Fracture of floating glacier ice upon sidewall decoupling

Abstract:

When the floating termini of tidewater glaciers protrude past confining lateral boundaries, fractures typically propagate from the glacier’s lateral margins toward the centerline. These rifts cut inward from the location where lateral contact is lost, often, ultimately, resulting in the production of large icebergs. We call attention to this type of rifting, which we term “detachment rifting,” and we show that the phenomenon of detachment rifting (and the formation of icebergs resulting from detachment rifts) is common to both the Greenland and Antarctic Ice Sheets. Through examination of Ryder, Zachariae, 79 North, and C. H. Ostenfeld Glaciers in northern Greenland, we explain this mechanism of fracture by invoking a rigorous force-balance argument which allows for longitudinally decreasing coupling between a glacier and its sidewalls. We find that, for forces to balance, the loss of shear stress against lateral boundaries results in significant flow-parallel extension and flow-perpendicular compression. Flow-parallel extension likely helps explain our observations of detachment rifting at sites where sidewall coupling is lost. The reliability of our force balance analysis is supported by our finding that the stresses we calculate can qualitatively reproduce observed trends in ice velocity through the zone of sidewall decoupling. Ultimately, we find evidence that the loss of contact of glacier ice with confining sidewalls produces significant extension and rifting near the margins at locations corresponding to those of observed detachment rifts. Via the generation of detachment rifts, the loss of glacier-sidewall coupling is likely a critical component of preconditioning ice tongues and shelves for calving.