JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B10, PAGES 10,246-10,258, SEPTEMBER 10, 1986 Origin of Convex AccretionaryWedges' Evidence From Barbados W.-L. ZHAO! Department of Geological and Geophysical Sciences, PrincetonUniversity Princeton, New Jersey D. M. DAVIS2 Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York F. A. DAHLEN AND J. SUPPE Department of Geological and Geophysical Sciences, PrincetonUniversity Princeton, New Jersey The surface slopes of submarine accretionary wedges generally decrease away from the toe or defor- mation front toward the arc. This paper presents evidence that this characteristic convexcross-sectional shape is a direct mechanical response to the lithification of sediments during accretion.The recently accretedsediments near the toe, which are typically unconsolidated and therefore very weak, must deformuntil a critical taper is attainedat which they are stable in the presence of the horizontaltectonic compression. This critical wedge taper, and thus the bathymetricslope of the accretionary prism, is relatively high for the porous, weak sediments nearthe toe.In contrast, the compacted sediments farther back in the wedgeare stronger, so they require a relatively modestcritical taper and thereforea lower bathymetric slope to be stable under the same tectonic compression. The result is a convex cross- sectional wedge shape. We investigate several possible causes of the increase in sediment strength within the wedgeand conclude that the primary cause is an increase in cohesive strength with decreasing porosity. A straightforward generalization of the exactcriticaltaper model is combined with empirical cohesion-porosity-velocity relations to infer the distributionof porosityand P wave velocitywithin the Barbados accretionary wedge. We estimate that the porosity decreases from its near-toevalue of 70% to approximately 30% at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge. INTRODUCTION Recent theoreticalstudies suggest that the overall mechan- ics of accretionary wedges is similar to the mechanics of a wedge of snow or soilin frontof a moving bulldozer [Chappie, 1978; Davis et al., 1983]. The material within such wedges deforms until a criticaltaper is attained, afterwhichthe wedge slides stablyover its base as long as no fresh material is accre- ted at the toe. Subcritical wedges with a taper narrower than the critical value cannot slidestably when pushed but deform internally, shortening and thickening until the criticaltaperis attained. If additional material is encountered at the toe, an accretionary or bulldozer wedge will growself-similarly, main- taining its critical taper. The development of critically tapered compressional wedges is a general phenomenon, and theoretical analyses havebeen developed for a varietyof rheologies, including materials that are either perfectly plastic [Stockntal, 1983] or viscous [Ent- erman and Turcotte, 1983]. Laboratory measurements of rock •Now at Supercomputer Institute and Department of Geology and Geophysics, University of Minnesota, Minneapolis. '•Now at Department of Earthand Space Sciences, State University of New York at StoneyBrook. Copyright 1986 by theAmerican Geophysical Union. Paper number 5B5816. 0148-0227/86/005B-5816505.00 strength suggest, however, that friction and brittle failure mechanisms control the deformation and state of stress in the upper10-15 km of the crust[BraceandKohlstedt, 1980].For this reasona Coulomb theory provides the most appropriate model of accretionary wedge mechanics in the upper crust, unless evaporites are involved [DavisandEngelder, 1985]. However,many accretionary wedges have neitherthe con- stant taper of a homogeneous noncohesive Coulombwedge [Dahlen, 1984] nor the slightlyconcave surface slope of a uniformly cohesive wedge [Dahlenet al., 1984]. Insteadthey often display a convex upper surface, asillustrated in Figure 1. In this paperwe evaluate quantitatively a number of possible mechanisms for the development of this convexity, usingthe Barbadosaccretionary wedge as an example.We conclude that the characteristicconvex shape is a necessary conse- quence of the increase in cohesive strength that accompanies the lithification of the highly porousaccreted sediments. Karig [1986] reacheda similar but not identicalconclusion in a study of the accretionary wedge along the Nankai Trough; he suggests that the convexity can be explained in terms of an increase in internal friction rather than an increase in cohe- sion,and we address this distinction below. BARBADOS ACCRETIoNARY WEDGE The Barbados wedge lies along the convergent boundary between the Caribbean and North American plates, where 65-m.y.-old oceaniccrust is subducting to the west. Minster and Jordan [1978] found the present-day convergence rate to 10,246