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 preferred 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). Our results suggest that ice at WIS preserves upstream fabric caused by palaeo-deformation developed over at least the past 450 years, implying that changes in the shape of WIS occurs on timescales shorter than COF re-equilibration. The “palaeo-fabric” can somewhat control present-day ice flow, which we suggest may somewhat counteract 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.