The effects of fluid escape on accretionary wedges 1. Variable porosity and wedge convexity

Abstract

Fluid flow out of accretionary wedges is a consequence of the porosity reduction of wedge material both with depth and with distance from the toe of the wedge. Changes in porosity result in changes in bulk density, because bulk density is a function of solid density and porosity. These variations in bulk density in turn affect the critical taper of the wedge. The critical taper of a wedge with spatially varying strength, pore pressure, and/or bulk density is dependent not only on the value of these parameters, but on their first derivatives as well [Dahlen, 1990]. Using the equations derived by Dahlen we find that the arc ward decrease in porosity results in the commonly observed convex wedge surface. A hypothetical model using a density‐depth function generated from a synthesis of accretionary wedge data [Bray and Karig, 1985] demonstrates that the surface angle can decrease between 0.5 and 1° in the first 60 km from the toe of the wedge. Steeper wedges exhibit a greater degree of convexity (greater changes in slope). The amount of change in slope is relatively insensitive to the internal strength of the wedge. Wide angle seismic data from the Barbados accretionary wedge [Bangs et al., 1990] imaged velocity variations from the surface down to the décollement for a distance of 100 km from the toe. These data can be used to map the spatial variations in bulk density within the Barbados accretionary wedge. Holding strength and pore pressure values constant, and allowing only bulk density to vary, results in a model that closely matches the observed bathymetry both in overall convexity and in shorter wavelength bathymetrie features. The model allows a limited range of acceptable basal pore pressure values for the wedge. The ability of the model to accurately match the majority of the observed bathymetry suggests that bulk density variations are of first order importance for determining not only the wedge shape but also the shorter wavelength features. The model, however, does not produce the most pronounced changes in slope at the ten kilometers nearest the toe. This suggests that other factors, such as variations in pore pressure, sediment strength, or cohesion, are more important influences on the wedge taper nearest the toe.