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.

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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 supranormal 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 several pre-Cretaceous South African continental 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 islands 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 interpretation and whose true nature is still in some doubt. A characteristic of supranormal volcanic structures is a thickened layer 3 and a thickened section of layer 2, sometimes composite, with a large range of velocities Figure 1. General bathymetry of Agulhas Plateau and environs, showing location of shooting pro- — 3.8 to about 6.0 km/s (see references in files reported in this study. Map has been modified after Simpson (1970). Cretaceous core sites are caption of Fig. 2, plus Raitt, 1957; Shor, from Saito and others (1974). Previously reported refraction stations are shown in inset. 1960, 1964; Menard, 1964; Francis and

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Raitt, 1967). This material is taken to be eastern Iceland is also shown. The layer comparison with extinct, or at least more lava piles built up through volcanic activity. with velocity 6.5 km/s is probably to be quiescent, features, should not be taken too The 4.84-km/s layer of the Agulhas Plateau equated to the oceanic layer, and the layers far. is also thickened relative to "normal" above this are Quaternary volcanic rocks oceanic crust (Fig. 2, at left). The plateau and Tertiary flood basalts whose velocities PROFILER AND resembles the other features in its general and total thickness are similar to the vol- MAGNETIC MEASUREMENTS structure, particularly the Maldive Ridge canic rocks of the other sections. The 7.2- and Gardner Pinnacles. We propose that it km/s layer is probably equivalent to the The traverses used in this study are is also of oceanic origin. anomalous upper mantle observed under shown in Figure 3. The geophysical Palmason (1971) has noted certain the crestal zones of mid-ocean ridges. Many parameters available from these traverses similarities between Iceland and the data suggest that the crust and upper man- are given in Table 3. Hawaiian Islands, and a section across tle in Iceland are in a dynamic state, and TABLE 2. VELOCITIES AND TABLE 1. VELOCITY DETERMINATIONS LAYER THICKNESSES FROM AGULHAS PLATEAU Layer Velocity Thickness Depth Layer Apparent velocity True Poisson's (km/s) (km) (km) North (T* South cr* velocity ratio Profile 4, model A Profile 4, model A Water 1.53 5.35 0.00 A 1.70 A 1.70 0.29 5.35 B 4.55 0.06 4.53 0.08 4.54 ±0.11 B 4.54 0.82 5.64 C shear 3.83 0.02 3.89 0.03 3.86 ± 0.03 1.75 0.26 C 6.77 3.67 6.46 C 6.68 0.05 6.87 0.05 6.77 ± 0.06 D 8.04 10.13 D 7.93 0.08 8.15 0.88 8.04 ± 0.11 Profile 4, model B Profile 4, model B§ Water 1.53 5.35 0.00 CI 6.60 0.07 6.78 0.10 6.69 ± 0.12 1.73 0.25 A 1.70 0.29 5.35 C2 7.06 0.09 7.29 0.12 7.12 ± 0.14 B 4.54 0.78 5.64 D 7.90 0.08 8.09 0.09 7.99 ± 0.12 CI 6.69 1.48 6.42 Profile S C2 7.17 2.65 7.91 A 1.60 D 7.99 10.56 B 4.65 0.06 4.43 0.07 4.54 ± 0.08 Profile 5 C shear 3.68 0.03 3.71 0.04 3.69 ± 0.05 1.86 0.30 Water 1.53 5.31 0.00 C 6.85 0.08 6.87 0.05 6.86 ± 0.09 A 1.60 0.20 5.31 D shear 4.82 0.06 B 4.54 1.91 5.51 D 7.98 0.08 1.66 0.22 C 6.86 2.36 7.42 Profiles 8 and 9 D 7.98* 9.77 A 1.80 Profiles 81 and 9 B 4.80 0.06 4.89 0.05 4.84 ± 0.08 Water 1.50 3.00 0.00 C 6.77 0.07 6.67 0.06 6.72 ± 0.09 A 1.80 0.53 3.00 D >7.6? B 4.84 3.83 3.53 C 6.6 -7.0 7-11 7.35 Note: All velocities are in km/s. Errors in the true velocity are quoted at the 95% confidence level. D S7.6? 14-18 * cr = standard deviation. f = rat * Determined from one side of a split profile Vp/Vs '° compressional to shear velocity. § A, B, and C shear are the same as for model A. only.

NORMAL ICELAND MALDIVE SAYA DE CHAGOS GARDNER MOZAM- AGULHAS OCEANIC RIDGE MALHA BANK PINNACLES BIQUE PLATEAU DEPTH CRUST BANK RIDGE (km) 0 Figure 2. Simplified 2.8 3.3 4.7 seismic sections beneath 4.1 4.4 3.9 some oceanic features. 5.1 5.3 Sources of data are Shor 6.5 5.0 5.6 7.0 and Raitt (1969) for normal 5.8 oceanic crust, Palmason 6.8 6.7 (1971) for Iceland, Francis 6.8 and Shor (1966) for Mal- 7.0 dive Ridge, Saya de Malha 18.1 Bank, and Chagos Bank, 15 Shor (1960) for Gardner Pinnacles (Hawaiian I 8.2 8.0 Chain), Hales and Nation (1973) for Mozambique 20 Ridge, and this work for WATER H VOLCANICS • 18.2 Agulhas Plateau. SEDIMENTS, H OCEANIC LAYER • CORAL 25 I MANTLE

