BRITTLE AND DUCTILE DEFORMATIONAL FABRICS OF GRAPHITE BEARING MARBLES IN HIGH STRAIN ZONES OF THE CENTRAL DAMARA BELT, NAMIBIA

Graphite bearing calcite marbles are widespread in regional shear zones of the central Damara Belt in western Namibia. They build up a series of crustal scale dome structures which are generally developed as two types in the area. The first type developed by the superimposition of isoclinal folds and the second type developed as granitic domes due to the massive regional intrusion of granitic melts. Along the rims of both of these types of dome structures, the graphite bearing marbles show complex brittle and ductile deformation fabrics within narrow high strain zones. These high strain zones show complex macro- and microfabrics and occur in a network of conjugated shear zones around the dome structures.

The investigation of the microstructures of the graphite bearing marbles show that three main deformation mechanisms appear in a close range next to each other within the high strain zones: Ductile deformation, cataclastic deformation and pressure solution. Minor static recrystallization can be observed within the domains of ultra-fine grained calcite. Texture analysis by neutron diffraction reveals very similar crystallographic preferred orientations in the mylonitic and cataclastic deformation. Graphite shows in the mylonitically deformed marbles a preferred orientation of the basal plains parallel to the foliation. In the cataclastic domains ultra-fine grained graphite forms graphite veins and network structures of varying density, length and orientation. As the mylonitic and cataclastic deformation show partially cross-cutting relationships, no clear progressive deformation from ductile to brittle deformation can be deduced for these marbles on a regional scale.

From the fabric analysis, calcite-graphite geothermometry and the field relationships, it is obvious that the mylonitic and cataclastic deformation occurred isochronal and mutually overprint each other, which is still preserved in certain domains of the shear zones, whereas other domains are completely overprinted by dynamic creep. The investigated fabrics document a seismic-aseismic transition zone deformation in marble rocks.