Magma flow pattern in a giant dolerite sill and implications for the Karoo mantle plume hypothesis
Abstract
The Karoo Large Igneous Province was formed over a period of 10 Myr, between 184 and 174 Ma, in Southern Africa. Thick series of continental flood basalts (CFB) fed by a network of sills and dikes were emplaced in the Carboniferous to Jurassic Karoo Basin. The large volume (106 km3) and geochemical signature of CFB suggest a mantle plume origin, although a sub-continental lithospheric mantle source has also been proposed (e.g., Duncan et al., 1984; Ellam & Cox, 1989; Ellam & Cox, 1991; Ellam et al., 1992; Sweeney et al., 1994). The generally accepted diameter of mantle plumes is about 1,000 km and plumes tend to form a mushroom head. The actual size of the Karoo plume beneath the base of the lithosphere (BOL) is likely to affect the flow pattern of basaltic magmas above the plume. For example, a wider plume would lead to a consistently vertical flow pattern across a wide area, whereas a smaller plume would generate a diverging upward flow pattern. The distribution and orientation of Karoo dikes outside the Karoo basin appears to be significantly controlled by Precambrian basement structures (e.g., Jourdan et al., 2006). Hence the dike pattern should not be used to constrain the Karoo plume location, size or shape. Alternatively, the magma flow directions in the dolerite sills emplaced within the Karoo Basin could potentially provide valuable constrains on the location of the plume head. The lower section of the Karoo Basin is characterized by a large, continuous, 10-20 m thick basal dolerite sill. We propose several theoretical magma flow models that would be expected from contrasting locations of the Karoo plume head: (1) radial, plume underneath the Drakensberg mountains, (2) radial, plume underneath between Namibia-Botswana or (3) parallel, driven by northerly continental convergence. These models are currently being tested using the anisotropy of magnetic susceptibility (AMS) as a proxy for magma flow direction. Oriented samples are systematically collected from the top of the sill where the AMS fabrics have been shown to accurately mimic magma flow.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V23E2155L
- Keywords:
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- 1518 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic fabrics and anisotropy;
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8450 VOLCANOLOGY / Planetary volcanism;
- 8499 VOLCANOLOGY / General or miscellaneous