Submarine Landslides, Canyons, and Morphological Evolution of the East Australian Continental Margin

Australia’s continental margins were generally assumed as relatively inactive; offshore eastern Australia, the Tasman Basin’s margin experiences low rates of sedimentation, seismicity, and geological changes. Bathymetry collected over two voyages on the RV Southern Surveyor have revealed ubiquitous mass failure scars along the East Australian Continental Margin (EACM), with the slopes presenting landslides, canyons, gullies, and other erosional features. These range from shallow-seated upper slope slides mobilising volumes of ~<0.5 km3, to larger middle-slope slump scars incised deeply into the slope that probably displaced volumes of several km3. Dredge samples have been collected from failure scarps and slide. The material is composed of siliciclastic and carbonate silt/sand; biostratigraphic ages indicate that the margin’s geomorphic features formed during the Neogene. Mechanically, the sediment samples are consolidated, rarely weakly-cemented, but mostly unlithified. Numerical modelling based on their geotechnical properties indicates that the slopes are statically stable, and external triggers such as a large earthquake (≥M7) are required to induce failure. None of the other commonly invoked triggers (e.g. rapid sedimentation, isostatic rebound, weak layers, gas hydrate dissociation) appear to be applicable to the setting. Thus the EACM presents a paradox: a tectonically-stable margin with low sedimentation, presenting evidence of large and widespread failure. The dredge samples and slope geometry strongly suggest that the continental slopes are inherently stable. An integrative model posits that the combination of increased seismicity and the intensified oceanic currents during the Late Miocene and Pliocene caused extensive slope dissection and failure. This thesis will investigate the morphological evolution of the EACM using bathymetric data and sediment samples, and examine various aspects of the integrative model through a series of case studies.