A craton ( / ˈ k r eɪ t ɒ n / KRAYT -on , / ˈ k r æ t ɒ n / KRAT -on , or / ˈ k r eɪ t ən / KRAY -tən ; from ‹See Tfd› Greek : κράτος kratos "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 continents, cratons are generally found in the interiors of tectonic plates ; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock , which may be covered by younger sedimentary rock . They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle.
32-707: Krishna Godavari Basin is a peri- cratonic passive margin basin in India. It is spread across more than 50,000 square kilometres in the Krishna River and Godavari River basins in Andhra Pradesh . The site is known for the D-6 block where Reliance Industries discovered the biggest natural gas reserves in India in 2003. The first gas discovery in the basin was in 1983, in Razole Well No 1, when ONGC had
64-400: A rising plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons. A third model suggests that successive slabs of subducting oceanic lithosphere became lodged beneath a proto-craton, underplating the craton with chemically depleted rock. A fourth theory presented in a 2015 publication suggests that the origin of
96-581: A "fixist". Part of Stille's work dealt with massifs and sedimentary basins in Central Europe; differing from Suess' interpretations for the same area showing that between the Bohemian and Rhine massifs Mesozoic rocks were folded . A central tenet in Stille's geology was that geosynclines became depressions without any faulting with any fault found being the product of later processes like
128-560: A small office in Rajahmundry and Narsapur. Since that discovery Reliance and others have joined the exploration effort. KG inland and offshore basins have good prospects of unconventional tight oil and tight gas reserves from the conducted field studies. Most of the conventional wells drilled and operated have a shorter lifespan than envisaged life and with erratic production. This may be due to drilling of conventional wells in tight oil and gas fields without horizontal drilling in
160-457: A solid residue very close in composition to Archean lithospheric mantle, but continental shields do not contain enough komatiite to match the expected depletion. Either much of the komatiite never reached the surface, or other processes aided craton root formation. There are many competing hypotheses of how cratons have been formed. Jordan's model suggests that further cratonization was a result of repeated continental collisions. The thickening of
192-415: Is calculated already at maximum price cap of Brent crude at $ 60 per barrel ($ 380/ m ) under the applicable formula linking the price of gas per million Btu (GP) to the price of oil : GP = 2.5 + (OP – 25) ^0.15 where OP is the annual average Brent crude price for the previous FY, with a floor of $ 25 per barrel ($ 160/m) and a cap of $ 60 per barrel ($ 380/m). As the annual Brent price has always been above
224-452: Is strongly influenced by the inclusion of moisture. Craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium and iron. Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent
256-653: The Baltic Shield had been eroded into a subdued terrain already during the Late Mesoproterozoic when the rapakivi granites intruded. Hans Stille Hans Wilhelm Stille (8 October 1876 – 26 December 1966) was an influential German geologist working primarily on tectonics and the collation of tectonic events during the Phanerozoic . Stille adhered to the contracting Earth hypothesis and together with Leopold Kober he worked on
288-1097: The Dharwar Craton in India, North China Craton , the East European Craton , the Amazonian Craton in South America, the Kaapvaal Craton in South Africa, the North American Craton (also called the Laurentia Craton), and the Gawler Craton in South Australia. Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice
320-443: The geosyncline theory to explain orogeny . Stille's ideas emerged in the aftermath of Eduard Suess ' book Das Antlitz der Erde (1883–1909). Stille's and Kober's school of thought was one of two that emerged in the post-Suess era the other being headed by Alfred Wegener and Émile Argand . This competing view rejected Earth contraction and argued for continental drift . As Stille opposed continental drift he came to be labelled
352-406: The "cratonic regime". It involves processes of pediplanation and etchplanation that lead to the formation of flattish surfaces known as peneplains . While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over geological time leads to the formation of so-called polygenetic peneplains of mixed origin. Another result of
SECTION 10
#1732791231755384-512: The basement rock crops out at the surface, and a platform which overlays the shield in some areas with sedimentary rock . The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen , referring to stable continental platforms, and orogen as a term for mountain or orogenic belts . Later Hans Stille shortened the former term to Kraton , from which craton derives. Examples of cratons are
416-417: The cap price since 2007, the revised gas price at $ 5.61 per million British thermal units ($ 19.1/MWh) during the year 2014 is in excess of the applicable price by nearly 20%. Craton The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons are composed of two layers: a continental shield , in which
448-502: The craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years. Rock fragments ( xenoliths ) carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites . These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotite
480-465: The cratons is similar to crustal plateaus observed on Venus, which may have been created by large asteroid impacts. In this model, large impacts on the Earth's early lithosphere penetrated deep into the mantle and created enormous lava ponds. The paper suggests these lava ponds cooled to form the craton's root. The chemistry of xenoliths and seismic tomography both favor the two accretional models over
512-399: The crust associated with these collisions may have been balanced by craton root thickening according to the principle of isostacy . Jordan likens this model to "kneading" of the cratons, allowing low density material to move up and higher density to move down, creating stable cratonic roots as deep as 400 km (250 mi). A second model suggests that the surface crust was thickened by
544-481: The crystalline residues after extraction of melts of compositions like basalt and komatiite . The process by which cratons were formed is called cratonization . There is much about this process that remains uncertain, with very little consensus in the scientific community. However, the first cratonic landmasses likely formed during the Archean eon. This is indicated by the age of diamonds , which originate in
