以數值模擬討論台灣西南地區地表變形和斷層發育

Numerical Simulation on the Surface Deformation and Fault Development in Southwest Taiwan

李芳儀(Fang-Yi Lee)

指導教授 ： 張翠玉

共同指導教授 ： 譚諤

國立臺灣大學/理學院/海洋研究所/碩士(2017年)

https://doi.org/10.6342/NTU201704113

摘要

本論文使用三維的數值模擬以彈塑性變形模式討論台灣西南地區之地表變型和斷層發育，區域涵蓋台灣西部麓山帶南端、西部海岸平原南部一直到高屏上部斜坡。
在菲律賓海板塊與歐亞大陸邊緣斜向碰撞下，台灣自新生代後期開始進行造山運動，造山帶由北向南發育並由東向西在西部麓山帶形成一系列南北向的摺皺逆衝帶。在各種野外觀察及觀測資料下得知，受板塊側向擠壓應力影響，西南台灣的平原區的地層正在發育逆衝斷層以及摺皺變形。而此區域的西側為被動大陸邊緣，構造上為擠壓變形的後擋，相對的，西南側為開放的邊界，許多的地球物理資料皆顯示台灣西南地區在側向擠壓下存在往西南方運動的逃脫構造。根據現代GPS的測量值推論，逃脫構造造成屏東平原逆時鐘旋轉的速度場以及讓旗山斷層帶右移的運動分量。而砂箱模擬演繹了台灣西南地區的逆衝斷層多帶左移分量並被右移斷層截切。豐富的地質地物觀測資料及相對簡單的邊界幾何讓西南台灣成為討論板塊聚合初始變形的合適地點。
本論文使用DynEarthSol3D程式模擬地層聚合變形的行為，在符合Lagrangian力學方程式的前提下，以彈塑性模式模擬在不同的幾何、邊界條件下的地層變形行為。在模型的設計上我們參考了斷層分布、新生代沉積物厚度、布蓋重力異常、震測剖面和地質圖來設計各個實驗的幾何。潮州斷層被設定為實驗的東邊邊界，相對澎湖以每年約5公分的速度向正西移動；北邊邊界設在潮州斷層周遭有以每年5公分向西速度的最北端；在不同實驗中我們分別將旗山斷層、變形前緣及位於北港高地東南邊的義竹斷層設為西邊邊界。除了考慮簡單的單層物質變形，本論文也設計了雙層及三層的物質模型，在雙層的模型的下層及三層模型的中層所設置的分層為分離上下層變形的滑脫面層。透過調整滑脫面層的摩擦角便可測試滑脫面上不同摩擦係數對變形的影響。而本研究也根據震測剖面，於滑脫面上設計斷坡(ramp)及複堆疊(duplex)。另外，在北邊和西邊邊界外圍，實驗中設置了一層5公里厚的物質設定不同摩擦角來控制討論區域與週遭物質之間的藕合強度。
根據模擬的結果，西南臺灣聚合變形有下列主要的構造：(1)一組平行於東邊邊界的共軛逆衝斷層會由南往北發育，隨著滑脫面層摩擦角及北邊、西邊界上的摩擦力增大，這組斷層系統會往北即往陸地的方向延伸。(2)在滑脫面上設置的斷坡和複堆疊構造，會發育由北往南的斷層系統，可以對應旗山斷層以及其週遭一系列與之平行的斷層(如：龍船斷層、古亭坑斷層等)。(3)當滑脫面層摩擦角較低時，在模型的東南角會發育逆時鐘旋轉的左移斷層，在一些實驗中會同時存在數個左移斷層呈扇狀分布。(4) 永安線型和崙後背斜為此區域中不可忽略的構造。

關鍵詞：西南台灣、數值模擬、斷層發育、地表變形、逃脫構造

關鍵字

西南台灣 ； 數值模擬 ； 斷層發育 ； 地表變形 ； 逃脫構造

並列摘要

This study uses 3D numerical simulation to discuss the fault development and surface deformation in Southwest Taiwan. The studies area comprises the southernmost part of fold-and-thrust belt of Taiwan, the southern part of the coastal plain and the offshore Kaoping upper slope.
Under the oblique collision of Philippine Sea plate against the Eurasian continental margin, active orogeny takes place around Taiwan since the late Cenozoic. The consequent lithospheric deformation is considered to propagate from east to west and has generated a series of folds and thrusts in N-S direction. As the obliquity, the orogenic belt developed from north to south. Also, the observations proved that the SW Taiwan is undergoing strong crustal deformation, rapid uplift and high denudation and erosion rate in neotectonics, which are typical condition for compressional region. The geometry of Chinese continental margin and the collision direction of Luzon arc to the continent, make southwest of the research area as a relative open boundary and many geophysical data also prove that under lateral collision with an open boundary, Southwest Taiwan is undergoing tectonic escape toward southwest. The dextral motion of Chishan fault and the counterclockwise-rotation velocity field in the Pingtung plain are regarded as an evidence for tectonic escape. Furthermore, the sandbox experiments show that in this area most of the thrusts exhibit left-lateral oblique slip while being truncated by the right-lateral faults. All these reveal a deformation partition in response to the plates’ convergence acting with the local indentation of the Peikang high. The abundant observations and relative simple boundary make SW Taiwan as a good place to study the initial deformation of orogeny.
With the program DynEarthSol3D, we do numerical simulation in elastoplastic mode in Lagrangian mechanic form using unstructured meshes. We adopt the local faults’ geometries and also consider the Cenozoic sediment isopach, Bouguer gravity anomalies, seismic profiles and geology map of this area to set our experiments. The Chaochou fault is considered as the eastern boundary, which moves westward in a velocity of ~ 5 cm/yr referenced to Penghu. In our experiments, we try to set the Chishan fault, the deformation front and Yichu fault (a fault lying along the SE border of Peikang high) as the western boundary in different trials. The northern boundary is set in the northernmost place where the GPS observations show 5 cm/yr westward along Chaochou fault to separate the complexity in central Taiwan. A fine zone parameterized in varied frictions is attached to the western front and the northern boundary as to discuss the coupling of the material flow in the boundaries. A soft layer with low friction angle is sandwiched or lying beneath the model to mimic the existence of decollement and separates the deformation of upper layer and lower layer; we named these low-friction-angle layers as decollement layers. Following the balanced cross section which is based on the seismic profile in north of Pingtung Plain, the decollement is not flat, ramps and duplexes are set in our decollement. We modify the friction angle of material in the decollement layer to test the effect of the friction coefficient to the development of faults and surface deformation.
After this study, the tectonic patterns subject to the plate collision can be drawn as, (1) There are a pair of conjugated longitudinal faults generated beside the eastern boundary and this fault system would develop more northward as the coupling of the discussed area with lower layer or surrounding material gets larger. (2) A fault system related to the ramp and duplex develops from north to south with reverse and dextral motion may related to a series of faults around Chishan and Lungchuan fault. (3) The sinistral strike-slip faults are seen in the southeast in our experiments. When the basal friction is low enough, the strike of these faults would rotate counterclockwise repeatedly. In some cases, there would be a series of sinistral strike-slip faults distribute in a fan pattern. (4) In our experiments, we found that the existence of Lunhou anticline and Yung-An lineament is important to the neo- crustal deformation in the SW Taiwan.

Keyword: Southwest Taiwan, numerical simulation, fault development, surface deformation, tectonic escape

並列關鍵字

Southwest Taiwan ； numerical simulation ； fault development ； surface deformation ； tectonic escape

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