Stratigraphic and sedimentological evidence bearing on structural repetition in early Archean rocks of the Barberton greenstone belt, South Africa

Abstract

The 3.5–3.3 Ga-old Swaziland Supergroup in the Barberton greenstone belt, South Africa, includes a lower, predominantly volcanic unit, the Onverwacht Group, and an upper, largely sedimentary section, the Fig Tree and Moodies Groups. Although Viljoen and Viljoen concluded that the Onverwacht Group exceeds 15 km in stratigraphic thicknesses, more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting. Our mapping indicates that the upper half of the Group includes at least 8 km of unrepeated stratigraphic section. Inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the Onverwacht Group on the west limb of the Onverwacht anticline. Shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform, highly complex zone of deformation up to 2 km wide. The displacement across the zone is unknown, but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the Group has not been significantly altered. Structures previously mapped as major fold hinges within the upper part of the Onverwacht Group can be shown to have formed by local deformation and reflect neither large-scale folding nor structural repetition of the rocks.Several kilometers higher in the section, rocks of the uppermost Onverwacht and Fig Tree Groups are stacked and repeated in a series of thrust nappes. Individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks. They were apparently emplaced by faulting or gravity sliding at or near the Earth's surface. Their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying, much weaker, serpentinized ultramafic komatiites. Locally, the sheets were deformed to form fold nappes. The lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact. The thrust faults are the loci of small hypabyssal igneous bodies that appear to be co-magmatic with a suite of upper Fig Tree volcanic rocks. This style of faulting has not affected rocks of the overlying Moodies Group. Although estimates of the timing of deformation are as yet imprecise, these features suggest that it occurred during a late Fig Tree phase of tectonism and volcanism.