褶皺逆衝帶上衝褶皺塊體的構造演化：台灣中部國姓地區褶皺、斷層、及節理分析

Abstract

本研究以台灣中部國姓地區雙冬斷層上盤出露的河床連續露頭為研究對象，試圖解析逆衝塊體在造山抬升過程中產生的一系列變形構造，包括節理、斷層及褶皺，並探討過程中應力與應變狀態的演變，進而重建雙冬斷層系統的演化過程。本研究透過露頭觀察、地層對比等方式以進行地質構造剖面的建立，並量測節理及斷層等脆性構造，同時利用其與褶皺的關係進行脆性構造演化史的建構。研究地區為雙冬斷層上盤的上衝塊體，主要為中新世岩層。野外調查的結果顯示，上盤地層在近斷層帶時形成2-3個覆瓦狀堆疊，並伴隨3-4個向北傾沒的短波長連續褶皺。顯示其為一應力集中之區域。 節理與斷層擦痕的初步分析結果得到數個主要構造事件如下：一期同沈積時期的伸張性應力（造成同沉積斷層或節理）與至少三期與雙冬斷層演化相關的東西向擠壓應力。在擠壓斷層滑移事件中，早期的共軛逆斷層系統是在地層還維持在接近水平位態時形成的（造山作用初期，Stage 1），約在地下4-6公里深。爾後隨著雙冬斷層上盤上衝塊體持續滑移抬升，使得地層褶皺傾斜與地層堆疊，同時也產生晚期的逆衝斷層及平移斷層系統（Stage 2, 3）。較晚期（Stage 2, 3）的斷層擦痕破裂，似乎是在上衝地塊在淺部（2-4公里深？）形成連續短波長褶皺的同時期所伴隨的脆性構造，而分析也顯示部分錯動是沿早期形成的既有破裂面滑動。從岩層大致走向和大區域來比較，本區域似乎沒有明顯的水平方向塊體旋轉，所計算之應力場/應變場亦未特別顯示朝順時針或逆時針水平旋轉之特色。 應力與應變分析顯示，三期擠壓性構造事件皆為東西向到西北-東南向的擠壓機制，每一期皆有主要的擠壓方向與若干組次要的擠壓方向，本研究提供另一個可能的解釋：在雙冬斷層系統演化的過程中，常伴隨許多地震滑移事件，地震循環的記錄隨著雙冬斷層演化及上衝地塊抬升，持續了數十萬年，並保存在斷層帶周遭的岩體破裂中，意即這些斷層擦痕不僅記錄了大地震時期或間震期的應力狀態，同時也記錄了震後時期餘震的應力狀態，而非東西向的最大擠壓應力也許是主震後應力調整期發生的餘震滑移。 此外，從矩陣張量所計算之軸差比來看，應力軸差比在演化過程中逐漸上升，從0.13到0.35；應變軸差比則穩定保持在0.5左右。無論何種計算方法，此結果皆顯示上衝褶皺塊體主要受到東西向強勢的水平擠壓應力及應變，並在爬衝過程中維持著強勢的鉛直方向最小主應力，在演化過程中水平圍壓應力（第二主應力軸）一直維持大於鉛直圍壓應力，暗示台灣中部地區的褶皺逆衝帶在造山作用過程中，水平側向還是有明顯的擠壓應力，這也許與台灣中部仍處於造山帶中段有關。

Based on tectonic analysis on the well-exposed riverbed outcrops in the Guoshing area, central Taiwan, we elucidate and characterize the deformation structures, including joint, fault, and fold, and its evolution during the upward propagation of a thrust sheet in fold-thrust belt. We mainly focused on a section of continuous outcrops about 1 km long in the hanging wall of the Shuangtung Fault, a major thrust fault revealing a duplex structure in the western foothills of the Taiwan mountain belt. By detailed field investigation we established a local geological 3-D architecture of deformed Miocene strata, which is characterized by two to three duplex structures accompanied by multiple folds, which plunge exclusively toward the North, suggesting stress localization near the main Shuangtung fault zone. We conducted fault slip data and fracture analysis via Faultkin and T-tecto software. By further comparing the brittle fractures with fold structure, our results show that slickenside faulting occurred on multiple phase throughout the propagation of the thrust sheet on the hanging wall of the Shuangtung fault and that the multiple wave folding seemingly developed in the late stage during the exhumation of the thrust sheet. We are finally able to summarize the evolution of the deformation structures in Guoshing area as following: 1）one early extensional stress event, which makes syn-sedimentary faulting or joint; 2）three phases of compressional thrusting events; one is early conjugate thrust system with E-W compression which may indicate the beginning of thrusting when the strata of this thrust sheet still kept horizontal at depth of about 4-6 km（Stage 1）. Then, the hanging wall strata have been tilted through upward movement of the thrust sheet along the Shuangtung Fault. During this period, a late conjugate thrust system （with E-W to NE-SW compression）developed at the shallow depth of 2-4 km（Stage 2, 3）. We found that the late stage of slickenside faulting events is likely syn-folding with the multiple folds. Our observations and analyses also show that parts of the late stage thrusting slickenside followed the pre-existing fractures developed in the earlier stages. Comparing with regional pattern and strata attitude, it seems that the strata have not experienced block rotation. In addition, for each stages of compressional stress events, we found one mainly compressive direction and several secondary compressive direction, we interpret as following: numerous faulting （slickenside slip）events occurred within the hanging-wall thrust sheet during the evolution of the Shuangtung Fault system, which might represent several seismic cycles. We tend to interpret the variations on the orientation of the maximum principal stress axis to be aftershocks transient stress state under stress perturbation following the major earthquakes. We calculated the stress ratio for each stages of stress event, the stress ratio gradually increasing from Stage 1 to Stage 3, during the Shuangtung Fault evolution, averaging from 0.13 to 0.35. The results show that 1） strong compressive maximum principal stress （σ1）in the horizontal direction, 2） minimum principal stress in vertical direction （σ3=σv）, and 3）The second principal stress（σ2）always larger than the minimum principle stress（σ3=σv）. It suggested that there is still have obvious compressive lateral stress during the mountain building process.