A giant Early Holocene tsunamigenic rock-ice avalanche in South Greenland preconditioned by glacial debuttressing

Deposits of a giant (more than 1 km 3 ) rock-ice avalanche with a runout of 15.8 km and an associated failure scar have been discovered in Tupaasat Valley, South Greenland ( Kalaallit Nunaat ). To study its formation, a geomorphological map of the area has been produced using mainly remote sensing, while the age of the landforms has been constrained based on 10 Be cosmogenic nuclide exposure dating. The rock-ice avalanche landforms include a 1100 m wide mound of displaced material located approximately 6.4 km from the scar, at a change in slope from around 4° to 1.5°. A boulder field containing boulders up to 45 m across lies 0.5 km farther down the valley. Beyond the boulder field are kettle holes up to 45 m in diameter and debris cones interpreted as molards reaching heights up to 36 m and diameters up to 140 m. The source area of the rock-ice avalanche is on a mountain crest above a present-day glacier. The rock-ice avalanche landforms yielded 10 Be ages ranging from c. 12.3 to 9.5 ka with a weighted mean of 10.9 ± 0.5 ka, which coincides with the generally known deglaciation age of the valley. Based on our results, we suggest that the rock-ice avalanche was preconditioned by glacial debuttressing during the deglaciation of the valley and created a tsunami when it impacted a nearby fjord. Such events are expected to be more frequent due to climate change and future ice loss and pose a hazard for populations located near glaciated valleys. A better understanding of past events can help mitigate future large rock-slope failures. • Mapping of a giant paleo rock-ice avalanche in Tupaasat Valley, South Greenland. • 10 Be ages of the avalanche deposit align with the deglaciation of the valley. • Large volume and deposit in the fjord suggest the landslide was tsunamigenic.