The paleogeography of the India–Asia collision system is the reconstructed geological and geomorphological evolution within the collision zone of the Himalayan orogenic belt. The continental collision between the Indian Plate and Eurasian Plate is one of the world's most renowned and most studied convergent systems. However, many mechanisms remain controversial. Some of the highly debated issues include the onset timing of continental collision, the time at which the Tibetan plateau reached its present elevation and how tectonic processes interacted with other geological mechanisms. These mechanisms are crucial for the understanding of Mesozoic and Cenozoic tectonic evolution, paleoclimate and paleontology, such as the interaction between the Himalayas orogenic growth and the Asian monsoon system, as well as the dispersal and speciation of fauna. Various hypotheses have been put forward to explain how the paleogeography of the collision system could have developed. Important ideas include the synchronous collision hypothesis, the Lhasa-plano hypothesis and the southward draining of major river systems.
Timing of collision onset
Definition The onset of continental collision is determined by any point along the plate boundary where the oceanic lithosphere is completely subducted and two continental plates first come into contact. In the case of the India–Asia collision, it would be defined by the first point of disappearance of the Neo-Tethys oceanic crust, where the India and Asia continent come into contact with each other. Such process is defined by a point since the shape of continental margins is irregular. The complete consumption of the oceanic crust could occur non-synchronously along the collision front. Different methods can be used to constrain the age of collision onset. Commonly used geological evidences include stratigraphy, sedimentology and paleomagnetic data. Stratigraphy and sedimentology indicates the transfer of materials from one continent to another when two continents, meet, as well as the change in depositional environment after the oceanic basin is closed and sea water is completely expelled. Paleomagnetic data indicates collision when the paleolatitudes of both continental margins overlap.
The onset of the India–Asia collision has been poorly constrained from Late Cretaceous to Oligo-Miocene due to different interpretations of geological evidences by different researchers.
Diachronous collision hypothesis The diachronous collision hypothesis involves mechanisms with two stages of collision, where the first stage starts during the Paleocene to Eocene.
Paleogene arc-continent collision hypothesis
The Paleogene arc-continent collision suggests that the Indian continent experienced a two-stage collision. The first stage involves the collision with an intraoceanic island arc in the Tethys Ocean at approximately 55 million years (Ma) ago. The second stage involves the collision between the Indian continent (together with the "merged" island arc) and the Asian continent at approximately 33 Ma.
This hypothesis is mainly based on the observation of lithostratigraphic patterns within and around the Yarlung-Zangbo suture zone (YZSZ). The YZSZ itself consists of ophiolite and basaltic to andesitic volcanic rocks, which is comparable to typical rock suites in an island arc subduction system. The north of the YZSZ is the Lhasa terrane of the Tibetan Plateau, while the south of the YZSZ is the Indian superterrane. The fact that the YZSZ separates two continental terrane suggests that it could have been an intraoceanic island arc in the past, locating in between the Asian continental margin (Lhasa terrane) and the Indian continental margin (Indian superterrane) before collision occurred.
Volcanic rocks in the Zedong terrane, which belongs to the YZSZ, has high K2O content and are classified as shoshonites. Shoshonites are potassium-rich basaltic andesite which are commonly found in modern intraoceanic arc settings. It therefore favours the prediction of the YZSZ as a paleo-intraoceanic island. However, recent studies suggest that volcanic rocks in the Zedong terrane have been altered such that the mobile ion ratios (e.g. K and Na) are unreliable. Immobile elements such as Zr/TiO2 ratios should be used instead for classification. New data suggests that volcanic rocks in the Zedong terrane has a calc-alkaline composition, which is common for volcanic island arc but not necessarily intraoceanic island. Moreover, volcanic rocks in the Zedong terrane share a similar geochemical pattern with Lower Jurassic-aged volcanic rocks from southern Lhasa terrane of the Tibetan Plateau. This suggests that the Yarlung-Zangbo suture zone is part of the Asian continental margin instead of a separate intraoceanic island.
Greater India Basin hypothesis
The Greater India Basin hypothesis suggests that there was a two-stage collision between India and Asia continent. The first stage occurred at approximately 50 Ma, where a microcontinent from the Indian plate collided with the Asian continent. It was followed by the subduction of the oceanic Great India Basin, which was located in between the microcontinent and the major Indian craton, under the Asian continent. The second stage of collision occurred after the oceanic crust of the Great India Basin had been consumed, where the major Indian craton finally came into contact and collided with the Asian continental margin (including the previously "merged" microcontinent, which was interpreted to be the modern Tibetan Plateau) at 25–20 Ma.
