views updated May 29 2018


Cratons are large regions of continental crust that have remained tectonically stable for a prolonged period of time, often a billion years or more. Precambrian cratons are commonly cored by Archaean granite-greenstone terrains and may be partly covered by sedimentary platform sequences. The North American Craton, Laurentia, which constitutes much of North America , formed by the assembly of smaller cratons in the Archean and Paleoproterozoic. Cratons are surrounded by orogenic (mountain building) or mobile belts, within which deformation has been localized. For example, cratons comprised of Archean granite-greenstone terrains and Paleoproterozoic sedimentary sequences in Africa , central India and Western Australia are rimmed by Mesoproterozoic and Neoproterozoic orogenic belts, many of which have been subsequently reactivated during rifting and the formation of Paleozoic sedimentary basins. Only minor reactivation of older structural weaknesses occurs in craton interiors during deformation on their margins.

Cratons have thick lithospheric roots or keels. Lithospheric thicknesses for Archean cratons show a bimodal distribution, with thicknesses of approximately 137 mi (220 km) and 218 mi (350 km) predominating. Larger cratons generally have thicker lithospheres. In contrast, post-Archean lithosphere is generally 62124 mi (100200 km) thick. The physical and/or chemical properties of the deep roots of cratons enable them to resist recycling into the underlying asthenopsheric mantle. This may be responsible for the stability of cratons. Isotopic signatures obtained from mantle lithosphere-derived, peridotite xenoliths and inclusions in diamonds imply that roots to Precambrian cratons have been isolated from the convecting mantle for billions of years. Archean subcontinental lithospheric mantle is buoyant relative to the underlying asthenosphere . It is therefore not easily delaminated and assimilated into the asthenosphere and will tend to be preserved. Geochemical changes may also impart stability to cratonic roots. Mantle plumes are generally unable to break and induce rifting of thick, cratonic lithosphere and may be deflected around cratons. Despite their buoyancy, the margins of cratons can be deformed at subduction zones, however, due to the development of detachments at the interface between crust and mantle lithospheric root. Numerical modeling suggests that the dominant stabilizing factor for the preservation of cratons is the relatively high brittle yield stress of cratonic lithosphere. As the strength of the continental lithosphere resides primarily within the crust, the physical properties of the crust may also play a role in the longevity of cratonic lithosphere.

Favorable pressures and temperatures for the formation and preservation of diamond are found beneath cratons. Diamond crystallizes from liquid carbon between 1652°2192°F (9001200°C ) at pressures above 50 kbar. At this pressure, equating to a depth of 93 mi (150 km) or more, temperatures are generally too high for the formation of diamond except in the roots beneath cratons. Most of the world's diamonds come from deep, mantle-sourced intrusive bodies such as kimberlites or lamproites that intrude Archaean cratons. Kimberlite and lamproite magmas intrude extremely rapidly up deep fractures and may bring diamonds to shallow levels in the crust. Near the earth's surface, they erupt explosively due to their high gas pressures, creating breccia pipes (called diatremes) and craters.

See also Plate tectonics


views updated May 08 2018

craton (shield) (adj. cratonic) Area of the Earth's crust, invariably part of a continent, which is no longer affected by orogenic activity. This stability has existed for approximately 1000Ma. A classic example is the Canadian Shield.