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