Tektonik und Metamorphose im Grenzbereich Damara-Prädamara, südwestlich Windhoek (Südwestafrika)

Abstract

Abstract provided by author:

Investigation of garnets and chlorites from the Marienhof schists indicated a low-grade metamorphism in this complex. It is the same metamorphic grade as is known from the southern margin of the Damara schists. Therefore, it is possible to extend the isoreactiongrades 1, 2 and 3 (HOFFER & PUHAN 1972, 1972), which were established in the Damara schists, into the basement. Minerals such as staurolite, sillimanite and cordierite, which could be referred to an older metamorphism of a higher grade, are not found in the Marienhof schists. Therefore, if a pre-Damara metamorphism existed, it could not have reached a higher metamorphic grade than the Damara metamorphism in this region

summary provided by author:

An investigation of the tectonics and metamorphism near the southern margin of the Damara belt and in the adjacent basement was undertaken as part of the "Sonderforschungsbereich 48" - a special research programme sponsored by the DFG (Deutsche Forschungsgemeinschaft). The aim was to determine the nature of the interplay between the Damara tectonics and the basement and the effect of the Damara metamorphism on the pre-Damara sediments

Analysis of the tectonic elements indicated three phases of deformation in both the Damara Group and the Marienhof Group. The first phase of deformation of the Marienhof Group has a pre-Damara age. This is a modification of the description given in my preliminary report (1973). It is now interesting to note that the first phases of deformation in the Marienhof Group and in the Damara Group show the same style of deformation although they are of different ages

First Phases: isoclinal folding with associated planes s1 developed ± parallel to bedding (a fine layering). The marble synclines stratigraphically associated with the Damara Group and located in the Marienhof schists were produced during the first deformation of the Damara Group

Second Phases: open folds and kink-like folds with associated crenulation cleavage planes s2. These s2-planes show the same strike as the s1-planes, but are always steeper. North of the southern margin of the Damara belt s2 dips mainly northwestward but south of this margin so dips consistently southeastward

Third Phases: formation of s3-cleavage planes and a crenulation 13 without a development of folds. The direction of strike of s3, and 1 3 vary between N in the Damara Group and W in the Marienhof Group

During the first phase of deformation in the Damara Group the most distinct younger foliation was developed in the adjoining basement complex, comprising the Weener Tonalite and the Gamsberg Granite south of the Damara belt. This younger foliation, and also sediments within the older foliation, usually dips northward and strikes more or less parallel to the southern margin of the Damara belt

The influence of the basement, especially the Weener Tonalite and the Tamsberg Granite, on the Damara tectonics is seen primarily in the formation of deep synclines, folded into this complex probably during the first deformation phase. As early as the time of the Damara sedimentation a relief existed with shallow depressions, which determined the position of the later synclines. Because of the extreme infolding of these Damara synclines it is not possible to distinguish between the separate phases of deformation

The synclinal axes usually have the same strike as the synclinal axial planes. Only in the Picadilly syncline are these directions divergent. This syncline reacted differently to Orogenic stresses because of its position along the competence boundary between the tonalite and the Marienhof schists