NEW STRATIGRAPHIC ARCHITECTURE OF THE ONVERWACHT SUITE, BARBERTON GREENSTONE BELT, SOUTH AFRICA: FRESH FERMENT FOR MID-ARCHEAN TECTONIC MODELS

New field data identifies major shear zones and a major unconformity that separate the rock sequences of the southern Barberton greenstone belt into seven complexes with different geologic histories. The stratigraphic nomenclature presently applied to these sequences (e.g. formations) cannot incorporate these complexities. This requires changing the status of the traditional ‘Formations’ of the Onverwacht Group of the Barberton greenstone belt into ‘complexes’. The complexes are referred to collectively as the Onverwacht Suite. The total range in age and thickness of the suite is about 120 myrs and 15 km, respectively, but the precise ranges of each complex remain unknown. All complexes comprise significant volumes of intrusions into their volcanic and volcanoclastic host rocks. In some complexes the intrusions and volcanic rocks have similar geochemistry, and are comagmatic. Others provide ‘stiching’ ages between complexes. Six of the complexes have ophiolite-like affinities and formed predominantly in oceanic spreading- and intraoceanic arc- environments with water depth >2km. The complexes are tectonically stacked, and the original spatial relationships between them are uncertain. One complex was uplifted and deeply eroded/incised between about 3470 and 3460 Ma, with a minimum uplift rate (2-4 mm/yr) similar to that during emplacement of Phanerozoic ophiolites. Unconformably overlying subaerial coarse clastic sediments overlap in age with felsic-intermediate igneous activity, dated between 3460 to 3445 Ma, that, in turn, relate in age to intrusion of the tonalite-trondjemite plutons to the south of the belt. The emergence of this deep water complex above sealevel and the onset of the subaerial clastic sedimentation probably best represent the start of obduction and tectonic burial of the lower Onverwacht complexes. Their exhumation followed 200 myrs later during a second major episode of subduction/accretion, with up to 20 km unroofing at minimum early cooling rates in the order of 1.5-5 mm/yr, similar to those recorded in modern orogenic belts. Rates of related sedimentation and deformation were like those observed in modern transcurrent plate boundaries. Thus in this study area rates of Archean tectonic processes appear similar, within an order of magnitude, to those encountered today.