Sill complexes in the Karoo LIP: Emplacement controls and regional implications. (October 2019)

• Record Type: Journal Article
• Authors: Coetzee, A.; Kisters, A.F.M.; Chevallier, L.
• Is Part Of: Journal of African earth sciences. Volume 158(2019)
• Publication Date: 2019-10
• Subjects: Saucer-shaped sills -- Dyke swarms -- Karoo LIP -- Gondwana break-up -- Magma emplacement processes -- Sedimentary basin -- Rigidity contrasts; Earth sciences -- Africa -- Periodicals; Earth sciences -- Middle East -- Periodicals; Geology -- Africa -- Periodicals; Geology -- Middle East -- Periodicals; Sciences de la terre -- Afrique -- Périodiques; Sciences de la terre -- Moyen-Orient -- Périodiques; Géologie -- Afrique -- Périodiques; Géologie -- Moyen-Orient -- Périodiques; Earth sciences; Geology; Africa; Middle East; Periodicals; Electronic journals; 556.05
• DOI: 10.1016/j.jafrearsci.2019.103517
• Languages: English
• ISSNs: 1464-343X

Abstract

Abstract: Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km 2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof. The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180 Ma), mainly sill complexes to later (182-174 Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism. Highlights: Nested sill complex made up of five distinct, successively emplaced saucer-shaped sills. Underaccretion of magma beneath solidified sills suggest they serve as stress barriers to later magma pulses. Low deviatoric stresses or mild shortening prevailed across southern Africa prior to Gondwana break-up.