3.5 BILLION YEARS OF RESHAPING, DEPLETING AND RESTOCKING KAAPVAAL CRATON CRUST

The KCraton comprises 200-300 km thick Archean lithosphere. Its crust, in places 3.7 Ga old, is divided into a number of amalgamated domains separated by tectonic shear systems with listric geometry rooted in its middle and lower crust and in some instances its upper mantle. Complex stacked crust is 35-50km thick with as much local depth diversity as in adjacent younger terrains. Relatively flat Moho-sections at 38-40km depth truncate across intra- and inter-crustal boundaries and is not pristine Archean: it has been repeatedly reshaped by thermo-chemical ‘surgery’ during ultra-high-T tectono-metamorphism, melting and magmatic processes, leaving a lower quartz-rich granulite crust with a seismic anisotropy that it shares with its underlying mantle and younger surrounding terrains.

The KCraton also is a mineral-diversity hotspot encoded with metallogenic “fingerprints” of Fe Cr Ni PGE Au; and diamonds that range in age from Mesoarchean to Cenozoic. Their preservation in the depleted mantle confirms the resilience of cratons as Archean archives. KCraton domains first stabilized by 3.0 Ga and were then eroded and peneplained: in many places up to 10 km of ore- and diamond-bearing crust was removed before 2.9 Ga. Thereafter the peneplain was reworked repeatedly near sea-level until Cretaceous uplift by up to 2 km removed a further 2-7 km crust. Thus, both the top and the bottom of the Archean crust are long-lived complex palimpsests. Copious amount of mantle materials were transferred across the evolving Moho during repeated geothermal perturbations, resulting in large LIP-events at: ca. 2.7, 2.0, 1.1, 0.18, 0.13, 0.1 Ga. Diamonds retrieved from kimberlites and sediments indicate that the crust and the depleted mantle retained cohesion > 3.2 Gyrs.

A fundamental question is how these observations can be reconciled with thermal plumes impinging on the KCraton. Perhaps its Moho-transition acts as an episodic thermo-chemical enrichment zone along which downwards percolation of CO2-H2O-rich fluids stimulate melting and transfer of Archean mantle materials into the crust, leaving patches of newly depleted mantle and lower crust. One test of this model lies in imaging if fluids from the East African Rift presently propagating into the KCraton are exploiting this Moho-space and in doing so increase risks of future hazards.