COULOMB THEORY APPLIED TO ACCRETIONARY AND NONACCRETIONARY WEDGES - POSSIBLE CAUSES FOR TECTONIC EROSION AND OR FRONTAL ACCRETION

Based on observations from both modern convergent margins and sandbox modeling, we examine the possible conditions favoring frontal accretio n and/or frontal and basal tectonic erosion. Mean characteristic param eters (mu, mub and lambda) are used to discuss the mechanical stabili ty of 28 transects across the frontal part of convergent margins where the Coulomb theory is applicable. Natural observations reveal that '' typical accretionary wedges'' are characterized by low tapers with smo oth surface slope and subducting plate, low convergence rates and thic k trench sediment, while ''nonaccretionary wedges'' display large tape rs with irregular surface slopes and rough subducting plate, high conv ergence rates and almost no trench fill. Sandbox experiments were perf ormed to illustrate the effects of seamounts/ridges in the subduction zone on the deformation of an accretionary wedge. These experiments sh ow that a wedge of sand is first trapped and pushed in front of the se amount which acts as a moving bulldozer. This is followed by a tunnell ing effect of the subducting seamount through the frontal wedge materi al, which results in considerable sand reworking. At an advanced subdu ction stage, the decollement jumps back from a high level in the wedge to its former basal position. We conclude that a high trench sediment ation rate relative to the convergence rate leads to frontal accretion . In contrast, several conditions may favor tectonic erosion of the up per plate. First, oceanic features, such as grabens, seamounts or ridg es, may trap upper plate material during their subduction process. Sec ond, destabilization of the upper plate material by internal fluid ove rpressuring causing hydrofracturing is probably another important mech anism.