Constraining the conditions for soft-bed subglacial ruptures: rate-state friction parameters in ice-till slip experiments

Observations of subglacial ruptures provide important information about under-constrained bed conditions and processes that allow unstable slip behavior. As another geological shear zone which exhibits a range of failure modes (including seismic events of a range of magnitudes, slow slip and tremor, surges, as well as stable creep), these settings also offer useful analogs to fault frictional processes, which occur at deeper depths and thus harder to access locations. Although observations of these events have been growing over recent decades, there remain open questions about the mechanics of unstable slip, in particular for soft-bedded subglacial environments. None of the previous three laboratory studies that address ice-on-till slip report rate-state friction parameters. These studies, as well as till-on-till deformation experiments and fault gouge measurements with significant clay content, have not found velocity-weakening behavior, motivating other processes involving clasts, debris-laden ice, rock outcrops, and/or sub-freezing temperatures to explain seismic observations. We utilize a double direct shear apparatus and gouge confinement techniques with precise cryogenic temperature control and a central ice block to measure the rate-state friction properties of ice-on-till interfaces under a range of realistic subglacial conditions. There is a large parameter space of potentially relevant dependencies to explore (temperature, effective normal stress, saturation, drainage, slip-rate, till layer thickness ...), but, as has been done for fault gouge settings, we start by testing the effect of till composition using a range of relative contents of sand and clay standards. This synthetic till mimics the common bimodal grain-size distribution observed in natural glacial tills, which vary greatly in sand and clay fraction. Ice-till coupling, shear localization, and compaction/dilation processes are analyzed post-deformation using particle tracers. These findings will help constrain the relationship between bed conditions and frictional behavior, including seismicity, in till-based glacial systems.