Archaean cratons

Formation and evolution of Archaean cratons insights from southern Africa... 

Archaean cratons have long been prime targets for a broad array of scientific studies, partly because they form the oldest cores of the continents, but also because they have economic significance as a major source of the world s mineral wealth. (The term craton is confined here to Archaean crust, although the word is commonly applied to Proterozoic shields as well. The Archaean-Proterozoic boundary is, as always, imprecisely defined, but in southern Africa is marked by a sufficient hiatus in time that there is no difficulty... 

Tomographic results show that high-velocity mantle roots extend locally to depths of at least 250 km beneath undisturbed Archaean craton, with no comparable root structures beneath post-Archaean terranes. Neither body- nor surface-wave analyses have yet produced evidence of a low-velocity zone beneath the cratonic keel (Ritsema van Heijst 2000 Freybourger et al. 2001 ... 

Seismic heterogeneity and anisotropy in the Western Superior Province, Canada insights into the evolution of an Archaean craton... 

An enduring question in the study of the Earth concerns the origin and stability of Archaean cratons (Jordan 1978 Bickle 1986 Helmstaedt Schulze 1989 Hoffman 1990 Abbott 1991). They drift around the globe with remarkable resistance to tectonic deformation and thermal erosion. A key to understanding the nature of these cra-... 

In some ways the Superior Province appears different from other Archaean cratons worldwide. For example, it exhibits thicker than average crust and long uninterrupted geological belts, not commonly seen on other cratons. The Superior Province is also unusual in the sense that it exhibits very large shear-wave splitting values in the heart of the craton and very little in the surrounding Proterozoic Trans-Hudson shear zone. It is more normal for the surrounding mobile... 

James, D. E. Fouch, M. J. 2002. Formation and evolution of Archaean cratons insights from southern Afriea. In Fowler, C. M. R., Ebinger, C. J. Hawkesworth, C. J. (eds) The Early Earth ... 

The mineralogical and chemical composition of peridotite from subcontinental lithosphere differs from that of peridotite from other parts of the mantle (Boyd 1989 Berstein et al. 1997). Peridotites from subcontinental lithosphere is depleted , which means it contains only a small amount of clinopyroxene and an aluminous phase, which together make up the so-called basaltic component. The lithosphere beneath the oldest Archaean cratons has a composition markedly different from that of younger subcontinental lithosphere (Boyd Mertzman 1987 Griffin et al. 1999). Old unmetasomatized lithosphere is harzburgitic, a mixture of olivine... 

The Pb isotope data used here are from a worldwide compilation of samples from Archaean cratons (These data can be obtained from the Society Library or the British Library Document Supply Centre, Boston Spa, Wetherby, West... 

Fig. 7. Location of high-(i and low-(i Archaean cratons with respect to CFB provinces and present-day DUPAL anomalies. Contour lines for DUPAL anomalies represent equal A7/4 values (Hart 1984). pi values for CFB provinces are calculated from Pb isotopic ratios (Carlson 1984 EUam Cox 1989 Hergt el al. 1989, 1991 Sharma et al. 1992 Wooden et al. 1993 Thirlwall et al. 1994 Molzhan et al. 1996 Antonini et al. 1999).

Hawkesworth, C., Moorbath, S., O Nions, R. Wilson, J. 1975. Age relationships between greenstone belts and granites in the Rhodesian Archaean craton. Earth and Planetary Science Letters, 25, 251-262. 

Robertson, D. K. 1973. A model discussing the early history of the Earth based on a study of lead isotope ratios from veins in some Archaean cratons of Africa. Geochimica et Cosmochimica Acta, 37, 2099-2124. 

Seismic studies of cratonal lithosphere at both the global and local scale indicate that Archaean cratons are sites of unusually thick, cold lithosphere, which may have mantle roots extending to depths of200-400 km compared with normal continental lithosphere, which is less than 150 km thick (e.g. van der Lee Nolet 1997 Ritsema et al. 1998 Ritsema van Heijst 2000 James Fouch 2002). Heat-flow data place some bounds on lithospheric thickness, but regional comparisons require major assumptions as to the lower-crustal contribution to radioactive heating. 

We summarize the results of several plume simulations for a small craton, using the approach of Sleep (1997). We then analyse the effect of plume material emplacement beneath cratonic keels and channels of different dimensions. This work presents new simulations of plume flow beneath the African continent that include deep keels beneath Archaean cratons, which were not considered in the simple models of Ebinger Sleep (1998). Flow velocities and strains predicted from our preferred model allow us to estimate the magnitude and direction of SKS splitting by plume flow around a cratonic keel. These patterns are then compared with SKS splitting patterns from normal mantle flow around a keel (e.g. Fouch et al. 1999). 

Artemieva, I. Mooney, W. 2001. Petrological constraints on lithospheric geotherms and implications for Archaean cratons. Journal of Geophysical Research, 106, 16387-16414. 

Many Archaean cratons show marked lateral heterogeneity. This heterogeneity can be expressed by many different attributes ages and distribution of greenstone belts, plutonic and (or) metamorphic age domains, contrasting struc-... 

Among the many geological attributes of Archaean cratons that appear distinct, if not unique, two features stand out. These features are intimately related to each other and, ultimately, to significantly higher heat production in both crust and mantle. 

The lateral heterogeneity of Archaean cratons, together with their structural and stratigraphic complexity, favours a mobile tectonic regime (e.g. Burke et al. 1976) that, at least in some... 

Archaean Cratons contain three different types of lithological association, each of which provides important information about the early stages of the Earth System. These associations are Archaean greenstone belts, late Archaean sedimentary basins, and Archaean granite-gneiss terrains. We will consider each in turn. 

There are only a few Archaean cratons which preserve late Archaean sedimentary basins, which are unconformable on the craton. These sedimentary basins are very different from greenstone belts, for they are not strongly deformed, nor do they have the elongate shape and the abundance of basalts frequently associated with greenstone belts. Basins of this type have previously been described as "late Archaean cover successions" (Nisbet, 1987). In the past, their relatively undeformed state has misled workers into thinking that they were Proterozoic, rather than Archaean in age,... 

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