dc.description.abstract | The western part of the Western Foothills is located at the deformation front of Taiwan Orogen, while the southern section of the Western Foothills is a part of incipient collision zone, where the Plio-Pleistocene Gutingkeng Mudstone crops out extensively. The terraces, meanders, badlands and mud volcanoes are the most distinct landscapes in the Gutingkeng Mustone area, and this area has known to have high rates of erosion and landform changes. This study uses multiple time-scale tools to analyze the features of landform changes in the Gutingkeng Mudstone, and elaborate the landform change tendency in the incipient collision zone.
The Erhjen river is the main drainage flowing through the Gutingkeng Mudstone area. The upstream of the Erhjen basin has been tilted towards east probably begining from 8500 years ago. The tilting could be detected from its asymmetric distribution of the terraces, the river channel migration and unequally crustal uplift. This study attributes the unequally crustal uplifting rate and block tilting towards east to causing the asymmetric landscape features in upper reaches of the Erhjen River. In addition, the Gutingkeng Fault and Hsiaokunshui Anticline form an uplift axis and cause the river channel migrating towards northwest in the middle reach of the Erhjen River.
This study uses various geomorphic indices to evaluate the tectonic activities in southern section of the Gutingkeng Mudstone. The results show that the Chishan Fault in the southern part of Zhongliao Mountain, the Lungchuan Fault along the Lungchuan Mountain, and the Gutingkeng Fault north to the Hsiaokunshui, are structures with relative higher activity. The mud volcanoes are distributed along these active structures and their activities also are affected by the kinematics of these structures.
The erosion rate of the Gutingkeng mudstone is about 39 mm/yr in the last 60 years by subtracting the 1980’s DTM from the 1921’s ; The bald nudstone area has been extended at a rate of 2.5 km²/yr in the last 7 years estimated from satellite image analysis, and the bald mudstone area encroaches on the vegetated lands in a outward and backward fashion. Using 3D Ground-based Laser Scanner to monitor the mud volcanoes changes, the results show an high erosion rate in the Gutingkeng mudstone of at least 9.8 cm yearly, especially when attacked by the typhoons or/and torrential rainfalls.
The active structures, mud volcanoes and crustal tilting in the Gutingkeng mudstone concurrently indicate the mountain building is ongoing in southwest Taiwan. Using the HI to evaluate the relative activity of the Chishan Fault and Lungchuan Fault, the result shows that the Chishan Fault is more active than Lungchuan Fault in that the higher HI accompanying with higher uplift rate in Zhongliao Mountain surpasses that in Lungchuan Mountain. This feature coincides with the sequential development of thrust faults in Taiwan orogeny. The SE-NW tectonic compression activates the structures and helps reform the landscpes in the study area.
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