Preservation and transportation of large landslide deposits under decadal and millennial timescales in the Taiwan orogenic belt

Abstract

Landsliding is one of the primary surface processes shaping the landscapes of active mountain belts. The mobilization (or stabilization) of landslide deposits plays a crucial role in fluvial dynamics, carbon output, and topographic development of active orogens. Despite numerous previous efforts, the controls on erosion or storage of large landslide deposits remain yet to fully understand. Toward this end, we investigated eight large landslides at three locations in Taiwan. Our observations of the Jiufengershan and Tsaoling landslides, the two largest co-seismic landslides of the 1999 Chi-Chi earthquake, indicate that as high as 90 % of the original landslide volume is still preserved in Jiufengershan after two decades, whereas only less than half remains in Tsaoling. The striking difference illustrates the importance of fluvial connectivity in the preservation or removal of landslide deposits. In the longer timescale, our topographic reconstructions of six fan terrace systems along the Laonung River revealed that, even with direct connectivity with one of the largest rivers in Taiwan, approximately 10 % to 30 % of the landslide deposits are still preserved in-situ after millennia of erosion. Their preservation ratio is mainly controlled by their age, their original size, and the river/hillslope processes nearby. Wider riverbeds around these terraces imply the occurrence of large landslides and subsequent deposition on riverbeds as an important mechanism of river lateral erosion. Our results suggest that a substantial portion of large landslide deposits may persist in the hinterland over thousands of years, and such long-term preservation of landslide deposits would be important in the landscape evolution and the mass balance of active orogenic belts.