Geomorphologic and structural analyses of the Wanan alluvial fans near the Chihshang fault in eastern Taiwan

• Main Authors: Yi-Ruey Lee, 李易叡
• Other Authors: Jyr-Ching Hu
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/89210479777795530709

碩士 === 臺灣大學 === 地質科學研究所 === 95 === Based on high resolution geomorphology analyses solving it, including 2 meters resolution digital elevation model (DEM) and GPS - RTK measurement in the field, this study determine the elongate condition of the Chihshang fault in the Wanan area from observing the surface deformation of the Wanan alluvial fans produced by the Chihshang fault. Combining 40M DEM and 2M DEM, we can divide the Wanan alluvial fans into three parts, and they formed at different times. The F1 fan is located in the northern- most part of study area with the highest elevation. The F2 fan is located in the southernmost part of study area. The recent Wanan river alluvial fan which I called F3 is the lowest fan in study area. Based on the relative elevation of the three alluvial fans, I interpreted that the F1 originally deposited further north.Later, the Wanan river flowed along the southern side of the F1 alluvial fan. Before this or at the same time, the southern branches of the Wanan river also deposited the F2 alluvial fan in the southern portion. While the Chihshang falut continuously moved and the hanging-wall gradually raised, the Wanan river cut the F2 then formed F3 alluvial fan. From the DEM analysis, we can find obvious deformations in F3. There is a scarp which faces oppositely to the F3 alluvial fan in the fan head. The other scarp is located in the middle part of the F3 fan. These two scarps form a depression in F3. In the southern portion of the Wanan river, we can observe some alluvial cones in the F2 fan, and there is an obvious scarp in the edge of the F2 fan. In the northern portion of Wanan river, the surface traces of Chihshang fault follows an observed scarp. In the field, we found breaks and fissures of concrete wall on the Wanan river embankment along the surface scarps of the fault,consistent with geophysical surveys. We can’t clearly observe scarp near the Wanan river channel, and we suppose that the erosion rate of Wanan river is more rapid than the fault uplift rate in the southern portion of the Wanan river, we interpreted that the fault scarps were eroded by river. because the edges of terrace show a curve shape. Furthermore, the ancient map also showed that there was an old river channel flowing beside the scarp. The location of surface trace of the fault may be farther west than the geomorphic scarp.