Classification and characteristics of slow-moving subaqueous landslides

Subaqueous landslides are a geohazard that can transport large amounts of sediment down-slope and trigger tsunamis. They may transport sedimentary material either in a catastrophic way or slowly and incrementally. Slow-moving subaqueous landslides (SMSL) are known to destabilise the seafloor and can be a precursor of catastrophic slope failure, but their mechanics and depositional processes are poorly known. SMSL behave and deform differently to catastrophic subaqueous landslides, yet the SMSL-specific morphological and structural characteristics have never been defined, and existing landslide classifications do not differentiate between different SMSL morphologies or failure styles. In this thesis, a review of the morphological and structural characteristics displayed by SMSL deposits in acoustic datasets (bathymetry, 2D & 3D seismic reflection) is conducted, and has been used to develop a process-based classification of SMSL. Four SMSL classes are recognised: creep, spread, slow flank slump and slow mudflow. The Tuaheni Landslide Complex (TLC), located offshore New Zealand, on the Northern Hikurangi Subduction Margin, is an example of SMSL showing a morphology reminiscent of a slow earthflow or a glacier. It shows a complex internal structure, consisting of a faulted creep deposit, resting on top of a lateral spread. The TLC is used as a case study in this thesis, with the objective of improving our understanding of SMSL dynamics. Interpretation of 3D seismic data allowed the mapping of the lateral boundaries of each TLC unit, indicating they are not genetically linked. These units are separated in time by ~5-14 ka and have distinct deformation mechanisms.
Using core description, enhanced core image and borehole resistivity image logs acquired at International Ocean Discovery Program (IODP) drill Site U1517, the internal structure of the TLC is characterised at the mm- to m-scale. Resistivity image analysis provides azimuthally oriented bedding and fracture data, from which the internal structure of formations can be characterised. This thesis is the first study to use resistivity image logs to interpret the internal structure of subaqueous mass-transport deposits (MTDs). The analysis of resistivity images indicates the presence of vertically stacked blocks in the TLC’s lower unit, including an undeformed basal slab. Fracture orientation patterns suggest that TLC blocks were subjected to different stresses. Using data from four IODP drill sites from both active and passive margins, resistivity image logs are used to compare internal deformation in SMSL and in catastrophically emplaced MTDs. Although no significant SMSL-specific internal deformation patterns are found from image log analysis, MTDs show structural trends distinct from the background undeformed units. This novel approach may be used to identify MTDs in drilling data and provide information on emplacement style and pore fluid connectivity.