Abstract
Until the end of 37 years of war in Angola in 2002, the structural geology of the Kunene Complex (KC), an 18 000 km2 Mesoproterozoic massif-type anorthosite complex extending from SW Angola to NW Namibia, remained poorly documented. Three different structural components are observed in the KC. The two domains in Angola are N-S trending and contain heterogeneously developed N-S to NNE-SSW-striking primary and tectonic fabrics. Instead, the southern component in Namibia (i.e., the Zebra Lobe, ZL) is characterised by an ENE-trending structure with ENE-WSW-striking fabrics and is separated from the Angolan components by a highly tectonised zone. Although some structural constraints on some parts of the Angolan KC are documented, the structural geology and tectonic evolution of the ZL remains inadequately constrained. Whether the ZL is a magmatic or tectonic feature is still unclear. The architecture of the lobe was previously characterised as a structural dome or antiform formed during continuous emplacement of anorthositic magma as sills. It was also implied that NNW-SSE compressional forces existed in basement rocks during the emplacement of the ZL anorthosites. This characterisation is controversial as it does not explain why the ZL differs from the components in Angola. In this thesis, field mapping was therefore conducted to better constrain the formation and geometry of the ZL. Here we combine structural data with processed geophysical data to give better insights into the architecture and tectonic evolution of the ZL. Furthermore, we use X-ray Diffraction (XRD) and TESCAN Integrated Mineral Analyzer (TIMA) to characterise deformation, mineralogical and textural associations of fault-bounded rocks in the southern ZL.
Four deformation phases are recorded in the ZL and surrounding rocks. The amphibolite facies D1 forms a steep NE-striking gneissic foliation and stromatic layering in basement rocks (i.e., Epupa Metamorphic Complex, EMC). D2 produced a high-temperature (HT) solid-state steep N-S-striking gneissic foliation in KC anorthosites, with a steeply plunging pyroxene and hornblende lineation (L2). Open F2 folds in basement rocks preserve S1 in the hinge zones. D3 overprints S2 in the anorthosites by deflection along shear planes associated with top-to-the-NW kinematics. In basement rocks, D3 folded S1 into open folds with NW-dipping axial planes. Analyses of pi-plots from magmatic structures reveal a regional D3 inclined fold plunging moderately to the SW. The axial surface steeply dips to the SE and is associated with a steeply SE-dipping axial planar cleavage. A later D4 event formed a steep NW-striking cleavage at greenschist facies observed in the EMC, KC and the Nosib Group of the Damara Supergroup. D4 formed during the Pan-African Orogeny and is associated with NW-striking fault planes that mark the southern ZL-EMC tectonic contacts. Highly altered, fault-bounded leucogabbronoritic rocks in the southern ZL analysed by TIMA and XRD are also affected by D4 and developed a schistosity marked mainly by epidote, actinolite and chlorite. Rocks 15-20 m away from the faults are less foliated than rocks near the faults but are equally altered. High-
v
temperature fabrics in anorthosite clasts of the Nosib Group conglomerates are observed and suggestive of a pre-Nosib Group, syn- or post-KC deformation event.
On the aeromagnetic maps, the margins of the ZL exhibit strong positive anomalies which coincide with locations of peripheral mafic-ultramafic intrusions consisting mainly of altered harzburgite, peridotite, dunite and troctolite. Very strong positive magnetic anomalies coincide with the NW- to WNW-trending dolerite dykes that crosscut the ZL. The interior of the ZL is characterised by alternating high and low magnetic responses attributed to the alternating dark and white anorthosites that make up the ZL. The alternating high – low magnetic response is also seen extending into basement rocks up to 10 km to the south of the exposed ZL. The extension of the signal corresponding to the alternating dark and white anorthosites continues along the strike of primary fabrics in the ZL southern limb. This magnetic response is interpreted as sub-surface layered anorthosite under basement. Crosscutting the ZL layering are D4 faults along ZL-EMC contacts, which are expressed as negative anomalies. The nature of the sub-surface magmatic layering-like magnetic signal, whether it represents the roof of an intrusive portion of ZL anorthosites into basement or thrust anorthosites under basement rocks is unknown and requires further detailed work.
This study confirms that the ZL acquired its architecture during a NW-SE shortening event (D3) prior intrusion of 1100 Ma crosscutting dolerite dykes and sedimentation of the ~750 Ma Nosib Group siliciclastic sediments. This inference on the possible timing of the formation of the ZL geometry is supported by the high-temperature fabrics seen in the Nosib Group conglomerates, suggesting an earlier deformation event. Crosscutting dykes are also undeformed. Mesoscale field structural data are in line with the reconstructed NW-SE shortening paleostrain directions compatible with top to the NW kinematics. This deformation event reworked earlier D2 fabrics, which are concordant with the NNE trend of the KC in southern Angola. A localized D3 event in the ZL accounts for the orientation difference between the NNE-trending Angolan and ENE-trending Namibian components of the KC. The ZL is therefore interpreted as an ENE-WSW trending inclined SW-plunging fold with a steeply SE-dipping axial surface. Folding in the ZL likely occurred by layer-parallel shortening (buckling) as attested by D3 axial planar fabrics and the preserved N-S striking orientation of pre-existing S2 structures in the hinge zone. Densely populated high-temperature high strain solid-state microstructures in the northern limb imply enhanced recrystallisation. A top to the north thrust along the boundary between the Angolan and Namibian components possibly intensified recrystallisation in the northern limb of the ZL by localizing strain. The architecture, structural evolution, geophysical response of the ZL and the mineralogy of fault-bounded leucogabbronorites are established in this study with an attempt to determine the sequence of events that led to the modern day ZL geometry.