以數值模擬討論變形楔型體的彈塑性聚合變形

Abstract

本論文應用數值模擬來增進我們對岩石圈構造的理解，並使用DynEarthSol程式進行了兩個案例研究，利用該程式的非結構化拉格朗日網格來研究變形楔型體在彈塑性變形模式下的薄皮變形。案例研究一以二維模型探討褶皺逆衝帶(FTB)發育過程中應力場隨時空的變化。分析結果揭示了FTB發展過程中應力場的周期性變化。本研究顯示FTB的應力場通常處於臨界狀態，佔總周期時間的95%以上。其餘5%的周期時間處於超臨界狀態，期間引發了前驅逆衝斷層區（proto-thrust zone）的形成和FTB內所有既有逆衝斷層的重新活動。隨後，新的前緣逆衝斷層形成，應力恢復到臨界狀態。我們還發現原逆衝斷層區間的距離可能影響結構型態，並可能為遠場和近場應力條件下地震剖面觀測到的結構差異提供解釋。案例研究二以三維模型研究臺灣西南地區的逃脫構造。該研究使用震測剖面、上新世沉積物等厚線和GPS資料構建了具東西向橫向壓縮、一個位於淺層的斷坡(ramp)以及西南方的開放邊界的模型。模擬結果中，於早期階段在西南邊界附近發生地表下陷，中後期階段模型則發育出具逆時針旋轉的地表速度場、及沿斷坡發育的右移逆衝斷層。最後，經過不同的參數調整和比較，模型需有極低的底部摩擦和走向大約114°的開放邊界來產生與GPS觀測吻合的地表速度。總體而言，本論文展示了數值模擬在理解複雜岩石圈構造行為方面的能力。

This thesis explores the use of numerical simulation to enhance our understanding to lithosphere tectonics. Specifically, it examines two case studies using the DynEarthSol program, which utilizes an unstructured Lagrangian grid, to investigate thin-skinned convergent wedges under elastoplastic deformation. Case Study 1 focuses on the stress evolution during the development of two-dimentional fold-and-thrust belts (FTB). The analysis reveals a cyclic pattern in the evolution of the stress field throughout the FTB development process. Our results show that the stress of the FTB predominantly remains in a critical state for more than 95% of the total cycle duration. For the rest of the 5% cycle duration, the stress condition is under the over-critical state, leading to the formation of the proto-thrust zone and reactivation of pre-existing thrusts within the FTB. Later, new frontal thrusts localize, and the stress returns to the critical state. We also found that the spacing between the proto-thrust zones may affect the structural style and potentially offer an explanation for the structural disparities observed in seismic profiles under far-field and near-field stress conditions. Case Study 2 focuses on tectonic escape in southwest Taiwan using 3D modeling. A simplified tectonic setting of SW Taiwan is constructed using seismic profiles, Cenozoic sediment isopach, and GPS data. The models incorporate lateral shortening, a shallow ramp structure, and a southwestern open boundary. Our results show surface subsidence around the southwestern open boundary during the early stage of the simulation, and a counterclockwise rotation in the surface velocity, a dextral thrust along the ramp after the velocity field becomes stable. Comparisons between the models highlight the impact of the basal friction on fault zone width and ground-motion rotation. Finally, the study shows that low basal friction and an open boundary striking at about 114° are crucial to match the GPS observation. Overall, this thesis demonstrates the power of numerical simulation in understanding the complex behavior of lithosphere tectonics.