應用地形分析方法研究台灣中央山脈東翼地表抬升

Distribution of uplift along the eastern flank of the Central Range in Taiwan: Inferences from geomorphic analyses

楊萬慧(Wan-Huei Yang)

指導教授 ： 張中白

國立中央大學/地球科學學院/地球物理研究所/碩士(2007年)

摘要

地表的侵蝕作用通常會伴隨相對的地體抬升回饋；因此，陡峭的地形往往發生在地表抬升相對快速的地區。藉由河流會隨地形發育而調整其坡度和集水面積的特性，本研究萃取定量的河流參數並導入水力侵蝕物理模型及地形計測方法，嘗試藉由地形分析，探討地表抬升速率與大地構造之關係。
本研究以台灣中央山脈東翼的18條主要水系為研究區域。由於中央山脈東翼為台灣全區年齡最老的區域，因此，其地層完整記錄了台灣基盤抬升的歷史。雖然近年來應用大地測量技術探測地表變形的方法日漸普及，但是此法僅侷限於現今的變形觀測，對於長時間尺度的地表抬升觀測記錄卻依舊缺乏。水系可完整記錄地形的演變歷史，其觀測時間尺度可溯及數千年甚至數萬年，本研究因此利用河流水力基盤侵蝕模型(Stream power bedrock incision model)及集水區不對稱性方法(Drainage basin asymmetry method)，期望藉由一系列之計算分析，了解較長時間尺度的地體抬升資訊。
河流水力基盤侵蝕模型可以有效反映各個集水區的相對抬升資訊，而集水區不對稱性方法則可反應垂直於水流方向的相對抬升情形。綜合以上方法研究結果顯示，本研究區的北部及中部相對抬升較為快速。比對三角測量所測得之近期地表抬升率、鋯石核飛跡定年(zircon fission-track)所測得之長時間尺度地表剝蝕率以及現今的地表高程，皆可發現中央山脈北部及中部的抬升和剝蝕速率都相對較為快速。顯示不同時間尺度的抬升及剝蝕率有相當高的一致性。另外，此兩區相對抬升快速的區域其緯度與台灣西部的基盤高區(觀音高區及北港高區)相當，暗示此兩區的快速抬升與基盤高區可能有密切的關係。就地體構造的觀點，由於菲律賓海板塊向北隱沒與弧後(間)擴張作用，造成宜蘭平原陷落，連帶使得本研究區北側的地形向北傾斜、高差明顯，河流下切作用相對快速；再加上季風與地形雨的影響，使得此區地表侵蝕嚴重。綜合以上因素，河流的下切與季風及地形雨的影響，亦為觸發北部研究區地表相對抬升快速的可能原因。

關鍵字

集水區不對稱性方法 ； 河流水力基盤侵蝕模型

並列摘要

Mass redistribution by erosion represents a governing force in the tectonic evolution of orogenic systems and this process makes the steepest landscapes associated with regions of rapid rock uplift and it could exert a primary control revealing the uplift evolution. The eroded materials from the orogens are generally carried away by the numerous rivers originated in the mountain belts. Therefore, these rivers and their longitudinal profiles record and reflect the changes in landscape. Based on this theory, parameterization of the information of the streams could reveal meaningful geological information in the active tectonic regions.
This study chose 18 drainage basins in the eastern flank of the Central Range in Taiwan as the study area, which is the oldest part of Taiwan and recorded complex uplift history. Although many geodetic measurements have been recently applied to reveal the uplift rates in the Taiwan island, they are often difficult to obtain the uplift history of larger time scale. In order to understand this complex history and the relationship between the geomorphology and tectonics, two geomorphic analyses, Stream power bedrock incision model and Drainage basin asymmetry method, have been applied.
In Stream power bedrock incision model, the evaluated normalize steepness index (ksn) of each drainage basin is directly proportional to uplift rate of each drainage basin. Whereas in Drainage basin asymmetry method, the calculated asymmetry factor (AF) of each drainage basin is developed to detect tectonic tilting transverse to flow within each drainage basin. The results reveal that there are two obviously relative high uplift zones: the northern-and the central part of the eastern Central Range. The identified high uplift zones coincide with the high uplift zones identified from modern uplift rate obtained by triangulation network survey data, and long-term denudation rate inferred from zircon fission-track data. The fast uplift rates in these two zones could be attributed to the impinging basement highs of Taiwan orogen, the Kuanyin and Peikang highs. Aside from the two high uplift zones, the subduction of the Philippine Sea Plate cause the Okinawa Trough spreading which develops upon the attenuated crust of the foundered orogen and spears into the Taiwan orogen at the Ilan Plain. The subsidence of the Ilan Plain promote serious incision in the very northern part of the study area which make the landscape tilting northward in the northernmost Central Range. Couple with high annual precipitation in the same area, the erosion might feedback to the orogenic system and promote further uplift.

並列關鍵字

Stream power law ； Drainage basin asymmetry method

參考文獻

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