| dc.description.abstract | In Southwestern Taiwan, at the latitude of the TsengWen River, westward GPS horizontal velocities dramatically decrease from 25 mm/yr east of the WuShanTou anticline to about 4 mm/yr in the eastern coastal plain. Therefore, active structures are expected at the topographic front of the mountain belt. However, geometry and activity of structures are not very well addressed in the previous studies. It might be important to understand the geometry and the deformation rate in this area for seismic hazard assessment. This study aims to determine the geometry of the geological structures and to quantify the Holocene deformation rates based on geological cross-sections, deformed Holocene deposits, and geodesy. In this study, I use TsengWen River terraces and published shallow boreholes data in the eastern coastal plain to calculate the Holocene deformation rates. I also build the geological cross-sections to determine the geometry of the structures.
I use my TsengWen River terraces observation and the dating results and combine them with the previous studies along the TsengWen River. Further, I also take the published shallow boreholes data and try to compare these two data sets. Since the sedimentary environments are different, I do the calculations of the uplift rates separately. I estimate the uplift rates of the terraces based on the incision rates, sedimentation rates, and the footwall subsidence rate. I use geodesy data and the Isochrones selected from the shallow borehole to select a relatively stable location for sedimentation rates calculation. I choose the shallow boreholes in TsungYeh which is west of the deformation zone and around 10km away from the piedmont as the reference to obtain the terraces uplift rates. In the coastal plain, I take the dating results in the shallow boreholes and consider the sea level changes to obtain the uplift rates. Lastly, I combine the uplift rate from terraces and the shallow boreholes to obtain a profile that has a similar trend to the contemporary geodesy data. These uplift rates increase from the west to the middle of the deformation zone and keep rising in the foothills until the WuShanTou anticline and KouhSiaoLi fault. In parallel, I use the published surface geological data, nannostratigraphy, deep boreholes, seismic reflection lines that I did the depth-converted, and Isochrones which I selected from shallow boreholes to build the cross-sections at the north part and the south part of the study area. Further, I used the results of the profiles to understand the geometrical differences of the structures in different latitudes. I propose that at a depth of approximately 3 to 4 km in the foothills the KouhSiaoLi fault may extend westward. Furthermore, the KouhSiaoLi fault may be the main structure causing Holocene deformation in the foothills, and it extends to the coastal plain forming an anticline.
I compare the results from Holocene deformation rates and the geological cross-sections. Then, I separate them into three different areas; the coastal plain, the Tainan foothills, and the piedmont for discussion. In the coastal plain, it is an active anticline in the coastal plain area according to the seismic lines and variation of the Holocene uplift rate. The structure that forms the anticline may extend from the foothills. In the Tainan foothills, I propose that the Holocene deformation may be caused by the KouhSiaoLi Fault. Under this premise, I used the fault dip angles and the uplift rates of the TsengWen River terraces to estimate the fault slip rates based on the fault-bend fold model. The fault slip rates I obtained are 26.2-27.9 mm/yr. In addition, according to the current geodetic data, the peak of deformation rate in the northern region is not near the KouhSiaoLi fault but near the WuShanTou anticline fold axis, which may be due to the high-dip angle formations in the north restricting the slip of the KouhSiaoLi fault and transforming the fault slip into the folding at the WuShanTou anticline. At the piedmont, I determined the uplift rate to be about 4.5-4.8 mm/year based on the slip rate of the KouhSiaoLi fault in the foothills and the fault dip angle in the geological cross-sections I drew. Therefore, according to the results of the study area, the uplift rate changes in the coastal plain, to the piedmont, and to the WuShanTou fold belt in the Tainan foothills are -4 mm/yr, 4.5-5.2 mm/yr, and 19.3 mm/yr, respectively.
The research results support the westward extension of the KouhSiaoLi fault, which extends to the heavily populated coastal plain on the southwest side of the study area and forms an anticline. Such structures that are active and have a long deformation zone have a significant impact on seismic hazard assessment. | en_US |