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Folding of the Rustenburg layered suite adjacent to the Steelpoort pericline: implications for syn-Bushveld tectonism in the eastern Bushveld Complex

KwaZulu-Natal Unit, Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200, South Africa, School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: bmclarke{at}mweb.co.za

Ron Uken, Mike K. Watkeys and Jürgen Reinhardt

School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken{at}ukzn.ac.za; watkeys{at}ukzn.ac.za; reinhardtj{at}ukzn.ac.za

The northeastern Bushveld Complex contact aureole is characterised by numerous, subequally-spaced, kilometre-scale metasedimentary Pretoria Group floor rock domes that penetrate the overlying Rustenburg Layered Suite to varying stratigraphic levels. The origin of these floor domes has been the subject of much debate, with both regional folding and diapiric models invoked as causal processes. Of late, however, the diapiric model of deformation has gained support and has been successfully thermomechanically modelled. Here the Rustenburg Layered Suite adjacent to one of these floor domes, the Steelpoort pericline, is examined and high-strain zones in the magmatic rocks in close spatial proximity to the pericline are described. On the northeastern flank of the pericline, the lower part of the Rustenburg Layered Suite is gently folded but discordant to the floor rock structure against which it abuts. The folds are characterised by their curvilinear axial traces that are parallel to the contact between the metasediments of the floor dome and the encircling igneous rocks. On the southwestern flank, high strain shear zones, characterised by sheath folds and strong tectonic fabrics, indicate shedding of igneous material off the flanks of the dome. These deformation zones adjacent to the Steelpoort pericline add independent support to the diapiric model of deformation proposed for the domal structures. The folding on the northeastern flank is readily attributable to diapir-induced drag and compression associated with diapiric growth. The sheath folds on the southwestern flank formed by the gravity-induced shedding of partially crystallised igneous material off the dome flanks during diapiric uplift. In addition, the numerous transgressive and onlapping relationships observed between units of the Rustenburg Layered Suite adjacent to the floor domes is compatible with diapiric deformation of the floor rocks.

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