Evidence for stress redistribution beneath Black Rapids Glacier, Alaska

Abstract

Observations of surface motion and ice deformation from 2002-2003 were used to infer seasonal stress distributions in a cross-section of Black Rapids Glacier. During periods in summer, basal shear stresses in a well-defined zone 500 m north of the centerline were redistributed to the glacier margins, as inferred by a simple inverse model that incorporates a 2D finite element flow model. The flow model can also reproduce the high surface velocities associated with the rapid drainage of marginal lakes if the well-defined zone north of the centerline is decoupled from the bed as a result of high water pressure. This zone is highly susceptible to changes in water influx, owing to a localization of the drainage system and/or to a weak zone in the till. These results suggest that surface velocities are strongly controlled by the basal stress distribution, and therefore water pressure distribution, over large parts of the glacier bed. Thus the relationship between water pressure and surface velocity is highly nonlinear, potentially explaining the observation that mean summer surface velocity was higher than mean winter surface velocity, even though water pressure was, on average, lower in summer than in winter.