Late syndepositional deformation and detachment tectonics in the Barberton Greenstone Belt, South Africa

Abstract

The thick‐bedded, competent quartzose sandstones, conglomerates, and siltstones of the Moodies Group (<3,225 Ma) influence structural style in the ∼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 greenstone belt suggests that much of the volcanic‐sedimentary greenstone sequence was buckle‐folded above a detachment within altered ultramafic rocks located at a depth of 3–6 km. Early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones. Penetrative strain during folding was low. The sense of displacement along faults, the original fold vergence, and the provenance of Moodies Group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins. Orogenic contraction due to folding alone was at least −56% and subsequent ductile shortening strain accounted for an additional contraction of approximately −10%. Buoyant rise of the Kaap Valley Tonalite to the north at approximately 3.22 Ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3.1 and 3.0 Ga, respectively, imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold‐and‐thrust belts. The syn‐and post‐Moodies tectonic history of the central Barberton Greenstone Belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable “protocontinental” crust. The probable extensional origin of at least some of these basins and their subsequent shortening, buckle folding, and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late‐stage deformation of this Archean greenstone belt.