Structural Evolution in the Central Longitudinal Valley, Eastern Taiwan, From the Distribution and Properties of Quaternary Conglomerates

The island of Taiwan is produced by the ongoing arc-continent collision between the Philippine Sea plate and the Eurasian plate. In eastern Taiwan, the east-dipping Longitudinal Valley fault is a major active structure, along which the strata of the Coastal Range thrust onto younger Longitudinal Valley deposits. The activity of this structure also caused uplift and production of distinctive tectonic landforms. Along the valley, previous studies have utilized several uplifted and deformed Quaternary conglomerate units to identify and characterize active structures and their properties. However, in Hegang area in the central segment of the valley, some similar conglomerate units have been mapped, but their sedimentary environment and their relationship with active structures remain unknown. To understand the source of these uplifted and deformed conglomerates and their sedimentary environment, we examined the composition of the conglomerate grains and compared it with modern river sediment from different parts of the valley. Combined with a refined mapping of river terraces in the area, we were able to obtain a better understanding of the structural evolution in the central Longitudinal Valley.

Our results show that the conglomerates found in Hegang area is composed of materials from both the Coastal Range and the Central Range on the two sides of the Longitudinal Valley. This composition is similar to present-day river sediment found in the trunk rivers flowing in the Longitudinal Valley floor. Therefore, we suggest that the area used to be located at the footwall block of the Longitudinal Valley fault. However, these conglomerates are currently located at the hanging-wall block of the present-day Longitudinal Valley fault, uplifted and deformed by the activity of the fault. As a result, we propose that the Longitudinal Valley fault used to be located to the east of its current location, and the active fault trace today is a young frontal branch of the system. A strip of high, lateritic river terraces along the western flank of the Coastal Range may represent remnant valley surfaces when the area was at the footwall block, supporting our hypothesis. Our results would provide important additional constraints on the evolution of active structures and their influences on landform developments in this area.