The continental margin of southwestern Africa was investigated under morphotectonic aspects, due to its origin from plate tectonics. The following starting points of this treatise result from earlier studies of the authors in the southeastern Cape region, the Little Karoo and Namibia. 

1. The landforms of the continental margin (the onshore part of which is named continental ramp) are of polygenetic origin, induced by tectonics and isostasy and sculptured under different climatic conditions. Question: What is the role of these diverse morphogenetic factors?

2. Satellite images and morphographic maps (Fig. 1) reveal a distinct pattern of structural landforms. Unless being orogenic features, they provoke the question: What are the relations to post-Gondwana tectonics? 

3. The most impressive landform of the continental ramp is the Great Escarpment, which is caused by isostatic uplift. Question: To what extent is it a result of flexural bending or of erosional backwearing? 

4. Widespread planation surfaces and their remnants indicate an episodic ("cyclic") development. Question: How many phases can be diagnosed and how are they caused?

5. Essential characteristics of planation surfaces are duricrusts. Question: What types of duricrusts do exist and what is their age? 

To answer these questions, fieldwork was done in 1990, 1994, and 1995. The research program as well as the interpretation of the results were based on comprehensive evaluation of the literature, maps, and satellite imagery. Laboratory investigations were done with 63 samples of duricrusts (Table 2): by micromorphological (48 samples) or geochemical (27 samples) methods, or by X-ray diffraction method (41 samples) respectively. 

A key role in morphotectonics play former planation surfaces and their remnants. Considering all available descriptions of them, a system of planation surfaces was established with the African Surface as a reference datum  (Table 1). Our own investigations of palaeo-surfaces passed two steps: 1. their reconstruction; 2. their regional synopsis. 

Reconstruction: The knowledge of surface remnants was improved by mapping of summit and crest flats, slope shoulders, and pediments as relic landforms (Fig. 14). They were connected using height accordance of summits and of shoulders (Figs. 2, 3 and 4) and by duricrusts. The latter are differentiated according to their mode of origin: Autochthonous, allochthonous, syngenetic, successive, secondary, multiple (Figs. 7 - 9). The most important difference exists between autochthonous and allochthonous silcretes (Figs. 6 and 10). As the basic step of  correlation, the remnants of palaeo-surfaces were arranged as surface sequences including at least one reference level (Figs. 13 and 15). 

Due to the scarceness of datable sediments saprolites and duricrusts were widely used as defining features. Where possible, these features were related to more precise dates so as to get comparable sequences (Figs. 11, 12). 

Regional synopsis: Being the main precondition of morphotectonic conclusions, the regional synopsis of surface sequences was a problematic procedure. It was achieved by 1. relating the sequences to the system of planation surfaces considering all published findings; 2. by detailed examination in the field and from maps and satellite imagery (Figs. 2 - 5 ). Using this procedure nine topographic profiles from the coast to the highlands were constructed (Fig. 16). Their palaeo-surfaces were grouped into four main levels, referred to the African Surface (AS): postAS, AS, preAS 1, preAS 2. In some cases postAS can be divided into the (already known) stages of postAS 1 and postAS 2. The preAS 1 is a clearly distinct level in the coastal region and in the highlands. The sparse preAS 2 remnants may represent more than one level, and in the Cape Fold Belt a preAS 3 level can be assumed.

Analogous to the alterations of recent surfaces by epeirogenic bending and extensional breaking the tectonic deformations of surface sequences are defined as surface vergence and surface discontinuity. To analyse these deformations was the main purpose of our morphotectonic investigations.

Surface vergence means the slope of a reconstructed planation surface which exceeds the normal value of (generally) more or less 0,2 %. Gradients of 0,2 - l % are typical of surfaces which are tilted by uplift (Fig. 16 Profile 13). Vergence values greater than 0,5 - 1 % are in all probability the consequence of epeirogenic deformations, belonging either to upwarps/downwarps or to flexures (Fig. 5). Varying vergences within a surface sequence, i.e. convergence or divergence of surfaces, can be related to coincidences. Those of AS and postAS are widespread in the coastal zone and are also existent in the highlands.

Surface discontinuities are produced by ruptures, which appear as fissures or as fault scarps (Figs. 17, 19, 20), the latter ones being often related to depressions, i.e. grabens or half-grabens (Fig. 21). Most of the ruptures were recognized from satellite imagery (Figs. 17 and 19). 

