Two-dimensional Analog Modeling of Fold and Thrust Belts<subtitle>Dynamic Interactions with Syncontractional Sedimentation and Erosion</subtitle>

Abstract

The effects of syncontractional sedimentation and erosion on simple, critically tapered Coulomb wedges were evaluated by conducting twelve two-dimensional analog model sandbox experiments. All 12 models produced critically tapered Coulomb wedges with topographic slopes of 6–10 above horizontal basal detachments. The model without syncontractional sedimentation or erosion exhibited a general forward-breaking sequence with synchronous thrust activity. Syncontractional sedimentation produced longer wedges composed of fewer major forward-vergent thrusts and lowered thrust activities in the foreland. Syncontractional erosion inhibited forward propagation of the deformation front, decreased the number of major thrusts, and increased thrust activities in the hinterland. Where combined, the effects of syncontractional sedimentation and erosion were complementary. At the scale of individual folds, syncontractional sedimentation altered fold evolution by producing limb rotation and a front-limb trishear zone formed by tip-line thrust splays. At this scale, syncontractional erosion did not cause significant changes to the fold geometries as they developed. Comparisons of the model thrust wedges with natural fold and thrust wedges indicate that the Nankai accretionary prism, with its well-ordered array of closely spaced thrusts, would be typical of fold and thrust belts with low rates of surface processes. In contrast, the fold and thrust belts of offshore Niger Delta, the central Apennines, and the sub-Andes are characterized by buried, widely spaced, low-activity thrusts in the foreland that would be typical of high syncontractional sedimentation. High syncontractional erosion would produce very active hinterland thrusts resembling the present-day Taiwan fold and thrust belt. Changes in thrust-wedge dynamics caused by increased syncontractional erosion in the model wedges imply that subaerial fold and thrust belts, with higher erosion, would evolve differently from their submarine counterparts.