| Summary: | 碩士 === 國立中央大學 === 地球科學學系 === 105 === 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.
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