Abstract
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.
Highlights
Lohrmann et al (2003), based on experiments under 1-g conditions, identified that the sequence of wedges formed by convergence can be categorized into three zones: the frontaldeformation zone (FDZ), the frontal-imbrication zone (FIZ) and the internal-accumulation zone (IAZ)
The plate convergence was stopped when the thrust fault front began to approach the Penghu Islands, since no tectonic thrust faults have been discovered beyond the Penghu Islands
Concerning scale effect on internal structures, a comparison of such structures, profiles AA', BB', CC' and DD', of the two models is in shown Fig. 10
Summary
Sandbox experiments provide an effective means of simulating tectonic processes such as divergence or convergence of plates by stretching or compressing sand in a sandbox model (e.g., Davis et al 1983; Dahlen et al 1984; Malavieille 1984; Mulugeta 1988a, b, 2002; Bar and Dahlen 1989; Dahlen and Barr 1989; Dahlen 1990; Colletta et al 1991; Liu et al 1991; Lallemand et al 1992; Willett 1992; Willett et al 1993; Byrne et al 1993; Calassou et al 1993; Sassi et al 1993; Lu and Malavieille 1994; Lallemand et al 1994; Lu et al 1995; Koyi 1995; Gutscher et al 1996, 1998a, b; Mugnier et al 1997; Wand and Davis 1996; Storti et al 1997; Lu et al 1998; Bonini et al 2000; Wang and Hung 2002; Costa and Vendeville 2002; Lu et al 2002; Mouthereau et al 2002; Koyi and Vendeville 2003; Maillard et al 2003; CouzensSchultz et al 2003). Lu et al (1998) performed a series of sandbox experiments, and discussed the impact of the basement high on the structures and kinematics of the western Taiwan thrust wedge They found that most of the foreland structures were strongly influenced by the shape of the backstop and structure highs. Control of material properties - Rheological similarity must be fulfilled such that experimental results obtained from various tests can be analyzed and compared with nature Factors such as strength, grain size distribution, relative density and presence of water influencing the mechanical properties of sand must be given consideration; 5. In this paper, experiments that simulate sand deformation of different depth scales are presented
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