Does Retrogression Always Account for the Large Volume of Submarine Megaslides? Evidence to the Contrary From the Tampen Slide, Offshore Norway

Submarine landslides can be several orders of magnitude larger than their terrestrial counterparts and pose significant hazards across entire ocean basins. The landslide failure mechanism strongly controls the associated tsunami hazard. Tampen Slide offshore Norway is one largest on Earth but remains poorly understood due to its subsequent burial beneath up 450 m sediments. Here, we use laterally extensive (16,000 km2), high-resolution processed 3-D seismic reflection data characterize upper Slide. We identify longitudinal (downslope, movement-parallel) chutes ridges that are 40 high, as well extensional compressional (cross-slope) ridges. This first time such size have been imaged in a deep marine setting. phase involved simultaneous translation over 720 km3 sediments along single plane. was followed by spreading head- sidewall, formation retrogressive debris flow slump, volumes which insignificant compared failure. process responsible for movement large sediment volume glide plane differs significantly from other passive margin megaslides, typically comprise numerous smaller fail retrogressively multiple planes. trigger (e.g., an earthquake), presence mechanically strong obstructions volcanic structural high), number location weak layers may key factors determine whether megaslides develop or retrogressively.