Influence of sediment dilation on the initiation of surges in glaciers with deformable beds

Abstract:

Glacier surges are quasi-periodic episodes of exceptionally rapid ice-flow that arise from increases in the rate of slip along glacier beds. Flow speeds during surges are more than an order of magnitude faster than typical quiescent-phase speeds and can last from months to years, with individual surge events separated by years or decades. Known surge-type glaciers represent only about 1% of glaciers worldwide and these are clustered in a few geographic regions spread across the globe, share some comparable geological factors, and inhabit a variety of climates. Most models of glacier surges assume rigid, impermeable beds. However, many surge-type glaciers overly sediment-covered beds, and the mechanisms that trigger and sustain surges in these glaciers are not well understood. We introduce a new model for incipient surge motion in glaciers with deformable, permeable beds composed of water-saturated till. Our model builds on ideas from fault mechanics and represents the evolution of internal friction, porosity, and pore water pressure within the till through a rate-and-state friction framework. We couple the model of till mechanics to a simple ice-flow model to study the acceleration phase of glacier surges. In our model, changes in pore water pressure govern incipient surge motion, making permeability of the till an important factor in determining whether a glacier can surge. Less permeable till facilitates surges because reductions in pore water pressure due to dilation slow the rate at which the till evolves to a new steady state, allowing time for the glacier to thin dynamically through increases in the mass flux divergence. To sustain surge motion in our model, the effective pressure must decrease, meaning that the rate of reduction in overburden pressure caused by glacier thinning must exceed the rate of increase in pore water pressure from dilation. Net reduction of effective pressure ensures that driving stresses exceed resistive stresses at the bed and lateral shear margins, thereby allowing for acceleration. The need for changes in both the hydromechanical properties of the till and the thickness of the glacier leads to restrictive conditions for surge motion that are broadly consistent with the geographic clustering and relative sparsity of surge-type glaciers.