Ingeology,basement andcrystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment.[1] The basement rocks lie below asedimentaryplatform or cover, or more generally any rock belowsedimentary rocks orsedimentary basins that aremetamorphic origneous in origin. In the same way, thesediments or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".
Basement rock consists of continental crustal rock which has been modified several times through tectonic events including deformation, metamorphism, deposition, partial melting and magmatism.[1]
Basement rock is the thick foundation of ancient, and oldest,metamorphic andigneous rock that forms thecrust ofcontinents, often in the form ofgranite.[2] Basement rock is contrasted to overlyingsedimentary rocks which are laid down on top of the basement rocks after the continent was formed, such assandstone andlimestone. The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 5 kilometres (3 mi) thick. The basement rock of the crust can be 32–48 kilometres (20–30 mi) thick or more. The basement rock can be located under layers of sedimentary rock, or be visible at the surface.
Basement rock is visible, for example, at the bottom of theGrand Canyon, consisting of 1.7- to 2-billion-year-oldgranite (Zoroaster Granite) andschist (Vishnu Schist). The Vishnu Schist is believed to be highly metamorphosed igneous rocks andshale, frombasalt,mud andclay laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well.
The basement rocks of the continental crust tend to be much older than the oceanic crust.[3] The oceanic crust can be from 0–340 million years in age, with an average age of 64 million years.[4] Continental crust is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced atsubduction andoceanic rifting areas.
 | This sectionneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources in this section. Unsourced material may be challenged and removed.(January 2019) (Learn how and when to remove this message) |
The basement rocks are often highly metamorphosed and complex, and are usuallycrystalline.[5] They may consist of many different types of rock – volcanic, intrusive igneous and metamorphic. They may also containophiolites, which are fragments ofoceanic crust that became wedged between plates when aterrane was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may formunderplating, where the new igneous rock forms a layer on the underside of the crust. The majority of continental crust on the planet is around 1 to 3 billion years old, and it is theorised that there was at least one period of rapid expansion and accretion to the continents during the Precambrian.
Much of the basement rock may have originally been oceanic crust, but it was highly metamorphosed and converted intocontinental crust. It is possible for oceanic crust to be subducted down into theEarth's mantle, at subduction fronts, where oceanic crust is being pushed down into the mantle by an overriding plate of oceanic or continental crust.
When a plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it causes an upwelling of magma that can causevolcanism along the subduction front on the overriding plate. This produces an oceanicvolcanic arc, likeJapan. This volcanism causesmetamorphism, introducesigneous intrusions, and thickens the crust by depositing additional layers ofextrusive igneous rock from volcanoes. This tends to make the crust thicker and less dense, making it immune to subduction.[6]
Oceanic crust can be subducted, while continental crust cannot. Eventually, the subduction of the underthrusting oceanic crust can bring the volcanic arc close to a continent, with which it may collide. When this happens, instead of being subducted, it isaccreted to the edge of the continent and becomes part of it. Thin strips or fragments of the underthrusting oceanic plate may also remain attached to the edge of the continent so that they are wedged and tilted between the converging plates, creatingophiolites. In this manner, continents can grow over time as newterranes are accreted to their edges, and so continents can be composed of a complex quilt of terranes of varying ages.
As such, the basement rock can become younger going closer to the edge of the continent. There are exceptions, however, such asexotic terranes. Exotic terranes are pieces of other continents that have broken off from their original parent continent and have become accreted to a different continent.
Continents can consist of several continentalcratons – blocks of crust built around an initial original core of continents – that gradually grew and expanded as additional newly created terranes were added to their edges. For instance,Pangea consisted of most of the Earth's continents being accreted into one giantsupercontinent. Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents.
InEuropean geology, the basement generally refers to rocks older than theVariscan orogeny. On top of this older basementPermianevaporites andMesozoiclimestones were deposited. The evaporites formed aweak zone on which the harder (stronger) limestonecover was able to move over the hard basement, making the distinction between basement and cover even more pronounced.[citation needed]
InAndean geology the basement refers to theProterozoic,Paleozoic and earlyMesozoic (Triassic toJurassic) rock units as the basement to the late Mesozoic andCenozoic Andean sequences developed following the onset ofsubduction along the western margin of theSouth American Plate.[7]
When discussing theTrans-Mexican Volcanic Belt ofMexico thebasement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerreroterranes respectively.[8]
The term basement is used mostly in disciplines of geology likebasin geology,sedimentology andpetroleum geology in which the (typicallyPrecambrian) crystalline basement is not of interest as it rarely containspetroleum ornatural gas.[9] The termeconomic basement is also used to describe the deeper parts of a cover sequence that are of no economic interest.[10]
This article incorporatespublic domain material fromMap Shows Content and Origins of the Nation’s Geologic Basement.United States Geological Survey. April 23, 2015.{{cite book}}: CS1 maint: others (link)
The dictionary definition ofbasement at Wiktionary