Mechanical Role of Backstops in the Growth of Fore-Arcs

We use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones. A backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying far-ther trenchward; it can be thought of as the bulldozer behind an accretionary wedge. We calculate the stress and displacement fields within forearcs for various backstop models using the finite element method, and we simulate deformation over a backstop using a small-scale laboratory model. In this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures. We find that the growth of an outer arc high, the development of an inner deformation belt with landward vergence, and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it. The structures produced in our numerical models are quite insensitive to the rheology, boundary conditions, and exact mesh geometry employed. The general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop. These results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone. Backstops in which the contact with the accretionary wedge dips arcward rather than trenchward, however, should produce only slightly different forearc structures, with less development of both the outer arc high and the landward-vergent inner deformation belt. Although natural forearcs are far more complex than our simple models, they exhibit many of the same features, indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs.