Drivers of The Daggers: Backstress-triggered fracture propagation on the Thwaites Eastern Ice Shelf, West Antarctica

The destabilization of ice shelves around Antarctica is not just a sea-level-rise priority, but an opportunity to observe and understand the mechanisms of ice-shelf failure. Recent studies show that ice shelves fail against their pinning points or ridges once sufficiently weakened by thinning and damage---a "backstress-triggered" failure mechanism that is not represented in ice-shelf collapse criteria employed by ice-sheet models. We suggest that fractures extending upstream from a pinning point/ridge are a signature of backstress-triggered failure. In this study, we identify controls on the spatiotemporal evolution of five such fractures on the Thwaites Eastern Ice Shelf. Vertical propagation may have been limited by a heterogeneous layer above the fracture tip, which penetrates 70-80% of the ice column, as revealed by ice-penetrating radar. Horizontal propagation is correlated with relatively small, ~10 degree, fluctuations in the direction of ice flow in response to a weakening shear zone against the pinning ridge, possibly modulated by a change in ice thickness at the fracture tips. In hindsight, we suggest this style of fracture has occurred during other ice-shelf breakups around Antarctica. As an observable indicator that backstress-triggered ice-shelf failure is underway, these fractures may be a promising avenue towards a new collapse criterion.