藉離散元素法探討竹山槽溝中斷層引致褶皺之構造演育;Evolution of fault-induced fold at Chushan excavation site, central Taiwan, derived from numerical analysis of PFC simulations

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Title:	藉離散元素法探討竹山槽溝中斷層引致褶皺之構造演育;Evolution of fault-induced fold at Chushan excavation site, central Taiwan, derived from numerical analysis of PFC simulations
Authors:	徐家祥;Hsu,Chia-Hsiang
Contributors:	應用地質研究所
Keywords:	竹山槽溝;斷層引致褶皺;構造演育;離散元素法;PFC2D數值模型;Chushan excavation;fold induced by fault;structural evolution;district element method;PFC2D model
Date:	2016-07-27
Issue Date:	2016-10-13 14:55:48 (UTC+8)
Publisher:	國立中央大學
Abstract:	竹山槽溝總長40公尺，深10公尺，橫跨於1999年集集地震引致的兩公尺高的地形崖上，其長軸方向的兩個主要牆面，分別以南牆與北牆稱之。在南牆上，分支斷層沿褶皺背斜軸跡截切，截切傾角為32度，且截切的最大錯距約為4.2公尺；在北牆上，分支斷層沿著褶皺向斜軸跡截切，斷層傾角為24度，且最大錯距約為3.0公尺，然而兩牆只相距十四公尺。本研究的目的即在於以離散元素法，藉由已知的地質條件，設計類似基盤斷層作用 (i.e. basement faulting or forced folding) 的模型，探討兩牆構造演育發生變化前，形成純單斜構造的地質條件，再初步探討短造成距離下兩牆面構造變化的可能原因。本研究使用以離散元素法為基礎的Particle Flow Code (PFC) 程式，將模擬對象假設由圓形剛體顆粒 (particle) 與不同的鍵結模式 (bond model)組成。由於PFC的設定參數非直接的地質材料力學參數值，須先進行PFC直剪盒及雙軸試驗模擬，以得出相對應的參數值。基於槽溝土層力學特性，以及槽溝附近的鑽井資料，竹山槽溝的模型由兩層力學特性不同的覆土層組成，上層為七公尺厚的黏土層，下層由八到十五公尺厚的礫石層組成。結果顯示，在垂直錯距為3.6公尺的下，斷層傾角必須大於24度，且黏土層的凝聚力強度必須在11~12 kPa才能形成單斜構造。本研究進一步發現，兩槽溝剖面的構造差異，可能是由於主控斷層傾角的側向變化，在兩剖面下有所不同所造成。當凝聚力為 11 kPa，下伏斷層角度為24度時，黏土層形成單斜構造，而礫石層形成楔形狀突入黏土層並截切黏土層褶皺的背斜軸處，與現今在南牆上看到的構造相同；當下伏斷層角度為32度時，黏土層同樣形成單斜構造，此時褶皺的背斜軸處受到截切，與現今槽溝中南牆的構造剖面相似。;Exposures in the Chushan trench were 40 m long and 10 m deep, excavated across a 1999 earthquake-induced escarpment of 2 m high. There were two main structure profiles, called north wall and south wall. On south wall the fold was truncated through along the axial trace of its anticline by a fault branch with a dip angle of 32 degrees and a maximum separation of 4.2 m while on the north wall the steep limb of the fold was displaced up to 3.0 m along the axial trace of its syncline by another fault branch with a dip angle of 24 degrees. The distance between two walls was only 14 m. This study intends to explore the geometerail condition of the site when the site fromed the pure monocline which is before the heterogeneous structure and explore how this heterogeneous structure might form using distinct element simulation of basement faulting. This study uses Particle Flow Code (PFC) based on discrete element method, regarding material as assembled rigid particles. The rigid particles can be connected by two types of bond models. Because the PFC parameters are different from geomaterial mechanic properties, we cannot directly use the values of geomaterial mechanic properties. In order to attain the values of PFC parameters equailvent to the mechanic properties. PFC simulations of direct shear test and bilateral test are perfomed. All our models consist of two mechanical layers, including an upper clayey layer of 7 meters thick and a lower gravelly layer of 8-15 meters thick, as revealed by the excavation, a borehole nearby and soil tests. At the vertical displacement of 3.6 meters, Our results show that the monocline fold can be simulated by a low-angle reverse faulting similar to a subsurface dominant fault with a dip angle of 24 degrees derived from the trench site at the ground surface and in a borehole in the hanging wall, and the monocline structure can only generate when the cohesion of the clay layer is 11~12 kPa. Furthermore, the different structures on the two exposures were mainly controlled by the dip-angle variation of the upper part of the subsurface dominant fault. The simulation of reverse faulting with a dip angle of 24 degrees shows a monocline forms in the clayey layer, and then a gravelly wedge starts to protrude into the clayey layer and displace it along the axial trace of the syncline similar to the structure on the north wall, and the simulation of reverse faulting with a dip angle of 32 degrees shows a monocline forms in the clayey layer, and then this monoclinal clayey layer starts to be displaced along the axial trace of the anticline similar to the structure on the southern exposure.
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