臺灣東部鹿野溪變質岩區淺層地熱地質構造模型之研究：地表線型三維建構及裂隙再活化分析

Abstract

臺灣島位於歐亞板塊及菲律賓海板塊斜向聚合及隱沒反轉之交界帶上，活躍的構造活動促使原位於地下深處岩石被快速掘升至地表，同時也將地下蘊藏的豐富熱能攜帶上來，並以多種地熱徵兆呈現於地表，如溫泉、噴氣孔、泥漿泉…等。為了能更加瞭解臺灣東部地熱地質構造及後續地熱能源之可發展性，本研究以臺東鹿野溪紅葉谷溫泉地區為例，進行總長約2公里之地表地質調查並建構該地區之地質模型，同時利用數值高程模型判釋地表構造線型，並以三角網格法計算地表構造線型位態，搭配震源機制反演大地應力解算各構造線型面之活化趨勢以及研判潛在的地熱導水裂隙。地質調查結果顯示本研究區域地層主要岩性為板岩，偶夾薄層變質砂岩，區域主要中視地質構造以發達的板劈理為主，劈理平均位態為010/20E，並常伴隨急折帶構造。此區溫泉露頭多位於鹿野溪沿岸，調查顯示有熱水流出之溫泉裂隙多半隨正斷層構造出露，且石英脈常富集於斷層周圍。另一方面，震源機制解反演結果顯示，鹿野溪地區大地應力狀態以逆斷層應力場為主，最大主應力方向為近南－北方向擠壓。數值高程模型判釋之地表構造線型經球型變異分析及投影角篩選後，存在255條構造線型，主要位態分布為西北－東南方向及東北－西南方向。本研究區域構造線型擴張趨勢方位以東－西走向傾角三十至九十度為高活化值區，滑動趨勢及裂隙不安定性高活化值則以東－西走向為主，並依傾沒方向不同區分成高角度向南傾及低角度向北傾兩區。現地測量溫泉裂隙並計算其活化趨勢值，結果顯示擴張趨勢平均值為0.9、滑動趨勢值為0.35及裂隙不穩定性值為0.85，說明鹿野溪地區導水裂隙活化於裂隙不安定性及擴張趨勢數值較高地區。綜合遙測判釋、地質調查以及活化趨勢分析，本研究認為鹿野溪地區地熱潛能良好，經遙測三維地表構造線型及野外考察，研判具備地熱發展潛勢之導水裂隙位態以西北－東南方向裂隙為主。結合磷灰石核飛跡定年及井下溫度數據，結果顯示地下熱資源即使再使用兩百萬年依然可維持上百度。紅葉谷溫泉地區地熱地質構造以低角度正斷層為主要構造，早期形成低角度正斷層時，應有超額液壓驅使低角度正斷層產生滑動，平均超額液壓值為31MPa以及標準差為16MPa。Taiwan is located at a speedily uplifted orogenic belt of the oblique convergence and subduction zone between the Eurasian plate and the Philippine Sea plate. Active tectonic activities have prompted the rapid exhumation process, and carried the abundant residual geothermal heat closed to the surface. Various geothermal signs such as hot springs, fumaroles, and mud springs are exposed on the ground. In order to understand the geological structures associated with geothermal energy resources in eastern Taiwan, this study aimed to build a geological model via various methods by an example of the Taitung Luye River Hongye valley hot spring area which is a total length of about 2km riverside. We identified the surface structural lineaments based on the digital elevation model calculated the attitude of fractures by the Delaunay triangulation algorithm, evaluated the reactivation tendency of each structural lineament with tectonic stress inversed from focal mechanism. Combined with field survey result, we estimated the potential fractures of geothermal fluid conduct.The field work results show that the main lithology in this study area is slate with occasional thin layers of metamorphic sandstone. The main meso scale structure is dominated by well-developed slaty cleavage, with an average attitude of 010/20E, accompanied by kink band structure. The hot spring outcrops are along the riverside of Luye River with the outflow fractures mostly following the normal fault system enriched with quartz veins. On the other hand, the tectonic stress inversion resultting from the focal mechanism solution shows that the tectonic stress state in the Luye River area is dominated by the reverse fault stress field, and the maximum principal stress direction is around the N-S compression. Through the spherical variance analysis and the projection angle analysis, 255 structural lineaments dominated with NW-SE and NE-SW strikes. In this study area, the attitude of high dilation tendency is dominated by E-W strike with the reactivation tendency along the steep dip angle ranging from 30 to 90 degrees. The attitudes of high slip tendency and instability tendency are dominated by E-W strike with two dip angle ranges, one is to the steep south and the other is to the shallow north. The in-situ hot spring fracture reactivation tendency values are calculated with the average values in the dilation tendency of 0.9, the slip tendency of 0.35, and the instability value of 0.85. This illustrate that the fluid conduits in the study area are activated by high instability and dilation tendencies.Based on lineament identification, fracture attitude estimation, geological survey and reactivation tendency analysis, this study believes that the geothermal potential of the Luye Riverrea is excellent. According to the structural lineament and field investigation, the attitude of high potential geothermal fluid conduit is dominated by NW-SE strike. Combined with the Apatite fission-track dating and downhole temperature data, the results show that the underground geothermal resources can still be maintained at boiling temperature even if they are used for another two million years. The geological structure associated with geothermal fluid in the Hongye valley Hot Spring area is dominated by low-angle normal faults. When low-angle normal faults are formed in the early stage, overpressured fluids should drive the low-angle normal faults to slide. The average fluid overpressure compare with hydrostatic pressure is 31 MPa and the standard deviation is 16 MPa.