Signature of NEGIS shear margins in radar stratigraphy

The North East Greenland Ice Stream (NEGIS) is an essential part of the Greenland mass balance, reaching up to the central divide of the ice sheet. Its shape is dominated by very distinct shear margins that appear not to be linked to bedrock topography in the upper part of the ice stream. To understand the effective mass transport within an ice stream it is necessary to investigate the nature of the shear margins, in which very localized deformation decouples the inner ice stream from the slower-flowing surrounding ice sheet. We present results from an airborne radar survey with the Alfred Wegener Institute Ultra Wide Band Radar system in the vicinity of the EGRIP ice core location, focusing on the signatures of the shear margins in the radar data. In regions of localized shear, the internal reflections show disturbances in the form of steep undulations, which are intensified with ongoing shear. We show the evolution of these characteristic signatures over the survey area. Our data covers the main trunk and the shear margins over 200 km along the ice stream, in which the flow velocity in the center increases from 12 m a–1 to 75 m a–1. We analyze the change in the shape of the internal reflections in the shear zones in combination with a strain rate field calculated from high-resolution flow velocities derived from TerraSAR-X SAR interferometry. In the vicinity of the EGRIP drilling site, 240 km downstream of the divide, the NEGIS widens and the shear margin in the northwest steps 6 km outward, which is visible in optical satellite imagery as well as in the satellite-derived velocity field. A special focus of our analysis lies on the step in the shear margin whose clearly visible radar-backscatter signatures indicate a newly forming vertical disturbance caused by a draw-down of the layers.