The geometry of Karoo dolerite dykes and saucers in the Highveld Coalfield : constraints on emplacement processes of mafic magmas in the shallow crust

Coetzee, Andre (2016-12)

Thesis (MSc)--Stellenbosch University, 2016.


ENGLISH ABSTRACT: This dissertation is a compilation of two case studies that integrates an extensive underground mining and drilling data set on Karoo dolerite intrusions in the Highveld Coalfield to constrain the emplacement processes of the upper-crustal plumbing system of the Karoo large igneous province. Chapter 3 describes by means of a three-dimensional strata model the geometry of a regional-scale Karoo-age (ca. 180Ma) saucer complex locally referred to as the number 8 sill. The saucer complex consists of three saucers largely confined to the Karoo Supergroup underlain by a shallow dipping basement feeder. The model demonstrates the lateral emplacement of magma where a single basement feeder gives rise to several split level saucers that subsequently coalesces into one vast saucer complex. Furthermore, these relationships show a strong spatial and geometric dependency of saucers to their underlying feeders. Lithological interfaces and weak layers control and facilitate the lateral emplacement of magma during the development of saucers in the Karoo Supergroup. The common occurrence of localised dome- and ridge-shaped structures along the flat inner sill of the saucers are likely generated from lobate magma flow processes. Inflation of individual magma lobes induce overlying strata failure along multiple curved faults that facilitates the formation of circular inclined sheets feeding a flat lying roof seated at a higher stratigraphic level. Chapter 4 examines the spatial and temporal relationships between dolerite dykes and the saucers that make up the larger number 8 sill complex. These dykes have distinct short strike lengths, curved geometries and form interconnected and cross-cutting patterns. Moreover, the dykes are often rooted along the upper surface of underlying saucers and cannot be seen to extend below the base (inner sill) of the saucers. Contact relationships show a mainly coeval or contemporaneous emplacement of dykes to the inner sill and inclined sheets of the underlying saucer. The dykes can be described as two diverse sets, namely systematic and non-systematic dykes. Systematic dykes form a well-organised interconnected boxwork or ladder-like pattern of two near-orthogonal dyke sets confined to the inner sill of the underlying saucer. The formation of these dykes is related to the uplift and stretch of the strata directly overlying propagating magma lobes along the inner sill of the underlying saucer. Conversely, non-systematic dykes display a more irregular array of structures that not only overlie the inner sill but often cross-cut or extend outwards from the inclined sheets of the saucer. These dykes are likely the product of multi-directional stretch induced during the emplacement of coeval and adjacent saucers forming a so called “cracked lid” similar to field descriptions of stacked saucers in Antarctica. Dyke-saucer relationships of the Highveld Coalfield highlight the influence of magma emplacement processes and the deformation of host strata rather than far-field tectonic stresses.

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