Structure and Holocene evolution of an active creeping thrust fault: The Chihshang fault at Chinyuan (Taiwan)

We conducted a variety of measurements and analyses at Chihshang Active Fault Observatory in eastern Taiwan, including surface-rupture mapping, three shallow borehole core analysis and kinematic analysis of geodetic measurements. We found that the Chihshang fault exhibits a three-branch fault system in the Chinyuan alluvial fan, which is composed of at least 100 m thick gravel deposits. Outside of the Chinyuan River, the Chihshang fault shows a single fault system with a sharp lithological contact. Combining the leveling results and trench excavation, we interpret that the three fault branches are coupled with a 50−60-m-wide pop-up structure in the hangingwall. Based on the ratio between vertical and horizontal displacements, we obtained dip angles of 38°, 62° and 16° for two west-vergent thrusts and an east-vergent backthrust, respectively. This pop-up was estimated to develop at the uppermost 30–40 m unconsolidated gravels during the last few thousand years above the main fault with a dip angle of 42°. By compiling the available ages data, we obtained an uplift rate of 2.3 cm/yr of the Chihshang fault and an alluvial sedimentation rate of 1.1 cm/yr during the past thousands years. Consequently, the individual uplift rate for each fault branch at the Chihshang Observatory was slightly less than the deposition rate of the Chinyuan River. No geomorphic fault scarp can thus be observed in the three-branch fault system area.

► We reconstructed 2-D geological cross-sections at near surface level for an active thrust fault, which revealed a rapid seasonal creep of about 2 cm/yr – the Chihshang fault in eastern Taiwan.
► We characterized the structures of three-branch fault system of the Chihshang fault, which is coupled with a 50−60-m-wide pop up anticlinal fold in the hangingwall.
► This pop-up with three fault branches is an under growing young structure, which developed within an alluvial fan above the uppermost 30–40 m gravel deposits during the past a few thousand years.