Borehole?Based Characterization of Deep Mixed?Mode Crevasses at a Greenlandic Outlet Glacier

Optical televiewer borehole logging within a crevassed region of fast-moving Store Glacier, Greenland, revealed the presence 35 high-angle planes that cut across background primary stratification. These were composed bubble-free layer refrozen ice, most which hosted thin laminae bubble-rich “last frozen” consistent with being traces former open crevasses. Several such last-frozen observed in four traces, suggesting multiple episodes crevasse reactivation. The frequency generally decreased depth, deepest detectable trace 265 m below surface. This is extent warmer-than-modeled englacial ice area, extends from surface to depth ?400 m. Crevasse orientation was strongly clustered around dip 63° and strike offset by 71° orthogonal local direction principal extending strain. traces’ antecedent crevasses therefore interpreted have originated upglacier, probably ?8 km distant involving mixed-mode (I III) formation. We conclude deep crevassing pervasive also at all dynamically similar outlet glaciers. Once healed, their represent weakness subject reactivation during subsequent advection through glacier. Given it highly likely traces—particularly those formed downglacier—survive ablation reach glacier terminus, where they may foci for fracture iceberg calving.