LATE SYNDEPOSITIONAL DEFORMATION AND DETACHMENT TECTONICS IN THE BARBERTON GREENSTONE-BELT, SOUTH-AFRICA

The thick-bedded, competent quartzose sandstones, conglomerates, and s iltstones of the Moodies Group (<3,225 Ma) influence structural style in the similar to 3.1-3.5 Ga Barberton Greenstone Belt. Moodies Group rocks are folded into a train of tight, subparallel, doubly plunging, overturned synclines separated by narrow, strike-parallel fault zones. Structural analysis along a transect across the central part of the g reenstone belt suggests that much of the volcanic-sedimentary greensto ne sequence was buckle-folded above a detachment within altered ultram afic rocks located at a depth of 3-6 km. Early development of brittle structures concentrated shortening and displacement near the fold hing es and in the anticlinal fault zones. Penetrative strain during foldin g was low. The sense of displacement along faults, the original fold v ergence, and the provenance of Moodies Group sandstone and conglomerat e suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins. Orogenic cont raction due to folding alone was at least -56% and subsequent ductile shortening strain accounted for an additional contraction of approxima tely -10%. Buoyant rise of the Kaap Valley Tonalite to the north at ap proximately 3.22 Ga and lateral intrusions of sheetlike granitic pluto ns along both sides of the preserved greenstone belt at 3.1 and 3.0 Ga , respectively, imposed significant ductile strain on the greenstone b elt margins and obscured or displaced the orogenic hinterlands of the facing fold-and-thrust belts. The syn- and post-Moodies tectonic histo ry of the central Barberton Greenstone Belt involves the formation and deformation of several short-lived alluvial to shallow marine sedimen tary basins on unstable ''protocontinental'' crust. The probable exten sional origin of at least some of these basins and their subsequent sh ortening, buckle folding, and displacement above a subhorizontal detac hment emphasizes the dominance of horizontal tectonics in the late-sta ge deformation of this Archean greenstone belt.