以砂箱實驗探討增積岩體的前緣增積作用

Abstract

利用物理模型來進行地質構造模擬，能即時觀察到構造的型貌及構造的演化，而由於顆粒狀流體的特性，砂箱模型可以用來模擬非線性變形過程及岩層破裂前後的變形。本研究主要針對增積岩體的前緣增積行為進行分析，增積岩體為板塊聚合作用中重要的地質作用區域，在不同的地質條件下，存在影響增積岩體的變形行為不同的因素。經由實驗室尺度的模型設計，本研究更改部分參數以進行模擬，包括底部摩擦係數(μb)、傾角(β)以及原始弱面存在深度等參數；本研究並利用PIV(Particle Image Velocimetry，質點影像測速儀)技術分析砂箱實驗過程中的顆粒動態位移場，並將前緣的增積變形行分成逆衝斷層初始(thrust initiation)階段、俯衝(underthrusting)階段及再活化(re-activaction)階段，結合傳統的砂箱實驗分析方法探討各個參數主要的作用結果。本研究使用之底部摩擦係數為0.55及0.31，底部摩擦係數對於前緣增積行為的影響，最大差異發生於斷層俯衝階段。低底部摩擦係數時，底部滑脫作用不易將逆斷層下盤物質帶入增積岩體內，而使得背衝斷層漸次向後方發展，以維持臨界錐角，主要抬升區域集中於前緣；高摩擦係數時，底部互鎖作用造成逆衝斷層下盤物質受俯衝作用帶入楔形體，造成主要抬升區域分布於後方。傾角的變化在本研究中分別由0度測試至8度，傾角的增加直接加強了重力平行於滑脫面之作用，造成斷層俯衝作用顯著；而傾角加上摩擦係數影響，則由底部摩擦係數高低決定是否有疊瓦狀構造或是大型背衝斷層，例如，高摩擦係數下易形成疊瓦狀構造，低摩擦係數下則易形成大型背衝斷層。本研究使用之砂層厚度為4公分，並利用較弱之玻璃微珠層模擬原始弱面，深度則由1.5公分至3.5公分。深度於1.5~2.5公分時，由主要前衝斷層前端之玻璃微珠層形成小型逆衝斷層，造成俯衝作用的間斷，並且此小型逆衝斷層屬於主要斷層之分支，而造成間斷後俯衝作用的加強；而深度於3~3.5公分時，則由玻璃微珠層形成新的一組滑脫面，整體變形特性受到玻璃微珠層的摩擦係數影響，屬於含有背衝斷層之低底部摩係數特性。因此，整體而言，摩擦係數可被認為是否有背衝斷層及決定主要抬升區域之指標，傾角則直接反映重力作用，原始弱面於淺部可產生斷層分支作用並加強俯衝作用，於深部會成為滑脫面。綜合此三種參數分析結果可以發現，俯衝作用的階段為不同參數在前緣增積循環中造成最主要差異之部分，因此日後我們可以利用研究此作用階段的特性，當作定義不同參數影響之重要階段。

Simulation of geological structures by physical modeling provides real-time observations on the geometry and evolution of deformed structures. Based on the granular flow characteristics of quartz sand, sandbox experiments are frequently used to model non-linear deformation behavior and rock failure in the upper crustal deformation. This study focuses on the deformation behavior of frontal accretion in accretionary wedges using sandbox experiments. In order to investigate the influence of different parameters on the development of accretionary wedges, including basal friction coefficient (μb), basal detachment dip (β) and the depth of inherited weak layer, a series of sandbox experiments with proper scaling are performed. Particle Image Velocimetry (PIV) analysis is applied to the images of sandbox results to visualize the spatial and temporal deformation patterns for each experiment. Combined with conventional analysis method of sandbox experiments, the influences of the tested parameters are discussed. The frontal accretion cycle observed in the sandbox analogue experiments of accretionary wedges can generally be divided into three stages: thrust initiation, underthrusting and reactivation. In this study, basal friction coefficients are designated 0.31 and 0.55 by using plastic and sandpaper belt, respectively. The main difference between the contrasting basal frictions is the deformation within the underthrusting stage. When basal friction is lower, the footwall material beneath the frontal thrust can not be easily displaced into wedge due to low coupling between sands and basement. This leads to reterowedge-directed development of backthrusts backwards to maintain critical taper and main uplift is located in the deformation front. In contrast, when basal friction is high, footwall material is underthrusted into wedge due to strong basal coupling and main uplift located in the rear wedge. The detachment dip is designed with 0, 3, 6 and 8 degree. With increasing β angle, the underthrusting process becomes more dominant because of increasing gravity component parallel to the basal detachment. In high β angle cases, the value of basal friction determines whether imbricate structure or large-scale backthrust is dominant. For example, imbricate structures are always observed in high basal friction cases and large-scale backthrusts in the low basal friction cases. To investigate the difference of varying depths of inherited weak horizon, experiments are set with 0.1 cm glass microbeads layer as weak layer are added into the 4cm sand layer. The depth of weak layer ranges from 1.5cm to 3.5cm. For cases of layer depth in 1.5cm, 2cm and 2.5cm, the main thrust event is paused in the underthrusting stage when the external small thrusts are generated from the glass microbeads layer. However, the development of external thrusts will increase the degree of later underthrusting when the main thrust becomes reactivation. For the cases of weak layer level in 3.0 and 3.5 cm, the glass microbeads layer becomes a detachment, in stead of detachment in the base on other experiments. The deformation features within these cases are similar to that within low basal friction cases. In summary, basal friction is the factor to influence the existence of imbricate thrust or large-scale backthrust and the location of uplift region. The β angle has a direct influence on gravity component. The shallower weak layer would be the location to generate external small thrusts and enhance the role of later underthrusting. The deep weak layer would become a shallower detachment, not in the base. Comprehensive results of influences from these three parameters indicate that underthrusting is the most important stage to evaluate the parameter effects in the context of frontal accretion cycle. Consequently, in the future we can focus on studying the differences of deformation patterns and behaviors induced by parameter change in the underthrusting stage to evaluate the impact of different factors to the development of accretionary wedge.