Abstract
Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.
A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America.
The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s-1, underlain by a refracting layer in which the average velocity is 4.5 km s-1. The velocity of 6.6 km s-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin.
On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s-1. This is more than 1.0 km s-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.
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References
Barker, P. F. and Dalziel, I. W. D.: 1974, ‘Southwestern Atlantic Leg 36,’ Geotimes 19 (11).
Barrett, D. M.: 1974, ‘The Structure and Origin of the Agulhas Plateau,’ Ph.D. Thesis, University of the Witwatersrand, Johannesburg.
BryanG. M. and SimpsonE. S. W.: 1971, ‘Seismic Refraction Measurements on the Continental Shelf between the Orange River and Cape Twon, Inst. Geol. Sci. Rept. 70 (16), 187–198.
BullardE. C., EverettJ. E., and SmithA. G.: 1965, ‘The Fit of the Continents around the Atlantic,’ Phil. Trans. Roy. Soc. 258A, 41–51.
ChristensenN. I.: 1973, ‘Seismic Structure of the Upper Portion of Layer 2’ (Abstract), Trans Am. Geophys. Union 54, 1033.
ChristensenN. I. and SalisburyM. H.: 1972, ‘Sea Floor Spreading, Progressive Alteration of Layer 2 Basalts, and Associated Changes in Seismic Velocities,’ Earth Planetary Sci. Letters 15, 367–375.
ChristensenN. I. and SalisburyM. H.: 1973, ‘Velocities, Elastic Moduli and Weathering-Age Relations for Pacific Layer 2 Basalts,’ Earth Planetary Sci. Letters 19, 461–470.
DietzR. S. and SprollW. P.: 1970, ‘Fit between Africa and Antarctica. A Continental Drift Reconstruction,’ Science 167, 1612–1614.
duToitA. L.: 1937, Our Wandering Continents, Oliver and Boyd, London.
EwingM.: 1965, ‘The sediments of the Argentine Basin,’ Quart. J. Roy. Astron. Soc. 6, 10–27.
EwingM., LudwigW. J., and EwingJ. I.: 1964, ‘Sediment Distribution in the Oceans: the Argentine Basin,’ J. Geophys. Res. 69, 2003–2032.
EwingM., EittreimS., TruchanM., and EwingJ. I.: 1969, ‘Sediment Distribution in the Indian Ocean,’ Deep Sea Res. 16, 231–248.
EwingJ. R., LudwigW. J., EwingM. E., and EittreimS. L.: 1971, ‘Structure of the Scotia Sea and the Falkland Plateau,’ J. Geophys. Res. 76, 7118–7137.
Fatti, J. L.: 1970, ‘The Use of Seismic Reflection Techniques in the Karroo Basin,’ M.Sc. Thesis, University of the Witwatersrand, Johannesburg.
FrancisT. J. G. and RaittR. W.: 1967, ‘Seismic Refraction Measurements in Southern Indian Ocean,’ J. Geophys. Res. 72, 3015–3047.
FrancisT. J. G. and ShorG. G.: 1966, ‘Seismic Refraction Measurements in the Northwest Indian Ocean,’ J. Geophys. Res. 71, 427–450.
FrancisT. J. G., DaviesD., and HillM. N.: 1966, ‘Crustal Structure between Kenya and the Seychelles,’ Phil. Trans. Roy. Soc. 259A, 240–261.
FrancheteauJ. and LePichonX.: 1972, ‘Marginal Fracture Zones as Structural Framework of Continental Margins in South Atlantic Ocean,’ Am. Assoc. Petroleum Geologists Bull. 56 (6), 991–1007.
GaneP. G., AtkinsA. R., SellschopJ. P. F., and SeligmanP.: 1956, ‘Crustal Structure in the Transvaal,’ Bull. Seism. Soc. Am. 46 (4), 293–316.
GaskellT. F. and SwallowJ. C.: 1953, ‘Seismic Refraction Experiments in the Indian Ocean and in the Mediterranean Sea,’ Nature 172, 535–537.
