The geometry of Karoo dolerite dykes and saucers in the Highveld Coalfield : constraints on emplacement processes of mafic magmas in the shallow crust
Date
2016-12
Authors
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Stellenbosch : Stellenbosch University
Abstract
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.
AFRIKAANS OPSOMMING: Geen opsomming beskikbaar
AFRIKAANS OPSOMMING: Geen opsomming beskikbaar
Description
Thesis (MSc)--Stellenbosch University, 2016.
Keywords
Karoo large igneous province -- Upper crustal plumbing system, Highveld Coalfield -- Underground mining and drilling data set, Karoo dolerite dykes and saucers -- Geometry, UCTD