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| Titel |
Long-term landscape evolution of the South Atlantic passive continental margin along the Kaoko- and Damara Belts, NW-Namibia |
| VerfasserIn |
Daniel Menges, Ulrich Anton Glasmacher, Peter Hackspacher, Gabriele Schneider, Eric Salomon |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101656
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| Publikation (Nr.) |
 EGU/EGU2015-845.pdf |
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| Zusammenfassung |
| The Kaoko Belt in northwestern Namibia originates in the collision of the Rio de la Plata and Kongo Craton during the Pan-African Orogeny in the Neoproterozoic (1) and represents the northern arm of the Damara Orogen. NW-Namibias continental crust mainly consists of the NE-SW striking intracontinental branch of the Pan-African Damara mobile belt, which separates the Congo from the Kalahari craton. The Damara Orogen is divided into several tectonostratigraphic zones that are bounded by steeply dipping, ductile shear zones. These regional lineaments can be traced at least 150 km offshore (2). The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks (534 (7) Ma – 481 (25) Ma (3) as well as Mesozoic sedimentary and igneous rocks. From Permo-Carboniferous to Mid Jurassic northern Namibia was affected by deep erosion of the Damara Orogen, Permo-Triassic collisional processes along the southern margin of Gondwana and eastern margin of Africa (4), and the deposition of the Nama Group sediments and the Karoo megasequence (5). Between the Otjihorongo and the Omaruru Lineament-Waterberg Thrust early Mesozoic tectonic activity is recorded by coarse clastic sediments deposited within NE trending half-graben structures. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183±1 Ma (6). The Early Cretaceous Paraná-Etendeka flood basalts (132±1 Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (7). Early Cretaceous alkaline intrusions (137-124 Ma) occur preferentially along Mesozoic half-graben structures and are called the Damaraland Igneous Province (8). Late Cretaceous alkaline intrusions and kimberlite pipes occur in northern Namibia. Post Early Paleocene siliciclastic sedimentation in Namibia was largely restricted to a 150 km wide zone (9) and is represented by the Tsondab Sandstone Formation (~ 300 m thickness). The oldest part has an age of early Paleocene and the upper part span from middle Miocene (~13 Ma) to Pliocene (~2 Ma) (10). Cenozoic alkaline intrusions and kimberlite pipes are also known from the region.
The so-called "Great Escarpment" that reach elevation of up to 2350 m characterizes strongly the morphology of the passive continental margin in Namibia (11,12). In contrast to Brazil, the escarpment is more than 150 km inland of Namibia. Interesting enough the Brandenberg intrusive complex of ~130 Ma age clearly indicates the post-intrusion denudation of more than 4,000m (13). The Great Escarpment can be traced from central Angola to the eastern edge of South Africa. A considerable variation along its distribution reflects variations in tectonic history, in lithologies, and in the drainage system. In Namibia, the retreating model has dominated the genetic discussion (14,15,16). However, surface process modeling has suggested other possibilities11. In addition, apatite fission-track research, terrigenious cosmogenic nuclides (TCN) have been used on specific landscape elements to determine denudation rates. In the central Namib Desert, denudation rates calculated from 10Be and 26Al are in the range of ±5 m Ma-1 and might be representative for the last 103 - 106 a (17). The persistence of arid climatic conditions throughout the Cenozoic might even lead to such low denudation rates for the past 10-12 Ma. A low retreat rate of ~10 m Ma-1 representative for the last 1 Ma was determined for the Great Escarpment in central and southern Namibia. Considering all currently, available thermochronological data for the Namibian margin (18,19,20), the validity of the scarp retreat model is highly problematic.
Apatite fission-track ages revealed so far range between 390.9±17.9 Ma and 80.8±6.0 Ma. The large spread in ages is partly related to significant changes of ages at the NW-SE trending Purros Lineament and at the Sesfontein thrust. In general, the AFT-ages are older northeast of the Purros Lineament. Furthermore, all basalt samples of Etendeka age display the same AFT-age range within error, between 103.5±4.9 and 108.0±5.6 Ma. The oldest ages are revealed from metamorphic rocks of the Damara Group as well as sandstones and glacial deposits of the Permo-Carboniferous Karoo series.
