Paleogeography of the India–Asia collision system

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. The Paleogene arc-continent collision suggests that the Indian continent experienced a two-stage collision. and basaltic to andesitic volcanic rocks, 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. This suggests that the Yarlung-Zangbo suture zone is part of the Asian continental margin instead of a separate intraoceanic island. The first stage occurred at approximately 50 Ma, where a microcontinent from the Indian plate collided with the Asian continent. 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. Therefore, the hypothesized oceanic Greater India Basin could have existed and separated a microcontinent from the major India craton. No ophiolite obduction from the oceanic Basin nor typical rock suites from arc-trench subduction system are found. 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. 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. 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. Adakite is an intermediate to felsic rock which is commonly related to oceanic subduction. Geochemical analysis of the Lhasa Adakite suggests that it is originated from magmatic activities triggered by slab breakoff.|alt= The Mesozoic model suggested that southern Tibet experienced intense crustal shortening and thickening as early as in Jurassic to Cretaceous time. It is widely accepted that the Indian plate began to approach the Eurasian plate during the Mesozoic times as a result of the break up of Gondwana supercontinent. By the time when the Indian continent and the Asian continent collided, South Tibet has already reached 3–4 km elevation. == Paleo-drainage configuration ==

Drainage pattern responding to tectonic processes driven by different factor can result in different drainage patterns, where uplifting is the upward movement of landmass with reference to the Earth's center. reconstructed the evolution of major river systems of the Indian-Asian collision zone based on tectonic history of the area. It is suggested that the most significant changes in drainage patterns occurred during Pliocene to Quaternary (5.3 Ma onwards). Detail changes in fluvial processes will not be discussed here. Major focuses are how river systems of the area responded to changing geological processes through time, as well as how regional drainage patterns are capable of reflecting tectonic evolution. This shows that rivers are reliable indicators of crustal strain and useful in reconstructing regional tectonic history. Moreover, the Indus and the Ganges river originally flowed parallel to the regional thrust on the Asian continent, but are now flowing perpendicular to it. They crossed the thrust and extended onto the Indian continent. This is conformable to the above-mentioned model proposed by Burbank (1992). Since tectonic uplift has significantly slowed down nowadays compared to when the collision has just started, the present day Indian-Asian collision region is dominated by erosional processes. Rivers like the Indus and Ganges, which originated from the Lhasa block, are therefore able to flow as transverse rivers and reach beyond the proximal part of the Himalayas mountain range. == Paleogeography and paleoclimate ==

South Asian monsoon system and the debate The South Asian monsoon system primarily affects the continents of South Asia and their surrounding water bodies. In this particular system, summer monsoon blows as onshore northeasterly while winter monsoon blows as offshore westerly. The driving force of monsoon systems is the pressure difference between landmasses and waterbodies. This is most commonly a result of differential heating of land and sea due to specific heat capacity difference. However, in the case of the South Asia monsoon system, the huge pressure gradient force is induced by the Himalayas and Tibetan Plateau. The Himalaya orogenic belt the highest elevated mountain range on Earth. In summer, air mass across the South Asia is heated up in general. On the contrary, airmass above the Himalayas and Tibet experiences adiabatic cooling and sinks rapidly, forming an intense high pressure cell. This cell is therefore capable of facilitating landward airflow towards itself, thus sustaining the onshore summer monsoon. The onset mechanism has long been debated and remained poorly understood. On one hand, it is believed that the uplift of the Himalayas and Tibetan Plateau is the major trigger of South Asian monsoon onset, since only such elevated landmass can change regional airflow configurations. On the other, numerical modelling and thermalchronological data suggest that Eocene uplift of the Himalayas and Tibet is driven by monsoon-intensified denudation, i.e. erosional driven uplift. This gives rise to a "chicken or egg" paradox. The channel flow model As mentioned above, a lot has been done on examining how the uplift of the Himalayas and Tibetan Plateau has triggered the onset of the South Asian monsoon. The approach of most studies is to first establish or make use of pre-existing tectonic models to constrain the timing of uplift and topographic evolution, then evaluate the significance of topography in controlling regional climate by numerical modeling. Various significant tectonic models have been discussed in previous sections. However, the only quantitative model which has assigned a significant role for climate suggests the opposite, i.e. the exhumation of the southern flank of the Tibetan plateau is a result of monsoon-intensified denudation. The study uses computer model to simulate the growth and evolution of the South Asian monsoon under three conditions: (1) both the Himalayas and Tibet are present, (2) Only Tibet is present, (3) both the Himalayas and Tibet are absent. Results shows that both condition (1) and (2) are able to produce similar monsoonal climate patterns, meaning that the Himalayas is climatically insignificant. while Mesozoic climatic reconstructions are done by analyzing benthic foraminifera from paleo-oceanic basins. Little study has focused on the Tertiary period, at which the South Asian monsoon is thought to have initiated. Further studies on Tertiary carbon isotope composition of paleosols could be carried out to examine the shift in C3/ C4 vegetation ratio. C3 and C4 plants practice different carbon fixation mechanism. C4 fixation is more water-efficient and therefore favours plant adaptation to extreme climatic conditions. Therefore, C4 plants are generally more abundant in cold and arid-temperate regions. Carbon isotopes in paleosols are remains of dead plants and therefore accurately reflects climatic regime shifts. Phylogenetic reconstructions of animal taxa is also useful as climate change may promote speciation or trigger extinction. == See also ==