Abstract

The Lethakane shear zone is a transcurrent ductile shear zone with a dextral displacement of c. 50 km which developed under low amphibolite-to greenschist-facies conditions in the Limpopo orogenic belt of eastern Botswana. Three distinct minor structures occur within the shear zone: mylonitic foliation (Sa), strain-slip or fracture cleavage (Sb) and minor shear zones (Sc). The non-penetrative structures Sb and Sc have transcurrent movement senses and occur in conjugate sets orientated sub-parallel and sub-perpendicular to the shear zone margins. The penetrative fabric Sa is restricted to quartz-bearing rocks and has the same sigmoidal relationship to minor shear zones as that described for schistosity in ductile shear zones by Ramsay & Graham (1970). This relationship confirms the view, first proposed by Johnson (1967), that mylonitic foliation defines the XY plane of the finite strain ellipsoid and is thus analogous to schistosity. The restriction of Sa to quartz-bearing rocks indicates that, under low amphibolite-facies conditions, the contrast between the ductile behaviour of quartz and the brittle behaviour of feldspars is the essential factor controlling deformation processes in the Lethakane shear zone.

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