Physics
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9915359d&link_type=abstract
Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. B8, p. 15,359-15,374
Physics
72
Earth Crust, Earth Planetary Structure, Geochemistry, Planetary Evolution, Seismology, Basalt, Chemical Composition, Lava, Sediments, Temperature Effects, Thickness
Scientific paper
Several recent models of crustal evolution are based on the belief that the thickness of the continental crust is proportional to its age, with ancient crust being the thickest. A worldwide review of seismic structure contradicts this belief and falsifies these models, at least for the Archean. Proterozoic crust has a thickness of 40-55 km and a substantial high-velocity (greater than 7 km/s) layer at its base, while Archean crust is only 27-40 km thick (except at the site of younger rifts and collisional boundaries) and lacks the basal high-velocity layer. Seismology also provides evidence that the lithosphere is thickest beneath Archean cratons, while diamond ages show that this lithospheric keel must have already existed in the Archean. Geochemical data also indicate significant differences between Archean and Proterozoic lithosphere. Major and trace element studies of sediments show a change in upper crustal composition between the Archean and the Proterozoic. The secular change in the crust-forming process is attributed to a decline in mantle temperature, leading to a change in the composition of the lithospheric mantle. The higher temperature of the Archean mantle led to the eruption of komatiitic lavas, producing a refractory lithospheric mantle which is ultradepleted in FeO and volatiles. The resultant lithospheric keel is intrinsically less dense than the surrounding mantle and thus not suceptible to delamination. In contrast, Proterozoic crust developed above fertile mantle. The eruption of continental flood basalts and underplating of basaltic sills is attributed to subsequent heating and partial melting of the lithospheric mantle. Consequently, Proterozoic crust is thickened and has a high-velocity basal layer.
Durrheim Raymond J.
Mooney Walter D.
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