Archaean crust thickness

James et al. 2001A). Strong variations in crustal thickness based on receiver functions reveal significant differences in the nature of the crust and the crust-mantle boundary between Archaean and post-Archaean geological terranes. Both... 

Within the resolution of the data, the mantle structure of the Archaean Limpopo Belt does not differ significantly from that of the adjacent cratons. The similarity with cratonic mantle structure contrasts sharply with the results of crustal structure determinations (Nguuri et al. 2001), which show the Central Zone of the Limpopo Belt to be characterized by thick crust and poorly developed Moho relative to the adjacent cratons. Interestingly, the SKS splitting results for the southern Africa array show that the Limpopo Belt exhibits a consistent east-west mantle fabric, presumably acquired at the time of craton collision (Silver et al. 2001). 

If therefore, the modern subarc mantle is the site where Phanerozoic subcontinental lithosphere is created, we are still left with a large number of questions about the earlier history of the subcontinental lithosphere. Why for example is the Archaean subcontinental lithosphere so different in composition, heat production and thickness from more recent subcontinental mantle What different processes were operating early in Earth history which are recorded in this mantle domain Is there a link with komatiite extraction, as suggested by Boyd (1989), or with the extraction of basaltic melts Or, is there a close link between the formation of this type of mantle and the over-lying continental crust We will return to these issues when we discuss the origin of the continental crust in Chapter 4 (Section 4.5.1). 

If, as many suppose, the Archaean mantle had a higher potential temperature than the modern mantle, it is important to examine the implications of this for melt production during the early history of the Earth. The relationship between mantle potential temperature and melt thickness during adiabatic melting was outlined in Section 3.1.4.3 and may be briefly summarized by stating that as mantle potential temperature increases so will the melt production, as expressed in the depth of the melt column and the melt thickness. This is illustrated in Fig. 3.26, which shows how deeper, higher-temperature melting should lead to the formation of a thicker oceanic crust. 

Geophysical studies of the continental crust have raised the possibility that there are differences in both crustal thickness and heat production between Archaean continental crust and juvenile crust that has formed more recently. 

It was shown earlier that there is some discussion over the composition of the lower continental crust and whether it is felsic or basaltic. One possible solution is that lower continental crustal compositions have changed with time. Durrheim and Mooney (1991, 1994) studied seismic sections through Archaean and Proterozoic cratons and suggested that Archaean continental crust is between 27 and 40 km thick and has a thin mafic layer at its base, making up between 5 and 10% of the total crustal thickness. In contrast Proterozoic crust is between 40 and 55 km... 

A related debate focused on heat flow data from different regions of the continental crust. Nyblade and Pollack (1993) showed that average heat flow measurements in Archaean cratons are lower than those for Proterozoic cratons. This observation has, however, been interpreted in two quite different ways. On the one hand, it has been argued that cratons of different age have different bulk compositions, and so have different concentrations of heat producing elements (U, Th and K), hence different levels of heat production. Alternatively, the observed differences in heat flow do not derive from the crust but reflect different lithospheric thicknesses between Proterozoic and Archaean cratons reflecting different mantle heat flow contributions (Rudnick et al., 1998 Nyblade, 1999). 

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