Deciphering the Long-Term Evolution of the Chelungpu Thrust, Central Taiwan, by Dating Deformed Terraces

The Chelungpu thrust is one of the most important active faults in the western foothills of central Taiwan, which by preliminary estimate consumes with the adjacent Changhua thrust ~45% of the ~80mm/y plate rate. Its activity is well documented by the ~80km long surface rupture of the 1999 Mw7.6 Chi-Chi earthquake. In addition, there was substantial coseismic fold growth within the thrust sheet above fault-bends and at the northern termination of rupture.

Our study area, Hsinshe, is located in the hanging wall of the northern section of the Chelungpu thrust, where the N-S striking surface rupture of the 1999 earthquake merges into a NE-SW to E-W trending surface fold. Three distinct levels of fluvial terraces deposited by the northward running Tachia river are discernible in the area (Lai et al. 2006). The highest terrace has been OSL dated to 55.0+/-2.6ka (Chen et al. 2003). All three terraces show westward tilting and are displaced vertically <100 m by fold scarps. The hanging wall deformation is caused by the underlying thrust geometry, which ramps from ~3km in the east of the study area to the surface ~5km west of Hsinshe (Yue et al. 2005). Motion of the thrust sheet over these fault bends produces a hanging wall syncline with a northward bifurcating axial surface (Lai et al. 2006). These axial surfaces are the cause of the large-scale kink-bands in the terraces.

The aim of our ongoing study is a detailed understanding of the deformation history of the Chelungpu thrust in this area by combining field data, subsurface data and geochronological data. We therefore are dating the different terrace levels using OSL and 14C methods. The different deformation amount together with the ages will thus allow us to determine the long-term fault-slip rate of the Chelungpu thrust system over a ~55ka timescale.