臺灣弧前基盤缺失的熱-機械力學耦合數值模型

Abstract

臺灣地處歐亞大陸板塊邊緣並與菲律賓海板塊交界，菲律賓海板塊以每年70到80毫米的速度向西北方向移動並與歐亞大陸板塊聚合，形成呂宋島弧與臺灣造山帶。但在臺灣造山帶中段呂宋島弧與歐亞大陸邊緣直接相連，缺失弧前基盤，此弧前基盤缺失現象亦廣泛出現於類似板塊聚合區域。過去研究認為此弧前基盤已隱沒至於菲律賓海板塊與歐亞大陸板塊之間。為了探討弧前基盤隱沒的機制，我們利用熱-力學耦合數值模擬，並結合地質資料，模擬岩石圈尺度的板塊隱沒到碰撞過程來探討臺灣造山帶中段弧前基盤隱沒的動力學機制。 模擬結果表明，當隱沒板塊由海洋地殼過渡到大陸地殼時，因大陸地殼材質密度較輕，不易隱沒而造山形成的巨大塊體阻擋菲律賓海板塊向西北前進，讓弧前地殼撓曲下凹形成一個弧前盆地，盆地內開始堆積大量的沉積物。直到盆地中心破裂形成弧前盆地斷層，盆地東側島弧區域逆衝至盆地西側弧前基盤之上方，推擠盆地內沉積物快速抬升，最終弧前基盤隱沒至菲律賓海板塊之下並形成中央山脈斷層。在此模型解釋了弧前基盤缺失的原因、縱谷斷層形成的時間點，以及海岸山脈沉積物記錄到的劇烈上下運動。

Taiwan is located at the edge of the Eurasian plate and borders the Philippine Sea plate. The Philippine Sea plate is moving northwestward at a speed of 70 to 80 mm/yr and is converging with the Eurasian plate, forming the Luzon arc and the Taiwan orogenic belt. However, in the middle section of the Taiwan orogenic belt, the Luzon arc is directly adjacent to the edge of the Eurasian continental margin, and the forearc basement is missing. This phenomenon of missing forearc basement is also widely observed in similar plate convergence zones. Previous studies have suggested that this forearc basement has subducted between the Philippine Sea plate and the Eurasian plate. In order to explore the mechanism of forearc basement subduction, we used thermal-mechanical coupled numerical simulations combined with geological data to simulate the dynamic mechanism of forearc basement subduction in the middle section of the Taiwan orogenic belt. The simulation results show that when the subducting plate transitions from oceanic crust to continental crust, the continental crust has a lower density and is not easily subducted. The huge mass formed by the orogeny blocks the Philippine Sea plate from moving northwestward, causing the forearc crust to bend concavely and form a forearc basin. The basin begins to accumulate a large amount of sedimentary material. Later, the center of the basin breaks to form a fault, the island arc to the east of the basin thrusts over the forearc basement, pushing the basin sediment to uplift rapidly, and finally the forearc basement subducts below the Philippine Sea plate. This model explains the mechanism for the missing forearc basement, the timing of the formation of the Longitudinal Valley fault, and the dramatic up and down movements recorded in the sedimentary rocks of the Coastal Mountains.