Download Comparing and contrasting Cretaceous brittle deformation structures

Comparing and contrasting Cretaceous brittle deformation
structures in the upper continental crust of west and east
coast regions of southern Africa: dyke geometry along the
incipient Namibian margin (Henties Bay-Outjo Dyke
Swarms) and joint systems in the Table Mountain Group
and Karoo intrusions flanking the Cape margin
Muedi T1,3, de Wit M.J2 and Reinhard R.O3
1) AEON (Africa Earth Observatory Network), and Department of Geosciences, Faculty of Science,
Nelson Mandela Metropolitan University, South Africa, Email: [email protected]
2) AEON (Africa Earth Observatory Network), Faculty of Science, Nelson Mandela Metropolitan
University, South Africa, Email: [email protected]
3) Karlsruhe Institute of Technology, Karlsruhe, Germany, Email: [email protected]
ABSTRACT
Mafic dyke swarms are a major component of the South Atlantic Large Igneous Province that originated during the
break-up of Africa and South America ca. 130 Ma. We examined Cretaceous dyke geometry, brittle deformation
and geometric patterns of the Henties Bay-Outjo Dyke Swarm (HOD), Namibia. We also investigated the
relationship between the HOD and Ponta Grossa Dyke Swarm (PG) of Brazil intruded during West Gondwana
break-up. This was achieved by reconstructing Gondwana plates containing the PG and HOD swarms, using
ArcGIS and Gplates software. We also investigated joints systems in the Table Mountain Group quartzite (TMG)
and Golden Valley Sill (GVS) a major Karoo mafic intrusion, both in the Eastern Cape Province, South Africa to
compare geometric patterns and brittle structures with those of the HOD Swarm. Mapping of the dykes was
carried out by remote sensing using digital aerial and satellite images. The fractal analysis conducted within the
HOD indicates a fractal dimension of Df =1.7, while analysis of the TMG and GVS joint systems indicates an
averaged fractal dimension of Df=1.8. For that reason it appears that analysed fractal geometry in the HOD, TMG
and GVS originated under different tectonic stress conditions. The HOD and GVS dominant orientations of
NNE/NE seem to have undergone similar tectonic stress regime during Gondwana break-up. We can determine
that the HOD dykes are not always related to joints during continental break-up; but rather that the dykes initiate
fracture systems into which they subsequently migrate during intrusion.
KEYWORDS: Dyke swarm, joints, tectonic stress, Gondwana and fractals