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

Mass flows evolve longitudinally during emplacement, but they can also vary laterally by forming discrete, shear zone-bound intraflow cells with different rheological states. Despite being documented in several field and subsurface studies, the controls on the initiation, translation, and cessation of these flow cells remain unclear. We here use five, high-quality post-stack time-migrated (PSTM) 3D seismic reflection datasets to define the seismic expression and investigate the structure and evolution of flow cells in the Gorgon Slide, a near-seabed mass transport complex (MTC) on the Exmouth Plateau, offshore NW Australia. The slide originated from a 30 km-wide, NE-trending headwall scarp that dips steeply (c.30o) seaward and travelled northwestwards over a basal-shear surface that deepens downslope. The slide is dominated by chaotic seismic reflections, which are interpreted as debrite, containing seismically-imaged megaclasts (c.0.05-1 km-long) derived from the slide’s headwall. The morphology and orientation of the basal-shear surface focused slide transport, resulting in the clustering of megaclasts in proximal parts of the translation domain. The megaclast cluster became an obstacle to flow, which resulted in the formation of two flow cells (Cells A and B), separated by a longitudinal shear zone. The interaction between the two cells is recorded by the development of sinuous flow fabrics within, and pressure ridges on the top surface of, the slide. Along the longitudinal shear zone, the flow fabrics and ridges in Cell A were dragged downslope by the relatively faster and/or longer-lasting Cell B, which continued translating downslope in the absence of any intraflow obstacles. The transport processes of the Gorgon Slide show how entrainment and abrasion of megaclasts induced velocity perturbations during emplacement causing: (i) changes to the flow rheology, and (ii) the initiation and cessation of flow cells. A better understanding of how flow cells evolve during MTCs transport may help to refine modelling of the potential impact of MTCs on submarine infrastructure.