石門斷層與大平地斷層位於臺灣西北部麓山帶前緣，石門水庫西側，為研究區內兩條相鄰之逆衝斷層。前人繪製地質圖中雖已有石門斷層與大平地斷層地表斷層跡線，然而受限於斷層沿線缺乏明確且可連續追蹤之斷層露頭，使得斷層的精確位置、傾角、活動性及兩者地下連接關係仍然存在不確定性，斷層構造關係亦難以直接由地表判定。因此，本研究依據野外地質調查所得之岩性分布與位態資料，並結合高解析度DEM所產製之日照陰影圖，利用地形明暗與差異侵蝕特徵判釋與構造大致平行之地形線形，重新繪製研究區域內地表岩性分布，作為剖面建構之地表依據；並整理鄰近石門斷層之鑽井岩心資料，辨識岩心中破碎帶、斷層角礫岩以及剪切構造較為集中的區段以判定斷層帶於岩心中的位置，作為斷層地下淺部構造的限制條件。綜合地表地質分布、鑽井、前人研究與地層厚度資料及區域構造背景，本研究建構之地質剖面AA′與BB′顯示，石門斷層與大平地斷層於近地表雖呈現兩條相鄰之斷層跡，但兩者可能屬於同一逆衝斷層系統，並於深部連接至低角度滑脫面。此外，鑽井顯示中新世地層與鬆軟礫石沉積單元呈異常接觸關係，由於該鑽井位置位於大漢溪襲奪前古流域範圍內，因此可解釋為大漢溪所搬運之沉積物，但該礫石沉積物與古大漢溪之關聯性，以及其對石門斷層活動時間之推估，仍有待後續沉積來源分析與年代資料做進一步驗證。;The Shimen Fault and the Taipingti Fault are two adjacent thrust faults located on the western side of Shihmen Reservoir， along the frontal part of the Western Foothills in northwestern Taiwan. Although previous geological maps have shown the surface traces of both faults， their exact locations， fault dips， activity， and subsurface connection remain uncertain due to the lack of clear and continuously traceable fault outcrops along the fault zones. Therefore， the structural relationship between these two faults cannot be determined directly from surface observations alone. In this study， lithologic distributions and bedding attitudes obtained from field geological investigations were integrated with hillshade maps derived from high-resolution digital elevation models. Topographic shading， differential erosion features， and geomorphic lineaments roughly parallel to regional structures were used to reinterpret the surface lithologic distribution in the study area， providing surface constraints for geological cross-section construction. In addition， borehole core data near the Shimen Fault were compiled to identify intervals with concentrated fractured zones， fault breccia， and shear structures. These intervals were used to constrain the possible positions of fault zones in boreholes and to provide near-surface subsurface constraints for fault geometry. By integrating surface geological distribution， borehole data， previous studies， stratigraphic thickness， and regional structural constraints， geological cross sections AA′ and BB′ were constructed. The results suggest that although the Shimen Fault and the Taipingti Fault appear as two adjacent fault traces near the surface， they may belong to the same thrust fault system and may connect to a low-angle detachment at depth. In addition， an anomalous contact relationship between Miocene strata and a loose gravelly sedimentary unit was identified in the borehole data. Because the borehole is located within the paleodrainage area of the Dahan River before river capture， this gravelly unit may be interpreted as sediment transported by the paleo-Dahan River. However， the relationship between this gravelly sedimentary unit and the paleo-Dahan River， as well as its implication for constraining the timing of Shimen Fault activity， requires further verification through sediment provenance analysis and geochronological data.
