The Northern Branch: Depositional Development and Timing of the Structural and Metamorphic Evolution within the Framework of the Damara Orogen

Abstract

The depositional history of the northern Damara branch (coastal branch) described on the basis of a cross-section from Kamanjab to Sesfontein and from there to the coast is controlled by the development and further evolution of rift (grabrn) structures. Four depositional domains (platform, continental rift, rift margin and oceanic basin) can be distinguished.

During the compressional stage these structures were partly reactivated thereby supporting the formation of major thrusts, e.g. Sesfontein Thrust. Metamorphism increases from very low-grade in the east to highgrade near the coast in the west. There is also an increase of metamorphism and deformation from the north to the south.

K-Ar isotopic studies on separated biotites and muscovites in the western part of the cross-section yielded age values between 530 Ma and 504 Ma. They are interpreted as cooling ages of a regional metamorphic event around 550 Ma ago. In the eastern part the same isotopic studies were carried out on the fine mineral fraction <2 μm of metasediments. Two distinct groups of ages were obtained, 490 Ma and 460 Ma respectively. The group of 490 Ma may be related to the main event in the west of about 550 Ma, while the group around 460 Ma represents a separate regional metamorphic event. The data are in favour of a polycyclic development of the coastal branch.

Along the northern margin of the intracontinental branch K-Ar ages between 380 Ma and 490 Ma were measured. This scattering is due to a rejuvenation of the ages locally connected with intense hydrothermal processes caused by late- to post-tectonic intrusions of granites.

An exception was found in the area of Tsumeb were only ages around 460 Ma occur. These ages are in accordance with the youngest group of ages in the Sesfontein area in the coastal branch and represent the last regional metamorphic event in the Damara Orogen.

From the now available data a more or less synchronous tectono-thermal history of the coastal and the intracontinental branch of the Damara Orogen can be assumed. At least three distinct tectono-metamorphic events may be distinguished in both branches of the Damara Orogen.