Ice-stream shutdown during deglaciation: Evidence from crevasse-squeeze ridges of the Iceland Ice Sheet

Both modern and palaeo-ice streams experience shutdown, which has critical implications for their mass balance and influence on relative sea level. Reconstructions of palaeo-ice streams have mainly focused on their phase of active flow and thus less is understood of their shutdown and style of deglaciation. Mapping of streamlined subglacial bedforms (SSBs), including drumlins and mega-scale glacial lineations, in northeast Iceland reveals cross-cutting flow-sets of palaeo-ice streams within the Iceland Ice Sheet (IIS) during and following the last glacial maximum (LGM). Here we map geometrical ridges (linear and reticular) within the Bakkafloi and thorn istilfjorour flow-sets and combine the morphological data with sedimentological analyses to increase our understanding of the dynamics of the IIS during deglaciation in northeast Iceland. We interpret the ridges as crevasse-squeeze ridges (CSRs), based on their interconnected network, primary orientation transverse and/or oblique to former ice flow and internal composition of homogenous subglacial till. In both areas, the CSRs are superimposed on the SSBs, indicating that they post-date the SSBs and signify the waning stage of ice streaming associated with the readvance of the IIS during the Younger Dryas period. The preservation of the CSRs suggests ice stagnation following the readvance and ice-stream shutdown. The morphological difference of the CSRs between the flow-sets is taken to indicate different kinematic setting within the ice streams; the linear CSRs in Bakkafloi formed further upstream where extensional forces parallel to ice flow were dominant, whereas the reticular CSRs in thorn istilfjorour are more indicative of transverse and longitudinal forces near the terminal zone. Future research reconstructing past ice-sheet behaviour and ice-stream dynamics would benefit from high-resolution bathymetric data from the adjoining shelf as well as enhanced geochronological constraints.