Evidence of extreme debris flows during early Holocene glacier retreat in the Cordillera Blanca (Peru) : paleo-GLOF hypothesis and implications for current hazard

The Peruvian Andes are among the world’s regions most affected by global warming, leading to rapid glacier retreat. The Cordillera Blanca (CB), the most glaciated tropical range, has lost ~41% of its glacier area since 1962. This retreat has preconditioned the formation and growth of numerous glacial lakes, increasing Glacial Lake Outburst Floods hazard and associated slope instability. The Santa River valley (western flank of the CB) is the second most exposed region worldwide to GLOFs, where steep topographic gradients favor high-magnitude floods toward downstream communities. Over the last decades, repeated catastrophic GLOFs in the Santa River valley have caused thousands of fatalities, including the 1941 Palcacocha GLOF (~1800 fatalities).

This study focuses on the Llullán and Parón valleys in the northern CB. These catchments include Laguna Parón (~60 × 10⁶ m³), considered one of the potentially dangerous glacial lakes, as well as Huandoy peak (6343 m a.s.l.). At the outlet of the Llullán valley lies an extensive fan-shaped deposit, ~4 km wide and 3 km long. The city of Caraz (~2300 m a.s.l.), with ~22,000 inhabitants, is built on this landform, where numerous very large boulders (up to 20 m in diameter) are scattered all across the surface. These features raise key questions about the magnitude and return periods of the extreme events. Our objective is to constrain the timing, magnitude, and topoclimatic conditions of the paleo-events that formed the Caraz fan deposit to assess exposure under current conditions.

Field and remote sensing observations show that the Caraz fan deposit is continuously connected to a sedimentary fill in the Llullán Valley. This surface exhibits a homogeneous longitudinal profile of ~11 km with ~5° slope, following an east–west trajectory. We inventoried hundreds of granodiorite boulders embedded across the deposit surface, which spatial and statistical distribution provide evidence of a high-energy event. Outcrops along the Santa River reveal vertical sections of a homogeneous debris-flow body characterized by subrounded granodiorite clasts (up to 5 m), some displaying jigsaw-clast structures, within a sand–gravel–silt matrix. Debris-flow run-up along the Llullán valley reached >50 m above the current riverbed, further indicating a high-energy event. The deposit ranges from 20 to 40 m in thickness, covers ~12 km², and has an estimated volume >400 × 10⁶ m³.

Twenty-four paired ages based on in-situ cosmogenic nuclide exposure dating (¹⁰Be, ²⁶Al) on eleven large boulders (including two replicates) reveal a consistent exposure history, indicating the Caraz paleo-event occurred as a single event at ~10 ka. This timing coincides with Early Holocene warming and rapid glacier retreat (horizontal velocity 4–8 km/ka) following the Younger Dryas glacial advance. We hypothesize that a paleo-lake, trapped behind the Younger Dryas moraine in the upper Parón Valley, at a location like the current Laguna Paron, may have been the source of an extreme GLOF that generated the Caraz debris-flow. Understanding the origin of the Caraz paleo-debris-flow provides key analogues to assess the hazard posed by Laguna Parón and its moraine dam, which has a volume of comparable magnitude.