HR: 1330h
AN: T43D-03    [Abstracts]
TI: Seismicity, Tectonics, and Lithospheric Structure of the Tibetan Plateau
AU: * Langin, W R
EM: William.Langin@Shell.com
AF: Cornell University, Snee Hall, Ithaca, NY 14853 United States
AU: * Langin, W R
EM: William.Langin@Shell.com
AF: Shell Exploration and Production, 701 Poydras St. Suite 3266, New Orleans, LA 70139 United States
AU: Brown, L D
EM: brown@geology.cornell.edu
AF: Cornell University, Snee Hall, Ithaca, NY 14853 United States
AB: The Tibetan Plateau has an average elevation of nearly five kilometers above sea level and constitutes the highest and most conspicuous region of such significantly elevated topography on earth. Formation of the Tibetan Plateau commenced when, by convergence of the Indian and Eurasian lithosphere, the Indian continent began colliding with the Eurasian continent approximately 50 million years ago. This continent collision is continuing today and many aspects of the formation and evolution of the Tibetan Plateau remain unresolved, including the present-day accommodation of deformation within the plateau and the configuration of the Eurasian and Indian sub-crustal lithosphere. We present 267 local earthquake locations for central Tibet. These earthquakes exhibit both spatial and temporal clustering that may indicate swarm-like activity in the Tibetan crust. Calculated focal depths indicate that seismicity is confined to the upper crust, with only a few events occurring more than 25 kilometers below the surface of the plateau. We found no earthquakes in the lower crust or uppermost mantle. These results suggest that the middle and lower crust is aseismic and undergoing ductile deformation, thereby supporting models for elevated temperatures at and beneath mid-crustal levels. A newly-compiled catalog of nearly 900 earthquake focal mechanisms for the Himalayas and Tibetan Plateau clearly defines four discrete zones of deformation within the collision zone. These data confirm previous suggestions of thrusting along the Himalayan Arc, while indicating that deformation within southern Tibet occurs largely by normal faulting. An abrupt transition to strike-slip faulting takes place near 32° N. In the northeast margin of the plateau, thrust faulting is the primary mode of deformation. The transition from normal faulting to strike-slip faulting in the center of the plateau is correlated with a two-to-three fold decrease in the thickness of the lithosphere, suggesting that the change in surface tectonics reflects stress concentration by lithospheric thinning. A new seismically-constrained model of the gravity field within Tibet supports the underthrusting of Indian lithosphere beneath the plateau to approximately 32° N, where it descends nearly vertically into the mantle.
UR: http://www.geo.cornell.edu/geology/indepth/indepth.html
DE: 7218 Lithosphere and upper mantle
DE: 7230 Seismicity and seismotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2005 Joint Assembly