以多重時間尺度分析古亭坑泥岩區千年來地形變動;Using multiple time scale tools to analyze the feature of landform change in Gutingkeng Mudstone from millennium years ago to present day

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题名:	以多重時間尺度分析古亭坑泥岩區千年來地形變動;Using multiple time scale tools to analyze the feature of landform change in Gutingkeng Mudstone from millennium years ago to present day
作者:	張鴻成;Chang,Hung-Cheng
贡献者:	地球物理研究所
关键词:	二仁溪;龍船斷層;古亭坑斷層;河階;地形指標;古亭坑泥岩區;旗山斷層;泥火山;river terraces;Lungchuan Fault;Gutingkeng Fault;Gutingkeng mudstone;Chishan Fault;mud volcanoes;geomorphic index;Erhjen River
日期:	2012-06-28
上传时间:	2012-09-11 18:26:28 (UTC+8)
出版者:	國立中央大學
摘要:	西部麓山帶西側位處台灣造山帶的變形前緣，而麓山帶南段是屬於碰撞初期的區域。台灣西南部古亭坑泥岩區中的河階、曲流、惡地與泥火山是最為特殊的地質景觀，而地形變化快速與高侵蝕率為泥岩區的一大特色。本研究利用不同時間尺度工具分析古亭坑泥岩區數千年來至今的地形變動特色，以了解初始碰撞區域受造山作用下的地形變動趨勢。二仁溪是古亭坑泥岩區中最主要的水系，上游集水區向東不對稱發育，從河階定年資料可追朔至8500年前左右開始形成；其現象可從河階的不對稱性分布、河流向東偏移與地殼不等量抬升等地形特徵觀察之；本研究認為地殼向東不等量抬升是造成地形不對稱性發育的主因。另外古亭坑斷層與小滾水背斜所形成的抬升中心是造成二仁溪中游河道朝西北方偏移的原因。以地形指標評估古亭坑泥岩區南段的構造活動度，認為旗山斷層於南二高中寮隧道以南、龍船斷層沿龍船山一帶、古亭坑斷層於小滾水以北，是具有較高活動度的區域。這些構造線亦是現今泥火山主要分布的位置，且泥火山活動型態深受構造運動形態所影響。以近六十年泥岩區的數位高程模型計算泥岩區的侵蝕率為39 mm/yr；以近七年衛星影像分析泥岩裸露地的面積每年增加2.5 km²。綜合上述評估方式，我們認為泥岩裸露地增加的方式是以現有裸露的區域向周圍、向稜線擴張為主。另外以三維雷射掃瞄監測泥火山變動，顯示強降雨是造成泥岩區快速被侵蝕的最主要原因。古亭坑泥岩區的活動構造、泥火山活動與地殼傾動情形顯示台灣島的造山活動正於初始碰撞區中進行。以HI評估旗山斷層與龍船斷層的構造活動度發現，中寮山具有較高的HI，顯示中寮山相對於龍船山有較高的地殼抬升率，符合台灣造山作用逆衝斷層活動由東向西發展的順序。東南-西北向的大地構造擠壓方向使研究區中接近南北向的構造從數千年以來至今保有高度活動性，同時也表現於地形特徵上。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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