Short, steep, and supercritical—Submarine fans in active rifts

High-resolution seafloor data from the Gulf of Corinth, Greece, reveal new insights into rift-margin submarine fans deposited at the base of fault scarps. The steep (13°) Sythas canyon cuts into the submarine Lykoporia fault and is carpeted by low-relief, elongate rhomboidal bed forms. Across the fault, the canyon passes abruptly into the 6-km-radius Sythas submarine fan. The proximal fan (∼3° gradient) contains multiple discontinuous channels, erosional cyclic steps, and scour fields. Channels shallow and broaden onto the medial fan (∼1° gradient), which is characterized by trains of depositional cyclic steps that pass into low-relief lobes on the distal fan (∼0.18° gradient). These morphological relationships compare to the submarine slopes of fan deltas and submarine fans in fjords but contrast markedly with established channel-levee distributary models of submarine fans. Sediment gravity flows were likely stratified, with a thin, concentrated, coarse-grained, and supercritical basal layer. Smooth interchannel areas suggest the basal layer (<5 m thick) was contained within erosional relief, before dissipating rapidly within ∼3 km of the canyon mouth, with flows transformed into dilute, subcritical turbidity currents on the distal fan. This is the first time a continuum of proximal to distal bed forms has been documented on an active base-of-fault-scarp submarine fan and linked to sediment-gravity-flow structure and transitions from supercritical through transcritical to subcritical conditions.