River Terraces along the Hsinwulu River and their Implications for Recent Tectonic Uplift of the Southern Central Range in Taiwan

Abstract:

River terraces are commonly distributed within the Central Range in Taiwan. Most of them are poorly preserved due to possibly the high rates of erosion and are not well geologically dated. The Hsinwulu River flows through the eastern flank of the southern Taiwan Central Range and its river terraces are well preserved in comparison with other parts of the Central Range. The river terraces may well record the recent tectonic uplift history of the most tectonically active region in Taiwan. Along the more than 20 km long river, there are three major groups of river terraces: the Hsinwu, Wulu and Lidau terrace groups. Field observations indicate that the river terraces are composed of both river sediments and debris flow deposits with unknown geologic ages. We mapped all the river terraces and some strath exposures along the river and projected them onto the longitudinal river profile. We estimated the ages of the strath exposures by plotting ages vs a range of possible incision rates, constrained by the vertical heights between the strath exposures to the river basement rocks. To further and independently evaluate the ages for the river terraces, we applied recent continuous GPS data along the upper, middle and lower reaches of the Hsinwulu River showing vertical displacements of 3.6, 8.0 and 9.1 mm/yr, respectively. Our results indicate that these terraces are much older than previously thought. The terraces range possibly from 20,000 to 60,000 years and their formation likely relates to the curve of sea level change during the period in addition to the tectonic uplift resulted from the plate convergence between the Eurasian and Philippine Sea plates.