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Project Summary
Personnel:
Simon Brocklehurst (Ph.D.)
Most mid- and high-latitude mountain ranges
have been sculpted dramatically by glacial erosion. The isolated
spires, narrow ridges, long bare-rock hillslopes, and broad valleys of
alpine terrain immediately give a qualitative impression that glaciers
are highly efficient erosive agents and that glacial erosion produces
considerable relief. However, little is known quantitatively about the
rates and patterns of glacial erosion, the factors that control them,
nor even the morphological consequences of a shift from fluvial
(non-glacial) to glacial erosion. This lack of knowledge is unfortunate
given the broad interest in developing a quantitative understanding of
the long-term evolution of topography in orogenic belts, and the
associated geodynamic and thermo-mechanical consequences of the rates
and patterns of erosional unloading (e.g. Koons, 1989; Molnar and
England, 1990; Beaumont et al., 1992; Isacks, 1992; Hoffman and
Grotzinger, 1993; Merritts and Ellis, 1994; Koons, 1995; Beaumont et
al., 1996; Batt and Braun, 1997). In this context, the potential for
strong dynamic coupling between climatic and tectonic processes has
sparked intense cross-disciplinary interest and debate (e.g., Raymo et
al., 1988; Hicks et al., 1990; Molnar and England, 1990; Hallet et al.,
1996; Brozovic et al., 1997). The issue of whether or not glaciers
primarily deepen valleys and increase topographic relief is central to
this debate and yet has never been studied in a systematic
way.
Given the many unknowns in the glacial
erosion problem (e.g., Hallet, 1979; Oerlemans, 1984; Hooke, 1991;
Harbor, 1992; Hallet, 1996; MacGregor et al., 1998) perhaps the
greatest challenge lies in the discovery of well-posed, tractable
problems directly relevant to the issue of the landscape-scale effects
of glacial erosion. One promising approach that we propose here is to
exploit natural laboratories at the southern limit of glaciation where
adjacent fully-, partially-, and un-glaciated, but otherwise similar,
basins can be studied. In the right field settings one may estimate
quantitatively the amount and pattern of topographic change
attributable to glacial erosion alone as a function of the intensity of
glaciation. We identify two such field settings in the eastern Sierra
Nevada, California, and the northern Sangre de Cristo range,
Colorado.
We propose a research program in these two
field settings combining field mapping, DEM analysis, and cosmogenic
isotope dating to test quantitatively 5 hypotheses developed during a
preliminary investigation of glacial landforms and erosion:
- Relief
production by glaciers involves (only)
lengthening hillslopes, widening valleys, and the formation of hanging
valleys
- Relief
produced by these mechanisms scales with
ice thickness (thus severely limiting total relief production)
- Glacial
erosion was less efficient below the
late-glacial equilibrium line altitude (ELA)
- Sub-ridgeline
relief increases are counteracted
by an overall lowering of peaks and ridges (due in part to the
apparently greater efficiency of glacial erosion above the ELA)
- Joint
spacing importantly influences the height and location of hanging valley
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