STRUCTURAL RELATIONSHIPS ALONG A GREENSTONE SHALLOW WATER SHELF CONTACT, BELINGWE GREENSTONE-BELT, ZIMBABWE/

The Belingwe greenstone belt, Zimbabwe, contains a thick tholeiitic ba salt and komatiite sequence structurally above shallow water shelf roc ks that in turn rest nonconformably over an older gneiss terrain. The nature of the contact between the basalt/komatiite sequence and underl ying shallow water shelf rocks represents one of the most critical rel ationships upon which general models for the tectonic setting of green stone 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-str ain zone, tectonites include ductile ultramafic mylonites, ultramafic ultramylonites, ultramafic phyllonites, and serpentinite schists, encl osing less-deformed ultramafic serpentinite blocks. In the part of the shear zone derived from shallow water sedimentary rocks, shear zone t ectonites include chert C-S mylonite, phyllonite, and iron-rich argill aceous shear bands. Both parts of the shear zone are cut by brittle fa ult 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 suggestion s, there is no evidence that the tholeiitic basalt and komatiite succe ssion of the Belingwe greenstone belt sits conformably over the shallo w water shelf rocks. Documentation of the shear zone at the base of th e Mberengwa allochthon lends support to the idea that the allochthon r epresents a fragment of an off-axis oceanic plateau, structurally empl aced over a shallow water shelf during a convergent plate interaction prior to 2.6 Ga.