576-426: The depleted "lid" formed by the first layer. The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either. All these proposed mechanisms rely on buoyant, viscous material separating from a denser residue due to mantle flow, and it is possible that more than one mechanism contributed to craton root formation. The long-term erosion of cratons has been labelled
608-693: The entire block was declared as a discovery area and RIL was allowed to retain it. In 2011, the Comptroller and Auditor General of India (CAG) criticised the Oil Ministry for this decision. The CAG also faulted RIL for limiting the competition in contracts, stating that RIL awarded a $ 1.1 billion contract to Aker on a single-bid basis. In May 2014, ONGC alleged that Reliance was illegally extracting gas from blocks owned by ONGC in Krishna Godavari Basin. The Indian government fixed
640-420: The final collapse of the geosyncline. In 1933 Stille would shorten Leopold Kober's concept of kratogen , that was used to describe those portions of the continental crust that were old and stable, into kraton (English: craton ). The Geotectonic Research journal was founded in 1937 by Hans Stille and Franz Lotze . The enormous influence that Stille's concepts had on tectonics is obvious from many of
672-471: The late Archean, accompanied by voluminous mafic magmatism. However, melt extraction alone cannot explain all the properties of craton roots. Jordan notes in his paper that this mechanism could be effective for constructing craton roots only down to a depth of 200 kilometers (120 mi). The great depths of craton roots required further explanation. The 30 to 40 percent partial melting of mantle rock at 4 to 10 GPa pressure produces komatiite magma and
SECTION 20
#1732791231755704-468: The longevity of cratons is that they may alternate between periods of high and low relative sea levels . High relative sea level leads to increased oceanicity, while the opposite leads to increased inland conditions . Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn Craton of Western Australia was flattish already by Middle Proterozoic times and
736-638: The natural gas price at producer end (on shore point) as US$ 5.61 per million British thermal units ($ 19.1/ MWh ) on net calorific value (NCV) basis compared to earlier price of $ 4.2 per million British thermal units ($ 14/ MWh ) on gross calorific value basis. However the enhanced price would be applicable only after compensating the shortfall gas in the previous years for the KG basin fields which are under exploration and production contract by Reliance Industries Ltd. The earlier price of $ 4.2 per million British thermal units ($ 14/MWh) on gross calorific value (GCV) basis
768-420: The plume model. However, other geochemical evidence favors mantle plumes. Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than 150 km (93 mi) and may be Archean, while the second is found at depths from 180 to 240 km (110 to 150 mi) and may be younger. The second layer may be a less depleted thermal boundary layer that stagnated against
800-550: The roots of cratons, and which are almost always over 2 billion years and often over 3 billion years in age. Rock of Archean age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, this suggests that only 5 to 40 percent of the present continental crust formed during the Archean. Cratonization likely was completed during the Proterozoic . Subsequent growth of continents
832-522: The shale rock formations and hydraulic fracturing . The basin is home to olive ridley sea turtle , a vulnerable species. KG-DWN-98/1 (KG-D6) - 8100 km. The total project is expected to cost $ 100 billion. 50 km off the coast of Kakinada . The Reliance Industries Limited (RIL) was supposed to relinquish 25% of the total area outside the discoveries in 2004 and 2005, as per the Production Sharing Contract (PSC). However,
864-515: The surrounding hotter, but more chemically dense, mantle. In addition to cooling the craton roots and lowering their chemical density, the extraction of magma also increased the viscosity and melting temperature of the craton roots and prevented mixing with the surrounding undepleted mantle. The resulting mantle roots have remained stable for billions of years. Jordan suggests that depletion occurred primarily in subduction zones and secondarily as flood basalts . This model of melt extraction from
896-527: The tectonic terms still in use today, despite the fact that almost everything he postulated was later shown to be completely erroneous. The Hans-Stille-Medaille of German Geological Society, awarded annually, is named after him. Also named for him is the mineral stilleite (ZnSe) and the wrinkle ridge Dorsa Stille on the Moon . This article about a German scientist is a stub . You can help Misplaced Pages by expanding it . This biographical article about
928-503: The typical 100 km (60 mi) thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into the asthenosphere , and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons. Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy and a low intrinsic density. This low density offsets density increases from geothermal contraction and prevents
960-455: The upper mantle has held up well with subsequent observations. The properties of mantle xenoliths confirm that the geothermal gradient is much lower beneath continents than oceans. The olivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity. Rhenium–osmium dating of xenoliths indicates that the oldest melting events took place in the early to middle Archean. Significant cratonization continued into
992-473: Was by accretion at continental margins. The origin of the roots of cratons is still debated. However, the present understanding of cratonization began with the publication in 1978 of a paper by Thomas H. Jordan in Nature . Jordan proposes that cratons formed from a high degree of partial melting of the upper mantle, with 30 to 40 percent of the source rock entering the melt. Such a high degree of melting
Krishna Godavari Basin - Misplaced Pages Continue
1024-423: Was possible because of the high mantle temperatures of the Archean. The extraction of so much magma left behind a solid peridotite residue that was enriched in lightweight magnesium and thus lower in chemical density than undepleted mantle. This lower chemical density compensated for the effects of thermal contraction as the craton and its roots cooled, so that the physical density of the cratonic roots matched that of
#754245