GreenR. W. E. and BlochS.: 1971, ‘The Ceres, South Africa, Earthquake of September 29, 1969. I. Report on some Aftershocks,’ Bull. Seism. Soc. Am. 61 (4), 851–859.
GreenR. W. E. and HalesA. L.: 1966, ‘Seismic Refraction Measurements in the Southwestern Indian Ocean,’ J. Geophys. Res. 71, 1637–1647.
HalesA. L. and GoughD. I.: 1960, ‘Isostatic Anomalies and Crustal Structure in the Southern Cape,’ Geophys. J. Roy. Astron. Soc. 3 (2), 225–236.
HalesA. L. and NationJ. B.: 1973, ‘A Seismic Refraction Study in the Southern Indian Ocean,’ Bull. Seism. Soc. Am. 63 (6), 1951–1966.
LarsonR. L. and LaddJ. W.: 1973, ‘Evidence for the Opening of the South Atlantic in the Early Cretaceous,’ Nature 246, 209–212.
LaughtonA. S., MatthewsD. H. and FisherR. L.: 1970, ‘The Structure of the Indian Ocean,’ in The Sea (ed. by A. E.Maxwell). Wiley-Interscience Publ., New York, Vol. 4, pp. 543–586.
LeydenR., EwingM., and SimpsonE. S. W.: 1971, ‘Geophysical Reconnaissance on African Shelf: I. Cape Town to East London,’ Am. Assoc. Petroleum Geologists Bull. 55 (5), 651–657.
LudwigW. J., NafeJ. E., SimpsonE. S. W., and SacksS.: 1968, ‘Seismic Refraction Measurements on the Southeast African Continental Margin,’ J. Geophys. Res. 73, 3707–3719.
Milner, B.: 1974, ‘A Geophysical Investigation of the Loading of the Hendrik Verwoerd Dam,’ M.Sc. Thesis, University of the Witwatersrand, Johannesburg.
RaittR. W.: 1963, ‘The Crustal Rocks,’ in The Sea (ed. by M. N.Hill), Interscience Publ., New York, Vol. 3, pp. 85–102.
Raitt, R. W. and Shor, G. G.: 1959, ‘Pacific Oceanic Crust,’ Preprints of Abstracts of Papers, First International Oceanographic Congress, New York, August 31-September 12, pp. 49.
Saaiman, C. H.: 1962, ‘Refraction and Reflection Seismic Tests Applied to Selected Geological Problems in South Africa,’ M.Sc. thesis, University of South Africa, Pretoria.
SchreiberE. and FoxP. J.: 1973, ‘Compressional Wave Velocities of Oceanic Rocks and the Geology of the Oceanic Crust: A Brief Summary’ (Abstract), Trans. Am. Geophys. Union 54, 1033–1035.
ScruttonR. A. and duPlessisA.: 1973, ‘Possible Marginal Fracture Ridge South of South Africa,’ Nature 242, 180–182.
SimpsonE. S. W., SchlichR., et al.: 1974, Initial Reports of the Deep Sea Drilling Project, Washington (U.S. Government Printing Office), Vol. 25, pp. 259–346.
Shor, G. G., Jr., Menard, H. W., and Raitt, R. W.: 1970, ‘Structure of the Pacific Basin,’ in The Sea (ed. by A. E. Maxwell), Vol. 4, pp. 3–25.
SmithA. G. and HallamA.: 1970, ‘The Fit of the Southern Continents,’ Nature 225, 139–144.
Spence, D. L.: 1972, ‘A Seismic Refraction Study of Sedimentary Structure of the Agulhas Bank, South of Cape Infanta,’ M.Sc. Thesis, University of the Witwatersrand, Johannesburg.
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Chetty, P., Green, R.W.E. Seismic refraction observations in the Transkei Basin and adjacent areas. Mar Geophys Res 3, 197–208 (1977). https://doi.org/10.1007/BF00310110
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DOI: https://doi.org/10.1007/BF00310110