TOPOGRAPHIC PROFILES OF ALPINE DEBRIS FLOWS IN THE CENTRAL FRONT RANGE, COLORADO: AN ANALOG FOR ‘MOJAVE' CRATER FANS ON MARS

In the Xanthe Terra region of Mars, 500-m long fan-shaped landforms associated with the walls of 60-km diameter ‘Mojave' Crater are rare among high-resolution Mars Orbiter Camera (MOC) images. Although channelized, dry mass movement may have formed these fans, the topographically controlled bifurcating pattern of tributary channels and the extensive dissection on the fan argues for a surface constrained fluid erosional process. The purpose of this investigation is to obtain accurate topographic measurements of terrestrial alluvial fans that will provide criteria for further evaluating hypothesized formation processes (debris versus fluvial flows) of the ‘Mojave' Crater fans.

Over 480 alpine debris flows were triggered by a 43 mm precipitation event on July 28, 1999, in the central Front Range, Colorado. In the deposits examined, talus was mobilized by overland flow concentrated in steep bedrock-lined channels (i.e. firehose effect) or by a system of coalescing rills; these initiation mechanisms are terrestrial analogs for transport of ejecta material down crater walls due to atmospheric precipitation, one hypothesized formation scheme for the ‘Mojave' Crater fans.

Longitudinal profiles for four alpine debris flow deposits in the central Front Range of Colorado were acquired using Differential Global Positioning System (DGPS). The Trimble R8 Total Station and roving receiver yields topographic data with 4 cm horizontal and vertical accuracy. These data compliment an existing dataset of topographic profiles for alluvial fans in southwestern California documenting the morphology of deposits from a range of formation processes. Consistent with our earlier results, we find the power law regression of upstream slope versus distance plot is the preferred approach for assessing concavity; the best-fit exponential, termed the concavity index, is a measure of the magnitude of concavity. The alpine debris flows have a concave-upward longitudinal profile and relatively large concavity index values (0.4-0.75), similar to those determined for fluvially-formed fans. In contrast to the shallow slope of fluvial alluvial fans, the alpine debris flows have steep average radial slopes (9-15°). Preliminary results suggest that the combination of slope and concavity index is a signature of flow process.