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Craton

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   World geologic provinces. (USGS) Oceanic crust      0-20 Ma      20-65
   Ma      >65 Ma Geologic province      Shield      Platform      Orogen
        Basin      Large igneous province      Extended crust
   World geologic provinces. ( USGS)
   Oceanic crust      0-20 Ma      20-65 Ma      >65 Ma Geologic province
        Shield      Platform      Orogen      Basin      Large igneous
   province      Extended crust

   A craton (kratos; Greek for strength) is an old and stable part of the
   continental crust that has survived the merging and splitting of
   continents and supercontinents for at least 500 million years. Some are
   over 2 billion years old. Cratons are generally found in the interiors
   of continents and are characteristically composed of ancient
   crystalline basement crust of lightweight felsic igneous rock such as
   granite. They have a thick crust and deep roots that extend into the
   mantle beneath to depths of 200 km.

   The term craton is used to distinguish the stable interior portion of
   the continental crust from such regions as mobile geosynclinal troughs,
   which are linear belts of sediment accumulations subject to subsidence,
   or downwarping. The extensive central cratons of continents may consist
   of both shields and platforms, and the crystalline basement. A shield
   is that part of a craton in which the usually Precambrian basement
   rocks crop out extensively at the surface. In contrast, the platform of
   the basement is overlain by horizontal or subhorizontal sediments.

   Cratons are subdivided geographically into geologic provinces. A
   geologic province is a spatial entity with common geologic attributes.
   A province may include a single dominant structural element such as a
   basin or a fold belt, or a number of contiguous related elements.
   Adjoining provinces may be similar in structure but be considered
   separate due to differing histories. There are several meanings of
   geologic provinces, as used in specific contexts.

   Continental cratons have deep roots that extend down into the mantle.
   Mantle tomography shows that cratons are underlain by anomalously cold
   mantle corresponding to lithosphere more than twice the approximately
   60 mile (100 km) thickness of mature oceanic or noncratonic continental
   lithosphere. Thus at that depth, it could be argued that some cratons
   might even be anchored in the asthenosphere. Mantle roots must be
   chemically distinct because cratons have a neutral or positive
   buoyancy, and a low intrinsic density that is required to offset any
   density increases due to geothermal contraction. Rock samples of mantle
   roots contain peridotites, and have been delivered to the surface as
   inclusions in diamond-bearing subvolcanic pipes called kimberlite
   pipes. These inclusions have densities consistent with craton
   composition and are composed of mantle material residual from high
   degrees of partial melt. 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 the crystalline residues after extraction of
   melts of compositions like basalt and komatiite. Alpine peridotites are
   slabs of uppermost mantle, many from oceanic lithosphere, also residues
   after extraction of partial melt, but they were subsequently emplaced
   together with oceanic crust along thrust faults up into the Alpine
   mountain belts. An associated class of inclusions called eclogites,
   consists of rocks corresponding compositionally to oceanic crust (
   basalt), but that metamorphosed under deep mantle conditions. Isotopic
   studies reveal that many eclogite inclusions are samples of ancient
   oceanic crust subducted billions of years ago to depths exceeding 90 mi
   (150 km) into the deep kimberlite diamond areas. They remained fixed
   there within the drifting tectonic plates until carried to the surface
   by deep-rooted magmatic eruptions. If peridotite and eclogite
   inclusions are of the same temporal origin, then peridotite must have
   also originated from sea-floor spreading ridges billions of years ago,
   or from mantle affected by subduction of oceanic crust then. During the
   early begins, when the Earth was much hotter, greater degrees of
   melting at oceanic spreading ridges generated oceanic lithosphere with
   thick crust, much thicker than 12 miles (20 km), and a highly depleted
   mantle. Such a lithosphere would not sink deeply or subduct because of
   its buoyancy, and because of the removal of denser melt that in turn
   lowered the density of the residual mantle. Accordingly, cratonic
   mantle roots are probably composed of buoyantly subducted slabs of a
   highly depleted oceanic lithosphere. These deep mantle roots increase
   the stability, anchoring and survivability of cratons and makes them
   much less susceptible to tectonic thickening by collisions, or
   destruction by sediment subduction.

   The word craton was first proposed by the German geologist L. Kober in
   1921 as "Kratogen," referring to stable continental platforms, and
   "orogen" as a term for mountain or orogenic belts. Later authors
   shortened the former term to kraton and then to craton.

Craton formation

   The process by which cratons are formed from early rock is called
   cratonization. The first large cratonic landmasses formed during the
   Archean eon. During the Early Archean the Earth's heat flow was nearly
   three times higher than it is today because of the greater
   concentration of radioactive isotopes and the residual heat from the
   Earth's accretion. Tectonic and volcanic activity were considerably
   more active than they are today; the mantle was much more fluid and the
   crust much thinner. This resulted in rapid formation of oceanic crust
   at ridges and hot spots, and rapid recycling of oceanic crust at
   subduction zones. The Earth's surface was probably broken up into many
   small plates with volcanic islands and arcs in great abundance. Small
   protocontinents (cratons) formed as crustal rock was melted and
   remelted by hot spots and recycled in subduction zones.

   There were no large continents in the Early Archean, and small
   protocontinents were probably the norm in the Mesoarchean because they
   were probably prevented from coalescing into larger units by the high
   rate of geologic activity. These felsic protocontinents (cratons)
   probably formed at hot spots from a variety of sources: mafic magma
   melting more felsic rocks, partial melting of mafic rock, and from the
   metamorphic alteration of felsic sedimentary rocks. Although the first
   continents formed during the Archean, rock of this age makes up only 7%
   of the world's current cratons; even allowing for erosion and
   destruction of past formations, evidence suggests that only 5-40% of
   the present continental crust formed during the Archean. (Stanley,
   1999).

   One evolutionary perspective of how the cratonization process "might"
   have first begun in the Archean is given by Hamilton (1999):

                "Very thick sections of mostly submarine mafic, and
                subordinate ultramafic, volcanic rocks, and mostly younger
                subaerial and submarine felsic volcanic rocks and
                sediments were oppressed into complex synforms between
                rising young domiform felsic batholiths mobilized by
                hydrous partial melting in the lower crust. Upper-crust
                granite-and-greenstone terrains underwent moderate
                regional shortening, decoupled from the lower crust,
                during compositional inversion accompanying doming, but
                cratonization soon followed. Tonalitic basement is
                preserved beneath some greenstone sections but
                supracrustal rocks commonly give way downward to
                correlative or younger plutonic rocks... Mantle plumes
                probably did not yet exist, and developing continents were
                concentrated in cool regions. Hot-region upper mantle was
                partly molten, and voluminous magmas, mostly ultramafic,
                erupted through many ephemeral submarine vents and rifts
                focussed at the thinnest crust.... Surviving Archean crust
                is from regions of cooler, and more depleted, mantle,
                wherein greater stability permitted uncommonly thick
                volcanic accumulations from which voluminous partial-melt,
                low-density felsic rocks could be generated."

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