How greenstone belts deform: Tectonic style along the southern margin of the Barberton Greenstone Belt, Eswatini

The Barberton Greenstone Belt (BGB) and many other orogenic belts in cratonic basements have been classically thought to be large, first-order synclinal structures, thus supporting vertically-dominated models of early Earth lithosphere formation. We here argue for a late phase of BGB deformation involving significant horizontal shortening along the long, polymetamorphosed linear southern margin of the BGB with the gneisses of the Ancient Gneiss Complex of central Eswatini by investigating the Ntaba Mhlope Syncline, a small, steeply doubly (inwardly) plunging, highly strained syncline of Moodies Group (ca. 3.21 Ga) strata, about 3 km long and 0.6 km wide. This structure represents the lone remnant of Moodies Group strata in the central segment of the southern BGB. Its subvertically dipping, upper-greenschist-facies-metamorphosed strata consist of (from base to top) conglomerates, quartzose sandstones and subordinate shales and siltstones a few hundred m thick. Age spectra of detrital zircons from sandstones suggest a depositional age of ca. 3232+-36 (2σ) Ma, similar to Moodies depositional ages elsewhere in the BGB. Deformation of conglomerate clasts is markedly constrictional; plunge is uniform and steep throughout the tight syncline, suggesting that strain followed folding. Geologic setting, fold style and prolate strain suggest that deformation occurred during substantial north-northwestward-directed shortening by folding and thrusting in high-strain zones at elevated temperatures, perpendicular to the BGB margin. These findings are comparable with similar deformation patterns along strike in the Malolotsha region further south but contrast with observations of regionally dominant oblate strain along the entire northwestern margin of the BGB.