Subsurface architecture of alpine icy debris fans: Integration of ground-penetrating radar and surface observations in Alaska and New Zealand

Abstract Icy debris fans (IDFs) are extremely dynamic supraglacial landforms at the mouths of bedrock catchments between valley glaciers and icecaps. Recent studies quantified the nature, pace, and volume of mass flow processes contributing ice and sediment to IDFs by integrating field observations, drone and time-lapse imagery, and terrestrial laser scanning. New geophysical data presented herein characterize the subsurface architecture of IDFs along the McCarthy Glacier in Alaska and the Douglas, La Perouse, and Mueller Glaciers in New Zealand. Ground Penetrating Radar (GPR) profiles and soundings from field surveys during 2013–2015 provide stratigraphic evidence of the following subsurface processes important in the delivery of ice/sediment through IDFs to glaciers: (1) transitional merging of fan IDF deposits with valley glacier ice in cirque settings, (2) progressive rotation (slumping) of IDF deposits preserved in the hanging walls of downfan-dipping crevasses/faults, (3) progradation of IDF deposits onto valley glaciers, and (4) buttressing of IDF deposits along the margins of valley glaciers. The GPR-based variations in subsurface architecture/processes are attributable to previously documented variations in ice/sediment supply from overlying icecaps/neves and topographic interaction with valley glacier junctions. GPR profiles document major depositional units separated by prominent reflectors, interpreted as annual ablation lag surfaces where lithic sediments are relatively concentrated. Most IDFs contain less than five depositional units, which may reflect the transitory nature of ice/sediment in these dynamic landforms. The uppermost depositional package on IDFs revealed by GPR is interpreted as deposition from the year preceding the GPR survey because it appears unconsolidated and lacks diffractions common in ice imaged in older (lower) units. The thickness of the upper package is typically less than or equal to the thickness of new deposits recorded by time-lapse cameras spread evenly over fan surfaces, which is expected accounting for compaction and ablation. The new geophysical datasets, together with previous surface observations, bear directly on managing hazards and evaluating glacial budgets in alpine regions. In particular, documentation of progressive rotation, buttressing, and progradation in IDF subsurface stratigraphies indicates protracted episodes of landform instability during deglaciation, consistent with previous surface observations documenting exceptionally high rates of depositional resurfacing and slumping. GPR-based evidence for subsurface deformation suggests ice/sediment is transferred to valley glaciers through IDFs. This interpretation is supported by previous visual evidence, including surface sliding/glacial flow and exceptionally high rates of deposition on IDFs coeval with little change in fan size. Integrating the role of IDFs in glacier budget models may result in forecasting a lower rate of deglaciation than traditionally recognized for some glaciers decoupled from icecaps.