The damara-ribeira orogen of the Pan-Africanbrasiliano cycle in Namibia (Southwest Africa) and Brazil as interpreted in terms of continental collision

Abstract

The Upper Proterozoic—Lower Palaeozoic Damara and Gariep belts of Namibia and South Africa and the Ribeira belt of Brazil and Uruguay are regarded as integral parts of a major “Damara—Ribeira orogen” which developed during the contemporaneous Pan-African and Brasiliano cycles, respectively. The “Damara—Ribeira orogen” is interpreted as an orogen/aulacogen system similar to the mid-Proterozoic Coronation geosyncline/ Athapuscow aulacogen of Canada (Hoffman, 1978). Starting from a complex radial three-armed continental-rift system with at least eight individual grabens about 1000 m.y. ago, continental separation, ocean opening, and development of continental-margin geosynclines occurred along two of the arms. The third arm remained as an abandoned rift extending into the continent and evolved towards an aulacogen as described by Martin and Porada (1978). Referred to the present geographical position, the opening of the Upper Precambrian proto-South Atlantic Ocean took place along the N-S trending coastal branch of the Damara belt, in the Gariep belt, and parallel to the Ribeira belt, while the aulacogen developed in the northeast trending intracontinental branch of the Damara belt. Closing of the proto-South Atlantic Ocean led to continental collision, with the South American Ribeira belt on the leading plate and the coastal Damara branch and Gariep belt on the consumed plate. Major parts of the “Damara—Ribeira orogen” resulting from this collision, including the westward inclined collisional suture, are hidden underneath the shelves of the present South Atlantic Ocean. In accordance with Dewey and Burke's (1973) continental collision model deformation on the consumed plate is characterized by a marked structural vergence and nappe transport towards the east in the northern coastal Damara branch, while slices of possibly obducted oceanic crust containing glaucophane-bearing assemblages occur in the Gariep belt . On the leading plate the sedimentary sequence of the Ribeira belt is affected by a low to intermediate pressure-type of metamorphism of the greeschist to ambhibolite facies including local anatexis. The style of deformation is characterized by open to isoclinal folds with steeply dipping axial planes of varying vergences. Crustal thickeing after continental collision resulted in arise of isotherms leading to widespread production of granitoid melts which intruded higher levels syn- to late tectonically.Upon completion of continental convergence, intermontane basins developed and were filled by molasse-type deposits and acid to intermediate extrusions. Extent and width of the Upper Precambrian proto-South Atlantic Ocean remain unknown. It may have been linked to a North African ocean armsituated in the region of the Dahomeyan and Pharusian belts by major transform faults in the area of the North-eastern Fold Region (Almeida et al., 1976) in northeast Brazil.