Definición

Four sites from the Hooggenoeg Formation measured in this study gave a site mean direc- tion which clustered fairly steeply to the south and was generally isolated above 480°C. This is in agreement with the HT1 direction isolated by Biggin et al. (2011) in two Hoogge- noeg Formation sites and a further five Kromberg Formation sites (which directly overlies the Hooggenoeg Formation and dated at 335-3445 Ma) . In the interest of comparing like for like, in the first instance, when plotting the HT1 directions on an equal area projection, only the results obtained from the Hooggenoeg Formation have been plotted. It is clear, from Figure 5.21 (a), that the site mean directions from the LHG and HG sites overlap (despite the LHG samples having a larger uncertainty) and that the directions from both studies should be combined to produce a new mean direction and pole. The new in-situ HT1 direction calculated from the Hooggenoeg Formation sites gives (D/I) = 168.7°/63.3°, ƙ= 343, α95=3.6°, N=6, with a pole of latitude (λ) and longitude (ɸ) of (λ/ ɸ) = -69.0°/53.7,

A95= 18.5, N=10. When this direction is compared to the direction mean calculated by also including the Kromberg site mean directions of Biggin et al., (2011) it becomes obvi- ous there is no clear reason to disregard the Kromberg directions, as the HT1 direction mean is given by (D/I) = 168.7°/63.3°, ƙ= 91.9, α95=5.06°, N=10, which is clearly compara-

ble to the HT1 direction calculated only from the Hooggenoeg sites. From this, a new pole is calculated for the HT1 of (λ/ ɸ) = -69.0°/53.7°. The new pole is plotted in Figure 5.25 and can be compared to other poles of the Kaapaval Craton as well as the results of other Onverwacht Group Formations of Biggin et al., (2011).

Interestingly, this direction is only observed in sites from the southern limb of the OF. In the case of Biggin et al., (2011) the most obvious explanation is that this is due to sam- pling bias, as no samples were collected in the northern limb of the fold as part of that study. Biggin et al., (2011) highlight that the HT1 pole is comparable to the younger 180Ma Karoo Large Igneous Province pole which caused wide spread overprinting throughout the Kaapval Craton (Strik et al., 2007). If this is the source of the direction, then it is younger than the LT2 and MT directions and would require a chemical remag- netisation which would record a more ancient magnetic field at lower blocking tempera- tures (Biggin et al., 2011). Alternatively, it is possible the similarity between the HT1 direc- tion and the younger poles is coincidental, and that, while HT1 was acquired subsequent to the formation of the OF and is therefore older than LT2 and MT (Biggin et al., 2011). The thermal events at 3.1Ga (Toulkeridis et al., 1994) or the large igneous events at 2.9 and 3.0Ga (de Wit et al., 2011; Schoene et al., 2008b) are likely candidates for imparting the direction (Biggin et al., 2011). From the new, improved HT1 direction presented in this work (which agrees well with the direction of Biggin et al., (2011)), there is no further evi- dence that would favour either of these interpretations and therefore they both remain valid. However, in either situation the implication is that the event which caused the HT1 magnetisation was widespread and so unusual that its signature is not seen in the rocks of the northern limb of the OF. There are indications that the magnetic carriers in either limb of the fold could be somewhat different. The rock magnetic indications seem to suggest that, at least some of the sites (e.g. LHG11), in the northern limb of the OF are dominated by ferromagnetic minerals whilst the southern limb sites are dominated by the paramag- netic contribution. NRM intensities in the southern limb of the fold are lower (10-4 to 10-3 A/m) than in the northern limb of the fold (10-2 to 10-1 A/m). At the same time, the mi- croscopy work did not indicate any obvious mineralogical differences between the two limbs of the fold and personal communication with M.J. de Wit on this subject clarified there are no significant lateral heterogeneities in the basalts across the OF. A shortcoming of the work presented here, however, is the fact that no southern limb samples were studied under the SEM and this would be recommended in any future work, as the optical investigations are limited in the resolution of the grain sizes that can be studied and it is Figure 5.21. Equal area plots showing HT1 site mean directions in geographic coordinates. (a) Site mean directions from this study are shown in yellow. Site mean directions calculated by Biggin et al., (2011) are shown in black. (b) Site mean directions are shown again, in this equal area plot. In addition the Hoogge- noeg Formation site means, the Kromberg Formation means, as calculated by Biggin et al., (2011), are in- cluded (shown in pink). The HT1 direction show good clustering. Filled points represent the lower hemi- sphere. Ellipses indicating α 95 cones of confidence.

possible that differences regarding the magnetic carriers would only be observed at a mi- cron or submicron scales. With the available data set it is not possible to fully clarify why the HT1 direction is only observed in the southern OF.