Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Pan-African Damara Mobile Belt has previously been described as ensialic, possibly resulting from a modified aulacogen. Three features of the Damara Mobile Belt are difficult to reconcile with ensialic models. Firstly, the complex asymmetrical structural pattern of linear zones, with up to 80% shortening across the belt. Secondly, the markedly asymmetric metamorphic pattern broadly follows the structural pattern forming two distinct, parallel metamorphic belts of relatively high (northern belt) and low (southern belt) geothermal gradients, respectively. Abundant granitic intrusions occur in the high-grade metamorphic belt. Thirdly, the evolution of the Damara igneous rocks; the early (Nosib) igneous rocks are alkali; mid-Damara (Matchless Member) amphibolites resemble oceanic-floor basalts. Depleted upper-mantle material representing oceanic lithosphere was tectonically emplaced into the Damara metasediments during early tectonism. An extensive calc-alkali suite (the Salem Suite) intruded the high-grade metamorphic belt during a long period spanning most of the Damara tectonism. A model invoking the formation of alkali rocks, followed by the development of oceanic crust, initiation of northwestward subduction and ocean closure terminating in continental collision is considered to explain the major features.