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Agulhas Plateau Off Southern Africa: a Geophysical Study

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Agulhas Plateau Off Southern Africa: a Geophysical Study Agulhas Plateau off southern Africa: A geophysical study DOUGLAS M. BARRETT Bernard Price Institute of Geophysical Research, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2001, South Africa ABSTRACT Broken Ridge (Francis and Raitt, 1967), are dence for the existence of a fracture ridge continental in structure and are thought to along the southeastern margin of South Af- Refraction data from the Agulhas Basin, be fragments resulting from the breakup rica, and Scrutton (1973) has suggested an south of southern Africa, show a crustal and dispersion of Gondwanaland. Others, origin for the Agulhas Plateau in terms of structure compatible with deep-water such as the Chagos-Laccadive Ridge (Fisher the separation of the African and South marine stations. Moho is about 10 km and others, 1971) and the Ninetyeast Ridge American plates along this fracture zone. deep, and the crust is believed to be of (Francis and Raitt, 1967) appear to be of He proposed that the Agulhas Plateau may oceanic origin. On the Agulhas Plateau, a oceanic genesis. McKenzie and Sclater be an abandoned spreading center. Emery basement layer having a seismic velocity of (1971) have suggested an oceanic origin for and others (1975) have also suggested an 4.84 km/s overlies the main crustal layer the Crozet Plateau and a continental struc- oceanic structure for this feature. with velocity 6.72 km/s. This structure is ture for the Kerguelen Plateau. The nature In 1968, as part of a program of marine not continental hut resembles that of cer- of the Agulhas Plateau has remained refraction work in the southwest Indian tain volcanic features in the Indian and obscure, but it occupies a key geographical Ocean, profiles were shot on the Agulhas Pacific Oceans, such as the Chagos- position in any proposed reconstructional Plateau and also in the deep tongue of the Laccadive and Hawaiian Ridges. The arrangement of the Gondwana fragments. Agulhas Basin, which separates the Agulhas Agulhas Plateau is interpreted to be of It is therefore important to ascertain its na- Bank from the Agulhas Plateau. The pur- oceanic origin. The plateau can be divided ture and origin in relation to the breakup of pose of the Agulhas Basin lines was to ex- into two physiographic provinces. The Gondwanaland. amine the nature of the structural connec- southern province is characterized by a The Agulhas Plateau (Fig. 1) rises about tion between the continental Agulhas Bank smooth basement overlain by relatively un- 2.5 km above the surrounding deep-sea and the Agulhas Plateau. This program was disturbed sediment 0.5 to 1.0 km thick. In floor. Heezen and Tharp (1965) charted its undertaken jointly by the Southwest Center the northern province the basement topog- general morphology and noted that the for Advanced Studies, Dallas, Texas (now raphy is rough. The origin of the relief is northern part is topographically rougher the University of Texas at Dallas) and the not clear, but several possible models are than the southern part. Graham and Hales Bernard Price Institute of Geophysical Re- suggested. The influence of bottom currents (1965) calculated the crustal thickness con- search, University of the Witwatersrand, is marked in this region, and the sediments sistent with their gravity data and obtained Johannesburg. The first part of this paper are more disturbed. Large magnetic a Moho depth of 21 km below the Agulhas describes the interpretation of the profiles anomalies are found over the plateau, many Plateau, thus attributing almost continental on the Agulhas Plateau and in the Agulhas of which are generated by basement topog- thickness to it, whereas isolated crossings Basin. Reflection profiling and magnetic raphy. Remanent reversal stripes cannot be (see, for example, Le Pichon and Heirtzler, data from two further cruises to the plateau identified with certainty. Magnetic models 1968) found large magnetic anomalies are reported in the second section, and a that incorporate the basement relief suggest more reminiscent of oceanic crust. Ewing synthesis of all available data is presented. that the basement material is basalt. The and others (1969) reported it to be capped magnetic results support the refraction in- by relatively unstratified sediment, 0.4 to GENERAL MORPHOLOGY terpretation of a volcanic constitution. The 0.5 s thick. Ships of the Lamont-Doherty Agulhas Plateau was apparently formed Geological Observatory have taken 15 A general bathymetric map is shown in during or after the separation of the Falk- pre-Quaternary cores from the Agulhas Figure 1. The Agulhas Plateau is about 750 land Plateau from southern Africa. Plateau, and five of these yielded Cretace- km long in a north-south direction and 400 ous ages, the oldest being mid-Cretaceous km wide. Its central zone is