GEOPHYSICAL INVESTIGATION OF ICY DEBRIS FANS WITH GROUND PENETRATING RADAR ON THE LA PEROUSE AND DOUGLAS GLACIERS, SOUTHERN ALPS, NEW ZEALAND

Degradation of ice caps has begun to alter alpine periglacial regions, expose bedrock escarpments, and increase the occurrence of mass-flow processes, including ice avalanches, rockfalls, slushflows, and icy debris flows. At the base of escarpments along the margins of valley glaciers, these processes may produce icy debris fans—recently described mass wasting landforms that are poorly understood.

Noninvasive Ground Penetrating Radar (GPR) profiles, including Common Mid-Point (CMP) soundings, were used to characterize the subsurface architecture of four icy debris fans along the La Perouse and Douglas Glaciers in the Southern Alps of New Zealand. More broadly, these measurements allow documentation of the volume of fan deposits and contributions to glacial budgets.

In the distal half of fans, six CMPs and 15 profiles ranging between ~25-220 m long were collected. Fan surfaces are dominated by ice and lithic material derived from ice avalanches, which constitute >90% of mass flows annually. Depositional processes over 8-9 months range from 15-300 events per fan. Semblance analyses of CMP soundings indicate the GPR signal velocity is 0.162 m/ns above a reflector at a depth of ~16 m, consistent with the presence of dominantly icy materials to this depth, similar to those observed at the surface.

This velocity and concurrent RTK-GPS elevations allows for the interpretation of elevations for subsurface stratigraphic interfaces, and cross-sectional analysis of reflections. The data indicates that ~25-75 m thick packages of lenticular fan deposits thin toward the perimeter of the icy debris fans. Multiple packages of reflectors with varying geometries indicate complex fan histories, including changes in fan volumes. Interpretation of GPR surveys provides information on the sedimentary architecture of the fans which can be compared to surface deposits. Preliminary interpretations indicate the presence of bedrock and/or valley glacier underlying the icy debris fans. Together with Terrestrial Laser Scanning (TLS) surveys and time-lapse imagery, GPR profiles allow estimates of fan volume and provide a better understanding of their process evolution over time, important in understanding their contribution to valley glacier budgets.