臺灣南部及東部之弧陸碰撞環境下惰性氣體特徵研究

Abstract

本研究利用台灣南部及東部兩個斷層系統附近的地下水、溫泉和岩石樣本之惰性氣體結果，討論在弧陸碰撞環境下，區域地體構造對惰性氣體分布特徵的影響。在中央山脈南端的下方，土龍灣－潮州－恆春斷層系統分隔出不同的地質區域，例如增積地殼區及沉積層區。本研究在地表出露的樣本中發現，其氦同位素組成證明上部地函衍生流體存在，表明此活動斷層系統可能延伸至地球深部。本研究推測上部地函流體沿著可達地球深部的斷層系統逸出；前人研究顯示，深層非火山型微震、電導度異常區亦分布於沿斷層走向，這些樣本所在區域之下方。這些以地球物理方法得到的證據，先前被認為與主要活動斷層的表面軌跡及傾角相關，本研究結果亦支持此結論。 台灣東部為歐亞板塊與菲律賓海板塊之間的板塊邊界縫合帶，此區南部的歐亞板塊在菲律賓海板塊的下方向東傾斜，而島弧與增積岩體之間的邊界則以池上斷層系統為標誌，池上斷層的軌跡由地震活動定義，往下延伸至少25 公里，此斷層也在島弧下向東傾斜。本研究採集花東縱谷兩側的地下水、溫泉和岩石樣品並分析其稀有氣體特徵，討論縫合帶中的區域地體構造對惰性氣體分佈特徵的影響。樣本的結果表明，池上斷層系統靠近地殼－上部地函邊界，為上部地函流體向上傳輸的主要管道。在2018花蓮地震前後，北部地下水的低氦同位素證明地函信號較弱，因在此區北部，隱沒方向從東傾的歐亞板塊轉變為向北傾的菲律賓海板塊，使板塊邊界斷層更為複雜所致。本研究認為隨著隱沒方向反轉，斷層曲折度增加，從而減少了深部流體上湧並限制了上部地函衍生氣體的釋放。

Active arc-continental collisions are one type of convergent margin where the subducting plate is composed of continental crust. Few direct geochemical records in such tectonically active area limit our understanding of the involvement of interconnected fluids from upper mantle. Isotopic signatures of noble gases in the mantle and crust and in the atmosphere afford exceptional insight into the evolution through time of these geochemical reservoirs. These proxies were applied to two cases in southern and eastern Taiwan in an arc-continental collision zone in this thesis. Noble gas signatures of groundwater, hot springs, and bedrock samples were studied from a major fault system that separates regional-scale blocks of accreted, continental materials in southern Taiwan. Despite the continental setting, the isotopic signatures argue for the presence of mantle derived fluids, suggesting that the active fault system is deep-seated. This is consistent with deep, non-volcanic tremors identified in the same area. The mantle fluids are speculated to escape along a crustal-scale fault marked by clusters of non-volcanic tremors directly beneath the southern Central Range. The evidence of these tremors and electrical conductivity anomalies along the strike of the fault recognized previously correlated up dip with the surface trace of a major active fault support the hypothesis. Another set of noble gas signatures of groundwaters, hot springs, and bedrock samples from two major fault systems in eastern Taiwan, forming the plate boundary suture zone between the Eurasian and the Philippines Sea Plates, was analyzed. In the south of eastern Taiwan, the Eurasian Plate dips east beneath the Philippine Sea Plate and the boundary between the arc and accretionary prism is marked by the Chihshang Fault, which also dips east beneath the arc. The fault is well defined by seismicity and extends to at least 25 km, close to the crust-mantle boundary. This fault appears to be the main conduit for mantle helium along the suture zone. In the north, as the subduction direction flips from the east-dipping Eurasian Plate to the north-dipping Philippine Sea Plate, the plate boundary fault is represented by a more complex system of east and west dipping structures. Low helium isotopes in this area evidence the weaker mantle signal, both before and after a Mw 6.4 earthquake. As the subduction direction flips from south to north along strike, the tortuosity of the fault-related conduits increases, reducing the flow and limiting the release of mantle-derived gases.