HOME

TheInfoList



OR:

The paleogeography of the India–Asia collision system is the reconstructed
geological Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...
and
geomorphological Geomorphology () is the scientific study of the origin and evolution of topography, topographic and bathymetry, bathymetric features generated by physical, chemical or biological processes operating at or near Earth#Surface, Earth's surface. Ge ...
evolution within the collision zone of the Himalayan
orogenic belt An orogenic belt, orogen, or mobile belt, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological proc ...
. The
continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroy ...
between the
Indian Plate The Indian plate (or India plate) is or was a minor tectonic plate straddling the equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, the Indian plate broke away from the other fragments of Gondwana an ...
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 The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
and
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
tectonic evolution,
paleoclimate Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...
and
paleontology Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure ge ...
, such as the interaction between the
Himalayas The Himalayas, or Himalaya ( ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the Earth's highest peaks, including the highest, Mount Everest. More than list of h ...
orogenic growth and the Asian monsoon system, as well as the dispersal and speciation of
fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...
. 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 Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
where the
oceanic lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time sc ...
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 Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
,
sedimentology Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understanding of m ...
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 In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
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 The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
to Oligo-
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
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 The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
to
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
.


Paleogene arc-continent collision hypothesis

The
Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...
arc-continent collision suggests that the Indian continent experienced a two-stage collision. The first stage involves the collision with an intraoceanic
island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...
in the
Tethys Ocean The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...
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 Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...
patterns within and around the Yarlung-Zangbo suture zone (YZSZ). The YZSZ itself consists of
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...
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 The Lhasa terrane is a terrane, or fragment of crustal material, sutured to the Eurasian plate during the Cretaceous that forms present-day southern Tibet. It takes its name from the city of Lhasa in the Tibet Autonomous Region, China. The north ...
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 Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Ameri ...
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 The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic ...
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 The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
-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 Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Caus ...
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 A craton ( , , or ; from "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of contine ...
, 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 A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental marg ...
(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 Obduction is a geological process whereby denser oceanic crust (and even upper mantle) is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, ...
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
turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...
s formed on the
passive margin A passive margin is the transition between Lithosphere#Oceanic lithosphere, oceanic and Lithosphere#Continental lithosphere, continental lithosphere that is not an active plate continental margin, margin. A passive margin forms by sedimentatio ...
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 The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
to early
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
across the NE-SW orientation of the Himalayas shows that the change in
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
is similar in time, with no
unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...
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 The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...
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 The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...
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 Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informati ...
and geochemistry. For example, Ingalls et al. (2018) uses δ18O ( oxygen-isotope) in
meteoric water Meteoric water, derived from precipitation such as snow and rain, includes water from lakes, rivers, and ice melts, all of which indirectly originate from precipitation. The journey of meteoric water from the atmosphere to the Earth's surface is a ...
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

The
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
model suggested that the Indian-Asian collision is the major cause for Tibet's uplift, which is likely to be wrong due to reasons discussed above. In this model, the Lhasa tectonic block, equivalent to the southern Tibet, experienced initial uplift due to compressional force created when the Indian and Asian continent collided and the Tethys oceanic slab broke off (45—30 Ma). This is supported by the presence of Adakite in the Lhasa block. Adakite is an intermediate to
felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...
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. This further reinforces the hypothesis that Lhasa block is uplifted during the initial continental collision phase. Later, magmatic activity ceased as the continent collision occurred. Denser materials in the Indian and Asian continental crust sank to the bottom part of the crust, making the lower crust extremely dense and heavy. It thus broke off and sank into the mantle. The removal of the dense lower crust reduced gravitational pull on the Lhasa block and allowed it to rise (30—26 Ma). Together with the intense compressional force and
thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
it experienced amidst collision, intense crustal thickening occurred, resulting in the major phase of uplift in South Tibet. As the collision proceed (26—13 Ma), the Northern Tibet continental block experienced compression, thrusting and shortening as well. This interpretation is supported by the thermochronological data of apatite fission tracks from the North Tibetan Plateau, which indicate phases of rapid exhumation and compression from 20 Ma onwards.


