Justin Leung, Thomas Samuel Hudson , John-Michael Kendall, Grace Barcheck

Understanding deformation and slip at ice streams, which are responsible for 90% of Antarctic ice loss, is vital for accurately modelling large-scale ice flow. Ice crystal orientation fabric (COF) has a first-order effect on ice stream deformation. For the first time, we use shear-wave splitting (SWS) measurements of basal icequakes at Whillans Ice Stream (WIS), Antarctica, to determine a shear-wave anisotropy with an average delay time of 7 ms and fast S-wave polarisation (φ) of 29.3º. The polarisation is expected to align perpendicular to ice flow, whereas our observation is oblique to the current ice flow direction (~280º). This suggests that ice at WIS preserves upstream fabric caused by palaeo-deformation developed over at least the past 450 years, which provides evidence of the concept of Microstructural Fading Memory. Our results imply that changes in the shape of WIS occur on timescales shorter than COF re-equilibration. The “palaeo-fabric” can somewhat control present-day ice flow, which we suggest may somewhat contribute to the long-term slowdown at WIS. Our findings suggest that seismic anisotropy can provide information on past ice sheet dynamics, and how past ice dynamics can play a role in controlling current deformation.