Geodynamic Model for the Geosynclinal Development of the Damara Orogen, Namibia, South West Africa1

Abstract

The Damara Orogen forms part of the Upper Proterozoic-Early Palaeozoic Pan-African belt system, parts of which have passed through a geosynclinal stage. The Damara Orogen comprising a N-S trending (“coastal”) and SW-NE trending (“intracratonic”) branch belongs to this category.

Under geodynamic and sedimentary aspects, the depositional development of the Damara Orogen may be subdivided into four successive stages, namely the graben, the downwarping, the synorogenic, and the lateorogenic stage.

The development of the Damara geosyncline is controlled largely by the specific evolutions of the non-contemporaneous active and inactive rift systems. Rifting started about 900 to 1000 Ma ago, when the “northern graben” was formed. This rift system remained active, as indicated by volcanic activity, until about 840 to 750 Ma ago. Due to a subsequent cooling and thermal contraction of the assumed asthenolith situated below the rift, the graben subsided differentially. This led to the development of a trough in which a thick turbidite sequence was deposited (Okonguarri turbidites).

When the rate of subsidence decreased the trough was filled up by sediments and eventually overstepped by carbonates which gradually formed an extensive platform throughout the central and northern Damara Orogen (“Karibib Platform”).

Rifting in the southern Damara Orogen started somewhat later than farther north. A hypothetic rift (“Khomas rift”) was formed in the crest of a major rift dome, while two subsidiary half-grabens developed at the northern and southern flanks of the dome (“central” and “southern” rifts”). The Khomas rift remained active throughout the entire geosynclinal development. Continued rifting and extension in the southern Damara Orogen was accompanied by mafic igneous activity and considerable crustal thinning and may have led to continental separation and formation of an ocean. Crustal thinning in the southern Damara Orogen initiated the development of a major basin (“Khomas Trough”). Sediments were supplied by turbidity currents, irom both the northern and the southern margins (Tinkas and Chausib turbidites) and later mainly from the east where the basin probably shallowed and perhaps closed (Kuiseb schists).

The Khomas Trough may have been connected with the Gariep Belt in southern Namibia/SWA where ocean opening appears to have occurred. It is assumed that this ocean gradually narrowed towards the Khomas Trough finally to end in a continental rift at the eastern closure of the Khomas Trough. The Khomas rift may thus be interpreted as a mid-ocean rift propagating into and ending in a continent.

After crustal separation in the Khomas Trough or due to a thermal relaxation of the stretched lithosphere, the entire Damara Orogen was affected by a final phase of regional subsidence. At this stage the Karibib Platform of the central and northern Damara Orogen subsided and the Kuiseb schists attained their maximum distribution in an extensive depository.

The development of the coastal branch corresponds largely to that of the intracratonic branch, with the “Sesfontein graben” corresponding to the northern graben and a hypothetic rift west of the present Atlantic coast corresponding to the Khomas rift.