Mesozoic uplift model

The
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
model suggested that southern Tibet experienced intense crustal shortening and thickening as early as in
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
to
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
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. In the Mesozoic time, there was an
oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, most of the ocean basins are large Structural basin, geologic basins that are below sea level. Most commonly the ocea ...
in between the Lhasa block and the North Tibet continental block.
Subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
of the oceanic slab underneath the North Tibet block started in the
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
. In Jurassic to Cretaceous, the Mesozoic ocean is closed. The Lhasa continental block and the North Tibet continental block collided with each other, resulting in intense crustal shortening and thickening of the Lhasa block, i.e. South Tibet. The closing of Mesozoic ocean, the continental collision between Lhasa block and North Tibet block and the early crustal thickening of Lhasa block is indicated by the presence of ultra-high pressure metamorphic rocks in the Qiangtang metamorphic belt in Central Tibet. By the time when the Indian continent and the Asian continent collided, South Tibet has already reached 3–4 km elevation. The compressional force resulted from the Indian-Asian collision further topped up Lhasa block's elevation and triggered crustal thickening in the North Tibet as the Indian continent proceed northwards. Although the timing of Lhasa block thickening in this model is conformable with geological evidences available, details remained debated.


Common consensus

Although the actual timing of occurrence of various geological events involving the Tibetan Plateau remains widely debated, there is a common consensus on the evolution of continental block configuration through time among what different studies have put forward. Royden et al. (2008) suggested a tectonic reconstruction model to illustrate how continental blocks of North and South Tibet has evolved throughout the Indian-Asian collision. This model also emphasizes the point that the Lhasa block is first deformed, followed by the North Tibet block. Moreover, the collision between the Lhasa block and the North Tibet block occurred later in the East than in the South. This suggests that detail collision mechanisms could be complicated and require further investigation. A single tectonic model is not likely to be able to explain the whole process. For example, although the above-mentioned Mesozoic uplift model is consistent with the onset timing of South Tibet crustal shortening, other details need to be refined.


Paleo-drainage configuration


Drainage pattern responding to tectonic processes

River A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
s are features formed by water eroding into the land surface. Drainage patterns provide clues not only to
hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...
conditions, but also to geology and tectonic evolution. Burbank (1992) proposed a model to explain how uplift 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. In the case of tectonic driven uplift, an active
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
front is present, constantly driving crustal materials upwards. This adds weight to the Earth's surface, causing land subsidence. Since the nearer a spot is to the active thrust front, the greater the effect of weight the uplifted crust has on the land surface, asymmetric subsidence is resulted. Groundmass nearer to the uplifted crust subside more, while those which are further subside less. This is reflected by the asymmetrical fan shape of sedimentary strata deposited during subsiding, where columns closer to the point of maximum subsidence are thicker while columns further are thinner. Tectonic driven uplift results in longitudinal rivers dominating the area instead of transverse rivers. Transverse rivers are rivers cutting at right angle to mountain ridges, while longitudinal rivers flow parallel to them. During active uplift and subsidence, accommodation space is created quickly and continually, while erosion rate remains relatively slow. Therefore, transverse rivers developed on the uplifted mountain range are not able to extend beyond the area nearest to the thrust front, where subsidence is the most intense. Instead, longitudinal rivers dominated most of the area. On the contrary, in the case of erosional driven uplift, active thrust front is absent. Uplifting of the crust is driven by
isostatic rebound Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...
. The fact that materials are constantly eroded and removed reduces weight adding on the Earth's crust, causing it to "bounce" higher. Since erosion dominates the whole area, uplifting is not limited to sections near to the mountain range. The uplifting rate of the whole drainage basin is rather equal, as reflected by symmetrical shape and equal thickness of sedimentary stratum deposited during uplifting. Erosion driven uplift results in transverse rivers dominating the area instead of longitudinal rivers. During active erosion and isostatic rebound, accommodation space is reduced quickly and continually, while sedimentation rate is also high. Therefore, transverse rivers developed on the uplifted mountain range are able to extend way beyond the foot of the mountain range. Longitudinal rivers only dominate distal parts of the drainage basin.


Evolution of major river systems and their implications

Brookfield (1998) 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 The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
(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. Before the continental collision occurred (which is defined as 50 Ma or before in Brookfield's model), longitudinal river system had dominated the Asian continent, where major river systems run parallel to the approaching regional thrust. Amidst the collision (which is referred as 20 Ma in Brookfield's model), the shape of river channels were affected by the approaching Indian continent. Although major river systems still flowed parallel to the thrust, they bent around both sides of the Indian continent since the collision exerted compressional force to the drainage basin. Such effect is most obviously reflected by the Indus river and the
Ganges The Ganges ( ; in India: Ganga, ; in Bangladesh: Padma, ). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international which goes through India, Bangladesh, Nepal and China." is a trans-boundary rive ...
river. The westward flowing
Indus The Indus ( ) is a transboundary river of Asia and a trans- Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in the Western Tibet region of China, flows northwest through the dis ...
river wraps around the western boundary of the thrust while the eastward flowing Ganges wraps around the eastern boundary of the thrust. In present days, the regional drainage configuration is very different from how it originally was. River systems were eastward flowing, with the Indus as an exception, before the continental collision started. At present, most rivers are flowing south to southeast. The
Salween The Salween is a Southeast Asian river, about long, flowing from the Tibetan Plateau south into the Andaman Sea. The Salween flows primarily within southwest China and eastern Myanmar, with a short section forming the border of Myanmar and Tha ...
,
Yom Yom () is a Biblical Hebrew word which occurs in the Hebrew Bible. The word means "day" in both Modern and Biblical Hebrew. Overview Although ''yom'' is commonly rendered as day in English translations, the word can be used in different ways to ...
,
Mekong The Mekong or Mekong River ( , ) is a transboundary river in East Asia and Southeast Asia. It is the world's twelfth-longest river and the third-longest in Asia with an estimated length of and a drainage area of , discharging of wat ...
and
Red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–750 nanometres. It is a primary color in the RGB color model and a seconda ...
river are drastically bent around the northeastern "tip" of the Indian continent. By further examining and studying the deformation patterns in these river basins, a two-phase deformation model in the East Himalayas is verified. 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 A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...
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 In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...
difference. However, in the case of the South Asia monsoon system, the huge
pressure gradient force In fluid mechanics, the pressure-gradient force is the force that results when there is a difference in pressure across a surface. In general, a pressure is a force per unit area across a surface. A difference in pressure across a surface then ...
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 An adiabatic process (''adiabatic'' ) is a type of thermodynamic process that occurs without transferring heat between the thermodynamic system and its environment. Unlike an isothermal process, an adiabatic process transfers energy to the s ...
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 of South Asian monsoon is poorly constrained since limited paleoclimatic data is available. It is generally accepted to have occurred during the Eocene-Oligocene climate transition (33.9 Ma onwards). 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 Denudation is the geological process in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interchang ...
, 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 Burial, also known as interment or inhumation, is a method of final disposition whereby a dead body is placed into the ground, sometimes with objects. This is usually accomplished by excavating a pit or trench, placing the deceased and object ...
of the southern flank of the Tibetan plateau is a result of monsoon-intensified
denudation Denudation is the geological process in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interchang ...
. The channel flow model explains the South Tibetan uplift in two stages. The first stage took place during
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
to
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
. It is hypothesized that the middle part of the Tibet continental crust was partially melted at that time and was bounded by a "channel" formed from the rigid upper and lower crust. The molten middle crust is thought to be represented by high-temperature rock suites in the Greater Himalayan Crystalline Complex. Since the upper crust was rather strong, the melt cannot propagate towards the surface. The second stage took place during early to mid
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
. The South Asian monsoon developed and the regional climatic condition was changed. Rainfall and wind intensified denudation and weakened the upper crust mechanically (but not thermally). The molten middle crust was therefore able to break through the upper crust and flow outward to the surface. The dilemma is that the South Asian monsoon was believed to have originated from topographic rise of the Himalayas and Tibetan Plateau. The channel flow model predicts that the rise of Tibetan Plateau requires the presence of South Asian monsoon, which leaves the Himalayas as the only possible candidate responsible for initiating the monsoon system. However, a study done by Boos & Kuang (2010) eliminated such possibility. 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.


Directions for future studies


Slab dynamics

Webb et al. (2017) proposed a model to explain Himalayan topographic evolution by taking slab dynamics into account. The model suggests temporal differences in topographic evolution in the East-central and Western Himalayas. Such differences allowed a series of positive climatic feedbacks to occur sequentially and remain sustainable. Feedback mechanisms include topographically-induced monsoon, monsoon-intensified erosion, and erosional-driven uplift (isostatic rebound). Although the discussion of this model is limited to 20 Ma onwards, such concept can be implemented to future studies focusing on the
Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
period so as to better understand how Tibet and the South Asian monsoon co-evolved.


Climatic proxies

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
climatic reconstructions of the Tibetan Plateau area are mostly based on
pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...
analysis, while Mesozoic climatic reconstructions are done by analyzing
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
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 Carbon (6C) has 14 known isotopes, from to as well as , of which only and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature, as trace quantities are formed ...
composition of
paleosol In Earth science, geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science. In geo ...
s 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 In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...
regions. Carbon isotopes in paleosols are remains of dead plants and therefore accurately reflects climatic regime shifts.
Phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
reconstructions of animal taxa is also useful as climate change may promote speciation or trigger extinction.


See also

*
Indian subcontinent The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...
* Precollision Himalayas *
Tibetan plateau The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...


References

{{DEFAULTSORT:Paleogeography of the India-Asia collision system Geology of the Himalaya Paleogeography Geology of India