The Cretaceous-Tertiary Deformation of the Lhasa Block and its Implications for Crustal Thickening in Tibet

The strong Tertiary deformation of the northern margin of the Indian plate and of the Tsangpo suture zone is not matched by comparable deformation within the Lhasa block, yet the southern margin of the block was the site of large-scale intrusive activity (the Gangdese, or Transhimalayan, batholith) throughout the late Cretaceous and was, therefore, presumably weaker than its surroundings. This poses a problem for any approach that interprets the deformation of Tibet in terms of diffuse crustal thickening. However, there is evidence that the Lhasa block was, in elevation as well as in magma type, an Andean margin by the end of the Cretaceous, and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern Lhasa block. This suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast. When subjected to boundary conditions representing the collision between India and Asia, the sheet accommodates convergence by diffuse deformation over a region comparable in size with the Tibetan plateau; in the absence of an elevation contrast this deformation consists of approximately 100% thickening strain over the region. A precollision elevation contrast of 3000 m in the Lhasa block would have resulted in its experiencing roughly 40%, rather than 100%, postcollisional thickening; smaller elevation contrasts would have resulted in greater thickening, and vice versa. These calculations have implications for the mode of crustal thickening in northern Tibet: As the available evidence suggests that the topographic expression of the earlier Mesozoic tectonic activity in southern Asia was erased before the India-Asia collision, we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the Lhasa block during the Tertiary.