Deformation in part of the Archaean Kaapvaal Craton, South Africa

Abstract

The results of a regional structural study of some major shear zones, which occur in the Archaean terrain of the northern and eastern Transvaal, are summarized. These zones are major, curvilinear features which are particularly well developed at the northern margins of the Barberton, Murchison and Sutherland greenstone belts, and tend to parallel the general ENE-WSW strikes of these belts. At these margins mylonitic fabrics are developed in both the greenstones near the contacts, and within the adjacent granitic gneisses up to 10 km away from the contacts. Individual, relatively narrow zones of striped mylonites and ultramylonites occur as an anastamosing network within these major zones of intense heterogeneous deformation. Breccia zones and thrusts containing pseudotachylite and ultracataclasite at Murchison and Barberton suggest an extended tectonic history for these zones, at successively higher crustal levels. These zones are relatively late in the tectonic history of the greenstone belts, and modify all pre-existing structures. The base of the Barberton greenstone belt is interpreted as a major, low-angle south-dipping sole thrust associated with the shears at the northern margin, which indicate a northward motion for the Barberton belt. The shear zones have been instrumental in establishing the greenstone belts as discrete entities, in exposing the high-grade gneisses of the Limpopo Mobile Belt to the north, and have influenced Proterozoic sedimentation.