HR: 10:35h
AN: T12D-02    [Abstracts]
TI: The role of crustal strength variations in shaping collisional orogens
AU: * Cook, K L
EM: klcook@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Ave., Cambridge, MA 02139, United States
AU: Royden, L H
EM: lhroyden@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Ave., Cambridge, MA 02139, United States
AB: Continental crust is often composed of regions with different lithologies, histories, thermal structures, and pre- existing anisotropies. Such large-scale variations in crustal properties are likely to play an important role in defining the mode and localization of crustal deformation in a continental collision zone. Using a three- dimensional semi-analytical numerical model of deformation in a viscous crust, we investigate the effects of lateral heterogeneities in the strength of both the upper and lower crust on the patterns of uplift and surface deformation in collisional orogens. The model includes a two layer crust and allows for flow of a weak lower crust as well as lateral and temporal variation of viscosity in both the upper and lower crust. Model results indicate that variations in crustal strength have a dramatic effect on the morphology and dynamics of a developing orogenic plateau, and influence not only the location and steepness of plateau margins, but also the transport of material throughout much of the plateau. As a developing plateau encounters regions of strong crust, the adjacent plateau margin becomes steep, remains localized along the boundaries of the strong crustal block, and is commonly concave in map view. These margins do not accommodate significant shortening strain, and velocities in both the upper and lower crust are generally directed subparallel to the plateau margin. In contrast, a weak crustal region develops a gently sloping margin that propagates rapidly across the weak zone and accommodates large amounts of shortening. Crustal material within the plateau moves towards the low- strength region, with rapid flow towards and across the low-gradient plateau margin. The contrasting styles of deformation and morphology that accompany these lateral strength variations enable us to recognize crustal strength variations in actual orogens. The morphology of the northern and eastern margins of the Tibetan plateau are consistent with the presence of strong crust in the regions of the Sichuan and Tarim Basins and weak crust beneath the southeastern margin. With a relatively simple distribution of strength variations, our model is able to reproduce much of the large-scale topography and surface motions of Tibet, particularly in eastern Tibet. Analysis of model results and plateau morphology suggests that variations in crustal strength have played a fundamental role in shaping the plateau and have likely influenced the development of major structures such as the Altyn Tagh and Xianshuihe Faults.
DE: 8104 Continental margins: convergent
DE: 8108 Continental tectonics: compressional
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting