Tectonic evolution of the south-eastern Mwanesi Greenstone Belt, Zimbabwe Craton: insights from structural geology and 40Ar/39Ar age constraints

The south-eastern Mwanesi Greenstone Belt (MGB) comprises variably deformed pillowed and amygdaloidal basalt, variolitic pyroclastic breccia and minor felsic volcanic rocks intercalated with phyllite and iron formation. It is bound to the east and underlain by granitic orthogneiss. New structural analysis combined with mica ⁴⁰Ar/³⁹Ar geochronology allows for the reconstruction of the tectonic history of the area. The data reveal three tectonic events, D₁-D₃.

D₁ is constrained at ca. 2680 Ma. In the phyllite of the MGB, it is characterized by recumbent F₁ folds and an axial planar cleavage S₁ that developed under greenschist facies metamorphic conditions. In the structurally underlying granite orthogneiss, D₁ is marked by a shallowly SW-dipping gneissic foliation S₁ and NW-plunging stretching lineation L₁, associated shear sense indicators suggesting top-to-the-NW movement. We interpret D₁ as recording NW-directed transport of the MGB over the granite orthogneiss. Regionally, D₁ is likely related to a thrusting event that affected the entire craton.

The D₂ event, dated at ca. 2575 Ma, led to the development of the N-S-striking Mhou Shear Zone (MSZ)—a one km-thick, steeply west-dipping structure that formed under upper greenschist to lower amphibolite facies metamorphic conditions. The shear zone is mainly localized within the granite orthogneiss and, to a lesser extent, within the MGB. The mylonitic foliation S₂ carries a subhorizontal stretching lineation L₂ with associated shear sense indicators suggesting sinistral movement. Outside the shear zone, the intensity of D₂ deformation progressively decreases eastward within the granite orthogneiss. This decrease is reflected by a change in fold style from isoclinal to open and gentle folds, associated with steep axial planar S₂ foliations, as well as steeply-dipping N-S-striking reverse faults. This suggests that the MSZ is a wrench-dominated sinistral transpressional shear zone which formed during regional NNW-SSE shortening. The parallel orientation of F₂ small-scale folds in the granite orthogneiss and the doubly plunging, NNE-trending MGB syncline supports its classification as a D₂ structure. At the regional scale, the D₂ event is considered a far-field effect of NNW-SSE thrusting in the Northern Marginal Zone of the Limpopo Belt.

Local and weak Paleoproterozoic tectonic overprint D₃, tentatively dated at ca. 2.0 Ga, is characterised by steeply NE-dipping axial planar cleavage S₃ and F₃ box folds developed under lower greenschist facies conditions. Our work in the south-eastern MGB demonstrates that orogenic events that affected the Zimbabwe Craton along its margins largely influenced the tectono-thermal evolution of the MGB.