| 摘要: | 以三維地質模型探討台灣西部麓山帶濁水溪南北的地下構造特性
中文摘要
本研究位於台灣西部麓山帶濁水溪南北兩側，區域西側出露車籠埔斷層，東側為雙冬斷層，初鄉斷層介於其間。西部麓山帶自更新世造山運動發生以來，受到來自東南方的動力作用向西北推擠，新近紀地層受褶曲和逆斷層的作用疊擠在一起，造成顯著的覆瓦狀斷層系統。過往研究中已有數條剖面於西部麓山帶建立典型斷坡—斷坪構造模型，然而南投丘陵區因初鄉斷層斜向截切，致使跨濁水溪南北的剖面未能良好連接，構造在空間中的連續性與相互關係並不明確。此外，鑽井資料亦顯示桂竹林層在此區域中層厚隨空間變化差異可至數十倍。
本研究使用MOVE軟體作為平衡剖面回復與建立三維地質模型的工具，藉由三維地質模型瞭解此區域的地下構造形貌。首先蒐集前人地質剖面，並重建於MOVE軟體二維環境中，進行平衡剖面回復以檢視其合理性，依據回復結果修改沿濁水溪剖面，將初鄉斷層併入車籠埔斷層的位置，由車籠埔斷斷的下部斷坡底往東移3.3公里。隨後將地質剖面與地表地質資料共同彙整於三維空間中，並以此為基礎建立三維構造模型。依據三維模型顯示推測正斷層在濁水溪地下呈南北走向朝東傾，並在濁水溪以北約3至4公里處轉為東西至東北向，朝南傾。正斷層的轉向使車籠埔斷層在區域北方斷坪向東延伸變得更長，因此儘管剖面回復推估顯示車籠埔斷層於最北剖面有最大錯移量，然而濁水溪以南斷層上盤出露地層仍較北段古老。初鄉斷層北段錯移量同樣較南段大，但南段上盤地層亦較北段古老，主因是斷層發育活動時地層已受車籠埔斷層錯位變形，南段上新世地層已抬升至極為接近地表，北段則仍在較深的位置，隨後初鄉斷層的錯位變形，並未能逆轉所致。
;Three-Dimensional Visualization and Structural Analysis of the Area around the Zhuoshui River in the Western Foothill of Taiwan
Abstract
This study focuses on the area around the Zhuoshui River in the Western Foothills of Taiwan, where the Chelungpu Fault is exposed on the western side of the region, the Shuangtung Fault on the eastern side, and the Chusiang Fault in between. Since the Pleistocene orogeny, the Western Foothills of Taiwan have been subjected to northwestward compression due to forces coming from the southeast. This compression has caused the Neogene strata to fold and thrust together, forming a prominent imbricate fault system. Previous studies have established several cross-sections and create a typical ramp-flat fault model in the Western Foothills, but due to the Chusiang Fault cuts across the middle, the cross-sections at north and south of the Zhuoshui River in the Nantou Hill region do not connect well, leaving the spatial continuity and interrelations of the structures unclear. Well data also reveal that the thickness of the Kueichulin Formation varies by several tens of times across this region.
In this study, MOVE software is used as a tool for performing balanced cross-section restoration and constructing a three-dimensional geological model. The three-dimensional model helps us understand the subsurface structural morphology of this area and to connect geological models across the Zhuoshui River. Initially, geological cross-sections from previous studies are reconstructed in the 2D environment of MOVE software, where balanced cross-section restoration is performed to verify their validity. Based on the restoration results, modifications are made to the cross-section along the Zhuoshui River, moving the connection point of the Chusiang Fault to the base of the Chelungpu Fault ramp further east. Subsequently, the geological cross-sections and ground geological data are integrated into a three-dimensional space to build a three-dimensional structural model. According to the model, it is inferred that the normal fault strikes north-south beneath the Zhuoshui River area, dipping eastward, and then turns to a northeast to east-west to direction, dipping southward, about 3 to 4 kilometers north of the Zhuoshui River. This change in the strike of the fault makes the fault flat of the Chelungpu Fault to extend eastward and become wider in the northern part of the region. Therefore, despite the largest displacement on the northernmost cross-section of the Chelungpu Fault, the strata exposed in the hanging wall south of the Zhuoshui River are still older than those in the northern section. Similarly, the displacement along the northern section of the Chusiang Fault is greater than that in the southern section. Exposed strata in the hanging wall in the southern section of the Chusiang Fault are older than those in the northern section, as well. This is mainly because the strata had already been deformed by the Chelungpu Fault. The Pliocene strata was uplifted close to the ground in the southern section while those remained in the deeper in the northern section. However, the following deformation produced by the activity of the Chusiang fault does not reverse the status. |
