When Did a Giant Peak Collapse in the Annapurna Himalaya—Medieval or Latest Pleistocene? Geological Evidence From Debris Avalanche and Debris Flow Deposits

Abstract

A giant summit collapse in the Annapurna Himalaya was detected by Lavé et al. in 2023 and proposed to have happened at approximately 1190 AD. These authors concluded that the collapse transformed into a debris avalanche and subsequent debris flows, which reached the Pokhara Basin 60 km downstream to form the Pokhara Formation. Our geological investigations of the sediments in the Sabche Cirque and the valley-fill sediments in the Pokhara Basin demonstrate, however, that the Pokhara Formation is not equivalent to the mountain collapse deposit in Sabche Cirque. To the contrary, the Tallakot Formation—the oldest valley-fill formation, which is composed of a monomictic breccia of cataclastic texture, is equivalent to the sediments in the Sabche Cirque consisting entirely of the sediments derived from the Tethys Himalayan Sequence. The Ghachok Formation, which overlies the Tallakot Formation, is a wide-spread well-consolidated debris flow deposit also consisting of the debris derived from the Tethys belt. Several dating studies on the samples collected from the Ghachok Formation and overlying Phewa Formation, the dammed-up lacustrine deposits yielded¹⁴C IntCal20 ages between 15 and 10 ka, the oldest of which originates from a layer of humic soil at the base of the Ghachok Formation. These findings indicate that the series of events from the giant summit collapse to debris flows occurred at 15–14 ka. This timing coincides with the deglaciation period in the latest Pleistocene; it suggests that the melting of glaciers and permafrost weakened the rock strength and supported the mountain collapse. The main triggering agent of the collapse is attributed to an E-W extensional, normal fault-type earthquake that occurred in the Tethys belt. Unlike the Tallakot and Ghachok Formations, the Pokhara Formation is a polymictic heterometric almost nonconsolidated deposit that unconformably overlies the Ghachok Formation and is dated to be approximately 1250 AD.