Mylonitization in the Lethakane shear zone, eastern Botswana

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.