Here we present new measurements of an anisotropic ice fabric in a fast moving (377 ma−1) ice stream in West Antarctica. We use ∼6000 measurements of shear wave splitting observed in microseismic signals from the bed of Rutford Ice Stream, to show that in contrast to large‐scale ice flow models, which assume that ice is isotropic, the ice in Rutford Ice Stream is dominated by a previously unobserved type of partial girdle fabric. This fabric has a strong directional contrast in mechanical properties, shearing 9.1 times more easily along the ice flow direction than across flow. This observed fabric is likely to be widespread and representative of fabrics in other ice streams and large glaciers, suggesting it is essential to consider anisotropy in data‐driven models to correctly predict ice loss and future flow in these regions. We show how passive microseismic monitoring can be effectively used to provide these data.

Flow dynamics of the ice streams that drain the Antarctic Ice Sheet are heavily influenced by processes at the bed. Natural seismic activity generated beneath an ice stream is associated with the motion of the ice over its bed and can be used to map both the characteristics of the ice-bed interface and to understand these basal processes. Basal microseismicity was recorded over a 34 day period on Rutford Ice Stream, West Antarctica, using 10 three-component geophones 40 km upstream of the grounding line. Around 3000 microseismic events were located in discrete spatial clusters near the ice-bed interface. The activity of each cluster varies with time, and the source mechanism for the events is interpreted as subhorizontal, low-angle faulting, slipping in the ice flow direction. Cluster locations are interpreted as "sticky spots" of stiff basal sediment at the ice-bed interface, where ice movement is accommodated by stick-slip basal sliding. The sticky spots occur in areas where independent active-source seismic surveys show low porosity sediments at the bed. We show that the sticky spots probably accommodate only a small amount of the total basal motion. Our results suggest that most of the ice stream basal motion is accommodated by aseismic deformation of soft, dilatant basal sediment, or by a well-lubricated, stiffer bed.

The crystal orientation fabric of ice reflects its flow history, information which is required to better constrain projections of future ice sheet behavior. Here we present a novel combination of polarimetric phase-sensitive radar and seismic anisotropy measurements to provide independent and consistent constraints on ice fabric at Korff Ice Rise, within the Weddell Sea sector of West Antarctica. The nature and depth distribution of fabric in the ice column is constrained using the azimuthal variation in (1) the received power anomaly and phase difference of polarimetric vertical radar soundings and (2) seismic velocities and shear wave splitting measurements. Radar and seismic observations are modeled separately to determine the nature and strength of fabric within the ice column. Both methods indicate ice fabric above 200-m depth which is consistent with present-day ice-divide flow. However, both measurements also indicate an oblique girdle fabric below 230-m depth within the ice column, inconsistent with steady state divide flow. Our interpretation is that this deeper fabric is a remnant fabric from a previous episode of flow, which is currently being overwritten by ongoing fabric development associated with the present-day flow regime. The preexisting fabric is consistent with ice flow from the south prior to ice-divide formation, in agreement with models of Holocene ice sheet evolution. These findings apply new constraints to the flow history at Korff Ice Rise prior to divide formation and demonstrate the capacity of radar and seismic measurements to map fabric and thus constrain past ice flow.Plain Language Summary When ice flows its crystals become oriented in specific ways dependent on the way the ice is flowing, forming what we call a fabric. When this occurs in an ice stream the form of this fabric is determined by the flow direction and geometry of the ice stream bed. These fabrics remain imprinted within the ice but can become overwritten by new fabrics which result from a subsequent flow pattern. The study site here is Korff Ice Rise, which is an isolated area of grounded ice within the floating Ronne Ice Shelf in the Weddell Sea, West Antarctica. Here the ice flows in a very specific way, forming a characteristic fabric. We can measure this fabric using radar and seismic measurements. We also see an older fabric which must have formed when the ice was flowing differently. This older fabric is consistent with a time when the ice sheet was much thinner and ice was flowing from the continent of Antarctica further upstream, and over the current site. This study shows how measurements of fabric within Antarctic ice help us look at how the ice sheet behaved in the past. This past behavior would have affected global sea levels.