由砂箱模型透視台灣南部弧前基磐隱沒機制及前緣增積系統

Abstract

台灣島位於歐亞板塊與菲律賓海板塊斜向聚合帶上，為世界活躍的造山帶之一。台灣南部海域包含了呂宋島弧的弧前基磐，而此弧前基磐隱沒作用，亦對應著台灣島的板塊運動自隱沒作用轉變為碰撞的過渡帶。本研究中嘗試利用物理砂箱模型進行構造重演，以模擬台灣南部海域弧前基磐隱沒作用以及增積楔形體的弧前增積系統之循環。接著定義弧前增積系統發育時，各個活動階段增積楔形體的幾何特徵以及空間分佈，更進一步利用質點影像速度分析，分析實驗中質點瞬間變化、位移場向量以及剪應變量化等分析。 從模擬結果發現，增積楔形體的發展受到底部摩擦力不同的影響，斷層生長的幾何方式也有所差異。當底部為低摩擦力時，則斷層幾何型式為前緣增積與滑脫作用；而底部為高摩擦力時，會造成深部物質與底拖帶產生互鎖現象，造成俯衝作用，幾何型式則形成覆疊形式的逆衝斷層。此外，PIV分析結果，說明增積楔形體在初始逆衝斷層發育階段呈現應變增強；深部俯衝階段應力則呈現應變減弱。從剪應變的結果中了解，表面的侵蝕作用會使斷層發生脫序行為，以及影響斷層發展的週期。整個台灣的造山運動由板塊的隱沒增積揭開序幕，當弧前基磐開始隱沒時，造成新造山增積楔形體發展於後阻體與弧前基磐之間，並與原先的楔形體產生碰撞，衍生出兩個切過碰撞帶的背衝斷層構造。且藉由此背衝斷層可能用以解釋台灣東南部地區目前所觀察到地層倒轉，位於海床底下的物質亦可能透過此背衝作用的機制，出露至地表形成野外所觀察到的混同層。本研究主要觀察到的結果說明了台灣造山帶初期的板塊構造演化。首先，因板塊隱沒作用而造成增積楔形體的發展，繼之，在初期碰撞時，增積楔形體底部弧前基磐的隱沒拖曳作用造成了兩次碰撞現象。本研究的驗證結果幫助我們瞭解台灣大地構造的演變，為利用砂箱模擬探討構造演化的一項實例。

The active Taiwan orogenic belt is considered as formed by the Eurasian plate obliquely converging with the Philippine Sea plate. In southern offshore of Taiwan, the oceanic lithosphere subducted eastward beneath the Philippine Sea Plate along the Manila Trench. In southern Taiwan there was a slice of Luzon forearc basement as a consequence of the transition from oceanic to incipient continental subduction. In this study, we designed the experiments by the sandbox of physical models to examine the detail impact of forearc lithosphere subduction on forearc basin and accretionary wedge deformation. We also demonstrate that the geometry of the deformation and spatial distribution of surface uplift can be used to the currently active phase within a frontal accretion cycle. After that, we analyzed the displacement field of particles by PIV (Particle Image Velocimetry) to quantify the vector field and the shear strain of the model result. Our experiment results showed that the friction could be affected the fault geometry. In low friction basement, the fault geometry favors develop as push-out decollement, on the contrary, in the high friction basement, the fault geometry prefers develop underthrusting and imbricate structure. From the PIV result, We infer that the three phases of a frontal accretion cycle are controlled by the strain hardening and softening processes. The stress-strain evolution will harden in the thrust initiation phase and soften in the underthrusting phase. Furthermore, our modeling result from PIV suggests that the surface erosion promotes the possibility of out-of-sequence thrusts. The orogeny of Taiwan is starting when the plates subducted to product the accretionary wedge. When the forearc basement began to subduct under the Luzon arc, the new orogenic wedge was grown at the boundary between the backstop and the forearc basement. The previous accretionary wedge and the orogenic wedge were accreted to develop major backthrust and backfolding in the forearc basin. In our opinion, this major backthrust and back fold at retro-wedge probably consistents with the field observation of the overturn structures in the southeastern Central Range. And the material beneath the seafloor could derived from the thrust and became part of the Melange. In summary, our study results demonstrate early stage of Taiwan orogenic evolution. At the first phase, plates subducting form the accretionary wedge; at the second phase of the initial collision, the forearc basement is subducted under the arc to cause double collisions, which is consistent with the early stage of Taiwan orogenic evolution. Those helped us to understand more about Taiwan tectonic evolution and built a good example for the application of sandbox simulation.