Henno Martin

The intracratonic branch of the Damara Orogen in South West Africa I. Discussion of geodynamic models

Abstract The late Precambrian Damara Supergroup was deposited in a geosyncline. There is good evidence that the intracratonic branch of this geosyncline began its development with a stage of rifting which produced three widely spaced grabens in sialic crust. During a second stage of subsidence the grabens merged and formed a geosyncline. A third stage produced intense deformation associated with thrusting and nappe transport, high-grade metamorphism, anatexis and large-scale granodioritic to granitic plutonism. A discussion of lifely geodynamic interpretations leads to the conclusion that the development of the intracratonic geosyncline can be best explained by a multiple aulacogen model. The dynamics of the orogeny cannot be readily interpreted with the help of a plate tectonic subduction-collision model. Concepts based on Rambergs (1972) gravitational instability models are considered applicable. The hypothesis is advanced that grabens, aulacogens and mobile belts may represent diverse responses of the crust to astenoliths of different sizes.

The intracratonic branch of the Damara Orogen in South West Africa II. Discussion of relationships with the Pan-African mobile belt system☆

Abstract In Part I the conclusion was reached that the intracratonic branch of the Damara orogen cannot be explained by a plate-tectonics continent/continent collision model. The intracratonic branch is connected with the coastal branch and also with the Zambezi belt. There is a direct stratigraphic and structural connection with the coastal branch which shows some features suggestive of a continental margin geosyncline, and, by inference, an orogeny caused by subduction. Such an interpretation would make it possible to regard the intracratonic branch as a failed arm of a hypothetical triple junction situated to the west of the present-day coast. The evidence for this attractive hypothesis must, however, remain inconclusive because the greater part of the coastal belt is hidden underneath the Atlantic Ocean. The hypothetical ocean arm cannot have been broad and did probably not extent northwards beyond Gabon. The connection with the Zambezi belt and the Lufilian arc is covered by younger deposits. The Zambezi belt is composed of tectonically and thermally reconstituted Archaean basement. It contains no structure which could be interpreted as a continent/continent collision suture. This is also true for the Mozambique belt which is structurally connected with the Zambezi belt. The join between the two can therefore not be regarded as a triple junction in the plate tectonic sense. The Lufilian arc contains a “miogeosynclinal” sequence which is stratigraphically very similar to the Damara Supergroup, but it is still uncertain whether the two are strictly contemporaneous. In these belts vertical movements and transcurrent faults seem to have played a greater role than compressive shortening. The evolution of these belts was to a considerable degree guided by the older Irumide structures which seem to be connected with the approximately contemporaneous Rehoboth-Sinclair igneous province delimiting the southern margin of the Damara belt. A geodynamic model is proposed for the evolution of grabens, aulacogens, ensialic geosynclines and regions with tectono-thermal overprinting interpreting these structures as different reactions of the heterogeneous crust to mantle diapirs of different sizes.

GRAVITY TECTONICS IN THE NAUKLUFT MOUNTAINS OF SOUTH WEST AFRICA

The late Precambrian Nama beds of the Naukluft Mountains in South West Africa show intense Alpine-type deformation. The crystalline basement underlying these rocks is little disturbed. Its upper surface is basin-shaped. The intensity of deformation in the overlying Nama beds increases upward to a practically undedormed unconformity which separates the Nama into two folded units. The unconformity is overlain by a strikingly uniform and little disturbed dolomite bed. Above this dolomite the intensity of deformation once more increases; it reaches its maximum in this upper deformed unit. The folds delineate a cascading movement from the topographically higher northern hinterland and form a great arc in front of the steepest slope of the basin, overriding the undeformed Nama beds of the southern foreland in a number of thrust sheets and nappes. These observations, based on detailed mapping from exceptionally favorable exposures, bring out a movement pattern which is plainly controlled by the shape of the basin. Such a pattern is best interpreted as due to plastic flow under the influence of gravity.

