Seismic expression, structure and evolution of flow cells within a submarine landslide

Harya Dwi Nugraha , Christopher Aiden-Lee Jackson , Howard D. Johnson, David Hodgson

Submarine landslides are not a singular mass of sediment, but may instead comprise shear zone-bound, intra-flow cells of different sizes, rheological states, and strains. Constraining their evolution is important for assessing risks to subaqueous infrastructure and coastal communities, yet controls on their initiation, translation, and cessation remain unclear. We here use five, high-quality post-stack time-migrated (PSTM) 3D seismic reflection datasets to investigate the structure and evolution of flow cells in the Gorgon Slide, a near-seabed submarine landslide on the Exmouth Plateau, offshore NW Australia. Our data suggest that the slide originated from a c.18 km-long evacuation zone bounded updip by a NE-trending headwall scarp that dips steeply (c.30o) seaward, and travelled northwestward over a strongly erosional basal-shear surface that deepens downdip. The slide is dominated by chaotic seismic reflections, interpreted as a debrite, containing large clasts (up to 1 km-long) derived from the headwall and/or entrained from the layered slope substrate. The morphology of the basal-shear surface focused slide transport, resulting in clustering of megaclasts in the medial part of the still-translating flow. This cluster became an obstacle to subsequent flows, resulting in two flow cells (Cells A and B), separated by a longitudinal shear zone. Interaction between the flow cells is recorded by sinuous flow fabrics within, and pressure ridges on the top surface of, the slide. The intra-flow fabrics and top-surface ridges in Cell A were dragged downdip by the relatively more mobile Cell B, which continued translating downdip without intra-flow obstacles. The transport and emplacement processes inferred for the Gorgon Slide suggest that entrainment and abrasion of megaclasts can induce velocity perturbations during slide emplacement, causing changes in flow rheology, and the initiation and cessation of flow cells. This reconstruction of flow-cell evolution within a submarine landslide may provide geological constraints for future geohazard risk assessments.