Nature and extent of shear deformation along the eastern and southern terrane boundaries of the Motloutse Complex, eastern Botswana

Abstract

The Motloutse Complex at the southwestern margin of the Zimbabwe Craton is an Archean medium-grade allochthonous terrane in eastern Botswana. The high-grade gneisses of the Limpopo Complex lie east of the Motloutse Complex and those of the Mahalapye Complex to the south. A number of shear zones are postulated to delineate the aerial extent of the Motloutse Complex, and separate it from the cratonic domain and high-grade terranes. Except the northern margin, these terrane boundaries are poorly understood. This study aims to characterize the nature and extent of shear deformation along the eastern (Dikalate Shear Zone) and southern (Sunnyside Shear Zone) boundaries of the Motloutse Complex. Representative areas from the eastern (Masikate Hills), and southern (Radisele-Mogome) regions were selected for geologic mapping. In view of the poor exposures outside these areas, aeromagnetic data is used to delineate the regional extent of the shear zones. Shear deformation is prominent along the N–S elongate Masikate Hills made up of tonalite gneiss, amphibolite, anorthosite and ultramafic rocks. A dominant right lateral sense of movement, with minor left lateral movement is inferred for the shear deformation. The extent of the N–S trending Dikalate Shear Zone, and its branches, is delineated in the aeromagnetic imagery.In comparison, the Radisele-Mogome area, exposes two set of rocks – tonalite gneiss, amphibolite, ultramafic rocks (part of the Motloutse Complex) to the NE, and granodiorite, diorite gneiss, leucogranite (part of the Mahalapye Complex) to the SW. This is well exemplified in the aeromagnetic image, with change in magnetic signature between rocks in the NE and SW parts. Shear deformation variably overprints rocks along the contact zone. Mylonitic deformation is prominent in the SW part. As along the eastern terrane boundary, a dominant right lateral sense of shear movement is inferred. The extent of the NW-SE trending Sunnyside Shear Zone and its branch is delineated in the aeromagnetic imagery. The results highlight an example of using a combination of geological and geophysical methods to delineate the nature and extent of shear zones that constitute terrane boundaries.