| Summary: | 博士 === 國立中央大學 === 地球科學學系 === 107 === Taiwan Island is an active orogenic belt, as a result of the Philippine Sea plate colliding with the Eurasian plate with a speed of around 8.2 cm/yr. The rapidly convergent rate results in significant crustal deformation and ongoing mountain building process. Both long-term and short-term observations show a rapid uplift (~6-20 mm/yr) in the Central Range, but the uplift mechanism is still under debate. According to previous studies, the Central Range Fault could exist and serve as a boundary between the eastern flank of the Central Range and the Longitudinal Valley. However, few evidences from field investigations show the traces of the Central Range Fault, and the geometry of the Central Range Fault is still a mystery and remains many questions. In order to investigate the Central Range Fault comprehensively, I used the geomorphic analysis, structure analysis, co-seismic rupture analysis, and seismic data to understand the geometry of the Central Range Fault.
According to the geomorphology analysis, I observed many triangular facets along the eastern flank of the Central Range. As a result, the spatial variability of the uplift rate can be known by the slope change of the triangular facets over 103-105 year timescales. The uplift rate of the Liwu River -Mugua River and the Lele River -Xinwulu River are the faster than other investigated areas, and the uplift rate of the northern side of Fengping River is the slowest among the study areas. Also, some of active normal faults were found in the field, which reveal a vertical principal stress direction by a kinematic analysis. The relocated earthquakes by hypoDD with the focal mechanisms of ML ≧ 3.0 events in the eastern Taiwan show that the distribution of the earthquakes generally is a steeply west-dipping reverse fault plane under the eastern flank of Central Range, which could be the Central Range Fault. The ruptures of 2018 Hualien earthquake along the Milun Fault in the north of the Longitudinal Valley show macro-scale Riedel shear structures and the Milun Fault could be a backthrust developed from the Central Range Fault.
By combining the geomorphologic analysis, field observations and the distribution of earthquakes, we suggest that the Central Range Fault at deeper depths is a high angle reverse fault system, which leads to the rapid uplift of the Central Range, and forms the normal fault in shallow. However, influenced by different tectonic process and geological environments from north to south of the eastern flank of the Central Range, the Central Range Fault could develop as a new thrust, or connect the east-dipping normal fault near the surface.
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