FACTS AND THEORIES ON THE HIMALAYAS

During the classical exploration in the 19th and early 20th centuries the ratio between facts and theories was 1:0.5. Plate tectonics change d it to 1:3 and with geophysics, geochemistry and structural analysis the ratio became 1:5. The backbone of the High Himalayas is a crystall ine core, involving the Lesser Himalayas in the W.Exposed are Precambr ian structures and metamorphism, overprinted by a Himalayan phase, its istensity still highly disputed. Cambrian nonorogenic granites cut di scordantly rocks with preserved Precambrian structures. New ages from 2,000 to 900 my confirm this fact. Himalayan PT have not cancelled all the Precambrian elements and thrusting along the MCT has transported some relic structures. The MCT forms a zone of imbrication or can expo se a sharp contact. Disputed is the fact of reversed metamorphism towa rds the MCT and above. The hot overthrust theory is contradicted by Ju rassic palynomorphs just below the thrust. The Himalayan metamorphic o verprint ends with the intrusion of leucogranites. They stress the 500 my intrusive gap from the Cambrian granites, a fact repeated in the N orth-Himalayan crystalline, diapiric domes; a gap filled with nonoroge nic Tethyan sediments. Locally the crystalline/sediments contact can b e downfaulted, negligible in the Garhwal Himalayas where basal sedimen ts am over 5,000m thick. Theories to fact ratio increase when we appro ach the Indo/Yarlung suture (IYS), the obducted remnants of a large or small Tethyan ocean, outlining the collision between India and Tibet. Its timing, proven by intra-trappean Asian faunas of 67 my predates a ll previous assumptions, though collision was not synchronous and star ted earlier in the West-Himalayas. The regional outline is surprisingl y constant, but the details vary considerably: West-Ladakh exposes 3 v ertical ophiolitic melange belts. Eastwards they are capped by a large (40 km) ultramafic body, which retains a normal cover of gabbros and volcanics. Further east it is transported 50 km northwards to the trip le-junction at Tashigang, the most important but least known spot alon g the IYS. From hem starts the Shyok suture and borders the Karakorum to the S as a deep fracture or a subduction. In a postcollision phase ophiolite nappes were thrust southwards. Remnants am seen in the Spont ang nappe (470 km thrust), the Amlang-La nappe (80 km) and Shigatse na ppe (30 km). Similar nappes are eves knows along the suture on the wes t side of the Indian shield with 30-50 km thrusts towards the E. Is al l nappes ophiolitc mealanges, frequently with exotic blocks, form the base and ultramafic bodies the top. North of the IYS follow the Andean -type Transhimalayan plutons, subdivided into Gangdese, Kailas, Ladakh and Swat plutons. They range from 100 to 40 my in age and border the complex Tibetan continental margin, by which they have been mom or les s contaminated. This is documented by many xeaoliths from the Tibetan ''basement''. However, in the western Ladak pluton, between the Shyok suture and the IYS, NS aligned xenoliths seem to resemble the basement of the Nanga Parbat spur. Subsequent to the last intrusions, the Tran shimalayas were strongly uplifted and eroded, producing an Oligocene m olasse, from which the spectacular Kailas, the most sacred mountain in Asia, has been carved. All the great rivers in the wider Himalayas or iginate from the Kailas region, cut through the highest uplifts of the rising Himalayas and deposit their sediments in the Indus and Bengal fans, the largest submarine deltas known.