| Summary: | 碩士 === 國立中央大學 === 應用地質研究所 === 107 === The Taiwan orogenic belt is the result from the westward convergence of the Philippine Sea Plate towards the Eurasian Plate at a rate of ~8 cm/yr. Shortening is mostly consumed along the Longitudinal Valley in Eastern Taiwan, the plate suture, and across the Western Foothills, at the mountain piedmont. In the southern Western Foothills, this convergence caused a series of thrust systems and westward movement reaching up to 6 cm/yr and 2 cm/yr local uplift rate based on geodetic data during 2000 – 2010. The Erhjen River, which is located in the south part of the Western Foothills, flows across several geological structures which are the Gutingkeng fault, Lungchuan fault, and Hsiaokunshui anticline. We found that four groups and four sub-groups of terrace level were developed. By observing these terraces we aim at better understanding the Holocene deformation across the Gutingkeng fault zone. We observe that 10 geomorphic surfaces are tilted with slope values in the range 1º - 4º ± 0.02º and mostly tilted to the NW direction near the Gutingkeng fault zone. Based on radiocarbon dating from a previous study and from our work, we estimate the age of the oldest terrace (T1a) to more than 2200 Years BP, while the youngest terrace (T4a) is less than 800 Years BP. Euclidian topographic profile from upstream to downstream shows that the terrace slope increases from T4 to T1, with a slope of 1.98o for T4a and 4.28o for T1a. We obtain local incision rate of four geomorphic surface where the radiocarbon dates are consistent with the geomorphic mapping. The calculation results show that the local incision rate at three sites are similar, ranging from 25 – 27 mm/yr, while local incision rate is slower at the forth site, at 15.3 mm/yr. Leveling data during 2004 - 2016 shows uplift rates of the same order in this area reaching 25.1 – 34.2 mm/yr, with the west side uplifted relative to the east side, while based on geology Gutingkeng fault is an east dipping thrust. We propose three working hypotheses that could explain the Erhjen River local incision rate in our study. These hypotheses include a change in river slope, the existence of a west-dipping thrust fault, and/or a change in the fault activity.
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