DISTRIBUTION AND TECTONICS OF SOUTH-AFRIC AN CRETACEOUS KIMBERLITES -IMPLICATIONS FOR THE DYNAMICS OF THE MANTLE

Tectonics of Cretaceous kimberlites is studied in very details through accurate 1:50,000 mapping over an area some 700 km long by 300 km wid e and covering three different crustal environments: craton, craton ma rgin, mobile belt. A map is compiled at the subcontinent scale. A pecu liar tectonic style is found which consists of curved fracture swarms. Each swarm seems to have a specific emplacement age and apparently ca n be distinguished by their petrological signature. The tectonic style is independent of the underlying crustal environment and occur as wel l above the craton, the margin or the mobile belt. Although the differ ent structural features of the crust might have plaid a role in the di stribution of kimberlites, the tectonic pattern cannot be explained in term of classical plate tectonic. A model is proposed in which kimber lite swarms reflect the shape of the magma source i.e. tangential turb ulent flow in the shallow upper mantle.