INVERTED METAMORPHISM ALONG THE WEST-DIRECTED AND ISOCLINALLY FOLDED BASAL THRUST SHEAR ZONE OF THE PALEOPROTEZOIC (~2.0 GA) LIMPOPO OROGENIC BELT, SOUTHERN AFRICA

The > 600 km long and ~200 km wide Limpopo orogenic belt (LB) is expressed as an east-northeast trending granulite-facies deformation zone across South Africa, Zimbabwe, and Botswana between the Zimbabwe and Kaapvaal craton. LB consists of the Central Zone dominated by granulite facies metamorphism and the Northern (NMZ) and Southern (SMZ) Margin Zones characterized by granulite-to-amphibolite facies retrograde metamorphism. Right and left-slip ductile shears and outward decreasing metamorphic granulite to greenschist facies gradient dominate the NMZ and SMZ. New geochronological data show ~2.0 Ga deformation associated with granulite facies metamorphism without a genetically related magmatic arc likely formed LB. Field work and regional field relationships suggest LB was isoclinally folded by a NNW-SSE shortening event at ~1.9 Ga, expressed by sub-vertical foliation throughout the three zones parallel to the general trend of LB and post-orogenic isoclinal folds formed 1.87 Ga – 2.0 Ga. Himalayan-style intracontinental thrusting generating inverted metamorphism may explain field gradients observed in NMZ to SMZ. This model predicts (1) the NMZ is a right-slip shear zone with metamorphic grades decreasing northward, (2) the SMZ is a left-slip shear zone with metamorphic grades decreasing southward, (3) the western end of LB is a west-directed thrust shear zone with metamorphic grades decreasing westward, and (4) west-verging folds are present within margin zones when LB is unfolded. The last two predictions were tested through fieldwork and show the metamorphic grade at the western end of LB decreases outwards towards nearby cratonal rocks. This is expressed by a garnet amphibolite with relics of pyroxene psudomorphs in the inner western marginal zone. We also observed outward decrease in metamorphism grade from amphibolite facies to upper greenschist facies across the western marginal zone. The greenschist-facies metamorphic zone of LB lies above a brittle fault zone, dominated by west-directed brittle thrust faults interpreted as coeval with development of west-directed ductile shear zone at the western end. Progressive deepening of the folded thrust shear zone bounding the base of the west-directed Limpopo thrust sheet may explain lack of brittle faults along the NMZ and SMZ 200-300 km east of our area.