HR: 0800h
AN: V21A-0596    [Abstracts]
TI: Effect of Lateral Variations in Crustal Strength on Lower Crustal Flow in Collisional Orogens
AU: * Cook, K L
EM: klcook@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, M.I.T., 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Royden, L H
EM: lhroyden@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, M.I.T., 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Burchfiel, B C
EM: bcburch@MIT.EDU
AF: Department of Earth, Atmospheric and Planetary Sciences, M.I.T., 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: The development of large plateaus in collisional orogenic settings is thought by many to be related to ductile flow of a weak lower crust. The Tibetan plateau provides an ideal opportunity to compare predictions based on modeling of lower crustal flow to observations of surface deformation and morphology. The morphology of the eastern margin of the Tibetan plateau varies dramatically from the steep front of the Longmen Shan to the gentle poorly defined southeastern margin; it appears to be heavily influenced by the geometry of the Sichuan basin. Seismic and heat flow data, as well as the lack of deformation in the Sichuan basin, suggest that the crust beneath the basin is stronger than the crust beneath the plateau, and may act as an obstruction to flow of the lower crust. In order to investigate the interaction between crustal flow and crustal strength heterogeneities and their effect on the development of plateaus in collisional settings, we have developed a 3-dimensional numerical model of flow in a viscous crust. The model includes a two layer crust and allows for lateral variation of viscosity in the upper and lower layers. The incorporation of lateral viscosity variations simulates the presence of regions with varying crustal strength and the effects of particularly strong or weak areas on the evolution of topography. Model results indicate that variations in crustal strength have a dramatic effect on the development of a plateau in regions where lower crustal flow occurs. Deformation is diverted around stronger regions, leaving them as areas of low elevation bounded by extremely steep plateau margins. Areas of decreased initial crustal strength result in longer wavelength, low gradient plateau margins. Modeled surface velocities are comparable to surface velocities obtained with geodetic studies of Tibet, as well as to crustal motions inferred from geologic mapping in eastern Tibet. In particular, the model predicts rotation of material around the eastern Himalayan syntaxis, a zone of left lateral shear corresponding to the Xianshuihe fault, a lack of convergence in the Longmen Shan, and distributed right-lateral shear northwest of the Sichuan Basin.
DE: 8020 Mechanics, theory, and modeling
DE: 8108 Continental tectonics: compressional
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005