Evolution of the Holocene uplifted terraces along the Chihshang Fault, Eastern Taiwan: Interactions between tectonic vertical movement and fluvial sedimentation

Abstract:

The Chihshang terrace group is located on the hanging wall of the Chihshang Fault, one of the most active segments of the Longitudinal Valley Fault which represents the onland plate-suture fault between the Eurasian and Philippine Sea plates in eastern Taiwan. Being different from common river terraces usually developed on two sides of river channel, the Chihshang terrace group is located on the toe area of the large alluvial fan of east-flowing Xinwulyu River, which derived from the Central Range and crosses the 5-7-km-wide Longitudinal Valley to reach the Coastal Rang and has been cut through by the west-thrusting Chihshang Fault.

The 5-m DEM morphology analyses results show the Chihshang terrace group can be divided into ten levels of various height differences. The spatial distribution of terraces is uneven: higher levels (T5 to T10, >34 m high) and lower levels (T1 and T2, <23 m high) distribute on the northern part of alluvial fan, medium levels (T3 and T4, around 25-32 m high) on the middle part, and no terraces can be found on the southern part. Field investigations and the observations from six trenches on terraces surface show that the compositing units of terraces from top to bottom are: the Coastal Range colluviums deposits from local hills or tributaries, the Central Range deposits from Xinwulyu River channel deposits, and the bedrock. Based on these results it appears that the Chihshang terrace group was formed under influence of the continuous thrusting movement of the Chihshang Fault and the episodic lateral erosion and sedimentation of Xinwulyu River on to the hanging wall area. The uneven geographic distribution of terraces reveals the channel migration history of the Xinwulyu River.

¹⁴C dating result of the Coastal Range deposits in a high level terrace (T7, around 30 m high from the footwall) is 5,750-6,880 cal yr B.P. Combine with previous reported data (290-490 cal yr B.P. of lowest level terrace) it reveals that the Xinwulyu River flowed to the north at least seven thousand years ago, and changed its flowing direction to the south in few hundred years ago. Together with the sedimentation rate of the footwall (1.0-1.2 cm/yr) the estimated maximum long-term vertical rate of faulting for the past seven thousand years is 1.4-1.7 cm/yr, which is a factor of two lower than the present geodetic data (2.5-3.0 cm/yr).