Cosmogenic age constraints on rock avalanches in the Qinling Range associated with paleoearthquake activity, central China

Although it is generally assumed that understanding the trigger factor leading to the transition from a progressive rock failure toward sudden and rapid events is better to risk management, the ability to identify triggering factors responsible for prehistoric catastrophic rock avalanches is still debated. Here we describe two rock avalanches (Ganqiu and Shuiqiu) in the Cuihua Mountain area of the Qinling Range, China, providing new mapping of rock avalanche source areas and deposits and absolute age constraint from cosmogenic Be-10 surface exposure dating. Results reveal that the Ganqiu rock avalanche is controlled by three sets of joints, causing the 380 m high ridge to collapse. Debris filled roughly the entire V-shaped valley and has a width ranging from 250 to 470 m, extending similar to 1260 m from the base of the cliff. The Shuiqiu rock avalanche was sourced near the summit of Cuihua Mountain from a cliff above 60 degrees that is mainly controlled by inherited fault and joints. The rock mass fell similar to 250 m as a giant blocky stream across the width of the valley floor, with the deposit extending similar to 300 m from the base of the cliff. Both events caused the large-scale collapse of ridge crests and developed into high-mobility rock avalanches with long runout. Cosmogenic Be-10 surface exposure dating from seven different parts of the deposits revealed nearly coincident ages for both rock avalanches, 10.0 +/- 0.9 ka for the Ganqiu rock avalanche and 9.5 +/- 1.2 ka for the Shuiqiu rock avalanche. Because this region was not affected by valley glaciation, rock avalanches triggering by paraglacial stress release can be excluded. Moreover, without clear evidence for specific climatic conditions in the Qinling area circa 9-10 ka, a climatic trigger for the Cuihua Mountain rock avalanches is equivocal. Instead, we find coincidence between the concurrent exposure ages and paleoseismic records from the nearby Qinling North Piedmont fault (QNF). This coincidence of ages, combined with the fact that the M-w 8.0 1556 CE Huaxian earthquake triggered numerous large rock falls in the Huashan Mountains at the eastern end of the Qinling Range, suggest that a large-magnitude earthquake centered in the middle-east section of QNF at ca. 8-11 ka may have triggered the Ganqiu and Shuiqiu rock avalanches.