SEDIMENTARY HISTORY OF THE TETHYAN BASIN IN THE TIBETAN HIMALAYAS

After an epicontinental phase, the sedimentary rocks in the Tibetan Hi malayas document a complete Wilson cycle of the Neo-Tethyan (Tethys II I) evolution between the Gondwana supercontinent and its northward dri fting margin (Lhasa block) from the Late Permian to the Eocene. During the Triassic rift stage, the basin was filled with a huge, clastic-do minated sediment wedge with up to > 5 000 in of flysch in the northern zone. Widespread deltaic clastics and shallow-water carbonates of lat e Norian to earliest Jurassic age in the southern zone mark, in conjun ction with decreasing tectonic subsidence, the transition to the drift stage. Some 4500 m of Jurassic and Early Cretaceous shallow-water car bonates and siliciclastics accumulated on the Tethyan Indian passive m argin. Deepening-upward sequences with condensed beds at their tops al ternate with repeated progradational packages of shelf sediments. Exte nsive abyssal sediments with basaltic volcanics in the northern deep-w ater zone reflect continued ocean spreading and thermal subsidence. Pa leomagnetic data, gained separately for the northern Indian plate and the Lhasa block, indicate that the Neo-Tethys reached its maximum widt h about 110 Ma ago with a spreading rate of 4.8 cm/year, before it com menced to close again. During the remnant basin stage in the Late Cret aceous and Paleogene, a shallowing-upward megasequence, capped by a ca rbonate platform, developed in the southern inner shelf realm. In the northern slope/basin plain zone, turbidites and chaotic sediments, der ived from both the acretionary wedge and the steepening slope of the p assive margin, accumulated. The depositional center of the remnant bas in shifted southward as a result of flexural subsidence and southward overthrusting. The sediments from the Triassic to the Paleogene are te ntatively subdivided into five mega-sequences, which are controlled ma inly by regional tectonics. Climatic influence (e.g., carbonate deposi tion), due to northward plate motion, is partially subdued by terrigen ous input and/or increased water depth. During the Oligocene and Mioce ne, crustal shortening led to rapid uplift and the deposition of fluvi al molasse in limited basins.