Large-scale crustal deformation of the Tibetan Plateau

The topography, velocity, and strain fields calculated from a three-dimensi onal Newtonian viscous model for large-scale crustal deformation are genera lly in good agreement with results from geological, geodetic and earthquake studies in and around the Tibetan Plateau, provided that the model theolog y incorporates a weak zone within the deep crust beneath the plateau (equiv alent to a viscosity of 10(12) Pa s within a 250-m-thick channel or 10(18) Pa s within a 15-km-thick channel). Model studies and observations show a p lateau at similar to5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins. Rapid shortening strain is conce ntrated along the lower portions of the northern and southern plateau margi ns (at rates similar to 20 mm/yr). Model results show north-south shortenin g (similar to 10 mm/yr) in reasonable agreement with GPS data (5-8 mm/yr of north-south shortening across the northern two thirds of the plateau) and east-west stretching (10-15 mm/yr) across the eastern half of the high plat eau, in reasonable agreement with seismic, geologic, and GPS data. Upper cr ustal material moves eastward from the plateau proper into a lobe of elevat ed topography that extends to the south and east. Clockwise rotation of mat erial around the east Himalayan syntaxis (at rates up to similar to 10 mm/y r) occurs partly as a result of dextral shear between Indian and Asian mant le at depth and partly as a result of gravitational spreading from the high plateau to the south and east. There is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower cru st, except in southern and northern Tibet where margin-perpendicular extens ion occurs only for the case of an extremely weak lower crust. Our results suggest that the Tibetan Plateau is likely to have gone through a two-stage development. The first stage produced a long, narrow, high orogen whose he ight may have been comparable to the modern plateau. The second stage produ ced a plateau that grew progressively to the north and east. East-west stre tching, eastward plateau growth and dextral rotation around the east Himala yan syntaxis probably did not begin until well into the second stage of pla teau growth, perhaps becoming significant after similar to 20 m.y. of conve rgence.