The Lebombo monocline and associated feeder dyke swarm: Diagnostic of a successful and highly volcanic rifted margin?

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As part of the Karoo large igneous province (LIP), the ∼ 700 km-long and ∼ 182 Ma-old Lebombo monocline outcrops inland of a younger Mozambique Lowland cover that separates it from the Indian Ocean. Detailed mapping of a particularly well exposed Olifant River traverse across this monocline correlates basaltic and rhyodacitic lava units within the Lebombo Group to at least three hosted generations of intrusions within the monocline-parallel Northern Lebombo dyke swarm, implying that a much denser feeder dyke swarm is covered by younger flows. Measured attitudes and thicknesses of up to 197 lavas and 791 dykes statistically quantify a structural cross section, from which a properly weighted lava log of nearly 5 km basalts inter-bedded with an overlying > 3.5 km of rhyodacites is constructed. The geological section is compatible with a slightly deeper (sub-lava) section through the innermost ∼ 30 km's of a relatively narrow East Greenland margin, to which the Lebombo monocline can be rigorously compared. In both cases, field relationships are consistent with early tectonic extension being overwhelmed by dyke dilation within margin-parallel swarm segments that may have been injected laterally from central magma chambers. Thus, it is argued that the Lebombo monocline is part of a successfully rifted margin that, furthermore, belongs to a distinct class of highly volcanic and relatively narrow margins, which only form proximal to classical triple rift LIP centers, where an elevated magma flux sustains a magmatic mode of extension. For the Karoo LIP, this conforms with (1) a tight Gondwana reconstruction of Africa with Antarctica, (2) successful breakup during intense ∼ 182 Ma emplacement of the Lebombo Group, (3) triple rift formation, together with a short-lived (∼ 181–178 Ma) Okavango dyke swarm, and (4) anomalously thick igneous crust formation during continued ∼ 2.5 cm/yr separation into a loose ∼ 155 Ma plate configuration of Africa and Antarctica.