| dc.description.abstract | The Erhjen River in southwestern Taiwan is a dynamic region characterized by active deformation and uplift clustered in the footwall of the several reverse faults. This study focuses on understanding the geological processes driving this phenomenon. The region predominantly comprises the Late Miocene to Early Pleistocene Gutingkeng Mudstone (Gtk), which exhibits an impressive thickness exceeding 4000 meters. The fine-grained composition and enormous thickness may trigger abnormal overpressure at depths, potentially resulting in the ductile deformation of the mudstone (shale tectonics). Major faults in the study area are the Chishan Fault, Lungchuan Fault, Gutingkeng Fault, and Hsiaokangshan Fault, which all dip eastward. Geophysical data reveal the presence of folded structures, such as the Chungchou Anticline in the Coastal Plain. Geodetic measurements indicate that the midstream of the Erhjen River experiences compression at a rate of approximately 30 mm/yr. Additional evidence from InSAR, leveling data, and Holocene incision rates support significant uplift in the region. Notably, the high deformation concentrates along the footwall side of the mapped Gutingkeng Fault. To understand the structures responsible for this pronounced uplift, several research objectives were pursued. Radiocarbon dating of the Erhjen River terraces in the deforming footwall was carried out to quantify incision rates. A comprehensive structural analysis was undertaken in the highly deforming area using bedding and shear zone orientations and nannofossil data. Additionally, we constructed a geological cross-section using the kink method. We found that incision rates from the terraces located along the deforming area, T1a and T2a are 19 mm/yr and 34 mm/yr, respectively, indicating a rapid uplift rate. Our nannofossil age from terrace T2a revealed a Pliocene age (NN15). The age of hanging wall of Gutingkeng Fault is not similar along the fault trace, therefore we utilized broader scale bedding attitude to define that the nannofossil age of Pliocene (NN15) is located on the footwall side of the Gutingkeng Fault. Additionally, using published nannofossil data in the Gutingkeng Mudstone, we classify it into three age units: The Miocene Gtk, Pliocene Gtk, and Pleistocene Gtk. Field observations in the Gutingkeng Fault zone unveiled a prominent southeast dipping shear zone with its shear plane oriented 080°/51S°. This southeast dipping shear zone/ fault exhibited muddier facies adjacent to silty facies and might have undergone a displacement of tens of meters. It is proposed that this fault might be the Gutingkeng Fault which cross-cuts a west-dipping reverse fault. Furthermore, our cross-section revealed that the Lungchuan Fault and Gutingkeng Fault lie on the same detachment at a depth of ~4 km. This interpretation is based on the similar nannofossil age of the strata on the immediate hanging wall of both the faults. Furthermore, the Mucha Fault detaches from a depth of ~4.5 km, thrusting Miocene Gtk to surface and having Changchihkeng Formation (Mcc) of Middle-Late Miocene on its hanging wall. Greater thickness of the Miocene Gtk to the east of Gutingkeng Fault can be explained by the presence of a normal fault/faults in the Late Miocene that led to the thicker deposition of Miocene Gtk in a basin we called as ‘’Gutingkeng Basin’’. The presence of shallow and deep marine facies of the same age along the Lungchuan Ridge from north to south support the idea of normal-fault basin. ChungChou structure can be interpreted as a fault-propagation fold based on the asymmetry of the growth strata and the fault goes as deeper as 5 km. Furthermore, we propose the presence of a continental margin reactivated structure at a depth of 8-9 km that deforms the post rift strata and the foreland sequence and is probably active. Additionally, owing to the thicker Gutingkeng Mudstone to the west of Gutingkeng Fault, the phenomenon of shale tectonics might also be playing a role and the footwall might be folded in response to compression that could explain the anomalous footwall deformation. | en_US |