- 學位論文
運用砂箱模型模擬花東縱谷縫合帶內活動構造之時空互動過程
Spatial and Temporal Interaction Between the Active Structures in the Longitudinal Valley Suture: Insights From Sandbox Modeling
摘要
本研究使用二維砂箱模擬花東縱谷縫合帶中央山脈斷層系統(Central Range fault system, CRFs)與縱谷斷層系統(Longitudinal Valley fault system, LVFs)之間的幾何互動演化過程。花東縱谷是臺灣東部的板塊縫合帶，菲律賓海板塊於此相對中央山脈地塊往西北方運動，使其上的海岸山脈與中央山脈產生斜向的弧陸碰撞，形成活動的中央山脈斷層系統與縱谷斷層系統。由北向南的板塊斜向碰撞特色使花東縱谷縫合帶能夠以南北空間的地質證據驗證地體構造隨碰撞歷時的變化，讓花東縱谷成為適合研究斷層構造演化的重要地點。然而，由於位於花東縱谷縫合帶中的同震地表破裂及活動構造地形主要集中於瑞穗以南，瑞穗以北在中央山脈側的近地表處則無明顯的活動構造特徵，因此若僅從地表地質調查方式著手，於瑞穗以北區域則缺乏足夠的地質資料以判斷兩斷層系統間的空間幾何關係。 為進一步探討CRFs與LVFs於南北方向上可能的構造幾何演化，本研究使用二維砂箱模型建立花東縱谷縫合帶最南端的地質特徵，並觀察模型中兩斷層系統隨碰撞歷時的變化，以對應斷層幾何架構由南而北連續性的轉換過程。本研究在砂箱模型中使用不同的實驗材料以類比花東縱谷縫合帶多樣的岩性強度(例如：中央山脈變質岩、縱谷沉積岩，以及海岸山脈火成岩等)，並於實驗過程中加入與板塊聚合同時進行的沉積作用及侵蝕作用。實驗模擬結果以相片定時記錄，經由多次對比CRFs與LVFs在模型和真實花東縱谷中的活動特性，將模型的地質物理參數反覆修正為較適合花東縱谷縫合帶的參數組合。 本研究結果顯示，CRFs與LVFs之間的空間互動關係共可分為三個階段：於第一階段中，兩斷層系統因板塊間聚合而逐漸靠近相交，彼此相互截切形成與共軛斷層相似的幾何架構。當進入第二階段後，LVFs則於淺部截切並超覆於CRFs之上，成為縫合帶中的主導活動斷層系統。在第三階段中，縫合帶中的主導活動斷層系統則轉為CRFs，且可發現CRFs持續向東發育新的破裂面，除截切LVFs外並貫穿海岸山脈中的火成岩體。本研究所模擬的斷層互動演化過程，除能提供未來建立CRFs與LVFs近南北向延伸的構造模型參考之外，也期待模型中各項變因的參數可成為未來建立三維模型的基礎，以更有效率地了解花東縱谷的構造演育模型。
並列摘要
In this study, we utilize 2D sandbox modeling to simulate the spatial and temporal interaction between the Central Range fault system (CRFs) and the Longitudinal Valley fault system (LVFs) in eastern Taiwan. The CRFs and LVFs are two major active faults along the Longitudinal Valley suture, accommodating more than 3 cm/yr oblique plate convergence between the Philippine Sea Plate and the Eurasian Plate. South to Ruisui, both the coseismic ruptures and the active landforms of CRFs and LVFs expose clearly, providing clear evidence to illustrate the overall geometric interaction between these two fault systems. North to Ruisui, however, the active features associated to CRFs on the surface are limited and discontinuous, limiting our understanding of the structural interaction between CRFs and LVFs from the geological investigations. Thus, how these two structures interact and evolve from south to north remains controvertial. To address on these issues, we incorporate materials with different strength in the 2D sandbox model to simulate the structural deformation within the Longitudinal Valley suture. Both syn-tectonic erosion and sedimentation are introduced in our model to reflect the first order geological history along the valley. We also use particle image velocimetry (PIV) to analyze the deformation pattern and to objectively determine the fault activity. Our results suggest that the interaction between CRFs and LVFs can be divided into three stages. First, CRFs and LVFs crop to the surface separately and gradually form a conjugate-like fault system in the suture. In the second stage, LVFs crosscuts and overrides on CRFs, and LVFs takes dominance in our model. The CRFs regains dominance in the third stage as it propagates eastwards, crosscuts LVFs, and truncates the Coastal Range. These structural changes observed in our 2D model are comparable to seismological and geological observations along the valley, suggesting the plausible structural interaction within the Longitudinal Valley suture from south to north.