Spatially variable extension in southern Tibet based on GPS measurements

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Abstract

[1] We use Global Positioning System (GPS) data and kinematic block models to study the present-day deformation of southern Tibet. GPS data from 33 sites in southern Tibet and Nepal surveyed between 1991 and 2000 reveal 13 ± 2 mm/yr of N110°E extension between Lhasa and Shiquanhe (80°E–91°E), of which 9.7 ± 3.0 mm/yr represents permanent extension of the Tibetan crust. The remaining ∼3 mm/yr results from elastic deformation from the locked, curving Main Himalayan Thrust fault. This extension is spatially nonuniform. One half to two thirds of the permanent extension is concentrated across the Yadong-Gulu rift, with an opening rate of 6.5 ± 1.5 mm/yr; most of the remainder occurs on or near the Thakkola graben, with little extension across the rifts between them. Differential velocities of sites north and south of the Yarlung-Zangbo suture in western Tibet imply that the suture or an adjacent structure may be active as a strike-slip fault. A numerical model suggests that right-lateral strike slip on the Yarlung-Zangbo suture may extend from the Karakorum Fault Zone in the west at least to the Yadong-Gulu rift in the east with ∼3 mm/yr slip rate, accommodating part of the eastward extrusion of Tibet. The convergence directions inferred from GPS are consistent with slip vectors of earthquakes; however, the rate of slip of India beneath Tibet along the Himalaya is lower than those previously estimated. We estimate a slip rate of 12.4 ± 0.4 mm/yr between longitudes 83°E and 88°E and 17 ± 1 and 19 ± 1 mm/yr in the western and eastern Himalaya, respectively. The variable slip rate correlates with the variable extension rate in southern Tibet, and we suggest that it results from variation in the deformation rate of the overriding Tibetan crust. We infer that the slower convergence rate in the central Himalaya is significant.

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