ANALYSIS OF FOREARC MORPHOLOGY AND STRUCTURE ALONG ACCRETIONARY AND EROSIVE CONVERGENT MARGINS IN THE CONTEXT OF COULOMB WEDGE MECHANICS

Taper of outer forearc wedges varies systematically along convergent margins as a function of the rates of tectonic accretion vs. basal erosion. Along margins with slow convergence rates and thick incoming sediment piles (i.e., accretionary margins-- Sumatra, Makran, Cascadia, S. Barbados), surface slope and taper are generally lower than along margins with rapid convergence rates, thin sediment piles, and extensive slope aprons (i.e., erosional margins-- Middle America, S. America, Japan, Tonga). For any given surface slope (or basal dip) of a noncohesive Coulomb wedge, there are two possible solutions for critical taper (i.e., maximum and minimum tapers), or taper at which a wedge is everywhere at the verge of failure while sliding on a weak basal decollement. Along accretionary margins, trench sediments deform until the minimum taper is attained, the wedge slides at the verge of failure, and the locus of deformation shifts to the subcritical material of the protothrust zone and trench. Applicability of a critical taper solution for erosional margins is less clear. The margin wedge of erosive margins is composed of upper plate basement or old accreted material and is stronger than the sediments of accretionary prisms, yet the tapers and surface slopes for erosive margins are larger—an observation not predicted by Coulomb wedge theory for weak vs. strong critical wedges. Although this could be attributed to greater basal friction, another possibility is that erosive margins are not critical wedges but are rigid. Under these circumstances, steeper surface slopes could be due to preferential basal erosion updip along the plate boundary. Normal faults observed in slope deposits along erosive margins cut the underlying basement and produce modest amounts of extension. These normal faults could be a shallow manifestation of flexure of the wedge due to updip basal erosion. In this case, the shallow stress field related to extension in the upper part of the wedge is supplanted at depth by a subhorizontal sigma1. Alternatively, the normal faults could reflect steep sigma1 throughout the wedge in response to coseismic drops in shear stress along the basal decollement (Wang et al., 1010). These models could be tested by measurement of the deeper in situ interseismic stress field in areas of shallow forearc extension.