HR: 09:45h
AN: T11G-08    [Abstracts]
TI: Erosional Reduction of an Orogenic Wedge: Structural Response to Neogene Climate Change within the St. Elias Orogen, Alaska
AU: * Berger, A L
EM: alberger@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24060, United States
AU: Spotila, J A
EM: spotila@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24060, United States
AU: Chapman, J B
EM: jaychapman.v@gmail.com
AF: U. Texas El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Pavlis, T L
EM: tlpavlis@utep.edu
AF: U. Texas El Paso, Department of Geological Sciences, El Paso, TX 79968, United States
AU: Enkelmann, E
EM: eva.enkelmann@lehigh.edu
AF: Lehigh University, Earth and Environmental Sciences Department, Bethlehem, PA 18015, United States
AU: Buscher, J T
EM: jbuscher@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24060, United States
AB: The kinematics and architecture of orogenic systems may be heavily influenced by climate, but little research has focused on the long term effects of glacial erosion on orogenesis. Apatite and zircon (U-Th)/He thermochronometry on >75 bedrock samples across the St. Elias orogen, one of the best examples of a glaciated orogenic wedge, is the basis for a new kinematic model and demonstrates an association between glacial denudation and orogenic architecture. The spatial pattern of low temperature cooling indicates that exhumation and deformation are focused within a thin-skinned fold and thrust belt on the windward flank, whereas the leeward flank of the orogen functions as a deformational backstop. A previously unrecognized structure beneath the Bagley ice field must separate these domains with south-side-up motion. We propose this structure is a backthrust making the orogen doubly-vergent. Suggestive of accelerated backthrust motion in response to climate change, cooling rates within the hanging wall block and across the entire windward flank of the orogen accelerated ten-fold coeval with enhanced glaciation. As backthrust motion increased, glacial unroofing also coincided with a regional shift in deformation away from prominent forethrusts including the North American-Yakutat terrane suture (Chugach St. Elias fault) and the seaward deformation front (Pamplona zone). Across the windward flank of the orogen, exhumation, at rates of up to 5 mm/yr, is focused within a narrow zone, where the glacial equilibrium line altitude (ELA) intersects the orogenic wedge. This zone of rapid exhumation, not present prior to the onset of enhanced glaciation, cuts across the structural trend of the orogen and is more narrowly focused than orographic precipitation. Accelerated denudation at the ELA thus appears to have redistributed strain along a series of forethrusts that lie at the zone of heaviest glacial flux, while the backthrust progressively truncates the southward-vergent forethrusts. In a cause and effect response, the expansion of glaciers therefore appears to have resulted in a narrowing of the orogenic wedge due to increased backthrust motion and a landward propagation of deformation in order to preserve topographic slope. This focusing of long- term glacial erosion and deformation at the ELA matches predictions from analytical models of orogenic wedges (i.e. Tomkin and Braun, 2002) and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.
DE: 1140 Thermochronology
DE: 8005 Folds and folding
DE: 8104 Continental margins: convergent
DE: 8107 Continental neotectonics (8002)
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: 2007 Fall Meeting