HR: 1340h
AN: T43C-1337    [Abstracts]
TI: Shortening calculations at the western end of the Indo-Asian collision zone: Implications for the evolution of the Tibetan orogenic belt
AU: * Robinson, A C
EM: robinson@ess.ucla.edu
AF: Department of Earth and Space Sciences and Institute of Geophysics and Planetary Sciences, University of California, Los Angeles, Los Angeles, CA 90095 United States
AU: Yin, A
EM: yin@ess.ucla.edu
AF: Department of Earth and Space Sciences and Institute of Geophysics and Planetary Sciences, University of California, Los Angeles, Los Angeles, CA 90095 United States
AB: One of the most striking aspects of the Indo-Asian collision zone is the asymmetry of thickened and elevated crust north of the Indian continent, increasing in north-south width from $\sim$500 km in the Pamir/Western syntaxsis region, to $\sim$1,500 km along the central and eastern Tibetan Plateau. Determining whether this asymmetry is the result of east-west variation in the magnitude of convergence between India and Asia, or other processes and boundary conditions, is critical in understanding the evolution of the orogenic belt. To address this, we calculated the amount of shortening for the western end of the Indo-Asian collision zone by area balancing a cross-section along a north-south line at 73$\deg$E. Crustal thicknesses were interpreted to be $\sim$40 km thick for the Indian plate, increasing to $\sim$70 km beneath the Pamir-Karakoram-Kohistan region based on previously published studies. Our results yield a minimum of 735 km of north-south shortening partitioned between four processes: 1) 295 km of northward underthrusting of the Indian continent beneath the southern margin of Asia based on $\sim$200 km of upper crustal shortening within India (e.g. DiPietro and Pogue, 2004), and a minimum of 95 km of Early Eocene subduction along the Main Mantle Thrust (based on Early Eocene coesite bearing eclogites). 2) $\sim$130 km of internal shortening and crustal thickening within the Karakoram and Pamir, assuming a pre-Tertiary crustal thickness of 35 km. 3) ~280 km of southward subduction of Asian lithosphere (Tarim) beneath the Pamir to a depth of ~200 km. 4) ~30 km of crustal shortening and thickening in the Tian Shan. Results of this study have two primary implications: 1) 295 km of underthrusting suggests that Indian lithosphere has been inserted beneath the Pamir/Western syntaxsis region as far north as the southern Pamirs (37.5$\deg$N). Therefore, $\sim$2/3 of the thickened crust beneath the Pamirs/Western Syntaxsis region can be explained by underplating of Indian lithosphere, suggesting only $\sim$130 km of internal north-south shortening. 2) The total magnitude of shortening at the western end of the collision zone is similar to estimates of the amount of underthrusting of India beneath the Tibetan Plateau along the central Himalayas ($\sim$700 km, DeCelles et al., 2002). Assuming Cenozoic shortening north of the Indus Suture Zone of several hundred km, these results indicate a westward decrease in the total amount of convergence between India and Asia.
DE: 8102 Continental contractional orogenic belts
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting