| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 101 === The active Taiwan orogenic belt is considered as formed by the Eurasian plate obliquely converging with the Philippine Sea plate. In southern offshore of Taiwan, the oceanic lithosphere subducted eastward beneath the Philippine Sea Plate along the Manila Trench. In southern Taiwan there was a slice of Luzon forearc basement as a consequence of the transition from oceanic to incipient continental subduction. In this study, we designed the experiments by the sandbox of physical models to examine the detail impact of forearc lithosphere subduction on forearc basin and accretionary wedge deformation. We also demonstrate that the geometry of the deformation and spatial distribution of surface uplift can be used to the currently active phase within a frontal accretion cycle. After that, we analyzed the displacement field of particles by PIV (Particle Image Velocimetry) to quantify the vector field and the shear strain of the model result.
Our experiment results showed that the friction could be affected the fault geometry. In low friction basement, the fault geometry favors develop as push-out decollement, on the contrary, in the high friction basement, the fault geometry prefers develop underthrusting and imbricate structure. From the PIV result, We infer that the three phases of a frontal accretion cycle are controlled by the strain hardening and softening processes. The stress-strain evolution will harden in the thrust initiation phase and soften in the underthrusting phase. Furthermore, our modeling result from PIV suggests that the surface erosion promotes the possibility of out-of-sequence thrusts.
The orogeny of Taiwan is starting when the plates subducted to product the accretionary wedge. When the forearc basement began to subduct under the Luzon arc, the new orogenic wedge was grown at the boundary between the backstop and the forearc basement. The previous accretionary wedge and the orogenic wedge were accreted to develop major backthrust and backfolding in the forearc basin. In our opinion, this major backthrust and back fold at retro-wedge probably consistents with the field observation of the overturn structures in the southeastern Central Range. And the material beneath the seafloor could derived from the thrust and became part of the Melange.
In summary, our study results demonstrate early stage of Taiwan orogenic evolution. At the first phase, plates subducting form the accretionary wedge; at the second phase of the initial collision, the forearc basement is subducted under the arc to cause double collisions, which is consistent with the early stage of Taiwan orogenic evolution. Those helped us to understand more about Taiwan tectonic evolution and built a good example for the application of sandbox simulation.
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