台灣南部恆春西臺地構造與抬升機制之研究

Structure and Uplift Mechanism of the West Hengchun Hill in Southern Taiwan

劉冠亮(Guan-Liang Liu)

指導教授 ： 張中白 ； 洪日豪

國立中央大學/地球科學學院/地球科學系/碩士(2015年)

摘要

位於台灣島最南端的恆春半島主體是由一套中新世中期至晚期的深海濁流岩層序所構成，除了少數的背衝斷層發育之外，大多數的斷層與褶皺都是向西逆衝且呈南北伸向。此南北伸向構造線分佈，應是來自構造抬升時期的東西向壓應力所造成。然而在恆春半島的西側，存在一個第四紀的珊瑚礁臺地－恆春西臺地。恆春西臺地主要由第四紀之珊瑚礁所組成並緩向東傾，與恆春半島主體相比有著截然不同的變形特徵，西臺地並不存在壓應力造成之斷層與褶皺構造，因此恆春西臺地的抬升機制是一值得探討的主題。

前人研究認為整個恆春西臺地屬於一個抬升背斜構造的東翼，不過在恆春西臺地的南北段地形特徵上卻有所不同。地形分析顯示，北段之地形較接近線性崖，而南段之地形則呈現半穹丘狀(half-dome)，這說明了恆春西臺地南北段可能存在著不同的抬升機制。且本研究野外調查結果發現，出露於恆春西臺地南段區域之基盤岩層馬鞍山層常伴隨由冷泉(cold seep)作用所沉積之碳酸鹽岩出現。而碳氧同位素分析結果顯示，這些冷泉碳酸鹽在沉積之後應受過其他的例如生物或天水等作用，而非單純冷泉之沉積作用。冷泉碳酸鹽應是沿泥灌入體的入侵伴隨而生，而馬鞍山層在恆春西臺地出露所呈現的地形狀態皆為圓丘狀，也表示泥貫入體作用在馬鞍山層中是相當活躍的。根據此些證據，本研究推論恆春西臺地之抬升，北段是來自於背斜構造發育的影響，而南段的抬升是由泥貫入作用所造成。在恆春西臺地抬升過程中，整個背斜構造西翼連同生物礁主體的部分，沿軸部上一西北伸向的正斷層向西滑落，於前人發表的震測剖面中，恆春西臺地西部外海發現到被埋在沉積物之下的一些珊瑚礁的反射訊號可以做為解釋。此正斷層活動可解釋恆春西臺地不對稱之地形與特殊岩層分布。

關鍵字

馬鞍山層 ； 泥貫入體 ； 恆春西臺地

並列摘要

Taiwan, locted at the boundary between the Philippine Sea plate and Eurasian plate, is a product of arc-continental collision. Because this collision takes place in an oblique direction to the convergent boundery, the orogenic belt of Taiwan has been propagating southward. As a newly emerged accretionary prism at the southern tip of Taiwan orogen, the Hengchun peninsula exhibits the youngest, least deformed and most complete accretionary prism in Taiwan.
The main body of the Hengchun peninsula is composed of the Middle-Late Miocene turbidite sequences. Few backthrusts being left apart, most faults and folds of the Hengchun peninsula are western-verging. The N-S trending structural pattern of the peninsula suggests that most of the deformation resulted from an E-W directed compressional tectonic regime during the period of tectonic uplift. However, in the western part of the peninsula, the West Hengchun Hill, mainly composed of the Quaternary coral reef, is gentlely tilted to the east but without any compressional structures. The origin and the uplift mechanism of the West Hengchun Hill is thus still a debate.
From previous study, they think that the West Hengchun Hill is the eastern limb of an uplifted anticline. From our geomorphologic analysis, the topography in northern West Hengchun Hill shows a linear scarp feature but half-dome topography in southern part. The topographic difference in these northern and southern West Hengchun Hill might indicate the different uplift mechanism. Also from our field investigation, Maanshan Formation usually crops out with cold seep carbonates in southern West Hengchun Hill. The presence of cold-seep carbonates and the dorm-shaped topography reveal mud diapiric features in Maanshan. According to the the evidences mentioned before, we believe the uplifted mechanisms are different in the northern and southern West Hengchun Hill, the anticline uplifted the northern West Hengchun Hill and a series of mud diapiric structure triggerd the uplift of southern offshore part. Some previous seismic profiles show reflectional signals of coral reef can be found in the western offshore of the West Hengchun Hill. We also interpret these as the western limb of the the West Hengchun Hill anticline, that slide down to the west along a NW trending normal fault during the uplifting period.

並列關鍵字

Maanshan Formation ； Mud diapir ； West Hengchun Hill

參考文獻

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