| 摘要: | 菲律賓海板塊每年以約8cm/year的速率向歐亞板塊聚合，並且形成了台灣造山帶。板塊隱沒造成的縮短變形主要集中於台灣東部的花東縱谷、板塊縫合帶以及西部麓山帶前緣。此板塊聚合作用於西部麓山帶南側形成一系列的逆衝斷層，並藉由2000年至2010年的大地測量數據可觀察到每年6公分的速率向西移動以及每年2公分之速率抬升。本研究的目標為利用局部河流下切速率及水準測量資料來了解古亭坑斷層帶在全新世的變形行為。位於西部麓山帶南段的二仁溪流經數個地質構造，包含古亭坑斷層、龍船斷層及小滾水背斜，本研究將二仁溪河階區分為四個群集與四個子群集，在靠近古亭坑斷層處共有10處的地形面向西北傾斜1º至4º，誤差為0.02 º。根據河階沉積物中的碳十四定年結果得知最古老的河階(T1a)老於2200 B.P.、最年輕的河階(T4a)不超過800B.P.，上游至下游的地形剖面顯示河階坡度從T4至T1逐漸增加，T4a傾斜1.98°、T1a傾斜為4.28°。我們藉由一組與地形測繪符合之碳十四定年結果來計算二仁溪的河流局部下切速率，計算的結果顯示河流局部下切速率在2ka至1ka期間為2~6 mm/year，而1ka至今則劇增為25~27 mm/year。假設碳十四定年的結果能代表河階沉積物之真實年代，針對二仁溪在1 ka至今下切速率據增之現象，本研究提出了三個可能的假設: (1)河道坡度的改變、(2)存在向西傾斜的逆衝斷層以及(3)斷層活動性的改變。;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. |
