River terrace development in response to folding above active wedge thrusts in Houli, Central Taiwan

Folding at the leading edge of the western Taiwan thrust belt is well preserved by a flight of fluvial terraces at Houli. Strain in this region is driven by slip on the north trending Changhua and Sanyi blind wedge thrusts and occurs at the boundary between the Miaoli and Taichung neotectonic domains located respectively to the north and south. In the past century, two large historical earthquakes occurred in these two domains and produced surface ruptures within or close to the study area, indicating Houli is not only tectonically active but also critical for understanding the interaction between the neighboring active systems. Although abundant river terraces and their deformation by active structures have long been well known in the area, a kinematical model for this region has not yet been proposed. We have therefore remapped the geomorphic features using the 40m DEM and aerial photos to identify often subtle structural features in order to separate the effects of folding interference produced by different active structural systems. Besides the Tuntzuchiao fault (TTCF), which generated the 1935 earthquake, two other active regional scale structural systems have been reported in Houli area: the Tiechanshan (TCS) anticline and the Changhua fault (CHF). Our results demonstrate a number of tectonic scarps and warped surfaces in association with the CHF and its coupled back thrust, indicating that the CHF system is active. In contrast, the flight of fluvial terraces distributed in the middle of the study area cannot be related to the growth of the TCS anticline. Instead, terraces here record growth of another smaller and previously unknown anticline, Yuehmei (tentatively named in this study and shortened to YM in the text). Although it has developed parallel to the TCS anticline, relevant seismic profiles show the YM anticline as a local drag-fold attached to the southern limb of the TCS anticline and restricted in the Houli area. Based on the above observation, the TCS anticline can be interpreted as becoming less active where strain is instead transferred to the more rapidly uplifting YM anticline. This structural switch may be the result of interference between the newly developed southern Taichung system and the preexisting TCS anticline.