143 research outputs found
Overview of the Geosynclinal, Structural and Metamorphic Development of the Intracontinental Branch of the Damara Orogen
No full textThe Pan-African Damara Orogen in Namibia has, during the past 11 years, been the object of multidisciplinary studies aimed at elucidating the geodynamic development of the well-exposed intracontinental branch of this fold belt. It is the purpose of this overview to provide a guide to these investigations. The problems raised by previous work are set out.
The results of the sedimentological, structural, petrological, geochemical and geophysical investigations are shortly reviewed and discussed with respect to some of the problems posed by the geosynclinal, structural, metamorphic and magmatic development of the orogen. The different geodynamic models which have been proposed in the course of these studies are mentioned. A discussion of these models is given by Martin, this vol
Generalized Comparison of the Variscan with the Damara Orogen
No full textIn this brief comparison the articles of this volume are not specifically referred to. The reader will find figures showing the main structural subdivisons of the two belts in Martin, this vol., Overview Fig. 3, and Fngel, Franke & Langenstrassen, this vol., Fig. 1
Alternative Geodynamic Models for the Damara Orogeny. A Critical Discussion
No full textDuring the last five years a number of geodynamic models have been proposed for the intracontinental branch of the Pan-African Damara Orogen. All the models assume subduction processes of some kind. Two categories of subduction have been proposed: Continental subduction (Ampferer subdiction) and ocean floor subduction (Benioff subduction). The former is represented by an “aulacogen” model and a “delamination” model. The second category comprises two models assuming subduction of a wide ocean, one model proposing subduction of a narrow ocean arm, and one advocating processes of strike-slip faulting coupled with oblique subduction of small oceanic pullapart basins.
The essential features and arguments of the different models are briefly outlined, and the models critically discussed. None of the models integrates, in its hitherto presented form, all the known facts satisfactorily
The Chemical Composition of the Common Metamorphic Sediments of the Damara Orogen
No full textThe chemical composition of common rock types of the Damara Secquence has been investigated. About 300 well selected samples were analysed. The influence of recent weathering on the composition of the collected samples is small. Under the prevailing conditions mechanical decomposition of the rocks is much more important than any chemical process.
The processes of decomposition and transportation of the Pre-Damara rocks forming the sediments of the Damara geosyncline, can be traced by the trends of chemical composition in the common rock types. As sodium depleted pelitic rocks as well as pure quartzitic rocks are uncommon in the investigated sequences, repeated cycles of reworking of sediments in the geosynclinal state can be excluded.
Increasing metamorphism of the Damara rocks resulted in an increasing FeO/Fe2O3 ratio but constant total iron content; decreasing CO2 content and decreasing H2O content. Loss of sodium and potassium can be exluded. Some 4 % of carbonate detritus seems to be a maximum value for the sedimentation of average Damara pelitic rocks.
Rifting was the governing tectonic process in the evolution of the Damara. It is characterized by short transportation distances and high vertical subsidences — both favouring single step sedimentation processes — the prerequisite of sediment compositions found for the main rock types of the Damara Sequence
Geodynamic Model for the Geosynclinal Development of the Damara Orogen, Namibia, South West Africa1
No full textThe 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
Geochronology of the Damara Orogen — A Review
No full textA review of the geochronological literature on the Damara Orogen indicates an extended period of deposition on a rifting and stretching continental crust lasting several hundred million years until about 600 Ma ago. This relatively quiet period was followed by a second phase of rapid comression, crustal thickening, uplift, metamorphism and very voluminous granite intrusions between 550 and 510 Ma ago. The peak of regional metamorphism seems to have occurred around 530 Ma ago over the whole width of the orogen. An area of granites of relatively young Pb/Sr ages (around 470 Ma) exists in the northern Central Zone of the orogen
- …
