HR: 14:10h
AN: T33D-03 INVITED     [Abstracts]
TI: Testing Glacial Limits to Mountain Building: The Buzz Saw in the Chugach/St. Elias Range, Alaska
AU: * Spotila, J A
EM: spotila@vt.edu
AF: Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Meigs, A J
EM: meigsa@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 United States
AB: As effective agents of erosion, glaciers act as an "ultimate" erosional control on orogenic development. Widespread alpine glaciation occurs when a significant fraction of a mountain range lies above a local elevation threshold for glaciers, an occurrence which, by itself, implies fluvial systems are locally incapable of limiting peak elevation and relief in mountain belts. The glacial buzz saw hypothesis states that glaciers impose a limit to orogenic mean elevation, via feedback loops between topography, erosion rate, and glacial equilibrium line altitude (ELA). If rock uplift exceeds fluvial exhumation, mean elevation can rise to the threshold at which glaciers form. In a buzz saw world, glacial erosion and associated mass wasting and sediment transport are hyper effective, thus limiting further surface uplift. This implies that ELA, which varies in space and time depending on latitude, mean annual temperature, and precipitation, fundamentally limits topography and thereby influences orogenesis. Empirical tests of the glacial buzz saw hypothesis have mainly focused on topography, including comparisons of mean elevation, ELA, slope distribution, and relief. Although applicable to some ranges, not all active, glaciated mountain belts meet the predictions of the hypothesis. An alternate approach is to compare orogenic flux rates. If glacial erosion can keep pace with tectonic rock uplift, topography should be steady state, exhumation should equal rock uplift, and the rate of tectonic influx should be equal to the rate of erosional efflux at the orogen scale. Testing for a steady-state flux, however, is difficult, requiring absolute comparisons between disparate data sets. In the Chugach/St. Elias Range of southern Alaska, erosional efflux varies over the timescale of measurement. Long-term exhumation averages ~1-2 mm/yr, but this estimate is plagued by thermochronometric errors and uncertainties in geothermal conditions along crustal trajectories. The tectonic influx is more poorly constrained. Estimates of total volumetric influx can be made based on approximate dimensions of the colliding Yakutat microplate and rate of convergence. However, there are large uncertainties in crustal thickness, crustal composition, degree to which convergence is accommodated by accretion, and distribution of plate motion on individual structures. Based on existing data, inferring steady-state flux is permissible for this range, consistent with predictions of the buzz saw hypothesis. Yet, the uncertainties of this comparison demonstrate the difficulty of quantifying the coupling of erosion and tectonics in field settings. More data are required to further calibrate the applicability of the glacial buzz saw hypothesis in southern Alaska.
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting