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Ingeology,continental collision is aphenomenon ofplate tectonics that occurs atconvergent boundaries. Continental collision is a variation on the fundamental process ofsubduction, whereby the subduction zone is destroyed,mountains produced, and twocontinents sutured together. Continental collision is only known to occur on Earth.
Continental collision is not an instantaneous event, but may take several tens of millions of years before thefaulting andfolding caused by collisions stops.The collision betweenIndia andAsia has been going on for about 50 million years already and shows no signs of abating. Collision between East and WestGondwana to form theEast African Orogen took about 100 million years from beginning (610 Ma) to end (510 Ma). The collision between Gondwana andLaurasia to formPangea occurred in a relatively brief interval, about 50 million years long.
The process begins as twocontinents (different bits ofcontinental crust), separated across a tract of ocean (andoceanic crust), approach each other, while the oceanic crust is slowly consumed at asubduction zone. The subduction zone runs along the edge of one of the continents and dips under it, raising volcanic mountain chains at some distance behind it, such as theAndes ofSouth America today. Subduction involves the wholelithosphere, the density of which is largely controlled by the nature of the crust it carries. Oceanic crust is thin (~6 km thick) and dense (about 3.3 g/cm3), consisting ofbasalt,gabbro, andperidotite. Consequently, most oceanic crust is subducted easily at anoceanic trench. In contrast, continental crust is thick (~45 km thick) and buoyant, composed mostly ofgranitic rocks (average density about 2.5 g/cm3). Continental crust is subducted with difficulty, but it is subducted to depths of 90–150 km or more, as evidenced by ultra-high pressure (UHP)metamorphic suites. Normal subduction continues as long as the ocean exists, but the subduction system is disrupted as the continent carried by the downgoing plate enters the trench. Because it contains thick continental crust, this lithosphere is less dense than the underlyingasthenospheric mantle and normal subduction is disrupted. Thevolcanic arc on the upper plate is slowly extinguished. Resisting subduction, the continental crust buckles up and under, raising mountains where a trench used to be. However, a continental plate with subducted ocean lithosphere attached to it can start to be subducted from the pull the oceanic lithosphere's dense weight. A great example of this can be seen in the Himalayas where the amount of collisional shortening of India and Asia suggests that continental subduction is occurring.[1] The position of the trench becomes a zone that marks the suture between the two continentalterranes. Suture zones are often marked by fragments of the pre-existing oceanic crust and mantle rocks, known asophiolites.
Thecontinental crust on the downgoing plate is deeply subducted as part of the downgoing plate during collision, defined as buoyant crust entering a subduction zone. An unknown proportion of subducted continental crust returns to the surface as ultra-high pressure (UHP) metamorphic terranes, which contain metamorphiccoesite and/ordiamond plus or minus unusualsilicon-richgarnets and/orpotassium-bearingpyroxenes. The presence of these minerals demonstrate subduction of continental crust to at least 90–140 km deep. Examples of UHP terranes are known from the Dabie–Sulu belt of east-centralChina, the WesternAlps, theHimalaya ofIndia, theKokchetav Massif ofKazakhstan, theBohemian Massif of Europe, the North Qaidam ofNorthwestern China, theWestern Gneiss Region ofNorway, andMali. Most UHP terranes consist of an imbricated sheets ornappes. The fact that most UHP terranes consist of thin sheets suggests that much thicker, volumetrically dominant tracts of continental crust are more deeply subducted.
Anorogeny is underway when mountains begin to grow in the collision zone. There are other modes of mountain formation and orogeny but certainly continental collision is one of the most important.Rainfall andsnowfall increase on the mountains as these rise, perhaps at a rate of a few millimeters per year (at a growth rate of 1 mm/year, a 5,000 m tall mountain can form in 5 million years, a time period that is less than 10% of the life of a typical collision zone).River systems form, andglaciers may grow on the highest peaks.Erosion accelerates as the mountains rise, and great volumes ofsediment are shed into the rivers, which carry sediment away from the mountains to be deposited insedimentary basins in the surrounding lowlands. Crustal rocks arethrust faulted over the sediments and the mountain belt broadens as it rises in height. A crustal root also develops, as required byisostasy; mountains can be high if underlain by thicker crust. Crustal thickening may happen as a result of crustal shortening or when one crust overthrusts the other. Thickening is accompanied by heating, so the crust becomes weaker as it thickens. The lower crust begins to flow and collapse under the growing mountain mass, formingrifts near the crest of the mountain range. The lower crust may partiallymelt, forming anatectic granites which then rise into the overlying units, forminggraniteintrusions. Crustal thickening provides one of two negative feedbacks on mountain growth in collision zones, the other being erosion. The popular notion that erosion is responsible for destroying mountains is only half correct – viscous flow of weak lower mantle also reduces relief with time, especially once the collision is complete and the two continents are completely sutured. Convergence between the continents continues because the crust is still being pulled down by oceanic lithosphere sinking in the subduction zone to either side of the collision as well as beneath the impinging continent.
The pace of mountain building associated with the collision is measured byradiometric dating ofigneous rocks or units that have been metamorphosed during the collision and by examining the record of sediments shed from the rising mountains into the surrounding basins. The pace of ancient convergence can be determined withpaleomagnetic measurements, while the present rate of convergence can be measured withGPS.
The effects of the collision are felt far beyond the immediate site of collision and mountain-building. As convergence between the two continents continues, the region of crustal thickening and elevation will become broader. If there is an oceanic free face, the adjacent crustal blocks may move towards it. As an example of this, the collision of India with Asia forced large regions of crust to move south to form modernSoutheast Asia. Another example is the collision ofArabia withAsia, which is squeezing theAnatolian sub-plate (present dayTurkey). As a result, Turkey is moving west and south into theMediterranean Sea and away from the collision zone. These far-field effects may result in the formation of rifts, andrift valleys such as that occupied byLake Baikal, the deepest lake on Earth.
Continental collisions are a critical part of thesupercontinent cycle and have happened many times in the past. Ancient collision zones are deeply eroded but may still be recognized because these mark sites of intense deformation, metamorphism, and plutonic activity that separate tracts of continental crust having different geologic histories prior to the collision. Old collision zones are commonly called "suture zones" by geologists, because this is where two previous continents are joined orsutured together.
