Date of Thesis
Bachelor of Science
Robert W. Jacob
Jeffrey M. Trop
R. Craig Kochel
Icy debris fan, Geophysics, Ground penetrating radar, GPR, CMP, Paraglacial, Mass wasting, Alpine region
Degradation of ice caps has begun to alter alpine periglacial regions, resulting in the exposure of bedrock escarpments, and in the occurrences of mass-flow processes. These mass wasting processes include ice avalanches, rockfalls, slushflows, and icy debris flows, and can result in the development of poorly understood landforms known as icy debris fans. Icy debris fans are formed at the base of escarpments along the margins of valley glaciers. According to a 2013 study in New Zealand, 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. Subsurface information on icy debris fans has allowed for the relationship of these mass-wasting landforms and their underlying valley glacier to be better understood. Ground penetrating radar (GPR) profiles allow subsurface geometries and architectures to be interpreted, which in turn allows estimates of fan volumes to be made, as well as provides a better understanding of icy debris fans' process evolution over timeÂ¿all important in understanding their contribution to valley glacier budgets. Noninvasive 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. RTK-GPS coordinate and elevation data was used to document the surface topography of icy debris fans. Interpretation of GPR surveys provides information on the sedimentary architecture of the fans, which can be compared to surface deposits. Integrating surface measurements with subsurface information allows documentation of the volume of fan deposits through time. Six CMPs and 15 profiles ranging between 25-210 m long were collected in the distal half of four fans along the La Perouse and Douglas Glaciers. Semblance analyses of CMP soundings on the East Fan of La Perouse Glacier 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 elevations allows for the interpretation of elevations for subsurface stratigraphic interfaces, and cross-sectional analysis of reflections. The GPR 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. GPR interpretations indicate a transition area between the presence of valley glacier underlying the icy debris fans at depths increasing from 0 m at the toe of the fan to package thicknesses of >75 m in the center of fans. This transition area occurs beneath the package of primary reflectors in GPR profiles, where subsurface material is more homogenous in nature, and indicates a transition to glacial ice. GPR studies are important in understanding how icy debris fans contribute ice and lithic material annually to the glacial budget. These investigations provide subsurface geometries to be interpreted, allowing estimates of fan volumes to be made. These estimates may be very significant, especially for glaciers that are not connected to upslope glaciers. Understanding of fans' contributions to valley glacier budgets is also important as icy debris fans serve a method for monitoring study sites that will likely degrade if climate change progresses as forecasted.
Rubino, Erica May, "Geophysical Investigation of Icy Debris Fans With Ground Penetrating Radar, Southern Alps, New Zealand" (2015). Honors Theses. 327.