A map of the main morphotectonic features (Fig. 22) was compiled from various publication records and supplemented by own investigations. As a dominating landform the Great Escarpment is to be emphasized. It proved itself to be a monoclinal flexure. Its tectonic nature is evidenced by antecedent valleys (Fig. 17) and supported by AFTA dates from the literature. Being analogous phenomenons, the surface vergences of the Namaqualand Escarpment (Fig. 16 Profiles 15 and 16) and of the Piketberg (Figs. 5 and 16 Profile 19) can be correlated. The southwestern Highveld Escarpment is formed by the Komsberg and the Roggeveld Flexures. The latter one is interpreted as a counterpart of the Doringrivier Flexure and both of them are attributed to the Tankwa Basin.

A distinct framework of warp axes is revealed by intact planation surfaces as well as by surface remnants. The position of some of the upwarp axes (Kamieskroon Axis, Cederberg Axis) parallel to the escarpment flexures points to a syngenetic origin of both features. Running orthogonally to them is another set of warp axes (Fischflussrücken Axis, Griqualand Axis, Krom/Sout Rivier Axis), the pattern of which can be ascribed to pre-existing structural trends.

Tensional ruptures were formed as a result of the upwarp of the continental margin. The plate movement also caused extensional features which are denoted as depressions like grabens and half-grabens (Fischfluss Depression, Huns Depression, Aussenkehr Depression, Olifantsrivier Depression, Langeberg Depression). As evidenced by their valley history, most of them are of Tertiary age, but some are reactivated Mesozoic features. 

Two striking morphotectonic phenomenons are worth mentioning: One is the trend of the Franschhoek Upwarp Axis (east of Stellenbosch). It seems to be related to microplate tectonics of the southwestern Cape Region. The other one is the system of deformations between the Lower Orange River and the Karasberge (Lorelei Flexure and coaxial lineaments, Noordoewer Basin with Aussenkehr Depression and Fischfluss Depression). They are related to the crossing of the Bremen-Garub Line and the Windhoek Lineament and are interpreted as results of movements along old crustal structures. 

Considering morphogenetic consequences here, the continental uplift was the most effective process of crustal movements. Attributes of the uplift are coastal flexures (Fig. 16 Profiles 13 and 21), tilted surfaces (Fig. 16 Profiles 13, 17, 21) and escarpment flexures (Fig. 16 Profiles 15-19). The coastal flexures are mainly developed offshore (Fig. 24) and are therefore omitted from the morphotectonic features. From surface tilts and surface flexures an upheaval of 600 - 1000 m since preAS 2 can be estimated. The separation of isostatic and tectonic components (Tab. 5) of upheaval is only approximative, deduced from AFTA dates cited in the literature. Their interpretation is based on an unusual high palaeo-geothermal gradient of 45°C/km, which is adopted from southeastern Australia. It is well in accordance with a mean denudation value of 1000 m in the southwestern Cape Region since preAS 2, i.e. Upper Cretaceous (Fig. 16 Profiles 18 -21; Table 5). 

The chronology of the palaeo-surfaces is a problem of "actual and comparative dates" (in the sense of L.C. King) and is complicated by the convergence and coincidence of surfaces. The chronological order of palaeo-surfaces was established using pairs of termini post quem and termini ante quem originating from dated volcanics and sediments (Table 4). This chronology could be confirmed correlating the onshore sequences of surfaces with the offshore sedimentary record and its unconformities (Fig. 24). 

Considering the periods of surface development and the time span involved in the formation of weathering mantles and duricrusts, a succession of morphogenetic phases of activity and passivity was deduced (Fig. 25a). The phases of activity coincide with volcanic episodes of continental (or even global) extension (Fig. 25d). The relationship between the morphogenetic phases, the main unconformities and hiatusses of the shelf sediments and the episodes of volcanic activities (Fig. 25) are interpreted as the result of syngenetic developments.

With regard to the questions raised initially, the following answers result: 

5.  Autochthonous silcretes and ferricretes are the dominant duricrusts in the southwestern parts of the region, partly developed syngenetic. Hardpan calcretes and allochthonous silcretes are more widespread in the northern parts. The autochthonous silcretes are of Early Tertiary age, most of the allochthonous ones as well as most of the hardpan calcretes are of Late Tertiary age. 
4. At least four phases of planation can be diagnosed throughout the whole region. Being related to shelf features and volcanic episodes, they are interpreted as a consequence of crustal movements. 
3.  The Great Escarpment proved itself to be a polygenetic landform, large segments being primarily a monoclinal flexure. 
2. The most of epeirogenic axes and of ruptures run parallel to the Atlantic rift structures or to orthogonal lineaments. All of them are of post-Gondwana origin, but some are rejuvenated Mesozoic features and some follow pre-Cambrian structures. 
1. Exogenic factors have controlled the weathering and denudation processes and were responsible for the microscale landforms. The macrosystem of landforms of southwestern Africa was obviously developed by endogenic processes. They have induced the phases of morphogenetic activity and have also determined their range of action. 

