Active anticline and backthrust override a geological thrust: evidence from present and Holocene deformation rates within the foothills of southwestern Taiwan

In the classical evolution of a fold-and-thrust belt, the deformation front progressively propagates basin-ward and the activity of the newly developed structures modifies the geometry and, likely, the kinematics of the existing structures located on their hanging wall. The Taiwan mountain belt sits at the convergence boundary between the Luzon volcanic arc and the Chinese continental margin. The 8-cm/yr westward shortening across the island is partly accommodated at the western piedmont of the orogen by a series of folds and thrusts trending about N30˚E. This study investigates the Holocene activity of a geological structure located within the foothills of southwestern Taiwan. At this latitude, GPS observations indicate about 2 cm/yr of westward shortening across a 20-km-wide zone. The targeted geological structure is located about 12 km east of the deformation front. It includes, from west to east, a tight upright anticline, the Wushantou anticline, with early Pleistocene mudstone at the anticline axis, a significant thrust fault with a steep dip to the east, the Lunhou fault, that brought early Pliocene strata against Pleistocene strata, and an active west-dipping backthrust with limited total offset, the Kouhsiaoli fault. These structures are crossed over by the Tsengwen River. A flight of 11 Holocene river terraces was dated from 10 ka to 2 ka using radiocarbon dating. The highest terrace locates near the anticline fold axis and lies 140 m above the modern river. We determined bedrock incision rates across the investigated structure and accounted for sedimentation rates in the Holocene foreland basin to determine uplift rates that evolve from 5-7 mm/yr west of the anticline to 15-20 mm/yr from the anticline axis to the Kouhsiaoli backthrust, and decreasing to 5-7 mm/yr east of the backthrust. Present deformation observed by InSAR indicates a similar deformation pattern. These suggest that the backthrust and anticline are the main active structures at least since the early Holocene and that the major geological thrust has likely been inactive during this time period. On-going works are focused on the deep geometry of these geological and active structures, on the partition of shortening between the anticline and the backthrust and on how the entire structure may have evolved as deformation propagated westward.