Facts and Theories on the Himalayas
Authors: Gansser Augusto 
Published: November 2024

During the classical exploration in the 19th and early 20th centuries the ratio between facts and theories was 1:0.5. Plate tectonics changed it to 1:3 and with geophysics, geochemistry and structural analysis the ratio became 1:5.

The backbone of the High Himalayas is a crystalline core, involving the Lesser Himalayas in the W.Exposed are Precambrian structures and metamorphism, overprinted by a Himalayan phase, its intensity still highly disputed. Cambrian nonorogenic granites cut discordantly rocks with preserved Precambrian structures. New ages From 2,000 to 900 my confirm this fact. Himalayan PT have not cancelled all the Precambrian etenients and thrustingalong tlie MCThas transported some relic structures. The MCT Fornls a zone of imbrication or can expose a sharp contact.

Disputed is the fact of reversed metamorphism towards the MCT and above. The holoverthrust theory is contradicted by Jurassic palynomorphs just below the thrust. The Himalayan metamorphic overprint ends with the intrusion of leucogranites. They stress the 500 my intrusive gap from the Cambrian granites, a fact repeated in the North-Himalayan crystalline, diapiric domes; a gap filled with nonorogenic Tethyan sediments. Locally the crystaltine/sediments contact can be downfaulted, negligible in the Garhwal Himalayas where basal sediments are over 5,000m thick.

Theories to fact ratio increase when we approach the IndoNarlung suture (IYS), the obducled 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 all previous assumptions, though collision was not synchronous and started earlier in the West-Himalayas. The regional outline is surprisingly constant, but the detaits vary considerably: West-Ladakh exposes 3 vertical 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 triple-junction at Tashigang, the most important but least known spot along the IYS. From here 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 are seen in the Spontang nappe (40 km thrust), the Arnlang-La nappe (80 km) and Shigatse nappe (30 km). Similar nappes are even known along the suture on the west side of the Indian shield with 3-50 km thrusts towards the E. In all 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 niore or less contaminated. This is documented by many xenolithsfrom the Tibetan "basement". However, in the western Ladak pluton, between the Shyoksuture and the IYS, NS aligned xenoliths seem to resemble the basenlent of the Nanga Parbal spur.

Subsequent to the last intrusions, the Transhimalayas were strongly uplifted and eroded, producing an Oligocene molasse, from which the spectacular Kailas, the most sacred mountain in Asia, has been carved. All the great rivers in the wider Himalayas originate 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.