1 Introduction

The development of large rock slope failures within glaciated valleys is often assumed to be primed by steepening of topography from glacial erosion and by subsequent glacier retreat (McColl 2012). Glacier retreat removes confining stresses, changes hydrological, thermal, and weathering processes, and changes the interaction of co-seismic waves within the slope (e.g. Gischig et al. 2011; McColl et al. 2012; Grämiger et al. 2016), gradually lowering stability through strength degradation or a rise in destabilizing shear stresses. Few studies have documented the evolution of slope failure in glaciated valleys over historical time periods, and thus relatively little is known about how these processes interact to generate failures. For example, glacial debuttressing may cause catastrophic failures, as indicated by numerous post-glacial rock avalanche deposits in formerly glaciated valleys; but some of these failures may have occurred thousands of years following ice withdrawal (e.g. Ballantyne et al. 2014; Ostermann & Sanders, in press), suggesting that earthquake shaking or gradual strength degradation may also be responsible for triggering failure. It has also been suggested that the initial development of rock slope failures can proceed prior to the complete glacier withdrawal, and involve deformation of a buttressing glacier (McColl & Davies 2013).

This study takes the opportunity to investigate the long-term development of a large, active rockslope failure that is still partially-buttressed by a glacier, with the aim to explore how failure develops over time and to assess the factors controlling movement and failure evolution.

2. Study site

The study focusses on the Mueller Rockslide, in Aoraki Mt Cook National Park, in the central Southern Alps of New Zealand (Fig. 1). The rockslide, estimated to be ~150 Mm3 in volume, occupies the dip slope of an overturned anticline in Mesozoic greywacke sandstone (Lillie & Gunn 1964) (Fig. 2). The rockslide toe is partly buttressed by the Mueller Glacier, which has thinned by ~100 m since the Little Ice Age. The glacier becomes narrow adjacent to the landslide toe, perhaps in part due to squeezing by the Mueller Rockslide (McColl & Davies 2013).

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