桃園大溪位於臺灣西北部褶皺逆衝帶之前緣。大溪背斜為主控大溪地區地層分佈之構造，依形貌研判屬於斷層延展褶皺（fault-propagation fold）。此背斜向南南西方位傾沒，褶皺軸位態約為29°/209°。因大漢溪的流貫與下切，使得河床連續出露背斜西翼之褶皺岩層。本研究致力於調查大漢溪河床中視尺度地質構造的空間分佈，建構中視構造彼此之先後順序，並探討其與大溪背斜之關聯性。透過野外觀察與紀錄，繪製各段河床露頭的構造地質圖；使用無人飛行載具掛載鏡頭空拍大面積河床露頭，由Pix4Dmapper軟體生成地面解析度5米之正射空拍影像，大幅增加野外調查之效率與精度。調查結果顯示，大溪背斜西翼，發育具有與主背斜軸相同趨勢之次要褶皺。桂竹林層有一系列半波長500至600公尺的褶皺；楊梅層出露一道不對稱褶皺。這些次要褶皺可能發育於主要背斜發育階段之中後期。褶皺形貌可能反映了岩性組合的差異，桂竹林層厚層砂岩以長波長褶皺形式，楊梅層疏鬆砂頁互層則發育波長較小之褶皺。背斜北段岩層受多道共軛斷層所截，推測為褶皺作用晚期發育之褶皺關聯裂理。背斜西翼普遍受到走向010°至040°之左移斷層截切，斷層長度約數十至數百公尺，水平錯距至少數公尺。由截切定律得知在大溪背斜褶皺作用之後，尚有此左移斷層作用的發生。根據變形之地層年代以及未變形之最老河階年代，大溪背斜活動年代晚於1.75個百萬年前，並早於0.05個百萬年前。在區域尺度下，大溪背斜具有南北側向顯著的形貌差異，本研究以斷層延展褶皺的演化模型推論，認為此差異可能記錄了背斜發育過程中不同階段的樣貌。;Daxi is located in the frontal part of fold-and-thrust belt in northwestern Taiwan, Taoyuan. The Daxi Anticline is the main regional structure which controls the spatial distribution of stratigraphy in the Daxi area. The anticline has been interpreted as a fault-propagation fold based on its geometrical feature and it is a SSW-plunging fold with the fold axis of 29°/209°. Because of the downcutting of the Dahan River, folded rock layers of the Daxi Anticline west limb expose well and continuously along the riverbed of the Dahan River. This study aims to document the mesoscopic structures on the riverbed, and the forming sequence of the meso-structures and the structural relationship with the Daxi Anticline are discussed. To construct detailed structural mapping, we undergo field observations and data collections. Unmanned aerial vehicle with camera and Pix4Dmapper software are used to generate 5-m ortho-images for surveying wide riverbed area with efficiency and accuracy. The results show that, several sub-parallel minor folds develop in the west limb of the Daxi Anticline, including a series of fold with half-wavelength of 500 to 600 meters in the Kueichulin Formation, and asymmetrical folds develop in the Yangmei Formation. They are likely to develop in the middle to late stage during the development of the major anticline. The geometry of minor folds are highly influenced by the lithology and thickness of strata. The long-wavelength folds are composed of thick sandstone layers of Kueichulin Formation, while the interbeds of loose, weak sandstone and shale tend to form smaller wavelength. Two sets of conjugate faults cutting through steeply dipping strata in the northern Daxi Anticline, and they can be interpreted as fold-related fractures based on the fact that the principal stress axis is perpendicular to the major fold trend. Several sinistral strike-slip faults cut through folded rocks in the strike about 010° to 040° and can be observed all along the study area. The fault traces usually reach several hundred meters long with their strike separation are at least several meters. These faults are regarded as the post-folding sinistral faulting by cross-cutting relationship. According to the ages of deformed strata and undeformed terrace, the age of the development of the Daxi Anticline is after 1.75 Ma and before 0.05 Ma. In the regional scale, the geometry of the Daxi Anticline has significant lateral difference from North to South. This geometrical difference can be explained as historical records in the different stages in the development of the anticline by using the assumption of fault-propagation fold model.
