Structural relationships along a greenstone/shallow water shelf contact, Belingwe greenstone belt, Zimbabwe

The Belingwe greenstone belt, Zimbabwe, contains a thick tholeiitic basalt and komatiite sequence structurally above shallow water shelf rocks that in turn rest nonconformably over an older gneiss terrain. The nature of the contact between the basalt/komatiite sequence and underlying shallow water shelf rocks represents one of the most critical relationships upon which general models for the tectonic setting of greenstone belts and the petrotectonic environment of komatiites are based. This contact is a 20‐ to 200‐ m thick high‐strain zone characterized by a wide variety of shear zone tectonites derived from both the shelf and overlying lava succession. In the ultramafic part of the high‐strain zone, tectonites include ductile ultramafic mylonites, ultramafic ultramylonites, ultramafic phyllonites, and serpentinite schists, enclosing less‐deformed ultramafic serpentinite blocks. In the part of the shear zone derived from shallow water sedimentary rocks, shear zone tectonites include chert C‐S mylonite, phyllonite, and iron‐rich argillaceous shear bands. Both parts of the shear zone are cut by brittle fault gouge fabrics. We suggest that the tholeiitic basalt and komatiite sequence is allochthonous with respect to the shallow water shelf and refer to it as the Mberengwa allochthon. Contrary to prior suggestions, there is no evidence that the tholeiitic basalt and komatiite succession of the Belingwe greenstone belt sits conformably over the shallow water shelf rocks. Documentation of the shear zone at the base of the Mberengwa allochthon lends support to the idea that the allochthon represents a fragment of an off‐axis oceanic plateau, structurally emplaced over a shallow water shelf during a convergent plate interaction prior to 2.6 Ga.