| dc.description.abstract | The Lunhou fault (LHF) dips to the east and Kouhsiaoli fault (KSLF), which is to the east from LHF, dips to the west. These two faults are striking north-south and are located in southwestern Taiwan through Chiayi and Tainan area. InSAR Images show 20 mm/yr of uplift across these two faults. Therefore, this study aims to reveal if LHF and KSLF have any contribution to this vertical component, based on the river terraces along the Tsengwen River. Firstly, we used a 5m DEM and a Lidar DTM for terrain interpretation and terrace mapping. Then, we conducted field investigations to measure the elevation of terrace surface, terrace strath and dated samples using total station, VBS-RTK and UAV, and to collect samples for 14C dating to complete the terrace correlation. Secondly, we draw a geological profile based on geological data. We integrated these two approaches to better analyze and explain the structural relationship and activity of these two faults. Eleven levels of terraces, which are T0 (15-18 ka), T1 (8 ka), T1b, T2a, T2b, T3 (3-4 ka), T4a, T4b (2 ka), T4c, T4d and T5 from oldest to youngest, have been classified according to their elevation difference from the modern river, and radiocarbon dating. In the field, the LHF dip angle is high and the KSLF dip angle is 42°. Combining 14C ages from this study and previous studies, we found that the age of the terrace T4b on the hanging wall of LHF is 1998-1878 cal BP and the age of the terrace T4b on the footwall is 1867-1615 cal BP. Additionally, the strath elevation of T4b on both sides of LHF are similar. This indicates that LHF has been inactive since 2 thousand years. The age of the terrace T3 on the hanging wall of KSLF is 3-4 ka, which is similar to the footwall, but the elevation difference of the strath between them is 30 m. These results suggest that KSLF is active. The activity of the KSLF is further demonstrated by our observation of the Liuchenwan outcrop, where the KSLF cut through the river deposit. There, four 14C samples were collected from west to east in the river terrace sediments on the footwall of the KSLF. Ages of 15256-14854 cal BP and 18480-17887 cal BP were obtained near the strath, and 8976-8595 cal BP and 3212-3005 cal BP were obtained in the upper fine-grain part of the terrace deposits at the east side of fault at about 260 m distance. These four samples are all deposited in the same terrace sequence and we have two explanations for that. First, the fault activity led the bedrock to uplift, which caused the path of the Tsengwen River to migrate towards the east. Second, the riverbed here was quite stable, with limited incision and deposition between 18 ka and 3ka. The terrace and age dataset were also used to calculate river incision rates. For the terrace T3, we observed fairly similar incision rates of 6.0-8.4 mm/yr 6 km west of KSLF and of 3.8-5.3 mm/yr 500 m east of KSLF. But the terrace T3 located 2.2 km west of KSLF shows a faster incision rate of 15.0-21.0 mm/yr that indicates that this is area is affected by tectonic activity. Therefore, our observations indicate that the KSLF has been active since at least 3-4 ka, and we can classify it as a type I active fault. In a next step, we project the coordinates of the river terrace surface on the N75°W section and analyze the study area in combination with existing InSAR data. From our terrace deformation profile the distribution of river terraces exhibits an asymmetrical antiformal geometry, and there are signs of uplift within at least 6 km to the west of the KSLF. This study proposes two possible explanations. First, the LHF is locked because of the high dip angle at its shallow part and hence it cannot continue to move, forming the KSLF back thrust toward the east; Second, Wushantou anticline (WSTA) is adjacent to the west of the LHF. However, the anticline is 1 or 2 km below the surface, and the two limbs are too tight and locked, so that the interlayer sliding is at the flexural point of the anticline western limb. The KSLF expanded and cut through the LHF and terrace deposits. From the terrace section of this study, it can be clearly observed that the KSLF hanging wall uplifts relative to the footwall with a range of about 6 km across WSTA and LHF. To summarize, this study revealed that the KSLF is a reverse fault that cut through the WSTA and the LHF. The WSTA growth and KSLF activity generate surface uplift to the west of KSLF. According to the correlation of river terraces and the incision rate, the relative uplift rate between terrace T3 near the WSTA and terrace T3 east of KSLF is 13.7-15.7 mm/yr. Using the relative uplift rate of terrace T3 across the KSLF, the fault dip angle (42°) of the KSLF outcropping in Liuchenwan, we inferred the maximum fault slip rate to be 20.0-23.0 mm/yr. | en_US |