Along the lateral margins of floating ice shelves in Greenland and Antarctica, ice flow past confining margins and pinning points is often accompanied by extensive rifting. Rifts in zones of marginal decoupling (“detachment zones’) typically propagate inward from the margins and result in many of Earth’s largest calving events. Velocity maps of detachment zones indicate that flow through these regions is spatially transitioning from confined to unconfined shelf flow. We employ the software package \textit{icepack} to demonstrate that longitudinally decreasing marginal resistance reproduces observed transitions in flow regime, and we show that these spatial transitions are accompanied by near-margin tension sufficient to explain full-thickness rifts. Thus, we suggest that zones of progressive decoupling are a primary control on ice shelf calving. The steadiness of detachment zone positions may be a good indicator of ice shelf vulnerability, with migratory or thinning detachment zones indicating shelves at risk of dynamic speedup and increased fracture.

Seasonal variability in the Greenland Ice Sheet’s (GrIS) sliding speed is regulated by the response of the subglacial drainage system to meltwater inputs. However, the importance of channelization relative to the dewatering of isolated cavities in controlling seasonal ice deceleration remains unsolved. Using ice velocity, moulin hydraulic head, and glaciohydraulic tremor measurements we show the passing of a subglacial floodwave following the drainage of an up-glacier supraglacial lake slowed minimum sliding speeds to wintertime background values without increasing the hydraulic capacity of the moulin-connected drainage system. We interpret these results to reflect a persistent basal traction increase consistent with the dewatering of isolated cavities exert the dominant control on seasonal ice velocity decreases. Current predictions of the GrIS’s ice-dynamic response to increased surface melting hinges on the subglacial drainage system’s ability to increase its capacity to offset sustained meltwater influxes, which our results demonstrate may not be the case.