Some aspects of the tectonic history of the Khan-Swakop region of the Damara Belt, Namibia

Abstract

Abstract provided by author:

The Central Zone has been affected by 4 main deformation phases. Within the study area any large F1 structures have been obliterated by later deformation. The intense F2 deformation phase, under dominantly shear conditions, formed the main regional fabric, interfolded pre-Damaran and Damaran rocks and produced large areas of inverted rocks. The F3 phase exhibited a wide variation in strain intensity and deformation mechanisms, and produced open to sheath folds (with upright axial planes usually plunging NE). Within the overall NE/SW F3 trend there is a variation in F3 axial directions which may be explained by a combination of superimposed folding (F4), dome formation, granite intrusion and variation in F3 strain intensity. Long, upright F3 faults mark the boundaries of the zone of highest F3 strain. The much less intense F4 deformation phase produced upright N/NW trending folds, postdates all diapirism and igneous activity, and is the last regionally correlatable deformation phase. The combined effects of F1 and F2 produced at least 80 percent shortening and approximately 7 km of crustal thickening. F3 strain varied from less than 35 percent to more than 80 percent shortening. Diapirism was active during and especially after F3

Amphibolite facies was reached before the end of F1, whilst the peak of M1 metamorphism occurred between F1 and F2. The M2 metamorphic event began before the close of F3, but most M2 reactions post-dated it. F4 occurred after the attainment of post- M2 recrystallised stable grain boundary configurations. The microfabric of most igneous samples may be divided into the Primary Igneous Texture (formed. by crystallisation from a melt) and the Secondary Igneous Texture (subsolidus exsolution and migration of some components). The Primary Igneous Texture has stable, low energy grain shapes. The Secondary Igneous Texture approaches the optimum low energy grain boundary arrangement and is equivalent to the post-metamorphic recrystallised microfabric in metasedimentary samples

Upper amphibolite facies conditions prevailed at the peak of M1. M2 was closely associated with (and apparently triggered by) the intrusion of very large volumes of leucogranite which produced a rise in temperature and increased XH2O and f02. Widespread anatexis and remobilisation occurred during M2. As indicated by the pelitic samples, prevailing P/T conditions at the peak of M1 were 2. 8 to 3 kb and about 675 degrees C. At the peak of M2 conditions were approximately 3. 8 to 4. 25 kb and 660 to 675 degrees C. The increased pressure during M2 reflected up to 6 km. crustal thickening, most probably due to the F2 deformation phase. M1 conditions reflect a geothermal gradient of 60°C/km whilst M2 conditions suggest a geothermal gradient of 40 degrees C/km

In the study area, Damaran igneous intrusion was initiated simultaneously with the M1 metamorphic event and continued until the closing stages of the orogeny. A range of igneous rocks from tonalites to granites was emplaced before F2. Between F2 and F3_ porphyritic and aphyric biotite granites were predominantly intruded. During and after F3, mainly leucogranites were intruded

In terms of core rocks, the domes of the Central Zone may be divided into three groups (1) Granite-cored domes, in the West of the sample area; (2) mixed metasedimentary/migmatite/igneous-cored domes, in the central part of the sample area; and (3) pure meta-sedimentary-cored domes in the East, which reflects the Eastward decrease in the amount of F3, leucogranite intruded, with associated anatexis of metasedimentary host rocks. The internal structures and geometry of the domes vary from simple periclinal folds to domes with complex internal structures. This latter group consists of three main types: (1) domes containing early recumbent folds refolded about one main F3, antiformal axis - producing an overall domal shape; (2) those domes without an F3 antiformal axis - these are the 'false' domes and (3) those domes without an F3 antiformal axis which are the site of major intrusions of dome-age granites -these are the 'photogeological' domes. The domes were formed by the following mechanisms, both separately and in combination, forceful buoyant igneous intrusion, progressive F3 deformation and diapirism. The dominant control on the mechanism (s) which operated was the amount of granite intruded during and after F3

Overall, the conclusions of this study fit most easily into a plate tectonic model for the development of the Damara Orogen. Certain of the conclusions render the post-depositional stages of an aulocogen model untenable