Sandbox Experiments of Plate Convergence-Scale Effect and Associated Mechanisms

When using a sandbox to simulate tectonic activities, it is essential to thoroughly understand scale effect and its consequence so as to ensure an adequate model with representative simulation results. This paper investigates the topography, faulting lineaments, and internal structures of different scales associated with the simulation of plate convergence using sandbox modeling, based on three major test series. In the first test series, plate convergence is performed using simple models with flat bases to investigate fundamental behavior and observe the scale effect. The scales of sandbox models are altered either by varying the thickness of sand or by applying centrifugal conditions. The models have a relative depth scale of 1:2:20:40. In the second test series, the scale effects of a sandbox modeling given more realistic conditions are studied, i.e., models with underlain basement highs based on the configuration of western Taiwan. In the third test series, the impact of rougher base friction, and the effects of compaction are investigated to identify possible factors related to scale effect. Based on the observed scale effect, the possible factors accounting for scale effects are also discussed in this paper. Just like most sands, the sand tested is characterized by two phenomena: (1) its internal frictional angle increases from 34° to 41° under greater degrees of compaction, with a relative density ranging from 55 to 90%; and (2) it possesses nonlinear constitutive behavior, whereby the deformational modulus increases upon greater confining stress. Such behavior inherently accounts for scale effect in the way that the embedded stress of sand is proportional to its depth; however, the frictional angle and the stiffness of sand cannot be retained constant. Results obtained from the first test series indicate that models with greater depth scale tend to have less deformation concentration near the backstop, quicker thrust faulting and longer fault propagation distances. Meanwhile, internal structures of models with greater depth scales, characterized by more pop-up structures, differ from those of small-scale models. Based on the critical wedge theory, the smaller slope angle yielded under a greater depth scale indicates stronger material strength, which results from greater confining stress or compaction. Moreover, the hardening, or increased stiffness of sand may facilitate forward propagation of convergence stress thereby contributing to the occurrence of scale effect. For the second test series, the yielding of three major fault groups, and curved faults is consistent for different scales and scale effect seems to be diminished. Nevertheless, when looking at internal structures, scale effect does exist. Here, more pop-up structures were yielded under a greater depth scale. This indicates that model geometry may suppress some part of scale effect. The third test series shows that the deformation pattern of models with compacted sand or with smoother bases resembles that of a greater scale model. Correspondingly, sand compaction as a factor is highlighted among those possible factors.

並列關鍵字

Plate convergence ； Fold-and-thrust structures ； Sandbox simulation ； Depth scale effect ； Centrifuge

國際替代計量

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