A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...

Geologic Evolution of the Himalayan-Tibetan Orogen

Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

Gondwana dispersion and Asian accretion: Tectonic and palaeogeographic evolution of eastern Tethys

http://www2.ess.ucla.edu/~yin/05-Publications/papers/091-Yin-2006-ESR.pdf

Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation

Summary Field studies of active faulting in S Tibet indicate that Quaternary extension has been taking place at a rate of ≃1 cm yr−1 in a direction of ≃ 100°. This implies that underthrusting in the Himalayas now absorbs less than half of the total convergence between rigid India and Asia, the rest being taken up primarily by strike-slip faulting N of the collision belt. En échelon right-lateral, strike-slip faults in S Tibet now allow this corresponding eastward displacement of the plateau with respect to India. The reproducible pattern of faulting obtained from plane-strain indentation experiments on unilaterally confined blocks of plasticine suggests that this extrusion process has occurred during most of the collision history. The Tertiary geological record in SE Asia corroborates a polyphase extrusion model, with displacements in excess of 1000–1500 km, in which India has successively pushed Sundaland, then Tibet and S China towards the ESE. Most of the Middle Tertiary movements may have occurred along the then left-lateral Red River-Ailao Shan Fault Zone, together with the opening of most of the eastern S China Sea. Regional geology, stratigraphy and deformation observed in Yunnan are consistent with this inference, as well as the timing, geometry and rates of sea-floor spreading in the S China Sea. Fast spreading (5 cm yr−1) in that sea implies that the Tibetan highlands formed mostly after 17 Ma BP. Sideways movements can also account for the existence of large, conjugate but asymmetric, Tertiary strike-slip faults within Sundaland and the formation of Middle Tertiary pull-apart and rift basins on the Sunda Shelf. Changing directions of opening are predicted in the Mergui and Andaman Basins and the lowlands of Burma, as well as large right-lateral displacements along the Shan Scarp. Most of Sundaland probably lay initially in a frontal position with respect to impinging India and the Shan Plateau may have been a Middle Tertiary analogue of the present Tibetan Plateau. In contrast with dominant overthrusting in the Himalayas, Tertiary strike-slip faulting, with more subordinate folding and thrusting, appears to have been important along and N of the Zangbo Suture. This difference must be accounted for in all models of formation of the Tibet Plateau. The surface of the indentation mark, left by the impaction of India onto the presumably simpler Early Tertiary margin of Asia (> 6 million km2), implies that mountain building and strike-slip faulting have absorbed, perhaps alternately, roughly equal amounts of collisional shortening. Since analogous interplays of extrusion and thickening probably govern the evolution of most collision zones, the Tertiary tectonics of Asia may be the best guide to unravel the interactions between Palaeozoic and Precambrian plates, for which sea-floor spreading constraints are unattainable.

On the mechanics of the collision between India and Asia

The 1981 French–Chinese expedition to Tibet focused on the Lhasa block, extending earlier coverage 400 km north of the Tsangpo suture. The Lhasa block stood between 10 and 15° N latitude over most of the Upper Cretaceous and Eocene and, if Gondwanian in origin, had detached from Gondwana by early Permian. Seismic profiles reveal a complex Moho topography resulting both from multiple continental thrusting and large-scale strike-slip faulting. Subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated. This study emphasizes the role of smaller blocks in the accretion of the continental mosaic.

Structure and evolution of the Himalaya–Tibet orogenic belt

Results are reported from the 1980 joint French–Chinese field expedition in Tibet. The area covered was from the High Himalaya in the south, to the region of Nagqu ∼250 km north of Yangbajain. Ophiolites in the Zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the Gangdise granodiorite belt in the late Mesozoic. The ophiolites were thrust to the south onto the Cretaceous melange and Triassic flysch but the age of this event is unclear. In the Tertiary, sediments of the Indian margin and the Xigaze basin were folded with steeply dipping cleavage, which indicates some north–south shortening of the crust. However, the Tertiary volcanic sequence on the Lhasa block and further north shows comparatively little folding and thrusting. In the Quaternary, deformation of Tibet was characterized by east–west extension.

Den Wanming

Papers

Structure and evolution of the Himalaya–Tibet orogenic belt

The Tibetan side of the India–Eurasia collision