ABSTRACT: An Overview of Fore-Arc Bypassing: the Roles of Submarine Canyons and Unconfined Turbidity Currents

M.B. Underwood

Without question, large submarine canyons are effective conduits for moving coarse-grained siliciclastic sediment from the shoreline of a subduction margin to the trench. Prominent examples can be found along the Peru-Chile Trench, the Middle America Trench, the Cascadia margin, and the Nankai Trough. Typically, submarine fans and/or channel-levee complexes form where the canyons debouch onto the trench floor. The Aleutian arc-trench system, in contrast, represents an enigmatic case in which large volumes of sediment reach the trench floor without the aid of through-going erosional conduits. Sediment must be transported around or over bathymetric obstructions such as the outer high of a fore-arc basin. Laboratory experiments and numerical analysis demonstrate that density currents will transfer mass upslope whenever they encounter a reversal in sea-floor gradient. The basal part of a turbidity current typically will be deflected back into the adjacent basin. However, if the top of the current rises above the ridge crest, the process of flow stripping will transfer some of the suspended sediment over the obstruction, in a fashion similar to levee spillover. Upslope movement is enhanced if the density contrast between the flow and the ambient fluid is reduced. The maximum run-up elevation of a flow is proportional to its thickness (h); assuming subcritical conditions and roughly 33% frictional energy loss, the maximum run-up is approximately 1.53h. Thus, even where submarine canyons dominate, sediment dispersal and lithofacies distributions are complicated y the effects of unconfined upslope transfer, flow deflection, and flow stripping.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990