ABSTRACT: Accretionary Wedge Convexity and Submarine Canyon Development: Two Consequences of Fluid Escape

Daniel L. Orange, Nancy A. Breen

Fluid escape from accretionary wedges is supported directly by the observation of fluid venting in accretionary wedges and indirectly by measured and inferred decreases in porosity of accreted sediments as they

are deformed within the wedge. The fluid flow associated with wedge dewatering affects both the surface morphology and overall geometry of the accretionary wedge.

Any non-zero permeability system with a fluid flow necessarily causes seepage force. Seepage force acts on the solid matrix in the direction of fluid flow and is proportional to hydraulic gradient. The wedge surface may be modified by sufficient upward seepage force causing slope failure, especially around fluid vents. An example of seepage related canyon growth is seen in headless canyons with vents at their topographic inflection points observed on the modern-day Oregon margin.

Dewatering affects the gross wedge mechanics as well as surface processes. Published solutions of Dahlen (1989) include seepage force implicitly. Equations also allow for variable bulk density with position in the wedge (an inevitable consequence of progressive dewatering and porosity decrease). We apply this model to Barbados using estimates of porosity derived from multichannel seismic velocities. Most of the first-order bathymetric features of the wedge can be modeled as responses to spatial variations in bulk density. The model, however, does not reproduce the most pronounced changes of slope within 10 km of the deformation front. Other factors, such as variations in overpressuring due to tectonic loading and an increase in cohesion, may account for the remainder of the wedge conv xity near the toe.

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