References
1. Goscombe, B. D., Gray, D. R., 2008. Structure and strain variation at mid-crustal levels in a transpressional orogen: A review of Kaoko Belt structure and the character of west Gondwana amalgamation and dispersal. Gondwana Res. 13, 45–85.
2. Clemson, J., Cartwright, J., Booth, J., 1997. Structural segmentation and the influence of basement structure on the Namibian passive margin. J. Geol. Soc. London 154, 477-482.
3. Miller, R.M., 1983. Evolution of the Damara Orogen, Vol. 11, Geol. Soc., South Africa Spec. Pub..
4. Coward, M.P., Daly, M.C., 1984. Crustal lineaments and shear zones in Africa: Their relationships to plate movements, Precambrian Research 24: 27–45.
5. Stollhofen, H., 1999. Karoo Synrift-Sedimentation und ihre tektonische Kontrolle am entstehenden Kontinentalrand Namibias, Z.dt.geol.Ges. 149: 519-632.
6. Duncan, R., Hooper, P., Rehacek, J., March, J., Duncan, A., 1997. The timing and duration of the Karoo igneous event, southern Gondwana, J. Geophy. Res. 102: 18127–18138.
7. Renne, P.R., Glen, J.M., Milner, S.C., Duncan, A.R., 1996. Age of Etendeka flood volcanism and associated intrusions in southwestern Africa, Geology 24 (7): 659– 662.
8. Watkins, R.T., McDougall, I., le Roex, A. P., 1994. K-Ar ages of the Brandberg and Okenenya igneous complexes, north-western Namibia, Geol. Rund. 83: 348–356.
9. Ward, J.D., 1988. Geology of the Tsondab Sandstone Formation, Journal of Sedimentary Geology 55: 143–162.
10. Senut, B., Pickford, M., 1995. Fossil eggs and Cenozoic continental biostratigraphy of Namibia, Pal. Afr.,32: 33–37.
11. Gilchrist, A.R., Kooi, H., Beaumont, C.,1994. Post Gondwana geomorphic evolution of southwestern Africa: Implications for the controls on landscape development from observations and numerical experiments, J. Geophy. Res. 99: 12211–12228.
12. Brown, R. W., Gallagher, K. and Gleadow, A. J. W., 2000. Morphotectonic evolution of the South Atlantic margins of Africa and South America, in M. A. Summerfield (ed.), Geomorphology and Global Tectonics, JohnWiley and Sons Ltd., Chichester, pp. 255–281.
13. Raab, M. J., Brown, R. W., Gallagher, K., Weber, K., Gleadow, A. J. W., 2005. Denudational and thermal history of the Early Cretaceous Brandberg and Okenyenya igneous complexes on Namibia's Atlantic passive margin Tectonics 24: 1-15.
14. Guillocheau, F., Rouby, D., Robin, C. Helm, C., Rolland, N., Le Carlier de Veslud, C., Braun, J., 2012. Quantification and causes of the terrigeneous sediment budget at the scale of a continent margin: a new method applied to the Namibia-South Africa Margin. BasinRes. 24, 3-30.
15. Dauteuil, O., Rouby, D., Braun, J., Guillocheau, F., Deschamps, F., 2013. Post-breakup evolution of the margin of Namibia: constraints from numerical approach. Tectonophysics 604, 122-138.
16. Rouby, D., Braun, J., Dauteuil, O., Deschamps, F., Robin, C., 2013. Long-term stratigraphic evolution of Atlantic-type passive margins: a numerical approach of interactions between surface processes, flexural isostasy and 3D thermal subsidence. Tectonophysics 604, 83-103.
17. Cockburn, H. A. P., Brown, R. W., Summerfield, M. A. and Seidl, M. A., 2000. Quantifying passive margin denudation and landscape development using a combined fission-track thermochronology and cosmogenic isotope analysis approach, EPSL 179: 429–435.
18. Brown, R. W., 1992. A fission track thermochronology study of the tectonic and geomorphic development of the sub-aerial continental margins of southern Africa., PhD thesis, La Trobe University, Bundoora, Australia.
19. Gallagher, K. and Brown, R. W., 1999. Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa, Philosophical Transactions Royal Society London A 357: 835–859.
20. Raab, M. J., Brown, R. W., Gallagher, K., Carter, A., Weber, K., 2002. Late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis. Tectonophysics 349: 75-92. |
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