibeam bathymetric DEMs reveals details of deformation patterns in a ~ 145 km2 submarine landslide complex that are commonly associated with slow-moving earthflows in terrestrial settings. This mode of failure, where existing landslide debris is remobilised repeatedly along discrete shear boundaries and is progressively conveyed through the complex, has not previously been recognised in the submarine environment. The kinematics contrast with the more traditional models of submarine landslide complex development in which repeated catastrophic failures each mobilise new source material to form a composite stacked landslide deposit. In our study of the Tuaheni landslide complex on the Hikurangi Margin of New Zealand, remobilisation has formed boundary shear zones imaged at the seafloor surface in multibeam data, and at depth in multichannel seismic reflection data. A significant amount of internal deformation has occurred within the debris streams. Phases of deformation appear to be partitioned longitudinally as extensional and contractional zones rooted into a basal decollement, and laterally with strike–slip shears partitioning discrete debris streams. While slow-moving terrestrial earthflows are activated by fluctuating piezometric levels typically controlled by precipitation, different processes cause the equivalent mobility in a submarine earthflow. Elevated pore pressures in submarine earthflows are produced by processes such as earthquake-generated strong ground motion and/or gas/fluid release. Earthflow movement in submarine settings is prolonged by slow dissipation in pore pressure.