Geomorphology and geological controls of an active paraglacial rockslide in the New Zealand Southern Alps

Geological structures precondition hillslope stability as well as the processes and landslide mechanisms which develop in response to deglaciation. In areas experiencing glacier retreat and debuttressing, identifying landslide preconditions is fundamental for anticipating landslide development. Herein, the 150 M m(3) Mueller Rockslide in Aoraki/Mount Cook National Park, New Zealand, is described; and we document how preconditions have controlled its morphology and development in response to thinning of the adjacent Mueller Glacier. A combination of geomorphological and geotechnical mapping-based on field, geophysical and remote sensing data-was used to characterise the rock mass and morphology of the rockslide and surrounding hillslope. Mueller Rockslide is identified as a rock compound slide, undergoing dominantly translational failure on a dip slope. The crown of the rockslide is bounded by several discontinuous, stepped scarps whose orientation is controlled by joint sets; these scarps form a zone of toppling that is delivering rock debris to the main rockslide body. Surface and subsurface discontinuity mapping above the crown identified numerous joints, fractures and several scarps that may facilitate continued retrogressive enlargement of the ro kslide. The presence of lateral release structures, debuttressing of the rockslide toe and steeply dipping bedding suggest that the rockslide may be capable of evolving to a rapid failure.