141 / 2014-09-06 11:05:59

Correlationship Between Deformation And Various Shortening Velocity Of The Fold-and-thrust Belt: Evidence From Sandbox Modeling

sandbox model, wedge thrust, piggyback imbricate-thrust, critical velocity

Brittle deformation in shallow crust is generally characterized by a long tectonic history and complex geological features which results from interdependent parameters. Since the 1810s, tectonic sandbox modeling is a powerful and indispensable tool to help in providing a unified picture of the evolution of deformation in shallow crust with considerable spatial and temporal detail based on self-organization and unreasonable effectiveness of tectonics. Based on a similar initial conditions of sandbox model (pure quartz sand thickness: 15 mm, 10 mm, 10 mm and red quartz sand thickness: 1 mm), we unravel different geometry, kinematics and evolution of the thrust wedge with various shortening velocity in this paper, which indicate a correlationship between deformation and various shortening velocity of the fold-and-thrust belt.
With high shortening velocity (0.5~0.1 mm/s, e.g., gravity-driven and earthquake-related event), the thrust wedge in sandbox modeling is simple, characterized with piggyback imbricate-thrust structures. The wedge is relatively narrow and thick, with a steady-state wedge taper angle of 5°-14°, and wedge length of about 410 mm. Furthermore, there are always seven faults occurred in the wedge with various velocity, of which the ramp angle decrease with increasing shortening, finally to a steady-state angle of 33°-43° (Table 1 and Figure 1). During the evolution of thrust wedge, the faults did not progressively forward-thrust, but show retro-thrusting to cut existed thrust fault, resulting into multiphase activity of wedge and fault. With low shortening velocity (0.05~0.005 mm/s), the thrust wedge in sandbox modeling is relative complex, characterized with piggyback imbricate-thrust and superimposed pop-up structures. Thus, the wedge with a steady-state wedge taper angle of 6°-12°, is wider and thinner than that with high shortening velocity. There are always eight faults occurred in the wedge with various velocity, of which the ramp angle ranged among 30°-50°. In particular, the wedge length increases with increasing retrothrust faults, resulted to an out-of-sequence thrusting. When the shortening velocity is very low with 0.002 mm/s, the deformation of thrust wedge shows significant difference with that of others. It is characterized with Jura-style folds, with seven faults occurred with a steady-state ramp angle of 36°-43 °. In particular, the wedge taper angle ranges among 8°-11° much smaller than all the others. Which suggest that a deformation mechanism of Jura-style folds is under low shortening velocity with uniform material, rather than detachment as usually we thought.
Our sandbox modeling with various shortening velocity reveal the differences in deformation structure style in fold-and-thrust belt. Under high velocity, the thrust wedge shows strong shortening and deformation along the wedge back, which is characterized with piggyback imbricate-thrust structures. However, under low velocity, the thrust wedge is characterized with gentle deformation and piggyback imbricate thrusting at first, followed by out-of-sequence retro-thrusting and backward pop-up structure after a critical wedge has been achieved. Due to temporality of sandbox modeling under low shortening velocity (i.e., how to unravel the maximum likelihood, the most complex deformation process with a most effective time), the choice of an effective shortening velocity is vital for the mechanism sandbox model. Therefore, we suggest a suitable and low velocity (i.e. ~0.01mm/s) can set down in most of our modeling, as a critical velocity.