Crust–mantle decoupling and the growth of the Archaean Zimbabwe craton
Introduction
The nature of tectonic processes resulting in the formation of Archaean cratons remains a poorly understood topic in earth sciences. We know that at some stage during the Archaean the change occurred from vertical melt segregation leading to formation of early crust, to the emergence of strong plates affected by plate margin processes similar to modern-day plate tectonics. What is less obvious is how and when this transition took place, whether it was gradual or sudden, or separated by a period dominated by entirely different tectonic processes.
In this paper we present a summary of the geological, geochronological and geochemical data for the Zimbabwe craton, which can be used to build a convincing case for lateral continental growth through obduction/thrusting, and low-angle underplating and remelting of crustal fragments, which is similar to scenarios described for the Superior Province, Kaapvaal craton and Yilgarn Block (de Wit et al., 1992; Myers, 1995; Kusky and Polat, 1999). However, such a scenario of crustal thickening through shortening must be paralleled by corresponding shortening of the mantle lithosphere, a point generally not addressed in literature. At the high geotherms prevailing in the Archaean, decoupling of crust and mantle lithosphere is expected, allowing strain to be accommodated independently in both layers. Accretionary geometries in the Zimbabwe craton display large scale spatial alignments indicative of deep seated linear controls that can be used to infer Crust–mantle delamination.
With this paper we would like to start a discussion regarding the interplay between crust and mantle lithosphere during the Archaean, by offering some possible tectonic scenarios based on the Zimbabwe craton example.
Section snippets
Growth of the Zimbabwe craton: a case study
The Zimbabwe craton is a granite-greenstone terrain in southern Africa that formed during a timespan of almost 1000 Ma (3.55–2.58 Ga; Fig. 1). The evolution of this terrain is explained with a series of crustal growth stages that added greenstones and granitoids to a ≈3.5 Ga old nucleus (Wilson et al., 1995). These stages are commonly linked to the emplacement of mantle plumes below existing continental crust, causing intracontinental rifting and crustal melting (e.g. Bickle et al., 1994;
And what about the mantle lithosphere?
Geochronological, geochemical and structural observations on the formation of the Zimbabwe craton are all consistent with lateral growth, centred on old proto-cratonic nuclei, through processes that resemble active continental margin and collisional processes like subduction and thrust stacking in active orogens. In spite of such similarities, a uniformitarian application of modern-day accretion–subduction–collision models to Archaean terrains cannot be made. For example, crustal stacking in
The strength of the Archaean lower crust
The Archaean represents the time period during which Earth transformed from a hot, seething ball of molten lava covered by a thin crust, to something resembling a modern-day tectonic scenario with plate boundaries and plates strong enough to transmit stresses. Throughout the Archaean, but especially during the earlier parts, Earth was characterised by a far less differentiated and evolved mantle-crust architecture, and by much higher geothermal gradients than today. This was partly due to
Conclusion
Considering all of the above points we would like to suggest the following scenario for the evolution of the Zimbabwe craton, which may also be applied to other cratons. In the early Archaean thick mantle lithosphere was formed after segregation of a basaltic melt, which accumulated as oceanic crust. At the prevailing geotherms a thin layer of strong upper crust was separated from a much thicker layer of strong mantle lithosphere by highly ductile lower crust. Total decoupling between the crust
Acknowledgements
We would like to thank Stichting Schurmannfonds for financially supporting this research (grants 1996–2001/13). Dr. Kurt Stuewe and Dr. Henri Kampunzu are thanked for their supportive reviews.
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