Thomas M. Will

The Influence of Inherited Structures on Dike Emplacement during Gondwana Breakup in Southwestern Africa

A kinematic analysis of Cretaceous and pre-Cretaceous structures was carried out along the west coast of southern Africa extending from the Cape Town area into Namibia with the aim of investigating whether inherited structural discontinuities controlled Gondwana breakup and the associated opening of the South Atlantic in the Cretaceous. This was augmented by map and satellite image analyses of mainly Cretaceous mafic dikes exposed from SW Angola to the southern tip of Africa. The combined results provide consistent evidence of E-W- to NE-SW-directed extension during the Cretaceous. E-W extension dominated from southern Angola to the Namibian–South African border, and NE-SW-directed extension occurred in an area extending from west-central Namibia to the southwestern coast of South Africa. These two extension directions overlap from west-central Namibia to southernmost Namibia. The kinematic analysis of Pan-African structures provides evidence of ENE-WSW-directed shortening in the western Saldania Belt in SW South Africa and ENE-WSW- to ESE-WNW-oriented constriction in the Gariep Belt near the border between South Africa and Namibia. Thus, the Pan-African shortening directions that led to the Late Neoproterozoic/Early Cambrian amalgamation of SW Gondwana are parallel to the main extension directions during Early Cretaceous Gondwana breakup and the initiation of South Atlantic rifting in southwestern Africa. This implies that opening of the modern South Atlantic was controlled by Pan-African (or older) structural discontinuities that were reutilized during the Early Cretaceous. The inherited structural basement anisotropies, which are generally parallel to major lineaments and/or crustal-scale shear zones, apparently controlled dike emplacement in the Early Cretaceous and the location of rifting at that time.

Chapter 5.5 Orogenic Tectono-Thermal Evolution☆

The metamorphic zonation of the Damara and Kaoko belts is very similar. Both have paired medium-pressure, Barrovian-type and low-pressure–high-temperature, Buchan-type belts with abundant to scattered granites in the latter and large-scale nappe structures in the former. In each, minerals of the early, highest pressure assemblages are enclosed in decompression coronas that record as much a 3 kbar of decompression within 20–30 myr. Unique to the western Kaoko Belt is a granulite-facies metamorphic event with associated calc-alkine magmatism at approximately 650 Ma, thus predating the peak of metamorphism and deformation in the Damara and Gariep belts at about 542 Ma. During both the M1 and M2 phases of metamorphism in the Damara Belt, the southern Central Zone was the leading edge of the high-temperature–low pressure active continental margin, whereas the Southern Zone and Southern Marginal Zone of that belt formed the low-temperature–medium pressure regions of the accretionary wedge riding atop the subducting Kalahari plate. In contrast to the above, only low-grade metamorphism is recorded further south in the Gariep and Saldania belts. There the main structural imprint was caused by sinistral transpression with top-to-northeast transport, similar as in the Kaoko Belt.