Mechanical role of backstops in the growth of forearcs

Abstract

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 farther 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 theology, 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 1Now atPaper number 92TC00618.0278-7407/93/92TC-00618510.00 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. INTRODUCWIONWithin a forearc, which is that part of the upper plate seaward of the volcanic arc at a subduction zone, there typically exist materials ranging from loose, porous sediments to dense and well-lithified rock. We define a backstop as a region within a forearc that is significantly stronger than the region just trenchward of it (Figure 1). It deforms relatively little, although it can have a significant influence on the kinematics of the sediments above and immediately trenchward of it. In using the term backstop we do not mean to imply anything about its composition or origin: it may consist of igneous rock such as arc basement, previously accreted welllithified sediment, an accreted allocthonous terrane, or any other relatively strong geologic material. The defining attribute of a backstop is simply its ability to support larger deviatoric stresses than the forearc material directly above or seaward of itself. Backstops have been inferred to exist at a number of margins on the basis of strong acoustic refle...

Geometry and kinematics of experimental antiformal stacks

“…12d, e), which has produced the best antiformal stack simulation, demonstrates that its formation requires some amount of backstop-edge effects. This fact confirms the analysis made by Byrne et al (1993) about backstops within a forearc at subduction zones. They concluded that backstops are stronger than the rocks lying further trenchwards, deform little but may influence the kinematics of the trench rocks.…”

Calibrating the bulk rheology of active obliquely convergent thrust belts and forearc wedges from surface profiles and velocity distributions

“…The wedge is bounded to the rear by a region that is deforming less rapidly or not at all, known as the backstop [Byrne et al, 1993]. The backstop may be rigid, or the lack of deformation may be a result of high elevation, such that the loads generated by the topography roughly equal those exerted on its boundaries.…”

Life cycle of the East Carpathian orogen: Erosion history of a doubly vergent critical wedge assessed by fission track thermochronology

“…The use of backstops with finite height resulted in significantly different wedge geometries [e.g., Malavieille, 1984;Byrne et al, 1993;Willet et al, 1993;Koons, 1989Koons, , 1990, although the deformation mechanism remained the same (Figure lb). The frontal part (or prowedge) still behaves as in the minimum tapered bulldozer model (Figure la), but differences occur to the rear above the tip of the backstop.…”