The Malolotsha Klippe: Large-scale subhorizontal tectonics along the southern margin of the Archean Barberton Greenstone Belt, Eswatini

Whether Archean tectonics were horizontally or vertically dominated is debated because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the formation of the earliest continental crust. Deformed strata of greenstone belts figure prominently in this debate because they record long periods and multiple deformation phases. Among the best-preserved belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of its southern part in Eswatini (Swaziland), combined with U-Pb zircon dating, show that the region preserves a tightly re-folded imbricate thrust stack in which metavolcanic and -volcaniclastic strata of the Onverwacht Group, deposited at 3.34-3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1450 m thick. Forward modelling of a balanced cross section indicates that this thrust stack was part of a northwestward-verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present-day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, the late-stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically- and horizontally-dominated deformation styles that interfered with each other at small regional and short temporal scales.