A defining characteristic of the cratonic mantle lithosphere is a seismic velocity that is faster than normal subcontinental mantle to depths of at least 200 km and locally to depths of 250-300 km (Plate 11.1b,c between pp. 244 and 245). Many Proterozoic belts lack these fast velocity anomalies at similar depths. In addition, Archean cratons are characterized by the lowest surface heat flow of any region on Earth, with a heat flux that is lower than adjacent Proterozoic and Phanerozoic crust by some 20 mW m-2 (Jaupart & Mareschal, 1999; Artemieva & Mooney, 2002). Isotopic age determinations and Re-Os studies of mantle nodules (Pearson et al., 2002; Carlson et al., 2005) confirm that the mantle roots are Archean in age and indicate that most have remained thermally and mechanically stable over the past 2-3 Ga. These observations indicate that the roots of the Archean cratons are cool, strong, and composi-tionally distinct from the surrounding mantle.
In the ocean basins, the base of the oceanic lithosphere is marked by a strong decrease in the velocity of shear waves at depths generally less than 100 km beneath the crust (Sections 2.8.2, 2.12). Similar low velocity zones occur under tectonically active continental regions, such as the Basin and Range Province, but beneath the stable cratons low velocity zones are either extremely weak or entirely absent (Carlson et al., 2005). Consequently the base of the continental lithosphere is not well defined by seismological data. With increasing depth the high seismic velocities of the stable lithosphere gradually approach those of the convecting mantle across a broad, ill-defined transition zone below 200 km. Thermal modeling and geochemical data from mantle xenoliths have helped to define the location of the lithosphere-asthenosphere boundary. The results suggest that the base of the continental lithosphere is deepest (~250 km) in undisturbed cratonic areas and shallowest (~180 km) beneath Phanerozoic rifts and orogens (O'Reilly et al., 2001). This determination is in general agreement with seismic observations.
In addition to being cool and strong, studies of mantle xenoliths indicate that the Archean mantle roots are chemically buoyant and highly depleted in incompatible elements (O'Reilly et al., 2001; Pearson et al., 2002). When mantle melting occurs elements such as calcium, aluminum and certain radiogenic elements are concentrated into and extracted by the melt whereas other elements, particularly magnesium, selectively remain behind in the solid residue. Those elements that concentrate into the melt are known as incompatible (Section 2.4.1). Both buoyancy and chemical depletion are achieved simultaneously by partial melting and melt extraction, which, in the case of the mantle lithosphere, has left behind a residue composed of Mg-rich harzbur-gites, lherzolites, and peridotite (O'Reilly et al., 2001). Eclogite also appears to be present in the cratonic lithosphere. However, high velocity bodies consistent with large, dense masses of eclogite have not been observed in the continental mantle (James et al., 2001; Gao et al., 2002). An inventory of mantle xenoliths from the Kaapvaal craton suggests that eclogite reaches abundances of only 1% by volume in the continental mantle (Schulze, 1989). These characteristics have resulted in the mechanical and thermal stability of the cratons for up to three billion years (Section 11.4.2).
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