Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology, hydro-mechanical interactions and till formation

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Description/Abstract

The rheology and hydro-mechanical interactions at the iceebed interface form an important component of the glacier system, influencing glacier dynamics and the formation of till. We demonstrate that the sand-rich till at Briksdalsbreen in Norway, undergoes deformation throughout the year. On the bulk rheology scale, till deformation exhibits elastic behaviour during the winter, when water pressures are low; and linear viscous behaviour after a critical yield stress of 35 kPa, when water pressures are high during the spring and summer. On the clast and matrix scale, low water pressures, correspond with high case stress variability and till temperatures. Meltwater driven, stick-slip, glacier velocity increases were transmitted through a relatively strong till grain network, causing brittle deformation. Intermediate water pressures, during late summer were linked to intermediate case stress variability and high till temperatures associated with the heat generated from stick-slip motion. High water pressures in the till were associated with low case stress variability and low, meltwater controlled, till temperatures, and occurred in the spring and autumn. Once the till was saturated, the ductile till absorbed any stick-slip velocity increases. We discuss, with examples, the different till forming processes associated with these changing conditions, demonstrating that the resultant till will represent a complex amalgamation of all of these processes.