Early Archean foredeep sedimentation related to crustal shortening: a reinterpretation of the Barberton Sequence, Southern Africa

Abstract

The upper, dominantly terrigenous sedimentary successions of the Early Archean Barberton Sequence, Kaapvaal Province, record an evolution similar to that of Phanerozoic foredeeps and may represent the oldest known foredeep sequence, rather than a passive continental-margin sequence as previously proposed. Pre-foredeep ocean crust, the 3.5 Ga-old Onverwacht Group, consists of bimodal volcanics with intercalated shallow-water sediments. Gravity modeling and boundary shear zones indicate that deformed Onverwacht crust was obducted northward onto sialic continental crust immediately before, during or after burial by the synorogenic terrigenous clastic fill. Foredeep basin deepening due to crustal loading from the south is recorded by basinal iron-formation, chert, and mudstone of the basal Fig Tree Group which accumulated paraconformably on the Onverwacht. Overlying sediments of the Fig Tree Group consist of ~ 2 km of submarine-fan greywackes and mudstones. Shoaling of the basin is indicated by the transition to the overlying braided-alluvial and shallow-marine sediments of the ca. 3.3 Ga-old Moodies Group (~ 3 km thick). Changing REE patterns and relative abundances of Ni, Cr, Sr, Th and Ur in the sediments of the Fig Tree and Moodies Groups indicate a significant ultramafic-mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution. A similar trend is reflected in sandstone petrography, in which the ratio of extrusive to intrusive rock fragments decreases upwards in the Fig Tree Group whereas the Moodies Group sediments are enriched in quartz and potassium feldspar. A persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of Phanerozoic recycled orogens. The distribution of facies in the Fig Tree and Moodies Groups, and paleocurrent, petrographic and geochemical data, indicate progressive unroofing of a southerly source terrane consisting of the Onverwacht volcanics and intercalated sediments, and the 3.5-3.2 Ga-old Ancient Gneiss Complex (AGC). In addition, older components of the Fig Tree Group were recycled into overlying stratigraphie units. The AGC consists of mantle-derived 3.5 Ga-old bimodal metavolcanics and metasediments, a tonalite-gneiss batholith, metabasite dikes, and meta-anorthosites. Its structures record progressive northwestward uplift, beginning around 3.4 Ga ago. Intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large-scale northwestward simple shear, followed by imbricate faulting and pseudotachylite formation. By 3.3 Ga ago the Fig Tree and Moodies Groups had also shortened, with similar kinematics to those of the AGC, by (partly syndepositional) folding and volume loss during the formation of transecting cleavage. Further folding and slide formation followed during subsolidus diapiric rise of the initially tabular Kaap Valley tonalite by 3.2 Ga and emplacement of trondhjemite plutons from 3.3 to 3.0 Ga ago.