Timing and flow pattern of the Orta Glacier (European Alps) during the Last Glacial Maximum

Knowledge of the glacial chronologies for the Last Glacial Maximum (LGM) helps in understanding the interactions between climate, topography and glacier development. In this sense, the investigation of the Lake Orta moraine amphitheatre (Alpine foreland, northern Italy) allowed spatial and temporal reconstruction of the Orta Glacier. The end-moraine system was investigated by means of geomorphological field surveys, analysis of 13 rock samples for cosmogenic Be-10 and Cl-36 concentrations, and remote sensing analysis. The dating results indicate that the age of the outer moraine belt is concordant with the LGM culmination at 26.5-23 ka, as found in other amphitheatres in the Alps. This new age estimate of the outermost moraines shows that the maximum extent of the Orta Glacier during the LGM was significantly bigger than recently suggested. A younger stabilization phase of the glacier front at about 19 ka indicates that the onset of the withdrawal of glaciers from the lower Alpine valleys started later. Provenance analysis of the boulders shows that the greatest contribution of ice to the Orta Glacier came from the Anzasca Valley rather than the major Ossola Valley. This reflects the closeness (about 45 km) to the foreland of the high-elevated accumulation area of the Monte Rosa massif (4634 m a.s.l.), whose eastern glacier seems to have reached the lower valley faster than the trunk Toce Glacier. This fact underlines the key role played by high-elevation accumulation areas that are located close to the foreland in controlling the path and geometry of major glaciers in the Alps.