Before the Indian subcontinent, that included present day India, Pakistan, Bangladesh, Sri Lanka, Bhutan and Nepal, as well as parts of Afghanistan and Burma, drifted north and crashed into Eurasia it was situated at mid- to high latitudes centred around 60^o S. In the Permian, about 250 Ma, it was between Africa-Madagascar and Antarctica-Australia, being part of Pangaea at that time. Rock surfaces with glacial striae (scratches) have been found dating from this time, the Carboniferous-Permian glaciation, in the southern parts of India, South America, Africa and South Australia, the last time prior to the most recent ice age that Earth was subjected was subjected to ice house conditions.

The Indian Plate, later combining with the Australian Plate to form the Indo-Australian Plate, was the fragment of Pangaea, then Gondwana, following the separation of Gondwana from Laurasia at the breakup of Pangaea, that travelled the furthest at the fastest speed. A bit more than 100 Ma it rifted from Africa and Antarctica and began drifting north. The mid-ocean spreading centre that caused the drift continued making new oceanic plate and the Indian Ocean began to form. The author³ suggests that these new ocean ridges, with their high spreading rate, that have been implicated in the rise of the sea level to its all-time high in the Late Cretaceous, a time when water covered 82 % of the surface of the Earth. It had crossed the equator and began interacting with superplume spots that were responsible for the formation of the Deccan Traps of west-central India. The author³ suggests it probably contributed to the mass extinction event at the KT boundary. The leading edge of the Indian Plate is believed to have begun colliding with the Eurasian Plate about 50-45 Ma.

It has been suggested that before the collision of the plates began there may have been a line of archipelagos along the southern margin of Eurasia but these would have been overrun and any coastal seas would have been squeezed out of existence. When the plates collided the movement of the Indian Plate was slowed but not stopped. The ancestral Tibetan Range, a name given by the author³ to a mountain range along the coastline in the area of impact was subjected to the full impact of the Indian Plate. The Himalayas were raised to their present height by the collision that resulted in the inclusion of 10 of the world’s mountains that reach above 8,000 m in height. Tibet was raised to its present height as the highest plateau in the world in the same uplift, all parts of it being above 5,000 m high.

It has been estimated that India moved north at an average rate of 10 cm/yr, and its earlier rate has been estimated to be 15-20 cm/yr and since the collision to 5 cm/yr, as it is continuing to move and the Himalayas are continuing to grow.

Closing ocean, rising mountain

The very particular plate organisation of Yunnan region of China in which the area trends directly to the east from the high Tibetan Plateau then veers to the south in a gigantic, tight tectonic arc, with mountainous ridges and deep valleys. This arrangement has resulted from the collision between India and Eurasia and India still pushes northward. In the Tibetan Plateau 3 of the largest rivers have their source that have carved out canyons that are immensely deep as they plunged from the 5,000 m height on the plateau. In one section of their courses the rivers flow side by side, about 80 km apart in the Three Gorges (Sanxia) area. At one point of the Yangtze River it is so narrow that legends tell that a tiger that was being hunted leaped across it, the place now being called tiger-leaping gorge, Hutiaoxia. After dropping 3,000 m from the plateau to the valley floor the Yangtze heads out across the central Chinese fertile plains. The other 2 rivers are the Salween that flows through Burma and the Mekong that flows in a meandering manner through Laos, Cambodia and Vietnam. Extremely large loads of erosion debris from the mountains are carried by the 3 rivers to their flood plains and eventually the sea.

According to the author³ a common feature of mountain belts, especially young ones such as the Himalayas, require years of research before their complex geology can be sorted out, though there are some features common to all. They found mountains that were composed of granite, the rock type most common in continental rocks that had been formed under extreme pressure and heat deep in the subsurface where the buried rocks melted in molten magma. The magma rises to shallower depths beneath the mountain chain, because the hot rock is less dense than the same rock before it is heated, where it crystallises out as minerals such as glassy quartz, white or ping feldspar and mica.

There were sections of mountains that were composed of serpentine, a greenish black rock that had a sheen that was almost watery. This is the same rock as that found in the Betic Mountains near Rhonda, Spain. This rock is highly altered mantle rock from deep within the interior of the Earth that was forced up through the overlying layers by the immense forces generated by the collision with India. It is found together with disjointed slivers of layered gabbros (slivers of rocks from the lower crust) and lavas of ocean-floor origin on the Shan Plateau of western Yunnan. Part of the ophiolites assemblage, this juxtaposition of rocks is diagnostic of former ocean crust and mantle exhumed at the surface, most typically of the collision and grinding together of tectonic plates that occurs along suture lines that mark the collision.

These serpentines have been subjected to great tectonic pressure during emplacement resulting in rocks that can be shot through by fractures, though some fractures can be filled by mineralising fluids that can form a delicate tracery of white calcite veins throughout the rock as it is binding the sides of the fracture tightly together. It can be rendered too schistose (flaky and crumbly) to be effectively worked. It is common to find rocks showing signs of fracturing, such as slickenside faces of rocks that have slid past each other, crushed fault breccia and powdered fault gauge where the rocks have been broken and finely abraded along the line of movement; and mineralised veins in spider web networks. The author³ says the faults they encountered and the overall scale of the deformation they encountered in western Yunnan was beyond anything he had encountered previously, with juxtaposition of rocks of entirely different type and age without any apparent reason.  Between these solid, recognisable rock outcrops were valleys and gorges that were deeply eroded, in places displaying a mélange, a truly chaotic rock type. The Indian subcontinent and the landmass of Eurasia are sutured together in a suture zone represented by the 3-Gorges region.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.