The discrete element method (DEM) is becoming widely accepted as an effective method for addressing tectonic problems in granular materials. It is capable of reproducing structures observed in the analog model (AM). However, the previous experiments also pointed to variability among DEM models and AMs in the number of fault zones, their dip angle and spacing, and the evolution of the surface slope of a thrust wedge. The accuracy of the DEM depends on the input parameter values, so the calibration of the discrete element method is very important. Microscopic properties of particles and macroscopic properties of loose quartz sand were calibrated through a series of repose angle and biaxial tests. Furthermore, an AM was constructed to simulate the evolution of the thrust wedge to compare with DEM results. DEM and AM results indicate an encouraging overall agreement in model evolution. Based on a new automated wedge quantification method, DEM results were systematically compared with AM results on the number of fault zones, their dip angle and spacing, the evolution of the surface slope of a thrust wedge, and other parameters. Our study provides a necessary comparison between commonly applied modeling approaches, which is important for more confidently applying these methods to understand real fold and thrust belt systems.

The salt layer is critical for the structural deformation in the salt-bearing fold-and-thrust system, which not only acts as the efficient décollement layer but also flows to form salt tectonics. Kuqa Depression has a well-preserved thin-skinned fold-and-thrust system with the salt layer as the décollement. To investigate the effects of salt thickness on the structural deformation in the Kuqa Depression, three discrete element models with different salt thicknesses were constructed. The experiment without salt was controlled by several basal décollement dominant faults, forming several imbricate sheets. The experiments with salt developed the decoupled deformation with the salt layer as the upper décollement (subsalt, intrasalt, and suprasalt), significantly similar to the Kuqa Depression along the northern margin of Tarim Basin. Basal décollement dominant imbricated thrusts formed at the subsalt units, while the monoclinal structure formed at the suprasalt units. The decoupled deformation was also observed in the tectonic deformation graphics, distortional strain fields, and max shear stress fields. However, the salt layer was thickened in the thick salt model, and the salt thickness of the thin salt model varied slightly because the thin salt weakened the flowability of the salt. The lower max shear stress zone was easily formed in the distribution region of salt under the action of compression stress, which is conducive to the flow convergence of salt and the crumpled deformation of interlayer in salt. The results are well consistent with the natural characteristics of structural deformation in the Kuqa Depression. Our modeling result concerns the structural characteristics and evolution of salt-related structures and the effects of salt thickness on the structural deformation in the compressional stress field, which might be helpful for the investigations of salt-related structures in other salt-bearing fold-and-thrust belts.

In recent years, the discrete element method (DEM) has become widely used to solve a number of geological and geophysical problems. The basic idea of this method is to consider rock-soil bodies, in their discontinuous nature, as a set of discrete elements. It is generally recognized that this type of complex model must be validated through comparison with experimental results. Many researchers have addressed this in earlier publications. However, one important aspect has been given little attention, namely, testing the code to ensure that the computer program executes the model specification correctly. This paper describes in detail a DEM program for cohesionless particle vibration and shows four simple simulations used to verify the code. The results of these simulations are compared with PFC (Particle Flow Code 2D) and analytical solutions that are mathematically correct, showing that they have a good consistency. It is concluded that mathematical tests in artificial situations can uncover bugs in programs that appear to be running correctly, even if the programs simulate real experiments reasonably well. The simulation of relative error between numerical solutions and analytical solutions increases as artificial viscosity is introduced. These tests are published with the aim of helping others validate their programs (such as PFC) in similar applications.

Four groups of discrete element models (DEMs) were set-up to simulate and analyze the influence of regional erosion and sedimentary loading on the formation and spatial-temporal evolution of faults in the southern and central Longmen Shan (LMS) active fold-thrust belt. The interior characteristics of faults in the southern and central LMS fold-thrust belt were also evaluated during the interaction of tectonic processes and surface processes according to the stress-strain analysis from DEM results. The results showed that synkinematic erosion promoted the reactivation of pre-existing faults in thrust wedges and also retarded the formation and development of new incipient faults in the pre-wedge regions. Meanwhile, synkinematic sedimentation also delayed the development of new incipient faults in the pre-wedge regions by promoting the development of thrust faults in the front of thrust wedges, causing these thrust wedges in supercritical stages with relatively narrow wedge lengths. According to these DEM results, we infer that: 1) The characteristics of erosion and sedimentation in the central and southern LMS have important influences on the activities of large faults which are extended into the deep detachment layer; 2) Besides differential erosion, the differential sedimentary loading may also be one of the important factors for the along-strike differential evolution of the LMS fold-thrust belt. This kind of differential deposition may lead to differential fault activity and uplift in the interior thrust wedge and pre-wedge region in the central and southern LMS; 3) Compared to the northern LMS, the central LMS and southern LMS is more conducive to the occurrence of earthquakes, because of synkinematic sedimentation (such as the growth of Chengdu plain) has a greater blocking effect on the stress propagation and strain convergence on the fault planes of front faults of an active thrust wedge.