HR: 14:25h
AN: T33D-04 INVITED     [Abstracts]
TI: Contribution of Orography, Structure, and Geography to Deformation, Exhumation, and Topography of an Active Glaciated Collisional Orogen
AU: * Meigs, A J
EM: meigsa@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97330 United States
AU: Spotila, J
EM: meigsa@geo.oregonstate.edu
AF: Virginia Polytechnic Institute and State University, Department of Geological Sciences 4044 Derring Hall, Blacksburg, VA 24061 United States
AB: Accretion of the Yakutat terrane (YT) to North America (NA) across the Chugach- St. Elias thrust fault (CSE) is forming the Chugach/St. Elias Range in southern Alaska. Glacial erosion has accompanied development of the orogenic belt over the last $\sim$6 My. A windward southern and leeward northern flank defines the orographic configuration across the range. Glacier redistribution on the windward side of the range is large (10's of km up- and down-valley ) relative to the leeward side of the range during Holocene glacial cycles (10$^{3}$ yr ). Glacier distribution in the landscape in space and time affects the proportion of landscape covered by glaciers, which in turn modulates base level for hillslopes, rivers in unglaciated portions of valleys, and sediment production, routing, storage, and delivery out of the orogen. Within the windward flank, low-T cooling ages imply a progressive increase in exhumation (from $<$1 mm/yr to $\sim$2 mm/yr) northward across strike from the deformation front on the south to the CSE on the windward flank. The magnitude of exhumation is smaller on the leeward side of the range relative to the windward flank. Orography thus controls the magnitude and frequency of glacier coverage within the orogen, which appears linked to larger degree of exhumation of the windward relative to the leeward flank of the range. Whereas YT-NA convergence velocity is $\sim$40 mm/yr, restoration of an area-balanced cross-section indicates $\sim$55 km of internal shortening has been accommodated within the YT on the windward flank near the eastern edge of the collision. Assuming this internal deformation accumulated after 6 Ma, $\sim$25% of the YT tectonic influx has accreted at $\sim$10 mm/yr onto the NA upper plate. Progressive exposure of deeper structural levels constrains erosional denudation from $<$ 5 km to $>$ 7 km from south-to-north, respectively. Structural variables such as plate interface dip, detachment stratigraphic locations, and convergence obliquity control the relatively small tectonic influx, shortening distribution, and exhumation and rock uplift rates. Mean elevation corresponds closely with the modern equilibrium line altitude of glaciers (ELA). The relatively low mean elevation of the range (2500 to 1100 m) reflects the 60$^{\circ}$ latitude and maritime setting of active deformation. Geography apparently dictates the dominant erosional process and topographic amplitude. Together, orography controls long-term exhumation magnitude because of the contrast in glacier distribution in space and time between the windward and leeward flanks, structure paces long-term exhumation rates because they are linked to rock uplift patterns, and geography determines erosion process and topographic form of this active orogenic belt.
DE: 8107 Continental neotectonics
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1854 Precipitation (3354)
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting