Formation and evolution of Archaean cratons: insights from southern Africa

Abstract

Archaean cratons are the stable remnants of Earth's early continental lithosphere, and their structure, composition and survival over geological time make them unique features of the Earth's surface. The Kaapvaal Project of southern Africa was organized around a broadly diverse scientific collaboration to investigate 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 cratons of southern Africa in geological detail, and to use the 3D seismic and geochemical images of crustal and mantle heterogeneity to reconstruct the assembly history of the cratons. Seismic results confirm that the structure of crust and tectospheric mantle of the cratons differs significantly from that of post-Archaean terranes. Three-dimensional body-wave tomographic images reveal that high-velocity mantle roots extend to depths of at least 200 kin, and locally to depths of 250 300 km beneath cratonic terranes. No low-velocity channel has been identified beneath the cratonic root. The Kaapvaal Craton was modified approximately 2.05Ga by the Bushveld magmatic event, and the mantle beneath the Bushveld Province is characterized by relatively low seismic velocities. The crust beneath undisturbed Archaean craton is relatively thin (c. 35-40km), unlayered and characterized by a strong velocity contrast across a sharp Moho, whereas post-Archaean terranes and Archaean regions disrupted by large-scale Proterozoic magmatic or tectonic events are characterized by thicker crust, complex Moho structure and higher seismic velocities in the lower crust. A review of Re Os depletion model age determinations confirms that the mantle root beneath the cratons is Archaean in age. The data show also that there is no apparent age progression with depth in the mantle keel, indicating that its thickness has not increased over geological time. Both laboratory experiments and geochemical results from eclogite xenoliths suggest that subduction processes played a central role in the formation of Archaean crust, the melt depletion of Archaean mantle and the assembly of early continental lithosphere. Co-ordinated geochronological studies of crustal and mantle xenoliths have revealed that both crust and mantle have experienced a multi-stage history. The lower crust in particular retains a comprehensive record of the tectonothermal evolution of the lithosphere. 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. Cratonization thus occurred not as a discrete event, but in stages, with final stabilization postdating crustal formation.

Long lasting epeirogenic uplift from mantle plumes and the origin of the Southern African Plateau

“…The Archean nucleus is comprised of the Kaapvaal and Zimbabwe cratons sutured together by the Limpopo Belt [Hartnady et al, 1985;de Wit et al, 1992]; its interior was modified extensively by the 2.05 Ga Bushveld Complex [Hatton and Von Gruenewaldt, 1990], and this modification could have involved some thinning of the lithosphere [James and Fouch, 2002]. Significant Phanerozoic disruption of the Precambrian shield occurred as Gondwana split apart.…”

“…[20] The upper mantle P and S wave velocity structure beneath the Archean nucleus and the mobile belts to the southwest has been imaged tomographically using relative travel time residuals from teleseismic earthquakes recorded by the Kaapvaal Project [James et al, 2001;James and Fouch, 2002]. There appears to be little evidence for any anomalous structure, either within the lithospheric mantle or beneath it.…”

Thermal, hydrous, and mechanical states of the mantle transition zone beneath southern Africa

“…We documented the shoaling of the 410-km discontinuity and the deepening of the 660-km discontinuity beneath the oldest parts of the Archaean cratons. The relation between the transition zone discontinuities and tomographically imaged seismic velocities [15,16], which were unavailable in the study by Gao et al [20], reveals the presence of excess water in a relatively cold mantle transition beneath the Archaean cratons. Furthermore, the spatial correlation between surface geology, high velocities beneath the cratons, and the transition zone structure strongly indicates that the continental keels beneath southern Africa extend to the transition zone, substantially deeper than previously suggested.…”

“…High-velocity features have been veri¢ed to depths of at least 250 km, and perhaps up to 300 km in local regions beneath the cratons [15,16]. At greater depth, the spatial correlation between high-velocity anomalies and the cratons breaks down.…”

“…Locations of broadband seismic stations (triangles) and geologic provinces in southern Africa. The temporary seismic networks comprised the Kaapvaal project [15,16] and Kimberley array and included 113 sites. The networks operated from April 1997 to July 1999 and from December 1998 to July 1999, respectively, and extended from Cape Town, South Africa, to regions of Zimbabwe and Botswana.…”