Morphometric analysis of Simrad EM300 multibeam bathymetric DEMs reveals details of deformation patterns in a ~
145
km
2 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.
Although submarine landslides have been studied for decades, a persistent challenge is the integration of diverse geoscientific datasets to characterize failure processes. We present a ...core‐log‐seismic integration study of the Tuaheni Landslide Complex to investigate intact sediments beneath the undeformed seafloor as well as post‐failure landslide deposits. Beneath the undeformed seafloor are coherent reflections underlain by a weakly‐reflective and chaotic seismic unit. This chaotic unit is characterized by variable shear strength that correlates with density fluctuations. The basal shear zone of the Tuaheni landslide likely exploited one (or more) of the low shear strength intervals. Within the landslide deposits is a widespread “Intra‐debris Reflector”, previously interpreted as the landslide's basal shear zone. This reflector is a subtle impedance drop around the boundary between upper and lower landslide units. However, there is no pronounced shear strength change across this horizon. Rather, there is a pronounced reduction in shear strength ∼10–15 m above the Intra‐debris Reflector that presumably represents an induced weak layer that developed during failure. Free gas accumulates beneath some regions of the landslide and is widespread deeper in the sedimentary sequence, suggesting that free gas may have played a role in pre‐conditioning the slope to failure. Additional pre‐conditioning or failure triggers could have been seismic shaking and associated transient fluid pressure. Our study underscores the importance of detailed core‐log‐seismic integration approaches for investigating basal shear zone development in submarine landslides.
Plain Language Summary
Submarine landslides move enormous amounts of sediment across the seafloor and have the potential to generate damaging tsunamis. To understand how submarine landslides develop, we need to be able to image and sample beneath the seafloor in regions where landslides have occurred. To image beneath the seafloor we generate sound waves in the ocean and record reflections from those waves, enabling us to produce “seismic images” of sediment layers and structures beneath the seafloor. We then use scientific drilling to sample the sediment layers and measure physical properties. In this study, we combine seismic images and drilling results to investigate a submarine landslide east of New Zealand's North Island. Drilling next to the landslide revealed a ∼25 m‐thick layer of sediment (from ∼75–95 m below the seafloor) that has strong variations in sediment strength and density. We infer that intervals of relatively low strength within this layer developed into the main sliding surface of the landslide. Additionally, results from within the landslide suggest that the process of landslide emplacement has induced a zone of weak sediments closer to the seafloor. Our study demonstrates how combining seismic images and drilling data helps to understand submarine landslide processes.
Key Points
We integrate scientific drilling data with seismic reflection data to investigate the submarine Tuaheni Landslide Complex
Basal shear zone of the landslide likely exploited a relatively low shear strength interval within an older (buried) mass transport deposit
Landslide emplacement seems to have induced an additional weak zone that is shallower than the interpreted base of the landslide deposit
Abstract
Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs ...immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. (1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. (2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. (3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and intersediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a reevaluation of slope stability following future climate‐forced variation in bottom‐water temperature.
Key Points
Low‐velocity active landslides are proposed to occur on the seafloor
Gas hydrates provide a perturbation mechanism for ongoing landslide mobility
We propose an active, mixed hydrate‐sediment seafloor glacier
PDF
