Early Holocene catastrophic mass-wasting event and fan-delta development on the Hua-tung coast, eastern Taiwan

Abstract Landslides and debris flows rarely occurred during historical times in the tectonically active Coastal Range of eastern Taiwan. This topographic stability, however, contrasts greatly with the widespread existence of terraced alluvial fans and fan-deltas on the Hua-tung coast which fringes the range. This study focuses on the two largest fan–terrace systems on the Hua-tung coast, both of which consist of alluvial fans (plane-view areas up to 8 km 2 ) larger than their contributing catchments. Stratigraphic data show that both systems were in sandy, wave-dominated settings during de-glacial times. The systems were then disturbed by a catastrophic landslide/debris-flow event (or events), which brought enormous amounts of gravel (Facies Gm) into the systems, deforming previously-deposited marine sands (Facies Sm) and shallowing the seafloor. The combined Gm/Sm complex yields multiple radiocarbon dates ranging from 11.3 to 8.3 ka cal BP, with a cluster around 8.6 ka cal BP. This mass-wasting event has been unique since the emergence of its contributing catchment 0.2–0.3 Ma ago. The low frequency of such an event could reflect the great resistance of rock mass in the source areas to weathering and erosion. The common blockage of valley floors by giant-boulder piles, which limits channel incision and sediment transport, could also increase the apparent stability of the mountain. The trigger of landslides in the Coastal Range has been linked to large earthquakes. Additionally, we propose that the great magnitude and duration of the observed early Holocene event were caused by the contemporaneous prolonged rainfall (and/or high frequency of typhoons) associated with the East Asian summer monsoon maximum.