Late-Quaternary Geomorphic Surface Evolution in the Touliu Table Land, Frontal Fold-and-thrust Belt, Southwestern Taiwan

Taiwan is situated along a segment of the convergent boundary between the Philippine Sea and Eurasian plates. In consequence, the Taiwan orogen developed as the result of on-going collision between these plates since 5 Ma. In southwestern Taiwan, the Touliu hilly land is located at the frontal fold-and-thrust belt, which is over thrusting the Coastal Plain towards west. The most conspicuous geomorphic characteristic of the Touliu hilly land is the widespread distribution of river terraces in the drainage divides. There are three distinct levels of river terraces, namely, the high level river terrace t1, the middle level terrace t2 and the low level terrace t3. During the development stage of the high-level river terrace t1, the eastern side of Tachienshan Fault was generally low and gentle hilly land. The well-developed terrace land and the gentle riverbed slope were found during this period. After the t1 period, the Touliu hilly land began to be affected by the activated Tongshuhu Fault. The entire hanging wall of the Tongshuhu Fault was uplifted and underwent erosion. The middle river terrace t2 is widely distributed along the drainage divide of the major rivers. T1 is about fifty-to-sixty meters higher than the current riverbeds. After the t2 period, successive activities of the Tongshuhu Fault caused asymmetric folding of the Neiliu Anticline with an overturned western limb. During the same period, the low level terrace began to develop along the Toushuhu Fault. To the east of the Tongshuhu Fault, these low-level terraces t3 are 5 to 10 meters higher than the present stream. However, to the west of the Tongshuhu Fault, the low level terrace has already merged into the alluvial plain. The geomorphic surface development of the Touliu hilly land is closely linked to the active movement of the Tongshuhu Fault and the Taichienshan Fault of the Frontal fold-and thrust system.