Modeling the tor-dotted crests, bedrock edges, and parabolic profiles of high alpine surfaces of the Wind River Range, Wyoming

TitleModeling the tor-dotted crests, bedrock edges, and parabolic profiles of high alpine surfaces of the Wind River Range, Wyoming
Publication TypeJournal Article
Year of Publication2002
AuthorsAnderson RS
Date PublishedJul
ISBN Number0169-555X
Accession NumberISI:000176679400003

Broad, low curvature surfaces common to many alpine landscapes, and particularly well expressed in the Laramide ranges of the western USA, appear to be roughly in steady state with respect to the current surface processes. Uniform, thin (order I m) regolith covers the crystalline bedrock with the exception of occasional tors at ridgecrests. The surfaces are commonly edged by bedrock lips, at the tops of cliffs that bound the intervening glacial troughs. Regolith production is low (order 10 mum/year) on bare bedrock and reaches a maximum under tens of centimeters of regolith. All of these characteristics may be accounted for with a numerical model that is faithful to the periglacial surface processes presently operating on these surfaces. Hillslope transport is modeled assuming the dominant process is frost creep; transport efficiency depends linearly upon local slope and appropriately vanishes in the absence of regolith. Frost frequency scales the rate of motion, while the ratio of frost penetration depth to regolith thickness dictates the degree to which the process is curtailed by available regolith. Initial conditions include initial mean slope and bedrock roughness and a uniform regolith cover. Boundary conditions are imposed by dictating that the edges of the calculation space be lowered at a rate appropriate for glacial incision. Broad convex surfaces emerge over time scales of order 1 million years. All surfaces rapidly decouple from the bounding glacial troughs, as the high negative curvature at the edges disallows accumulation of regolith. These bedrock edges lower at a few microns per year, while the troughs lower at many times this rate. As the bedrock edges, therefore, serve as the effective boundary condition for the remaining surface, the surfaces operate independently of both lowering and widening of the glacial troughs. For a reasonable set of transport and weathering rates, the final steady-state landscape is sensitive to the initial condition. In particular, the steeper the initial landscape, the more likely tors are to develop, as the high curvature disallows accumulation of regolith. These tors, in turn, limit the lowering rate of the crest, and the landscape retains significant local relief Low relief initial conditions more likely lead to parabolic surfaces that lack tors. That tors are more likely on the crests reflects the high initial curvature at the crest. Tors on the side slopes are eliminated. The present high surfaces in the Laramide ranges of the western USA imply operation of periglacial surface processes over millions of years. The occasional lots imply a steeper rather than flatter initial landscape. The strong decoupling of surfaces from glacial troughs justifies their use as local geomorphic markers that allow calculation of relief production since glacial incision was initiated. On the other hand, their present low curvature does not require a landscape history in which these surfaces were once joined as an initially widespread erosion surface. (C) 2002 Elsevier Science B.V. All rights reserved.

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