Evolution and temporal constraints of a multiphase postglacial rock slope failure

Large rock slope failures are temporal processes which act to modify the landscape after glacial retreat. The slope failure process often shows a lag time of thousands of years after deglaciation, with multiple failure events possible. While global datasets constrain this lag time from extensive mapping and dating of paraglacial rock avalanches, the timeline is poorly refined in northern Norway. We present a case study of multiphase failure at Skredkallen on Vanna, one of a group of coastal islands in Troms, northern Norway. The site contains an actively deforming rock slope above a large rock avalanche deposit. The rock slope deformation (RSD) is a system of fractured and dislocated blocks up to 3 Mm³, and is moving slowly ~5 mm/yr downslope to the north-east. The metasedimentary rock mass contains four pervasive joint sets and a foliation, contributing to a compound structure failure mechanism. The rock mass is further weakened by foliation-parallel sheared mylonite, and the presence of a brittle fault in the immediate area, with evidence of hydrothermal fluid flow though the RSD. The rock avalanche deposit below the slope deformation is calculated to be 3 Mm³, and extends >1 km from the source area, displaying typical mobility for north Norwegian rock avalanches onto undrained sediments. The deposits showcase exceptional lobate morphology with elongated ridge-and-furrow features. Raised shorelines predating and postdating the deposit provide temporal constraints on the deposit and an opportunity to reconstruct a relative timeline for the slope evolution. The postglacial marine limit (>14 cal. ka BP) is obscured by the deposit, while shorelines corresponding to the early Younger Dryas (12.2 cal. ka BP) and the subsequent Tapes transgression maximum (7.6–7.2 cal. ka BP) are prominent across the deposits, implying that the avalanche was emplaced between 15 and 12.2 cal. ka BP. Failure occurred during a time of immense climate instability at the boundary to the early Holocene, consistent with global reports of mountain slope failure following glacial retreat. The avalanche was emplaced into what would have been the marine environment. The anomaly between the rock avalanche source area volume (35 Mm³), and the rock avalanche deposit implies previous failure events, the deposits of which were either removed due to failure of the underlying marine sediments into the fjord, by retreating glacial ice or scour. The initiation of movement at the RSD may be attributed to periods of local climate changes, such as the Holocene Thermal Maximum. Cosmogenic nuclide dating is suggested as the next step to fill gaps in the slope evolution story through the mid to late Holocene.