The elusive feeders of the Karoo Large Igneous Province and their structural controls

Abstract

Dolerite sills in the central parts of the Karoo large igneous province on the Kalahari Craton in South Africa record the emplacement of magic magmas over 350,000 km2, but the feeders to the voluminous magmatism have yet to be identified. Extensive exploration and mining data across a 7400 km2 area and to a depth of 2.5 km in the northern Karoo Basin provide unprecedented access to the subsurface geometry of dolerite sills and their relationship to underlying basement strata. The data show saucer-shaped sills in the Karoo Supergroup with distinctly funnel-shaped geometries along their base that are seated within basement rocks. The funnel-shaped dolerite structures are interpreted to represent the feeders to the overlying regional-scale sill-saucer complexes. Four distinct saucer geometries can be identified (1) cone-shaped saucers that extend outwards from a single funnel-shaped point (Type A), (2) elongated saucers that dip towards multiple aligned funnel-shaped points (Type B), (3) the more commonly described elongated saucers with flat inner sills (Type C) and (4) elongated saucers with a combination of Type B and C geometries (Type D). Type A and C saucer geometries and point-like contacts are consistent with analogue models of saucers for respective pipe-like and linear feeders. However, the multiple pipe-like feeders associated with Type B and combination thereof with sheet-like feeders (Type D) represent saucer-feeder relationships thus far not modelled. Notably, the lower dolerite funnels are commonly rooted in older faults and/or dykes within the basement rocks. This spatial correlation emphasizes the significance of pre-existing basement structures in the Kalahari craton for the long-range transfer of the magmas through the crust. The distributed occurrence of numerous magma feeders across much of the craton is more consistent with thermal incubation of the sub-lithospheric mantle beneath Gondwana than the impact of a mantle plume.