Zusammenfassung

Die Kontinentalrampe des südwestlichen Afrikas ist unter dem plattentektonischen Aspekt des passiven Kontinentalrandes untersucht worden. Als tektonisch bedingte Großform ist sie Gegenstand morphotektonischer Analysen, wobei die Große Randstufe eine Kennform darstellt.
Zur Klärung der Morphogenese sind zunächst ehemalige Rumpfflächen ermittelt worden. Sie haben eine Vertikalgliederung des Reliefs in vier morphogenetische Hauptstockwerke ergeben, die formal der Gondwana Surface, post-Gondwana Surface, African Surface und post-African Surface im Sinne von L.C. King entsprechen. Ihre Gültigkeit im gesamten Untersuchungsraum  von Heidelberg im Süden bis Lüderitz im Norden ist durch die Integration aller relevanten Erkenntnisse kontrolliert und im Prinzip bestätigt worden, wobei die ältesten Relikte jedoch nur teilweise Gondwana zugeordnet werden können. Die Korrelation der Altflächen basiert auf dem Vergleich von Vertikalsequenzen der Flächenstockwerke ("Flächensequenzen"), die mittels Referenzflächen zeitlich geordnet worden sind. Als chronologische Zeugen weiträumigen Zusammenhanges sind Verwitterungsprodukte, darunter vorrangig Silcretes und Calcretes, ausgewertet worden. Dabei hat sich die Unterscheidung autochthoner und allochthoner Silcretes als aussagekräftig erwiesen. Wichtigste Referenzfläche ist die African Surface (AS), mit der postAS- und preAS-1-Stockwerke regelhaft, preAS-2-Relikte nur selten assoziiert sind (und durch Gipfelfluren repräsentiert werden).
Deformationen der Flächensequenzen dienen als morphotektonische Indikatoren. Es sind:

1. Flächenvergenzen: Abdachungen ehemaliger Rumpfflächen, deren Gefälle das Normalmaß (etwa 0,2 %) weit übersteigt und zur Zertalung geführt hat; 

2. Flächendiskontinuitäten: Unterbrechungen der Rumpfflächen an Rupturen, die als Klüfte, Spalten oder Verwerfungen entstanden sind.

Aus den Flächenvergenzen sind als epirogene Deformationen Flexuren, Wölbungen und Mulden, aus den Flächendiskontinuitäten als bruchtektonische Formen Rupturspalten und Ruptursenken (Gräben und Halbgräben) erschlossen worden. Die Deformationen bilden ein Muster vorherrschend küstengleich und quer laufender Richtungen der Achsen und Störungen. Die bedeutendsten Flächenvergenzen sind Flexuren im Bereich der Großen Randstufe, deren primär tektonische Anlage damit bestätigt ist. Mit ihnen sind Wölbungen, Rupturspalten und -senken assoziiert, die einen wesentlichen Teil des Mesoreliefs bilden und den Grundriß der Talsysteme determinieren. Außer isostatischen Ausgleichsbewegungen  haben Krustenzerrungen das Relief des passiven Kontinentalrandes  entscheidend beeinflußt.
Die morphogenetisch dominierende Krustenbewegung im südlichen Afrika ist die Aufwölbung des Subkontinents, die phasenhaft vom späten Mesozoikum bis zum Känozoikum stattgefunden und die Vertikalgliederung des Reliefs durch Altflächen bewirkt hat. Die Flächensequenzen sind durch eine Kombination von termini post quem (vorwiegend radiometrisch datierte Vulkanite) und termini ante quem (datierte Sedimente und Verwitterungsbildungen) zeitlich geordnet und mit Vertikalgliederungen der Schelfsedimente (Diskordanzen und Schichtlücken) korreliert worden. Daraus ist eine Folge von morphogenetischen Aktivitäts- und Passivitätsphasen abgeleitet worden, die als Resultat episodischer Ereignisse der Plattentektonik interpretiert werden.