Active deformation front delineated by a blind thrust underneath the coastal plain, Southwestern Taiwan

We discovered a blind thrust underneath the Chaiyi coastal plain, which may indicate the deformation front of the fold-and-thrust belt of Taiwan, which is created by arc-continent collision between Eurasian continent and Philippine Sea plate. By using the geomorphic and drainage pattern analysis, we can divide the study area into three distinct N-S geomorphic domains. From shoreline mountainward, they are coastal plain, uplifted tableland, and low hill. The coastal plain domain is characterized by meandering rivers. The surface slope (Ss) and river gradient (Rg) are relatively low, but river sinuosity (S) is high [i.e., Ss: 0.7 m/km, Rg: 0.2 m/km, S: 1.3 ], showing no tectonically uplifting in this domain. The middle uplifted tableland domain is dominated by tablelands with surfaces slightly titled to the west and meanders on the top but more tributaries developed [ Ss: 11.8 m/km, Rg: 2.4 m/km, S: 1.5 ]. Anomaly high river sinuosity in this domain implies that there is an influence driven by active structure. To the east of low hill, scraggly relief landscape with higher surface slope is dominant. The river gradient increases but the sinuosity deceases [Ss: 89.2 m/km, Rg: 59.3 m/km, S: 1.2], indicating it has been uplifted for a while and is probably undergoing. To accommodate the strain of the crustal deformation among the geomorphic domains, we infer that a blind thrust exists between the coastal plain and uplifted tableland. Independently, we calculated the long-term uplifting rates by determining the terrace ages via the radiocarbon method. Our results reveal that the hanging-wall of the blind thrust is uplifting in rates of 0.2-3.1 mm/yr, while the footwall is subsiding in rates of 0.1-5.2 mm/yr. We also suggest that the blind thrust may be indicative to the active deformation front of the orogeny belt and foreland basin tectonic units in western Taiwan.