Seasonal variations in ice deformation and basal motion across the tongue of Haut Glacier d’Arolla, Switzerland Ian WILLIS, 1 Douglas MAIR, 2 Bryn HUBBARD, 3 Peter NIENOW, 4 Urs H. FISCHER, 5 Alun HUBBARD 6 1 Scott Polar Research Institute and Department of Geography, University of Cambridge, Cambridge CB2 1ER, England E-mail: iw102@cus.cam.ac.uk 2 Department of Geography and Environment, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF, Scotland 3 Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB,Wales 4 Department of Geography andTopographic Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland 5 Laboratory of Hydraulics, Hydrology and Glaciology, ETH-Zentrum, CH-8092, Zu« rich, Switzerland 6 Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, Scotland ABSTRACT . Records of surface motion, englacial tilt and repeat inclinometry are used to determine patterns of surface, internal and basal motion across the tongue of Haut Glacier d’Arolla, Switzerland, over temporal scales ranging from days to months. Findings are interpreted with reference to contemporaneous measurements of subglacial water pres- sures, and prior knowledge of the glacier’s subglacial drainage-system structure. Long-term inclinometry results show pronounced extrusion flow over a subglacial drainage axis, with basal velocities up to twice those measured at the glacier surface. Deformation profiles are more conventional away from the drainage axis, with basal velocities ¹60^70% of surface velocities. Comparison of long-term tilt rates from repeat inclinometry and englacial tilt- meters shows close correspondence. Englacialtiltmeter data are used to reconstruct internal velocity profiles and to split surface velocities into internal deformation and basal motion contributions over spring, summer and autumn/winter periods. Although, spatial patterns of surface movement are similar between periods, patterns of internal and basal motion are not. Results are interpreted in terms of the location of sticky and slippery spots, with tem- porally changing patterns of basal drag reflecting changing distributions of water pressure. INTRODUCTION Glaciers move by a combination of internal deformation and basal motion, the latter involving sliding at the ice^bed inter- face and/or the deformation of subglacial sediment. Know- ledge of the relative importance of these mechanisms, how they vary spatially and temporally, and what controls this variability is crucial to an understanding of glacier and ice- sheet dynamics and the representation of these mechanisms in numerical models. Several studies have measured surface motion, calculated internal deformation from Glen’s flow law, and estimated basal motion as the residual (see Copland and others,1997b; Gudmundsson and others,1999; and references therein).The problem with this method is that the accuracy of Glen’s flow law in these circumstances is unknown since it assumes that the local shear strain rate is controlled by the local shear stress, and it ignores variations in ice rheology and the effects of longitudinal stress gradients (Blatter and others, 1998). Other studies have measured surface motion and then used inverse force-balance techniques to calculate basal drag, as a surrogate for basal motion (Hooke and others, 1989; Van der Veen and Whillans,1993; Iken and Truf- fer,1997; Mair and others, 2001). Although longitudinal stress gradients are accounted for in this approach, they are cal- culated on the basis of surface strain rates that are assumed to be constant with depth. Consequently, the method can only be applied over spatial scales greater than a few ice depths. Other studies have attempted to measure basal motion directly in natural cavities, artificial tunnels or at the base of boreholes using photography, video or purpose- built instruments (see Gudmundsson and others, 1999 and references therein; Hubbard, 2002). Such studies tend to measure basal motion over a spatially restricted area and over time-scales of hours to weeks, and the results cannot easily be extrapolated to broader spatial scales or longer time-scales. Furthermore, such studies have not usually been combined with measurements of surface motion and have therefore not been able to determine the contributions of internal deformation and basal motion to overall flow. Repeat borehole inclinometry has successfully been used to examine the contribution of internal deformation (and, by subtraction, basal motion) to net glacier motion over time-scales of months to years (see Copland and others, 1997b and references therein; Harbor and others, 1997; Harper and others,1998). However, on slow-moving glaciers such as Haut Glacier d’Arolla, Switzerland, repeat inclinom- etry cannot be used to resolve the components of surface motion at time-scales less than about 1year (Copland and others, 1997b). Recently, englacial tiltmeter records col- lected by data logger at high temporal resolution have been used to reconstruct the ice-deformation profile and the con- tributions of ice deformation and basal motion to surface flow (Gudmundsson and others,1999). While the technique was applied to just one borehole on the centre line of Unter- Annals of Glaciology 36 2003 # International Glaciological Society 157