shallowest, be- INTRODUCTION (Saito and others, 1974). Their positions tween 2 and 2.5 km in depth. Except for a are plotted in Figure 1. narrow bridge to the east, it is entirely sur- Apart from seismically active mid-ocean Le Pichon and Hayes (1971) and Fran- rounded by water more than 4.5 km deep. ridges, regions of the oceans having depths cheteau and Le Pichon (1972) have It can be divided into two physiographic intermediate between those of continental suggested that the eastern continental mar- provinces separated approximately by lat shelves and the deep ocean basins are rare. gin of South Africa represents an ancient 38.5°S. Examples are found in all the oceans of the line of shear with the northern edge of the In the southern province there is still a world, but they are somewhat more com- Falkland Plateau. If this is so, the Falkland paucity of detailed bathymetric data, par- mon in the Indian Ocean (Laughton and Plateau previously covered the area now ticularly in the eastern section. Where others, 1970). Geological and geophysical occupied by the Agulhas Plateau, which ar- sufficient coverage exists, a generally investigations have established that some of gues against it being a microcontinent. smooth rise from the surrounding deep these aseismic rises, such as the Seychelles Talwani and Eldholm (1973) and Scrutton ocean to the central plateau is revealed. The Bank (Shor and Pollard, 1963) and possibly and Du Plessis (1973) have reported evi- western flank between lat 39° and 40°S, Geological Society of America Bulletin, v. 88, p. 749-763, 9 figs., June 1977, Doc. no. 70602. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/749/3429253/i0016-7606-88-6-749.pdf by guest on 25 September 2021 750 D. M. BARRETT which is the best known region, displays Interpretation of iments is typical of oceanic layer 2. Below some faults with throws of as much as 0.5 Refraction Measurements this layer, the velocities indicate oceanic km. These occur between depths of 4 and rather than continental material. This is 4.5 km and define the western extremity of The locations of the refraction lines are true even if the more complicated model B the feature here. The few lines that cross the shown in Figure 1. A discussion of the re- is accepted for profile 4. A double crustal southern limit into the Agulhas Basin do fraction data is contained in Appendix 1. layer has been reported in other oceanic re- not show any significant faulting at the Table 1 gives a summary of the velocity de- gions (see, for example, Sutton and others, boundary rise. terminations. 1971). The northern physiographic province is A bipartite crustal layer has also some- characterized by much rougher topog- Profiles 4 and 5 times been found below, and in the vicinity raphy. Generally, the northern boundary of, several features thought to have origi- with the Transkei Basin is more abrupt and The structural interpretations of these nated because of supranormal marine vol- irregular than the southern and western profiles are given in Table 2. In both pro- canism, resulting in an overthickened crust. edges. files, the velocity of the layer below the sed- These are Oahu, Hawaii (Furomoto and others, 1965, 1968); north of Maui, Hawaii (Shor and Pollard, 1964); Shatsky Rise (Den and others, 1969); the Canary Is- lands (Bosshard and MacFarlane, 1970) and Bowers Ridge (Ludwig and others, 1971). Evidence presented below supports a marine origin for the Agulhas Plateau. If model B is correct, the crustal structure of profile 4 may be associated with supranor- mal volcanism. Station 150 of Ludwig and others (1968) was shot in the same deep-water channel (Fig. 1), although on the continental rise (water depth, 3.84 km). These authors mentioned shooting difficulties due to large drift over rough terrain, and they stated that this result should be considered tenta- tive. Nevertheless, their section looks more oceanic than continental. Moho is about 10 km deep below profiles 4 and 5, which is also typical of the deep ocean. We interpret this part of the Agulhas Basin to be oceanic in structure and not thinned continental crust. Stations on Agulhas Plateau The velocity of 4.84 km/s for the layer below the sediment is representative of sev- eral pre-Cretaceous South African conti- nental rocks and also of oceanic layer 2. However, at least the upper part is def- initely magnetic (see below), and there is little doubt that it is basalt. The velocity of the next layer (6.72 km/s) is typical of the main oceanic crustal layer. Figure 2 shows several simplified sections through some aseismic rises and volcanic is- lands or their aprons, including the Mozambique Ridge and the Agulhas Plateau. The material with light shading is interpreted as basalt by the various workers whose results are depicted here, except the Mozambique Ridge, for which Hales and Nation (1973) gave no lithological interpre- tation and whose true nature is still in some doubt.
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