Mixtite deposits of the Damara sequence, Namibia, problems of interpretation

Abstract Mixtite deposits form extensive outcrops in the Pan-African Damara orogenic belt of Namibia. They occur at two different stratigraphic levels in the lower part of the Upper Proterozoic Damara Sequence (South African Committee for Stratigraphy 1980; Kroner, 1981) . The older Varianto mixtite is confined to the northern margin of the fold belt. No evidence indicating a glacigenic origin has been found, but on the southern foreland of the fold belt two, possibly coeval, mixtite deposits (Court and Blaubeker Formation) show fair evidence of such an origin. The younger Chuos mixtitesoccur in three separate zones of the belt. All these mixtites have been interpreted as glacigenic sediments, and the Chuos Formation has been used as a stratigraphic marker for the correlation between different parts of the fold belt. The present investigation shows that: (1) The mixtite deposits show no features indicating a glacial origin; (2) The sedimentary features indicate deposition by gravity-flow processes; (3) turbidites are in several areas closely associated with and even interbedded in the mixtite deposits; (4) mixtite deposition is in some areas not confined to the Chuos Formation proper, but began at lower stratigraphic levels or persisted to a higher level; (5) in some areas calcareous or dolomitic sediments are interbedded in mixtite, or mixtites are interbedded in such formations. The mixtites of the Chuos Formation are interpreted as various kinds of submarine gravity-flows (slump breccias, mass-flow, slurry, grain-flows, turbidites) which were probably triggered by tectonic activity during a stage of differential subsidence that affected the whole geosyncline, but was concentrated in three rift zones. Under this assumption the majority of the mixtites may have been deposited during a limited time spin. A different model assuming synorogenic deposition of the “pebbly schist” mixtites of the Southern Margin Zone is briefly discussed. It is concluded that the mixtite deposits are not of glacial origin and can therefore not be regarded as reliable chronostratigraphic markers for correlations between the different facies domains of the Damara sequence nor for correlation with other Upper Proterozoic sequences (e.g. Gariep belt, Katanga belt); such a correlation might, nevertheless, exist, if the extensive gravity-flows should have been caused by eustatic changes of the sealevel during a time of widespread glaciation on other parts of the globe. In this case local mountain glaciers might have contributed material to some of the Chuos mixtites.

Beobachtungen zum Problem der jung-Präkambrischen glazialen Ablagerungen in Südwestafrika

The Late Precambrian Numees Formation contains besides very thick deposits of tillite-like habit also laminated siltstones resembling glacial varved rocks. These enclose coarser sand grains, pebbles and boulders of all sizes which have dropped from above into the soft sediment. As only floating ice can have transported these large components, the conclusion is inescapable that the laminated siltstones are true glacial varves and that the tillite-like rocks are true tillites.This evidence for Late Precambrien glacial conditions makes it highly probable that other formations of a similar age, which have been described as tillites, are also of glacial origin. The Buschmannsklippe Formation begins locally with a tillite which seems to be more or less contemporaneous with the Numees Formation. A high percentage of facetted and deeply striated pebbles and boulders makes it probable that this tillite originated as a basal moraine.The Nama tillite in the Klein Karas Mountains is stratigraphically not very far removed and in time probably not very much younger than the Numees tillite. Striated and grooved floors are associated with this tillite.The Chuos tillite and the Otavi tillite of the Damara System are contemporaneous deposits of Late Precambrian age. Their age relative to the Numees tillite is not known. The Chuos- and the Otavi tillite are probably glaciomarine drifts. For these a glacial origin cannot be proved with the same degree of conclusiveness as for the above described deposits. However, the existence of glacial conditions in the Late Praecambrian having been proved for an adjoining area, a glacial origin may reasonably be assumed for the tillite-like rocks of the Damara System too.Both the Numees tillite and the Otavi tillite are intimately associated with sedimentary iron ore deposits. Oxygen deficiency in stagnating bottom waters, caused by an ice cover, is thought to be responsible for this peculiar combination of sediments.The excellent evidence for the existence of glacial conditions during parts of the Late Precambrian, found in South West Africa, strongly supports the assumption that the many tillite-like deposits of a similar age, which have been described from Central and West Africa are also of glacial origin.

Alternative Geodynamic Models for the Damara Orogeny. A Critical Discussion

During 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.

A CRITICAL REVIEW OF THE EVIDENCE FOR A FORMER DIRECT CONNECTION OF SOUTH AMERICA WITH AFRICA

The probabilities of a former connection of South America with Africa have been debated ever since F. E. Suess propounded the concept of a former supercontinent, called Gondwanaland embracing India and all the continents of the southern hemisphere. At first the idea did not seem very startling, because it was well known that large continental areas had in the past been repeatedly invaded by the sea and because it was unknown that continents and oceans had dissimilar crustal structures. It was then Wegener’S exposition of the hypothesis of continental drift which focused attention on the many similarities in the geological history of the two widely separated continents.

The Root Zone of the Naukluft Nappe Complex: Geodynamic Implications

The relationships between the Naukluft Nappe Complex and the Damara mobile belt are investigated with the help of lithofacies comparisons. Some metamorphic features and the origin of the Sole Dolomite which forms the sole of the upper nappe complex, are discussed. Before crystallization, the Sole Dolomite consisted of a mobilised mylonitic sludge with a high content of hot saline fluid. The saline fluid was probably derived from evaporite beds in the lower Damaran Duruchaus Formation which fills a deep basin some 90 km to the north-east of the nappe complex, but may originally have had a wider extent.