This hypothesis is mainly based on the observation of crustal shortening deficit in the Himalayas. The convergence of the Indian and Eurasian plate since the Cretaceous should have led to crustal shortening of approximately 3,600 ± 35 km. However, the observed shortening in the Himalayas and the Asian continent accounts for only 30–50% of the total convergence. The Greater India Basin model is therefore put forward to explain such observation, where the total amount of convergent has actually been dispersed into two separate stages of crustal thickening, i.e. the uplift of the microcontinent (Tibetan Plateau) and the Himalaya orogeny. The subduction and disappearance of the Great Indian Basin oceanic crust beneath the microcontinent reduces the measurable amount of total convergence expressed by crustal shortening at the surface.
Paleomagnetic data suggests that the Indian continent had experienced a N-S extension with minimum extension rates of 40–67 mm/y during 118 and 68 Ma. Such extensional rate is comparable to typical records of intracontinental rifting. Therefore, the hypothesized oceanic Greater India Basin could have existed and separated a microcontinent from the major India craton. However, rock records in the Greater Himalayan crystalline complex, which is located south to the Tibetan Plateau and should have contained remnants of the oceanic Greater Indian Basin if it had existed, do not show supporting evidences. No ophiolite obduction from the oceanic Basin nor typical rock suites from arc-trench subduction system are found.
Synchronous collision hypothesis
The synchronous collision hypothesis limits the age of collision onset at 59 Ma by dating the oldest turbidites formed on the passive margin of the India continent, which indicates the incoming of materials from the active Asian continental margin. Geological evidence of rocks younger than 59 Ma and deposited on top of the turbidite sequence can be considered as indicators to reconstruct tectonic evolution after collision had begun. Various evidence documented along NE-SW and NW-SE sections of the India–Asia collision zone synchronize with each other, being in favour of a "one-off" collision.
Facies changes (NE-SW): stratigraphic correlations of Paleocene to early Eocene across the NE-SW orientation of the Himalayas shows that the change in depositional environment is similar in time, with no unconformity and only a few tens of meters of vertical differences. This suggests that the whole Indian continental margin collided with the Asian continental margin at approximately the same time. Detrital zircon age patterns (NW-SE): a transect of paleo-syncollisonal basins (59-56 Ma) on the active Asian continental margin, the point of collision and the passive Indian continental margin is considered. Detrital zircons from these basins shares same age peaks at 50 and 100 Ma. This suggests that the origin of sediments and timing of basin infill along this NW-SE transect is the same, ruling out the possibility of presence of an island arc in between two continental margin and multiple stages of collision.
Kshiroda Plate
As per geological research conducted in 2015, there possibly existed two subduction zones between the Indian and Eurasian plates. A hypothetical lost oceanic plate called the Kshiroda Plate is supposed to have existed between the two subduction zones. It is now believed that this oceanic plate is actually a broken-off fragment of the above mentioned "Neo-Tethys oceanic basin". The bed of the Tethys sea lay on the Kshiroda Plate and was carried along with it towards Eurasia.
The southernmost part of the Eurasian plate was actually the Lhasa block, which itself had drifted north and joined the landmass, simultaneous to the drift of the Indian Plate. This, however, is not included in the hypothesis, as it does not gravely affect the tectonic activities.
According to this hypothesis, the Kshiroda Plate after being subducted under the Eurasian Plate caused the uplift of the Tibetan Plateau and also the delamination of the Indian Plate beneath the plateau.
Paleo-elevation of Tibetan Plateau
Evolution of Tibet's geomorphology
When and how did the Tibetan Plateau reach its present-day elevation has long been widely debated. Tibet has an average elevation of 5 km, which makes it the highest plateau and one of the highest topographic features on Earth. It is very rare to see the Earth's crust achieving such a large extent of thickening. This is why Tibet attracts scientific interest. It was previously believed that Tibet uplifting is solely resulted from the Indian-Asian continental collision. However, more and more studies revealed that Tibet might have reached its present-day elevation as early as in the Cretaceous period (145—66 Ma). Diversified scientific evidences have been put forward to support such hypothesis, such as paleomagnetic reconstruction, sedimentology and igneous petrology, structural geology and geochemistry. For example, Ingalls et al. (2018) uses δ18O (oxygen-isotope) in meteoric water and Δ47 (clumped-isotope) in non-marine carbonates to reconstruct paleotemperature and paleoprecipitation of the Tibetan Plateau. It is suggested that the southern part of Tibet is around 3–4 km high and have an average temperature of 10 °C as early as in Late Cretaceous (92 Ma). This shows that southern Tibet has to be already at its present-day sub-equatorial latitude, such that 10 °C, an extremely warm temperature for highly elevated regions, can be maintained.
It is now generally accepted that Tibet grew differentially, with its southern part reaching present day elevation first, followed by its northern part. For example, Fei et al. (2017) uses 40Ar/39Ar and (U-Th)/He thermochronology to track the growth of the Plateau through time and the results are positive. The figure below shows a generalized evolution model of when did different areas of the Tibetan Plateau reaches its present-day elevation. Although the age is not well-constrained, a clear north-younging trend can be observed.
Tectonic models for crustal thickening
Miocene uplift model