利用二維離散元素法探討增積岩體的變形模式

Abstract

因滑脫面摩擦係數(μb)在增積岩體的變形模式上扮演著重要的角色，所以，本研究利用二維離散元素法探討滑脫面摩擦係數(μb)的不同對於增積岩體發展過程所造成的影響並試圖找出高滑脫面摩擦係數與低滑脫面摩擦係數之間的臨界值(critical value)。在此研究中，所使用的分析方法為二維顆粒流軟體，其以離散元素法(distinct element method: DEM)為理論基礎，而圓形元素為基本單位的一套軟體。本研究的滑脫面摩擦係數範圍為0.05至0.9，而以顆粒之間的摩擦係數為0.5。經由不同滑脫面摩擦係數的模擬結果，可以清楚觀察到兩種增積岩體的變形模式，兩種型態的變形模式分別為低滑脫面摩擦係數，其滑脫面摩擦係數小於0.3~0.4，另一種為高滑脫面摩擦係數，其滑脫面摩擦係數則大於0.3~0.4，低滑脫面摩擦係數的變形模式，主要於楔形體或楔形體的腳邊增積形成前緣增積(frontal accretion)並伴隨著冒起(pop-up)構造的產生，當其滑脫面摩擦係數增加並轉為高滑脫面摩擦係數時，在變形過程中，俯衝(underthrusting)構造為其主要變形特徵並且穿透整個楔形體。不同摩擦係數所造成的差異性不僅能在演化過程中清楚辨別，也能在逆衝斷層傾角中觀測到其變化，逆衝斷層傾角由滑脫面摩擦係數0.05時的50度左右變化至滑脫面摩擦係數0.9時的10度左右。再者，生長率及抬升率的曲線明顯的顯示在滑脫面摩擦係數為0.3~0.4時，有一過渡帶存在。這提供我們以另一種方式去清楚得知高滑脫面摩擦係數與低滑脫面摩擦係數之間的臨界值(critical value)。另外，滑脫面摩擦係數為0.05時，楔形體地形面坡度(surface slope)為3°±1°，至滑脫面摩擦係數為0.9時，楔形體地形面坡度則為19°±1°，並且在滑脫面摩擦係數為0.35時，達到一個穩定的狀態。因此，依著生長率、抬升率及楔形體地形面坡度，我們可以清楚知道增積岩體的變形在滑脫面摩擦係數為0.3~0.4時，達到幾何形態上的平衡。

This study explores the dominant role played by décollement basal friction μb on the deformation style or pattern of accretionary wedges. The Particle Flow Code in 2 Dimensions (PFC2D), a special implementation of distinct element method (DEM) using circular elements, is applied. In this study, basal friction of the décollement is designated to range from 0.05 to 0.9, and the interparticle friction is fixed as 0.5. Based on the modeling results with different basal frictions, two modes of deformation are clearly observed. For the low basal friction case (μb ≦ 0.3~0.4), the frontal accretion is prominent and is dominated by ‘pop-up’ structures at or near the toe of the wedge. For the high basal friction case (μb≧0.3~0.4), underthrusting is the principal feature during deformation throughout the wedge. We can observe not only the difference in the evolution but also the variation of thrust angle in the accretionary wedge during the experiments. Thrust angle varies from about 50° at μb=0.05 to about 10° at μb=0.9. Furthermore, we find a transition mode of deformation presents in μb=0.3~0.4 observed from growth rate of distance to deformation front, deformation zone, and uplift rate of maximum height. The range of this transition zone gives us another way to distinguish the critical value of the transit from low to high basal friction. Moreover, the surface slope changes from 3°±1° at μb=0.05 to 19°±1° at μb=0.9 and reaches stable at μb=0.35. Based on analyses on growth rate, uplift rate, and surface slope in the numerical models, geometric steady state of accretionary wedges is achieved when μb=0.3~0.4.