DEFORMATION AND MOTION ALONG THE SOUTHERN MARGIN OF THE LHASA BLOCK (TIBET) PRIOR TO AND DURING THE INDIA ASIA COLLISION

Field studies in the Lhasa block (central and eastern S-Tibet, long. 88-degrees to 95-degrees-E and lat. 29.3-degrees to 30-degrees-N) demonstrate N-S shortening with coeval and consecutive E-W extension and wrenching. Cretaceous, pre-collisional deformation was both coaxial and non-coaxial with top-to-S displacement in central S-Tibet and top-to-SE displacement in eastern S-Tibet. Post-Eocene, probably Miocene shortening as calculated from restored sections across the Qiuwu molasse and the Xigaze forearc basins caused greater-than-or-equal-to 31% N-S contraction by folding and greater-than-or-equal-to 65% by folding, faulting and ductile deformation, with significant coeval E-W extension. Reduced stress tensors calculated from fault-striae data yield sigma1 trending subhorizontally 010-degrees(+/- 15-degrees) and sigma2 trending subhorizontally 101-degrees (+/- 17-degrees, azimuths of 22 sites in central S-Tibet). Sigma2 is tensional. Post-Eocene structures related to N-S shortening are overprinted by structures indicating E-W compression in eastern S-Tibet (sigma1: 068-degrees +/- 11-degrees, sigma3: 155-degrees +/- 16-degrees, 6 sites) and by distributed deformation involving rifting and predominantly dextral wrenching (sigma3: 40-degrees +/- 17-degrees, 9 sites in central S-Tibet; 075-degrees +/- 11-degrees, 6 sites in eastern S-Tibet). The eastern syntaxis forms an anticline (axis: 011-degrees/61-degrees), shows dextral-oblique normal faulting between Indian-plate and Lhasa-block rocks, and is cut in the NE by the active Karakoram-Jiali-Parlung fault zone. We discuss the geodynamics of the southern Lhasa block using all the data available: Pre-collisional deformation resulted from shortening during northward subduction and magmatic activity in an Andean-type continental margin. A major phase of exhumation in the early Miocene is correlated with major Tertiary folding and faulting. Associated strain accumulated pre-dominantly coaxilly, involved (major) subvertical and (minor) subhorizontal stretching, and reflects both thickening of Tibetan crust and bulk orogen-parallel (E-W) stretching. E-W extension is concentrated along distinct zones which coincide with margins of tectonostratigraphic units. The coaxially deformed part of S-Tibet is probably bound to the S by the Miocene, dextral-oblique North Himalayan (normal) Fault Zone. This zone connects the western and eastern syntaxes and allowed the Miocene deformation of the Himalaya-Tibet system to be partitioned into a Himalayan zone thickened by simple-shear stacking and a Tibetan zone thickened by predominantly coaxial flattening-type shortening. At present strong eastward flow of the area S of the Karakoram-Jiali-Parlung fault zone is inhibited by pinning at the eastern syntaxis. The E-W compression in eastern S-Tibet results from the ''dead comer'' W of the syntaxis. Recent strike-slip faulting truncates the syntaxis and will probably relieve the locking of S-Tibet.