Metal Deposits in Relation to Collision Events
Although the Alpine-Himalayan belt provides a clear example of the consequences of Cenozoic collision tectonics, and the Caledonide-Appalachian belt an example of Paleozoic collision tectonics, the interpretation of the tectonic history of many Paleozoic
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Metal Deposits in Relation to Collision Events
9.1 Introduction Although the Alpine-Himalayan belt provides a clear example of the consequences of Cenozoic collision tectonics, and the Caledonide-Appalachian belt an example of Paleozoic collision tectonics, the interpretation of the tectonic history of many PaleozoiC and older orogenic belts is fraught with uncertainty. Wilson Cycle concepts provide a cogent explanation for the history of Iapetus, but there are many orogenic belts where the bulk of the evidence seems to militate against the former operation of a full Wilson Cycle. The problem is exacerbated by the fact that the large majority of orogenic belts provide reasonably clear evidence of an initial extensional phase followed eventually by a compressional phase, although the two events may be separated by large time intervals. It is also now recognized that many orogenic events are caused by accretional collisions that may involve just a fragment of continental-type crust or a relatively young arc system (Hoffmann 1988). The formation and eventual subduction of an oceanic tract much wider than the present Red Sea can be expected to leave telltale evidence of its prior existence in the form of obducted ophiolites, provided erosion levels are not too deep. More significantly, irrespective of erosion levels, some evidence for the former presence of a calc-alkaline volcanoplutonic arc should be manifest along one side of the collision orogen. In some orogenic belts, such as the Damarides, much of this evidence is lacking, and paleomagnetic studies (McWilliams and Kroner 1981) provide no support for significant separation at any stage of the flanking, older cratonic blocks. For reasons of exposure, access, and mineral potential the Damara Province has been the subject of considerable study (see Tankard et al. 1982 and references therein), and represents a key area in terms of understanding certain Proterozoic and perhaps Paleozoic orogenic belts that do not appear to fit a simple Wilson Cycle model. Tankard et al. (1982) provide and excellent summary of the sedimentation and tectonics of the Damara Province and the Gariep Province to the south, and also review the various models proposed for it. The Damara Province represents a three-armed asymmetric orogenic system in which lithologic and structural continuity is observable between each arm (Fig. 9.1). Two of these arms parallel the Atlantic coast and may represent original depositional basins whose western flanks are now represented in part by lithosequences in the Ribera Province of Brazil (Porada F. J. Sawkins, Metal Deposits in Relation to Plate Tectonics © Springer-Verlag Berlin Heidelberg 1990
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