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

[摘要] ENGLISH ABSTRACT: This dissertation is a compilation of two case studies that integrates an extensive undergroundmining and drilling data set on Karoo dolerite intrusions in the Highveld Coalfield toconstrain the emplacement processes of the upper-crustal plumbing system of the Karoo largeigneous province.Chapter 3 describes by means of a three-dimensional strata model the geometry of aregional-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 Supergroupunderlain by a shallow dipping basement feeder. The model demonstrates the lateralemplacement of magma where a single basement feeder gives rise to several split levelsaucers that subsequently coalesces into one vast saucer complex. Furthermore, theserelationships show a strong spatial and geometric dependency of saucers to their underlyingfeeders. Lithological interfaces and weak layers control and facilitate the lateral emplacementof magma during the development of saucers in the Karoo Supergroup.The common occurrence of localised dome- and ridge-shaped structures along the flat innersill of the saucers are likely generated from lobate magma flow processes. Inflation ofindividual magma lobes induce overlying strata failure along multiple curved faults thatfacilitates the formation of circular inclined sheets feeding a flat lying roof seated at a higherstratigraphic level.Chapter 4 examines the spatial and temporal relationships between dolerite dykes and thesaucers that make up the larger number 8 sill complex. These dykes have distinct short strikelengths, curved geometries and form interconnected and cross-cutting patterns. Moreover, thedykes are often rooted along the upper surface of underlying saucers and cannot be seen toextend below the base (inner sill) of the saucers. Contact relationships show a mainly coevalor 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 andnon-systematic dykes. Systematic dykes form a well-organised interconnected boxwork orladder-like pattern of two near-orthogonal dyke sets confined to the inner sill of theunderlying saucer. The formation of these dykes is related to the uplift and stretch of thestrata directly overlying propagating magma lobes along the inner sill of the underlyingsaucer. Conversely, non-systematic dykes display a more irregular array of structures that notonly overlie the inner sill but often cross-cut or extend outwards from the inclined sheets ofthe saucer. These dykes are likely the product of multi-directional stretch induced during theemplacement of coeval and adjacent saucers forming a so called 'cracked lid similar to fielddescriptions of stacked saucers in Antarctica. Dyke-saucer relationships of the HighveldCoalfield highlight the influence of magma emplacement processes and the deformation ofhost strata rather than far-field tectonic stresses.

[发布机构] Stellenbosch University