| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 90 === ABSTRACT
The Chinshui fault is located at the western border of the Tatu terrace, west-central Taiwan. It is believed to be a continuous part of the Changhua fault extenting from the Pakua terrace in the south and also connecting to the Tachia fault developed along the Houli terrace in the north. These serial faults: the Tachia-Chinshui-Changhua fault represent the most western thrust fault system along the deformation front of western Taiwan foothill area. Photogeologic studies indicated that it is an active fault with apparent lineation and fault scarps. The seismic profiles provided by the petroleum company reveal that a standard ‘fault-bend fold’ may exist under the anticlines of the terraces. This study attempts to use the shallow reflection seismic method to map the shallow structures around the fault and to examine its relationship with the surrounding fold structures. This data may also be able to explore the generation mechanics of the terraces and the faults.
Over 50 seismic lines are distributed along the boundary between the plane and the terrace. It is surprised to find that most seismic sections are composed of flat layers even inside the area of low-level terrace. The places where the sedimentary layers start to bend are very close to the boundary of the Toukoshan formation exposed at the high-level terrace. The smoothly bending layers on the seismic sections support the model of fault-bend-fold. The following conclusions can be drawn:
1.The Chinshui fault can be segmented into two parts: the southern and the northern portions by the Ta-An fault which bisects them in the middle, close to the town of Shalu. These two portions of faults form an en-echlon arrangement of the faults.
2.The accompanied back-thrust fault of the main Chinshui fault, i.e., the Teichenshan fault, Tachiatung fault and the Hungshan fault, may not exist. The underground seismic images of these faults are quite flat, not show a single bit of evidences of bending. Their fault-like shapes on the surface may be resulted from the erosion of the rivers and the drainages, not from the fault activity.
3.The bending of the structural layers near the fault indicates that they are of the fault-bend-fold type. A hidden ‘blind’ thrust may exist at deep places and above which arises the fold as a result of thrusting. The theory of the fault-bend-fold nicely explains the fault behavior along the fault.
4.Apparent fold angle variation occurs along the south-north direction of the fault. The degrees of folding decrease from the south to the north in the southern portion, but increase again toward the north in the northern portion. It seems that the layer variations are largely reduced in the middle where the Tai-An fault passes. This deep sited fault acts as a prohibitor which may have relaxed the fold stresses in this area.
5.In certain places, the layers near the surface have the same degrees of folding with the deep layers. This means that the process of the folding is quite young and its reactivity in the future should not be under estimated.
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