DEFINITION OF CRATONS

Large areas of relatively thick continental crust that

have been stable for long periods of geological time, generally

since the Archean. Most cratons are characterized by low heat

flow, no or few earthquakes, and many have a thick mantle

root or tectosphere that is relatively cold and refractory, having

had a basaltic melt extracted from it during the Archean.

Understanding the origin of stable continental cratons

hinges upon recognizing which processes change the volume

and composition of continental crust with time, and how and

when juvenile crust evolved into stable continental crust. The

evidence from the preserved record suggests that the conticratons

nental landmass has been growing since the early Archean,

although the relative rates and mechanisms of crustal recycling

and crustal growth are not well known and have been

the focus of considerable geological debate. The oldest rocks

known on the planet are the circa 4.0 Ga Acasta gneisses

from the Anton terrane of the Slave Province. The Acasta

gneisses are chemically evolved and show trace and Rare

Earth Element (REE) patterns similar to rocks formed in

modern supra-subduction zone settings. Furthermore, the

3.8-billion-year-old Isua sequence from Greenland, the oldest

known sedimentary sequence, is an accretionary complex. A

few circa 4.2 Ga zircon grains have been found, but it is not

clear if these were ever parts of large continental landmasses.

Approximately half of the present mass of continental crust

was extracted from the mantle during the Archean.

Exposed portions of Archean cratons are broadly divisible

into two main categories. The first are the “granite-greenstone”

terranes, containing variably deformed assemblages of

mafic volcanic/plutonic rocks, metasedimentary sequences,

remnants of older quartzo-feldspathic gneissic rocks, and

abundant late granitoids. The second main class of preserved

Archean lithosphere is found in the high-grade quartzo-feldspathic

gneiss terranes. Relatively little deformed and metamorphosed

cratonic cover sequences are found over and

within both types of Archean terrain, but they are especially

abundant on southern Africa’s Kaapvaal craton. Also included

in this category are some thick and laterally extensive carbonate

platforms similar in aspect to Phanerozoic carbonate

platforms, indicating that parts of the Archean lithosphere

were stable, thermally subsiding platforms.

Although the rate of continental growth is a matter of

geological debate, most geological data indicates that the

continental crust has grown by accretionary and magmatic

processes taking place at convergent plate boundaries since

the early Archean. Arc-like trace element characteristics of

continental crust suggest that subduction zone magmatism

has played an important role in the generation of the continental

crust. Convergent margin accretionary processes that

contribute to the growth of the continental crust can be

divided into five major groups: (1) oceanic plateau accretion;

(2) oceanic island arc accretion; (3) normal ocean crust (midocean

ridge) accretion/ophiolite obduction; (4) back arc

basin accretion; and (5) arc-trench migration/Turkic-type

orogeny accretion. These early accretionary processes are

typically followed by intrusion of late stage anatectic granites,

late gravitational collapse, and late strike-slip faulting.

Together, these processes release volatiles from the lower

crust and mantle and help to stabilize young accreted crust

and form stable continents.

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