在褶皺逆衝帶前緣發展的過程當中，任何斷層相關構造代表最基本的特徵，而影響其構造演化。在台灣西北部外麓山帶，一系列的斷層相關褶皺被一組前造山時期的橫移斷層所分割，而位於此構造區域的最南段，為台灣陸域上最大的天然氣田場址-鐵砧山背斜，是斷層逆衝帶前緣中完整的斷層相關褶皺構造之一。然而，前人研究對形成該褶皺的地底下斷層幾何形貌仍然有許多不同的看法，並且無解釋整個鐵砧山完整的地下三維形貌。本研究主要的目的就是提出與前人不同的看法，並利用一糸列網狀的震測剖面和井下地質資料去重建褶皺的三維幾何形貌以及解釋褶皺底下的逆衝斷層。本研究將時間剖面轉換成一比一的深度剖面，並利用鑽井地質資料與震測剖面中反射線的形貌以及其上盤地層和斷層面的角度幾何關係去建立三維地下褶皺與逆衝斷層構造形貌。
地表上，鐵砧山構造為受橫移斷層所切割而形成南北兩段獨立背斜。然而，根據一系列的震測剖面所作的地下構造解釋，鐵砧山地下構造為兩對稱性相反的背斜所組成，而兩對稱性相反的背斜構造朝向其交會處逐漸消失，形成一轉移構造，相反的褶皺逆衝方向也暗示著兩背斜分別由兩條滑移方向相反的逆衝斷層所形成。在地表上，由褶皺軸面的延伸可以顯示在兩背斜構造之間的轉移構造為多重轉折的背斜形貌。
在鐵砧山構造北段，地下斷層朝西逆衝形成西翼陡而東翼緩的不對稱褶皺形貌。由反射面的截切位置可以定義斷層面的位置，並根據褶皺地層與斷層的交角關係以及其滑移量變化可以推論其構造形貌為具折彎的斷層延展褶皺。但在斷層面與後翼地層之間仍有明顯之交角，推論此斷層延展褶皺在初期發育時，其斷層底滑面並非與上盤地層平行。而斷層面與斷坪之交角可由前人所提出之理論模型加以驗證。另外，斷層底滑面深度由北向南至兩背斜之間的轉移構造而逐漸遞減。在鐵砧山構造南段，其褶皺形貌東翼短而陡，西翼長而緩。根據軸面的位置與兩翼的角度關係以及反射線的截切位置，解釋為斷層朝東逆衝之斷層延展褶皺。
由三角剪切模式的正向模擬結果顯示，應變橢圓拉張的區域主要分佈於褶皺兩翼以及靠近斷層處。此外，根據三維地下斷層形貌與頂部構造圖以及軸面分佈，可推論兩背斜相交處的地質構造均受到南北兩背斜的影響，形成多處轉折的褶皺。鐵砧山地區地下斷層的滑移量均向背斜交會處逐漸減小，且斷層面漸漸消失。然而，通霄背斜與鐵砧山背斜之地下斷層大致上均為東北─西南走向，暗示著鐵砧山地區之背斜走向同為東北─西南走向，結果與理論相符。

In the frontal part of a fold-and-thrust belt of in-sequence and ongoing development, any fault-related structures represent the initial features that may shed some influences on the following structural evolution. In the outer foothills belt of northwestern Taiwan, a train of fault-related folds are segmented and geometrically affected by a set of pre-existing transcurrent faults. In the southernmost part of the structural domain, the subsurface structure of the largest gas-field in Taiwan, the Tiechanshan anticline, appears as an intact fault-related fold in the frontal part of the fault-and-thrust belt. However, the subsurface fold-forming fault geometry is still unclear and in debate. The main purpose of this study is to use a grid of seismic sections and well bore data to reconstruct a 3-D geometry of the fold and the interpreted fold-forming thrust. We converted the time sections into depth sections of identical V/H scale and analyzed the angular relationship between each pair of limb, termination of reflectors and, the most importantly, the subsurface thrust constrained by drilled wells to build the 3-D geometry of subsurface fold and thrust structure.
On the surface, the Tiechanshan structure is characterized by two segmented anticlines offset by a tear fault striking at high angle to the fold axis. However, the subsurface geometry of the structure built by a series of seismic lines indicates that the frontal structure is composed of two folds of opposite vergence that are softly linked in the transfer zone. The opposite vergence implies that the separated folds are related to slip along thrusts with opposite dip direction. On the surface, the axial surfaces extending from both anticlines form multiple bend fold structure in the transfer zone.
For the fold segment of forelandward vergence in the northern part of the structure, the variety of displacement and the angular relationship and variable lengths among the limbs and crest favor a fault-propagation fold model with fault bend for the structure. The subsurface trajectory of the thrust can be defined by axial surface and reflector termination. Yet, there is a noticeable intersectional angle between the unparallel thrust ramp and backlimb, suggesting that the fault-propagation fold is not a simple step structure. The angle between fault ramp and flat can be calculated with the theoretical model of the previous studies. The depth of the lower flat of thrust decreases southward to the transfer zone. On the other hand, the fold segment of hinterlandward vergence in the southern part of the structure is interpreted as a fault-propagation fold. The interpretation is not only indicated by the fold shape but also supported by the reflection termination.
The simulation of trishear model illustrates that the strain distributes in the fold limbs and the area nearby the fault. Besides, according to the 3D subsurface fault plane, the top of structural map and the distribution of axial surfaces, the geological structure of the transfer zone is influenced by both of the anticlines. Besides, the displacement along the faults decreases and the fault planes gradually disappear toward the transfer zone in the Tiechanshan area. However, the thrusts below the Tongsiao anticline and the Tiehchanshan anticline are striking northeast-southwest. It implies that the anticlines are also striking northeast-southwest. The result is corresponds with the theory.

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