Abstract

The Mozambique Belt, a north-south trending orogenic zone along the eastern margin of the African continent, is characterized by isotopic ages of about 500 m.y. The age of the belt's predominantly gneissic rocks and the structural relationships with the older rocks to the west are in dispute. A comprehensive study therefore was made where the western front of the orogenic belt lies deep in rocks much older than 500 m.y.

Both the Archaean basement (>2650 m.y. old) and the sedimentary rock cover of the Umkondo System (>1100 m.y. old) were found to be deformed and metamorphosed about 500 m.y. ago. Across the orogenic front the increase in the intensity of the first phase of deformation is marked by three northwest- to north-northwest-trending zones: (1) an outer zone characterized by local shearing, (2) an intermediate zone in which the rocks are refoliated and folded due to a flattening-type deformation, and (3) an inner zone distinguished by constriction-type deformation. In both the outer zones the structures trend north-northwest to northwest, parallel to the orogenic front. In the intermediate and inner zones, however, are minor folds which plunge eastward in the extension direction. The different orientation of the folds is explained in terms of a deformation ellipsoid which maintained its orientation but changed its shape during a single episode of progressive deformation.

During the second phase of deformation the earlier structures were deflected by broad east-west-trending folds and by the emplacement of a diapiric granite. The diapir is believed to represent mobilized Archaean granite, which rose through heavier rocks under the influence of gravity and which was encircled by a rim syncline. Locally developed shear zones were the chief result of a third and final phase of deformation.

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