Frontal Accretion of Accretionary Wedges Based on Sandbox Experiments

Simulation of geological structures by physical modeling provides real-time observations on the geometry and evolution of deformed structures. Based on the granular flow characteristics of quartz sand, sandbox experiments are frequently used to model non-linear deformation behavior and rock failure in the upper crustal deformation. This study focuses on the deformation behavior of frontal accretion in accretionary wedges using sandbox experiments. In order to investigate the influence of different parameters on the development of accretionary wedges, including basal friction coefficient (μb), basal detachment dip (β) and the depth of inherited weak layer, a series of sandbox experiments with proper scaling are performed. Particle Image Velocimetry (PIV) analysis is applied to the images of sandbox results to visualize the spatial and temporal deformation patterns for each experiment. Combined with conventional analysis method of sandbox experiments, the influences of the tested parameters are discussed. The frontal accretion cycle observed in the sandbox analogue experiments of accretionary wedges can generally be divided into three stages: thrust initiation, underthrusting and reactivation. n this study, basal friction coefficients are designated 0.31 and 0.55 by using plastic and sandpaper belt, respectively. The main difference between the contrasting basal frictions is the deformation within the underthrusting stage. When basal friction is lower, the footwall material beneath the frontal thrust can not be easily displaced into wedge due to low coupling between sands and basement. This leads to reterowedge-directed development of backthrusts backwards to maintain critical taper and main uplift is located in the deformation front. In contrast, when basal friction is high, footwall material is underthrusted into wedge due to strong basal coupling and main uplift located in the rear wedge. The detachment dip is designed with 0, 3, 6 and 8 degree. With increasing β angle, the underthrusting process becomes more dominant because of increasing gravity component parallel to the basal detachment. In high β angle cases, the value of basal friction determines whether imbricate structure or large-scale backthrust is dominant. For example, imbricate structures are always observed in high basal friction cases and large-scale backthrusts in the low basal friction cases. To investigate the difference of varying depths of inherited weak horizon, experiments are set with 0.1 cm glass microbeads layer as weak layer are added into the 4cm sand layer. The depth of weak layer ranges from 1.5cm to 3.5cm. For cases of layer depth in 1.5cm, 2cm and 2.5cm, the main thrust event is paused in the underthrusting stage when the external small thrusts are generated from the glass microbeads layer. However, the development of external thrusts will increase the degree of later underthrusting when the main thrust becomes reactivation. For the cases of weak layer level in 3.0 and 3.5 cm, the glass microbeads layer becomes a detachment, in stead of detachment in the base on other experiments. The deformation features within these cases are similar to that within low basal friction cases. n summary, basal friction is the factor to influence the existence of imbricate thrust or large-scale backthrust and the location of uplift region. The β angle has a direct influence on gravity component. The shallower weak layer would be the location to generate external small thrusts and enhance the role of later underthrusting. The deep weak layer would become a shallower detachment, not in the base. Comprehensive results of influences from these three parameters indicate that underthrusting is the most important stage to evaluate the parameter effects in the context of frontal accretion cycle. Consequently, in the future we can focus on studying the differences of deformation patterns and behaviors induced by parameter change in the underthrusting stage to evaluate the impact of different factors to the development of accretionary wedge.