Chronology and morpho-climatic history of large rock slope failures in the Southern Swiss Alps

Alessandro, De Pedrini and Chantal, Del Siro and Christian, Ambrosi and Cristian, Scapozza (2023) Chronology and morpho-climatic history of large rock slope failures in the Southern Swiss Alps. In: XXI INQUA Congress 2023, 14-20.07.2023, Rome, Italy.

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Abstract

Large rock slope failures in an Alpine environment are influenced not only by structural weaknesses inherited from orogenetic tectonics and lithological characteristics of slopes but also by the climatic history. In particular, the glacial history plays a significant role in slope stability, as the glaciers’ pressure exerts a variable load on the valley flanks and reduces the slope strength often leading to rock slope failures. In the Southern Swiss Alps, in the territory between the five valleys north of Bellinzona (Riviera, Valle Leventina and Valle di Blenio in Canton of Ticino, Val Calanca and Valle Mesolcina in Canton of Graubünden), several debris accumulations of large rock slope failures can be observed. A detailed geochronological assessment of glacial retreat and the age of slope collapse is essential to understand the strong relationship between the two phenomena. This research aims to define the exposure age of the mapped rockslide/rock avalanche deposits through Schmidt hammer exposure-age dating (SHD) and analyze the achievements in relation to the collapse volume to obtain a morphodynamic interpretation during deglaciation. The exposure ages indicate a collapse just a few centuries after the deglaciation, which occurred for the lower and middle parts of the Valle Mesolcina, Valle di Blenio and Valle Leventina between 16.94 and 16.25/15.96 ka b2k. Both deglaciation and dated rock slope failures occurred during the Greenland Stadial GS-2.1a of the INTIMATE event stratigraphy, dated between 17.48 and 14.69 ka b2k (Rasmussen et al. 2014, Quat. Sci. Rev. 106) or, at least, at the beginning of the Greenland Interstadial GI-1 (14.69–12.90 ka b2k), in particular during the events GI-1e (14.69–14.08 ka b2k), GI-1d (14.08–13.95 ka b2k) and GI-1c (13.95–13.31 ka b2k), characterized by the first significant temperature increase after the Last Glacial Maximum. The exposure age dating highlights a lack of collapses between the Last Interstadial and the past 500 years, suggesting slope stability during the Holocene and increased activity after the little Ice Age. This fact agrees with the progradation and sedimentation rates of the Ticino river, which show a reduced magnitude during the time free of collapses. The comprehension of the timing and type of rock slope failure collapses in paraglacial environments help the definition of future scenarios of slope collapse.

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