Bajada de la Virgen Contexto y los buenos augurios Fecha: Veintidós de agosto.

One of the key aims of this study was to determine the significance of mylonites at the basement-cover transition. These were first recorded by Steven (1992), near Usakos at Sandmap Noord, who attributed them to magmatic ballooning deformation. Oliver (1994) later recognised mylonites as a widespread phenomenon around the domes in the Central Zone and suggested that the contact was a site of tectonic shearing, probably due to the inherent weakness of the basement-cover unconformity. Oliver (op. cit.) termed this the Khan River Detachment. Values of k and variation of fold styles between domains demonstrate that deformation was not homogenous. Higher strains may have been exclusive to this discrete site within the regional structure. Fabrics in the field were given a qualitative value to indicate variation in strain intensity from 1-5 i.e. L/S1 to L/S5. Visual observation of L>S5 mylonites in cover rocks 10-50m above the basement-cover contact (shown on Map 1 & 3) support the hypothesis that this was a zone of higher strains than rocks above or below. Furthermore this is consistent with observations made from semi-quantitative strain estimates from deformed pebbles and boudinage.

Previously the basement cover contact was regarded as a regional erosional unconformity. This was evidenced first by Gevers & Frommueruze (1929) who noted discordant basal conglomerates in contact with distinct augen gneisses. The age of the rocks below the unconformity in the Namibfontein area is supported by a zircon SHRIMP age of 1O38±58 (Kroner et al. (1991). Regionally the unconformity surface has a complicated folded geometry and is obscured by Damaran granite intrusions. It is evident from mapping that the basement was not a passive rock mass (cf. Argand 1922) because both cover and basement have undergone Damara deformation. It is unlikely therefore that mylonites at the basement-cover interface resulted solely from displacement of the cover over the basement.

In this study mylonites were identified in both the cover and basement consistent with the findings of Oliver (1994). In the Namibfontein area mylonites were recorded in three sub-areas. In the first area mylonites were observed as orthorhombic L & L/S tectonites in the core of the Valencia Dome. Figure 3.29 shows this area corresponds to the axial zone of the Valencia Dome.

Mylonites at the contact grade from S=L2 mylonites to L>S4. This is best observed from location N38.19 GR[038,014] where a c. 5m zone of S=L mylonites grades progressively into ubiquitous L>S tectonites. Plate 2.3b (see Chapter 2). shows a typical L tectonite cut in the z/y plane. In this dimension the Kfs augens tend to have a circular cross section such that Z=Y and k=°c, foliation is weak and disjunctive while lineation is pronounced. In the Y/Z plane augens are extremely elongated with typical x:y ratios >1:10. Augens form (j) type porphyroblasts with symmetric wings and simple shear sense cannot be deduced from these rocks. Cataclasis is not observed, Kfs feldspar is deformed plastically (cf. White & Knipe 1978). Deformation was therefore by diffusion creep and not dislocation creep (Tullis & Yund 1991), probably at temperatures >650°C.

In the second area mylonites outcrop along the northern flank of the Namibfontein Dome at the transition of the Basement and Etusis Formation, at location N42.15 GR[038,109]. Mylonites in this area display a 2-3mm spaced foliation which is both discrete and continuous. The fabric defines an S>L tectonite. Kfs augens and Qtz ribbons form symmetrical <)) type porphyroblasts. It is notable that at N42.15 outcrop is only 300m thick, considerably less than the southern flank of the Dome between location N14.10 GR[093,095] and N19.1 GR[079,096]. This is interpreted as a result of tectonic thinning.

In the third location, N17.9 near Twien Koppie (Figure 3.12). the basement-cover transition can be traced around a large granite koppie. The contact is discrete and defined by a strong c. 5m zone of L/S tectonites. The granite is intruded into and above this zone; above and below the contact the granite is unfoliated. The basement-cover transition dips at 45°-60° to the north-east, Lq plunges 35°-060°. The contact here can be traced west to location N42.21 GR [032,085]where a deformed basal conglomerate defines a highly deformed L-S mylonite.

In the Khan Mine area the tectonic nature of the basement cover contact is displayed clearly along its entire length. The surface is folded around the main macroscale structure. In addition it forms mappable mesoscale folds along the Nose Structure Anticline. The general nature of the transition is seen clearly at location K14.3 where a type section of the contact was mapped at a scale of 1-1000. Figure 3.30 shows a 100m transition from L tectonites in the basement to S>L tectonites of the Etusis Formation. Fabric strength generally increases towards a zone marked by a band of sillimanite schist, i.e. the basement-cover interface. This is demarcated as a major shear zone in Figure 3.27 which dips 42°-060°. Minor shear bands are observed cutting the quartzite 40m north of the major shear zone, suggesting that the main shear zone anastomises or is bounded by anastomising shear zones. Both top up to the south-west and top down to the north-east shear sense can be deduced.

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In the Nose Structure Anticline the basement-cover interface is folded and overturned. The general form of the surface is indicated in Figure 3.31, two geometric types of folds are found: i) reclined and ii) subhorizontal. Decimetre scale reclined folds were mapped in three locations, at GR[016,039], GR[021,040] K26.19 and GR[029,042]. Fold hinge plunge directions are subparallel to the tc axis of the Nose Structure Anticline.

Close examination of the fold form at location K26.19 shows they are defined by folded SrS0 and that mylonitic lineation Lb ~55°-074°, is unfolded and subparallel to the mesoscale F2axis ~52°-074°. This relationship is interpreted as syn-kinematic progressive folding of the basement-cover interface. Subhorizontal folds in Figure 3.31 are implied from minor monoclinal gently plunging folds seen in the quartzites at location K21.1 GR[020,044].

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Figure 3.28 Structural geology map of the basement-cover transition at location K14.1.

Etus is Q u a rt z c ® E ® co cd XI "O ® E ® o o ® ° <p o c 0- cn ® o ? o c “O CD CD E c CD O « CD CO CL X3 '— ■u o CD CO ? § Li- O ® O ? O

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Inspection of the plunge direction of both reclined and subhorizontal folds shows that they are all subparallel to domain K(B) tc axis. Since the only geometric difference is the amount of plunge, the folds are interpreted as contemporary.

In summary several lines of evidence suggest that the basement-cover interface is a tectonic shear zone. Firstly, it is evident that mylonites, mainly L-tectonites, are ubiquitous at the interface. According to the classifications of Ramsay (1980) and Shimamoto (1989) the basement-cover interface or Khan River Detachment is a heterogeneous and ductile shear zone; no boundary constraints can be applied to the Khan River Detachment and it is defined as a non­ discrete zone of more intense strain. Secondly, semi-quantitave estimates of finite strain from boudinage and pebble data support the notion that higher strains have occurred along this zone. Thirdly, sheath folds are recorded in this zone and probably originated from the focusing of strain in horizons near the interface. These folds may have resulted from pure shear since sense of shear markers within this zone are symmetrical. Asymmetrical structures may have been obliterated by late annealing due to thermal metamorphic effects but more likely due to the dominance of pure shear. Pressure solution shadows around some garnet porpyhroblasts do indicate a top to the south-west sense of shear. Plate 3.13 shows complex

a-5

type garnet porphyroblast (Figure 3.32) west of the Karub Syncline, S! dips to the east , sense of shear indicated top to the south west. Often, however, shear sense is ambiguous with near symmetrical quartz shadows occurring very commonly. A component of differential shear cannot be ruled out along the basement-cover interface since evidence of shear bands is occasionally seen (shear sense directions are shown in maps 1 & 2 Appendix).

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