HR: 11:15h
AN: S51F-04 INVITED     [PDF]
TI: Formation and Evolution of Cratons: Insights from the Kaapvaal Project of Southern Africa
AU: * James, D E
EM: james@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Schmitz, M D
EM: schmitz@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287 United States
AU: Shirey, S B
EM: shirey@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Carlson, R W
EM: carlson@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: The Kaapvaal Project examined fundamental questions of craton formation and mantle differentiation in the early Earth. The principal aim of the project was to characterize the physical and chemical nature of the crust and mantle of the Archean cratons of southern Africa in geologic detail, and to use the 3-D seismic and geochemical images of crustal and mantle heterogeneity to reconstruct cratonic evolution. 3-D body wave tomographic images reveal that high velocity mantle roots extend to depths of at least 200 km, and locally to depths of 250-300 km beneath cratonic terranes. No low velocity channel has been identified beneath the cratonic root. Seismic velocity determinations in both on-craton and off-craton mantle xenoliths are consistent with observed seismic structures. The Kaapvaal craton was modified ca. 2.05 Ga by the Bushveld magmatic event, and relatively low seismic velocities characterize the mantle beneath the Bushveld province. The crust beneath undisturbed Archean craton is comparatively thin (~35-40 km), unlayered, and characterized by a strong velocity contrast across a sharp Moho. Post-Archean terranes and Archean regions disrupted by large-scale Proterozoic events tend to be characterized by thicker crust, complex Moho structures and higher lower crustal velocities. Re-Os depletion model age determinations confirm that the mantle root beneath the cratons is Archean in age. No age progression with depth is observed in the mantle keel, indicating that it was at least 200 km thick even in Archean time. Both laboratory experiments and geochemical results from inclusions in diamonds, eclogite xenoliths, and coeval crustal rocks suggest that subduction processes played a central role in the initial formation of Archean crust, the melt depletion of Archean mantle, and the assembly of continental lithosphere from smaller cratonic blocks. Coordinated geochronological studies of crustal rocks and mantle xenoliths have also shown that the timing of final stabilization of the extant craton correlates with convergent margin processes and assembly of terranes along its northern and western ramparts. Numerous lines of evidence thus appear to support the petrogenetic coupling of continental crust and lithospheric mantle through a model of continental arc magmatism, subduction zone mantle wedge processing and terminal collisional advective thickening to form Archean continental tectosphere. Analysis of lower crustal xenoliths has shown that much of the deep craton experienced a dynamic and protracted history of tectonothermal activity that is temporally associated with events seen in the surface record. Recent studies show, moreover, that compositions of diamonds and diamond inclusions are correlated with seismic P-wave velocities at depths in the diamond stability field. Although the deep root structures beneath the southern Africa cratons have been basically stable since the Archean, they have been repeatedly subjected in post-Archean time to thermal perturbations and varying degrees of metasomatic modification.
UR: http://www.ciw.edu/kaapvaal
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 7218 Lithosphere and upper mantle
DE: 8180 Tomography
DE: 9305 Africa
SC: Seismology [S]
MN: 2003 Fall Meeting