Geometry and emplacement controls of dolerite sill complexes in the Karoo basin

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Stellenbosch : Stellenbosch University
ENGLISH ABSTRACT: This dissertation is a compilation of three case studies that integrates drilling, field and geophysical data on dolerite sill complexes across the Karoo basin to constrain the emplacement processes, feeder systems and regional controls during the formation of the Karoo Large Igneous Province (KLIP) at ca. 183 Ma. Chapter 4 describes the relationships between several saucer-shaped sills in the sedimentary strata of the northern Karoo basin and underlying Archaean to Neoproterozoic basement strata of the Kalahari craton. The three-dimensional geometry of numerous individual sills across an area of 7400 km² were reconstructed using extensive drill hole and mining data up to a depth of 2.5 km. Saucer-shaped sills, largely confined to the Karoo Supergroup and representing various geometries, dip towards distinct narrow, funnel-like dolerite structures seated exclusively within the basement layers that likely represent a connection to deep-seated feeders. Importantly, many of these funnel-shape structures are localised near or rooted in older, dykes and normal faults in the basement strata. This highlights the significance of preexisting basement structures in facilitating magma transport through the upper crust and into the Karoo basin. Furthermore, the wide spatial distribution of magma feeders across the Kalahari craton is more aligned with thermal incubation of the sub-lithospheric mantle than the development of mantle plumes beneath Gondwana. Chapter 5 constrains the emplacement controls of sills in the central Karoo basin through field and drill hole data from the Victoria West sill complex. The sill complex consists of five successively emplaced saucers-shaped sills across an area of 2000 km2 that formed though magmatic underaccretion beneath earlier sills. Rigid and fully crystallised earlier sills served as stress barriers to the upwards propagation of later sill feeders which resulted in a nested sill structure marked by a sill-in-sill configuration. The close spatial overlap between the individual saucer-shaped sills and NW-SE trend of the sill complex suggest the reutilization of the same feeder system, most likely the rim of a large underlying sill. Emplacement controls of the Victoria West sill complex indicates sill formation occurred under a mild compressional stress regime at the onset of Karoo magmatism. The changeover from sill complexes at 184-180 Ma to later dyke swarms at 182-174 Ma suggest a switch in the prevailing stress field prior to Gondwana break-up. Thermal loading followed by lithospheric subsidence due to the rapid emplacement of >350 000 km3 of sills and outpouring of >106 km3 of lavas across southern Africa and Antarctica may have triggered extension and the subsequent intrusion of dyke swarms along cratonic margins. Therefore, inherited basin and lithospheric architecture was crucial in the later development of Karoo magmatism. Chapter 6 represents a study of the distinct variations of sill geometries and associated intrusive structures across the Karoo basin in relation to the basal elevation of the Drakensberg Group lavas. These spatial variations in the mode of emplacement of sills are attributed to emplacement depth in addition to changes in sedimentary facies and driving magma pressures. The northern Karoo basin is characterised by shallow (<500m) emplacement depths with sills typically showing small (<10 km) diameters and thicknesses up to 40 m. The central Karoo basin represent intermediate emplacement depths of up to 700 m with sill diameters over 30 km and thicknesses below 100 m. Advanced emplacement depths of <2 km is found in the southern Karoo basin where sills reach sizes of 50-80 km and thicknesses of 35 m. Local dyke networks show similar depth dependent relationships across the Karoo basin. Dense systematic and non-systematic dyke networks are associated with respective shallow (<500 m) and intermediate (<700 m) emplacement depths whereas greater depths (<2 km) are largely devoid of dykes. Additionally, the asymmetric distribution of different sedimentary rock types and southward thickening of layers in addition to differential magma pressures across the Karoo basin further influenced sill emplacement processes and associated structural features. Generally, the wide spatial and stratigraphic occurrence of sills throughout the Karoo basin is more consistent with many distributed feeders emplaced in the underlying basement rocks than a single plume-related source along the rift margins of southern Africa.
Thesis (PhD)--Stellenbosch University, 2020.
Karoo dolerite dykes and saucers -- Geometry, Igneous rocks, Sills (Geology) -- South Africa -- Karoo, Dolerite -- South Africa -- Karoo, UCTD, Upper crustal plumbing system, Diabase -- South Africa -- Karoo, Sedimentary basins -- South Africa -- Karoo, Dykes (Geology)