Dyke-sill relationships in Karoo dolerites as indicators of propagation and emplacement processes of mafic magmas in the shallow crust. (April 2017)

• Record Type: Journal Article
• Authors: Coetzee, A.; Kisters, A.F.M.
• Is Part Of: Journal of structural geology. Volume 97(2017:Apr.)
• Page Start: 172
• Page End: 188
• Publication Date: 2017-04
• Subjects: Saucer-shaped sills -- Dolerite intrusions -- Sill-fed dykes -- Karoo Basin -- Mafic dyke patterns; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2017.03.002
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: This paper describes the spatial and temporal relationships between Karoo-age (ca. 180 Ma) dolerite dykes and a regional-scale saucer-sill complex from the Secunda (coal mine) Complex in the northeastern parts of the Karoo Basin of South Africa. Unlike parallel dyke swarms of regional extensional settings, mafic dykes commonly show curved geometries and highly variable orientations, short strike extents and complex cross-cutting and intersecting relationships. Importantly, the dyke networks originate from the upper contacts of the first-order dolerite sill-saucer structure and are not the feeders of the saucer complex. Cross-cutting relationships indicate the largely contemporaneous formation of dykes and the inner sill and inclined sheets of the underlying saucer. Systematic dykes form a distinct boxwork-type pattern of two high-angle, interconnected dyke sets. The formation and orientation of this dyke set is interpreted to be related to the stretching of roof strata above elongated magma lobes that facilitated the propagation of the inner sill, similar to the "cracked lid" model described for large saucer complexes in Antarctica. Dyke patterns generally reflect the saucer emplacement process and the associated deformation of wall rocks rather than far-field regional stresses. Highlights: Sill-fed dykes overlying saucer complexes in the Karoo Basin. Dykes are short, curved with variable strikes forming interconnected patterns. Systematic and non-systematic dyke patterns are linked to the stretch of strata above intruding magma lobes.