Abstract Active accretionary prisms at subduction margins generally include a horizontal detachment, décollement, within the sedimentary pile. The décollement, and its extension to undeformed regions (i.e., proto-décollement), corresponds to a layer of high fluid pressure. The deformation of the prisms, including such an anomalous layer, can be modelled and examined using analogue experiments and numerical simulations. Both these methods approximate the material under deformation as an assembly of partides (grains). The décollement layer is found to be best modelled by intercalating a layer with smaller internal frictional coefficient than the surrounding materials corresponding to the sediments. Our analogue experiments with dry sand and microglass beads reproduce structural geometry similar to that of interpreted seismic profiles at the toe of the prisms. Thrust faults originate from the horizontal beads layer and propagate upward with a constant angle of about 30°. Each of the fault bends produces a series of minor back thrusts. A particle image velocimetry (PIV) analysis revealed that the fault activity is characterized by intermittent reactivation and segmentation. The numerical simulations based on the distinct element method (DEM) were performed with similar kinematic settings and material properties as the analogue experiments. The numerical simulation results not only reproduce similar geometries as in the analogue experiments, but also show that the particle assembly experiences temporai variations in the deformation velocity and stress field as deformation propagates. This might be related to stick-slip motion of the frictional fault surfaces, which is a common feature of faulting during accretionary processes at subduction margins.

Abstract Structural deformation by faulting and folding has been analyzed by sandbox experiments that appropriately model the brittle behavior of the upper crust.This type of physical experiment using granular materials can also be done by numerical simulation (digital modeling) using the distinct element method (DEM).This chapter presents a set of two-dimensional simulation results of structural deformations using the DEM in basic tectonic settings of extension and contraction and also in the indentation tectonics as a consequence of continental collision of India. By comparing with analog experiments, the simulations reproduced fault systems similar to those of the experiments in terms of the overall deformation geometry and their development sequence. In particular, characteristic features of the Indian collision tectonics, such as fragmentation and rotation of continental blocks, were clearly identified in our DEM simulation. Because the DEM digital modeling is cheaper and faster than conventional sandbox experiments and it can incorporate discontinuity surfaces properly, the method can prove to be a powerful tool to simulate fault-related phenomena, such as structural traps in sedimentary basins. The DEM is a forward-modeling technique and can provide useful information on possible deformation pathways.