臺灣脊樑山脈南部橫貫公路之磁性組構研究與隱示

Abstract

臺灣位於歐亞板塊及菲律賓海板塊相互隱沒的交界帶，如此特殊的地體構造使臺灣擁有複雜的地質演化史。其中，脊樑山脈經歷多次且長時間的變動，但其變形歷史尚未釐清，由於磁性組構已可應用於造山帶應變史之研究，因此本研究藉由磁感率異向性實驗解析橫跨脊樑山脈南段的應變特徵，進而探究脊樑山脈的變形歷史。本研究區域位於臺灣南部橫貫公路的東段，由埡口向東至初來，橫跨畢祿山層、太魯閣帶、玉里帶、初來層四個地質單元。野外工作主要為沿線觀察露頭，記錄構造位態並採集定向岩石樣本。室內將定向樣本製備成每邊長2.2公分的正立方體，利用非均向磁感率測磁儀測得樣本的三軸磁感率方向與數值，且將測量所得的磁感率橢球體視作應變橢球使用，建立脊樑山脈南段的東-西向的應變剖面，並配合四種不同磁性礦物鑑定實驗加以確認磁性礦物種類，以評估磁感率橢球體變形是否可反應構造變形。透過磁性礦物的辨認，發現研究區域以順磁性礦物為主，鐵磁性礦物比例較少，其中磁黃鐵礦只分布於板岩區，磁鐵礦分布於板岩區及壽豐剪切帶圍岩處。此外，雖磁黃鐵礦的平均磁感率與異向性可能存在明顯正相關之結果，但鐵磁性礦物於樣本中的含量較少，可推測研究區域的磁感率橢球體變形並非受磁性礦物影響，而是構造變形所致。磁感率研究結果顯示，磁性組構與岩石組構之間位態相互吻合，由西到東，磁性線理傾向由東南向轉為東北方向與西南方向，磁性葉理由向東傾沒轉為向西傾沒，由此磁性組構位態方向特徵性可將研究區域劃分為A-E區段。整個南橫東段剔除高應變區的資料，隨著變形強度由西向東先略升後遞減，磁感率橢球體形狀參數變化，大致顯示A區段由橢球狀至平板狀，D區段由平板狀至橢球似雪茄狀，其餘B、C及E區段以平板狀為主。將Flinn diagram與T-Pj路徑演化圖結合觀察發現，隨著由西到東，因應力場改變使得K1軸轉換方向，且變形路徑於A－C－D區段不同。A區域的橢球形狀處於橢球偏平板狀並K1軸方向指向東南方，到C區域時變形強度較A區域略高，橢球形狀為平板狀並K1軸方向轉為東北－西南方向，最後至D區域時變形強度低於C區域，橢球形狀由平板狀變至橢球狀並K1軸方向指向東北－西南方向。綜合以上結果，磁感率橢球體的變形實驗可以提供南橫區域的變形演化歷程的資料。推測由板塊碰撞開始，形成第一期褶皺，並於最西側的A區域保留此期的構造資料，向東傾沒的劈理及東南方向的線理。之後弧陸碰撞加入造山，改變應力場形成第二期褶皺，於東側片岩區形成向西傾沒的劈理，並因南向側向擠壓於C及D區域產生東北－西南方向的線理，如此南橫區域才有現今的地質變形現象。

Taiwan is located at the convergent boundary between the Eurasian Plate and the Philippine Sea Plate, resulting in a unique tectonic setting that has led to a complex geological evolution. Among these features, the Central Range has undergone multiple tectonic events, yet its deformation history remains unclear. Magnetic fabrics studies have proven effective in studying the strain history of orogenic belts. Therefore, this research utilizes anisotropy of magnetic susceptibility experiments to decipher the strain characteristics across the southern segment of the Central Range, aiming to explore its deformation history.The study area spans the eastern section of the Southern Cross-Island Highway in southern Taiwan, from Yakou to Chulai, traversing four geological units: the Pilushan Formation, the Tailuko Belt, the Yuli Belt, and the Chulai Formation. Fieldwork primarily involved outcrop observations along the road, structure examination, and oriented samples collection. In the laboratory, these samples were prepared into 2.2 cm cubic specimens, and parameters of magnetic susceptibilities were measured using a Kappabridge KLY-3. The measured magnetic susceptibility ellipsoids were treated as strain ellipsoids, establishing an east-west strain profile across the southern Central Range. Additionally, four different magnetic mineral identification experiments were conducted to confirm the types of magnetic minerals present, assessing whether magnetic susceptibility ellipsoid can be representative of structural deformation.Through magnetic mineral identification, it was discovered that the study area is predominantly composed of paramagnetic minerals, with a lesser proportion of ferromagnetic minerals. Pyrrhotite is found only in slate areas, while magnetite is present in both slate areas and the surrounding rocks of the Shoufeng Shear Zone. Despite the apparent positive correlation between average magnetic susceptibility and anisotropy of pyrrhotite, the low content of ferromagnetic minerals in the samples suggests that the shape and orientation of magnetic susceptibility ellipsoid in the study area is more likely influenced by structural deformation rather than magnetic minerals.The results of magnetic susceptibility study indicate that the magnetic fabric aligns with the rock fabric. From west to east, the trend of magnetic lineations shifts from southeast to northeast and southwest directions, while the rock foliations change from eastward dipping to westward dipping, allowing dividing the study area into segments A to E. Excluding data from high-strain zones in the eastern section of the Southern Cross-Island Highway, the overall strain intensity increases slightly and then decrease from west to east, reflected in changes in magnetic susceptibility ellipsoid shape parameters: segment A changes from oblate to prolate, segment D from prolate to cigar-shaped, and segments B, C, and E predominantly oblate.Integration of Flinn diagrams and T-Pj evolutionary paths elucidates the deformation paths in the Southern Cross-Island region. As stress fields change from west to east, the orientation of the K1 axis shifts, and also deformation paths varies in segment A, C and D. Sequentially from segments A to C to D, the ellipsoidal shapes evolve: segment A exhibits an ellipsoid to oblate shape with K1 axis trending southeast, segment C shows slightly higher intensity than A with a oblate shape and K1 axis trending northeast-southwest, and segment D exhibits lower intensity than C with an oblate to ellipsoid shape and K1 axis trending northeast-southwest.Based on the results above, the deformation experiments using magnetic susceptibility ellipsoids have been provide essential data to constrain the deformation evolution in the Southern Cross-Island Highway region. The process began with plate collisions, initiating the first phase of folding. Structural data from this phase are preserved in the westernmost A area, characterized by east-dipping cleavages and southeast-trending lineations. Subsequently, arc-continent collisions became the dominant force in mountain building, altering the stress field and causing the second phase of folding. In the eastern schist areas, west-dipping cleavages are preserved developed. And due to southward lateral extrusion in areas C and D, northeast-southwest trending lineations have formed. These processes have shaped the current geological deformation pattern of the Southern Cross-Island Highway region.