Gravitational spreading of mountain ridges coeval with Late Weichselian deglaciation: impact on glacial landscapes in Tröllaskagi, northern Iceland

•Identification of Deep-Seated Gravitational Slope Deformation in Tröllaskagi mountains (northern Iceland).•DGSDS driven by debuttressing induced by Late Weichselian deglaciation.•DSGSD initiates before valley deglaciation has been completed.•Major role of the DSGSD on glacial trough widening over glacial/paraglacial cycles.

During the Late Weichselian deglaciation, the coastal mountains of northern central Iceland have experienced significant paraglacial readjustment processes in the form of conspicuous rock slope failures and deep-seated gravitational slope deformation (DSGSD). Local topographic slopes and ridges were deeply reshaped by these large scale paraglacial processes. Located on the eastern side of Skagafjörður, one of the largest fjords of northern Iceland, the Óslandshlíðarfjöll and Hnjúkar ridges (65°49N, 19°14W) exhibit geomorphic evidence of spectacular DSGDS. Several series of DSGSD-induced landforms such as crestal graben and troughs initiated by ridge-top splitting were investigated over a 30-km2 area. On the basis of geomorphological mapping we recognized: (i) a ridge-top splitting event mainly controlled by glacial debuttressing induced by a minimal 300 m lowering of the glacier surface in the Deildardalur valley; (ii) a rapid Late Weichselian deglaciation of the Deildardalur valley spanning a few thousands of years (ice-free stage probably reached around 14,000 years cal. BP); (iii) ridge-top splitting having an influence on large-scale glacial patterns by guiding and facilitating glacial erosion along ridge-top grabens, resulting in accelerated trough widening. Based on these interpretations, we propose an evolutionary sequence of both the kinematic stages of the DSGSD and the Late Weichselian deglaciation at the valley scale. This work provides new insights into (i) the patterns of the Late Weichselian deglaciation in the Skagafjörður area, especially in tributary valleys of the fjord, (ii) the timing of large-scale paraglacial ridge-top deformations in relation to the post-LGM deglaciation and (iii) the influence of paraglacial DSGSD features on large-scale glacial erosional patterns.