MORPHOLOGY AND DYNAMICS OF ICY DEBRIS FANS: LANDFORM EVOLUTION ALONG DEGRADING ESCARPMENTS UNDERGOING RAPID DEGLACIATION IN ALASKA AND NEW ZEALAND

Impacts of recent climate warming have been significant in alpine glaciated regions. Valley glaciers formerly linked to icecaps have decoupled at icefalls, exposing escarpments and generating a suite of dynamic landforms dominated by mass wasting. Ice-dominated landforms, termed icy debris fans, develop rapidly by ice avalanching, rockfall, and icy debris flow. Reconnaissance studies in the Wrangell Mountains, Alaska and Southern Alps, New Zealand, provide the basis for a morphogenetic model of spatial and temporal evolution of icy debris fans. The influence of these processes on landform evolution has been largely unrecognized in descriptions of rapid post-glacial landform adjustment. Within a few years of glacier decoupling, icy debris fans form along the base of bedrock escarpments and prograde onto valley glaciers. The presence of a distinct catchment, apex, and fan distinguishes these landforms from amorphous reconstituted glaciers. Ice avalanching is the most abundant process in icy debris fan formation. Typically, avalanches fall into fan catchments, transform into grain flows, and flow onto the fans. Once on the fans, deposits ablate rapidly, flatten and concentrate lithic fragments. Icy debris fans may thicken to become glaciers with splay crevasse systems. Where fan catchments are deeply incised, avalanche sediments are temporarily stored in bedrock basins, along with rockfall debris. Episodic outbursts of icecap meltwater mix with sediments and mobilize icy debris flows to the fans. Most icy debris flows contain >90% lithics but also include rounded ice clasts. Catchment morphology dictates spatial variability and dominance of depositional processes. Steep, small catchments expedite delivery of ice avalanches to the fan and generate large ice-dominated fans. Larger, complex catchments result in lower depositional frequency on fans but more icy debris flows and fewer avalanches. As ice supply wanes, fans rapidly downwaste and evolve into talus cones with ephemeral ice avalanches. The evolutionary cycle of these landforms probably occurs during an early paraglacial interval (i.e. 10-100 yrs). Observations comparing avalanche frequency, volume, and morphologic evolution at the Alaska site from 2006 to 2010 illustrate complex response between icy debris fans even within the same cirque.