The role of structural heterogeneity in glacier ice deformation: Insights from Planpincieux Glacier

Abstract. Natural glacier ice is not a monophasic, isotropic material as commonly assumed in models based on Glen-Nye's flow law. It can contain crevasses, develop crystallographic preferred orientations, and include mixtures of debris and interstitial water in temperate glaciers. Understanding the influence of such structural heterogeneities on deformation is therefore essential for accurately modeling glacier dynamics. In this study, we investigate the multi-scale evolution of structural heterogeneities with depth in the Planpincieux Glacier (Italian Mont Blanc massif) and evaluate their respective influence on ice deformation using a borehole instrumented with an optical televiewer, a full-waveform sonic logger, a piezometer, and an inclinometer chain. Complementary GNSS and seismic data provide additional constraints on hydrological activity and surface motion. Optical and sonic logging reveal two main families of heterogeneities: open and closed crevasses in the upper 60 m, and debris-rich layers near the bedrock interface. Acoustic data show continuous but opposite trends in both P- and Stoneley-wave velocities with depth, interpreted as reflecting an increase in water content but a decrease in permeability. Tiltmeter measurements indicate that roughly one-third of the surface velocity is accommodated by internal deformation, with pronounced strain localization near the bedrock, particularly within debris-rich layers. These layers exhibit enhanced strain following hydrological drainage events, suggesting a coupling between mechanical heterogeneity, basal hydrology, and strain localization. The results highlight that glacier friction laws may be significantly influenced by such heterogeneities, including the effects of interstitial water and debris on local mechanical behavior.