台灣南部造山運動的地體動力模擬

Geodynamic Modeling Of The Mountain Building In Southern Taiwan

陳劍祥(Chen, Jianxiang)

指導教授 ： 王維豪

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2012年)

摘要

本研究修改Gerya所設計的地體動力模擬程式，進行台灣南部(22.5°N以南)造山運動的模擬。雖然弧陸碰撞被認為是台灣造山的主要機制，但南中央山脈的隆起卻在弧陸碰撞之前且其與中、北部中央山脈的形狀迥異，同時南中央山脈西側的屏東平原存在一異常低的重力低區，此也非狹長的南中央山脈荷重所能解釋。因此我們根據地質與地球物理的資料，重建歐亞大陸與菲律賓海板塊在南台灣聚合的情形，利用節點內含標點的有限差分法（markers-in-cell finite difference method）去解算偶合力學與溫度擴散方程來探討台灣南部造山運動的可能機制。
本研究的模擬結果，不論是形狀、山脈高度、大陸地殼隱沒深度等，皆與實際觀測資料相符。根據我們模擬的結果顯示，南台灣的造山與弧陸碰撞無關，而是在大陸地殼隱沒時擠壓原本堆積於大陸邊緣的沉積物所致。在山脈西側的屏東平原，其成因是因為大陸隱沒時造成地殼撓曲的結果。當大陸與海洋聚合時，大部分的大陸地殼都隱沒至海洋板塊之下，因此南部中央山脈並沒有持續增長而停留在碰撞初期上衝構造(pop-up structure)的階段，加上海洋地殼向上撓曲，增加山脈向上抬升的動力，使得南部中央山脈西側的坡度大於最小臨界楔形體的坡度。同時因為菲律賓海板塊可延伸至南部中央山脈東側淺部地殼處，造成此處山脈出現罕見的正的布蓋重力異常值。由於要發生聚合帶轉移必須有適度的大陸地殼與隱沒帶內岩層的強度對比，所以在11 Ma發育的拉張盆地，其底部的正斷層或剪切帶極可能成為先存的弱帶，並控制此地的構造演化。

關鍵字

地體動力模擬

並列摘要

We explore the orogeny in southern Taiwan (south of 22°N) by employing a thermomechanical code modified from Gerya (2010). The southern Central Range has a shape quite different from the northern Central Range and it grew prior to arc-continent collision, which is considered to be the main mechanism of the Taiwan orogeny. West of the southern Central Range, the Pingtung Plain exhibits an unusually lower gravity, which is too low to be attributed to the loading of the relatively narrow and low-relief southern Central Range. To untangle these puzzles, we set up thermomechanical models based on both geological and geophysical data to simulate the convergence of the Eurasia and Philippine Sea Plates in the southern Taiwan. We use the markers-in-cell finite-difference method to solve the coupled mechanical and temperature diffusion equations, which allows us to explore the possible mechanism of the orogeny in southern Taiwan.
According to our simulation results, the orogeny in southern Taiwan may most likely result from deformation of the sediments deposited on the continental margin during subduction of continental crust rather than arc-continent collision. The subduction of the Asian continental margin also resulted in deflection in the Pingtung Plain west of the southern Central Range. The Southern Central Range does not continued to grow and remains a pop-up structure as most of the continental crust has been subduction beneath the oceanic plate. The upward deflection of the oceanic crust during subduction of the continental margin further and enhance the vertical movement of the southern Central Range. These two factors make the slope of western flank of the southern Central Range exceeds the minimum critical taper. Because the Philippine Sea plate may extend to the east side of the southern Central Range, the southern Central Range exhibits unusual positive Bouguer gravity anomalies. Our simulation results also suggest that jump of subduction zone may occur when the strength contrast between the continental crust and the material in the subduction channel is distinct. We suspect that the formation of the rift basin in southern Taiwan in about 11 Ma may have developed a weak shear zone at its base, which could affect the strength contrast and control the tectonic evolution in our study area.

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