Thrust-related accretion of an Archaean greenstone belt in the Midlands of Zimbabwe

Abstract

Detailed structural data from the Midlands greenstone belt show that, in contrast to pre-existing models, the evolution of the greenstone belt involved an early episode of thin-skinned thrusting affecting separate lithological domains. These display different sedimentary and structural-metamorphic histories prior to their juxtaposition across steep, west-directed thrust zones. The domains include 3.5Ga old gneiss, 2.67–2.88Ga, mafic–felsic volcanic units and early-syn-tectonic, clastic sedimentary sequences. Concomitant with thrusting along domain boundaries, upright folding and ‘minor’ shear accommodated strain within domains.The early thin-skinned, and later, steeper, thrusting events can be interpreted as progressive stages in an accretionary and crustal thickening sequence, possibly associated with under-plating or low-angle subduction. The clastic sedimentary sequences contain intraformational clasts and show coarsening upward cycles below major early thrust horizons, and may have formed on alluvial fans developing in front of west-moving nappes. Strike-slip motion on the main shear zones in the Midlands greenstone belt only occurred late in the tectonic history and there is no evidence that this resulted in large displacements. A shift of σ1, from E–W to N–S during the late stages of Archaean deformation of the greenstone belt can be attributed to extensional collapse following E–W shortening and crustal thickening of the Zimbabwe craton.