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SEDIMENT CHARACTER lated between these profiles. For conveni- cate erosion. Generally where the basement ence, it has been labeled layer P in Figure 4. is flat-lying and smooth, such as in the Line drawings of four reflection profiles Layer P is approximately conformable with southern physiographic province, the sedi- run from east to west on the western flank basement and shows evidence of differen- ments are relatively undistrubed and more of the Agulhas Plateau are shown in Figure tial compaction (profile 7). It crops out near or less conformable with it. 4A. The basement is overlain by as much as long 25.5°E on lines 4 and 5. The original In the northern physiographic province, 0.5 s of sediment that is mostly transparent record of the latter is shown in Figure 4B. the basement relief has had an appreciable to the frequencies of the airgun used. A The irregular surface of the layer P inlier effect upon the history of the sedimentary prominent reflector about halfway down and the truncation of bedding planes in the processes that have been affected by bottom the sedimentary succession can be corre- upper material to the west of the inlier indi- currents. Elevated areas (Fig. 5) have a thinner cover, and in some areas sediments are completely absent. It is difficult to de- cide whether this is a result of erosion or nondeposition but the occasional sharp truncation of bedding planes shows that certain erosional episodes have occurred.

BASEMENT

Southern Province

The available profiler traverses reveal a generally smooth basement, which is inter- rupted in places by ridges. Figure 4 shows that there are two basement ridges near long 24.7° and 25.8°E which are elongate in an approximate north-south direction. Near the plateau center and also in the de- eper water to the west are some smaller, steeper sided features. These may be ridges but more likely are basement hills. The faults near long 24°E cannot be linearly aligned and look like block faults.

Northern Province

The change from the smooth physiography of the southern province to the characteristically rough physiography of the northern province occurs near 38.5°S (Fig. 5). The dotted lines show two valleys, labeled D and E, which appear to correlate over several profiles. These valleys are 30 to 40 km wide. South of D but north of lat 38.5°S are a number of narrower troughs and ridges that are not as readily correct- able between tracks. The side wails of some of the features are quite steep, reaching slopes of 35° to 50°. The steepness of the slopes, sometimes in combination with linearity, suggests an east-west system of faults. However, the lack of precise navigation and the wide spacing of the tracks pre- clude certainty regarding the reality of these lineations. Space limitations prevent presentation of all the reflection profiles here. Further data may be found in Barrett (1974).

MAGNETIC MEASUREMENTS

A striking aspect of the Agulhas Plateau Figure 3. Ship's tracks over Agulhas Plateau. Heavy dotted lines mark tracks shown as airgun pro- is its attendant magnetic field, which dis- files in Figs 4 and 5. Solid line is 4.5-km isobath. For details of geophysical parameters available from plays large 1,000 y anomalies in places. The these tracks see Table 3. available magnetic measurements are

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shown in Figure 6 plotted along the mean magma along linear zones of weakness. (1971) indicate an Early to middle Creta- ship headings. The readings have been cor- Such a mechanism could produce linear ceous age for the Agulhas Plateau, and it rected for the International Geomagnetic structures having contrasting suscep- may be assumed that the magnetism was Reference Field (IAGA Commission 2, tibilities, or directions of magnetization, acquired around this time. We therefore Working Group 4, 1969), which leaves the different from the surrounding rock. also computed theoretical anomalies for data with a negative offset in this area. An An examination of the magnetic and remanence acquired in the mean Jurassic amount of 220 y was arbitrarily added to seismic profiles along most tracks shows a and Cretaceous paleofields (Creer, 1970). the plotted values. marked correlation between the total field There was an insignificant improvement of The edge anomaly shown by the double and basement relief. This is particularly fit for these fields. The lack of a definitive dashed line in Figure 6 has been interpreted true of the northern province where the to- result is a consequence of the unfortunate (Talwani and Eldholm, 1973) as marking pography is rough. To examine this interre- coincidence that the paleolatitude of South the division between continental material, lation, models were constructed from the Africa has changed little since Jurassic causing the magnetic quiet zone to the left basement relief interpreted from the profiler times, whereas the present-day declination of the line and oceanic crust to the right. records. In forming the models, a velocity is relatively large. Hence the inclinations Seaward of this line the anomalies are big- of 2 km/s in the sediment was used. The and declinations of the present-day and ger, with still larger fluctuations over the models were assumed to be two dimen- paleofields are not greatly different. How- Agulhas Plateau, in part a result of its shal- sional and induced, and remanent ever, the relatively high value of kapp = lower depth. The magnetic coverage is still anomalies were computed for various 0.036 suggests fresh basalt (Vacquier, too sparse for meaningful contouring, but magnetic parameters. Anomalies induced in 1972) in which the remanent intensity vec- some trends appear, and the more con- the present-day Earth's field for profiles tor approximately reinforces the induced spicuous ones are indicated. Although the T218 and 8, approximately east-west and vector. The occurrence of such a wide- northern province is dominated by topo- north-south respectively, are shown in Fig- spread basaltic layer is in agreement with graphically generated anomalies, there are ure 7. The violation of the two-dimensional the refraction interpretation of a marine some whose origin cannot be put down to assumption no doubt contributes substan- origin. this cause. The best example is the trend tially to the misfit, but the correspondence near lat 37.2°S. The two magnetic is remarkable. ORIGIN OF trends to the west are caused by basement An apparent susceptibility contrast NORTHERN MORPHOLOGY ridges. The northeast-southwest lineation (kapp) of 0.036 emu/cm3 or, alternatively, near the southeastern edge is of unknown an intensity of magnetization (Jn) of 0.0105 Some possible models for the northern orign (there are no profiler or bathymetric emu/cm3 duplicates the amplitudes of the physiography are discussed briefly below. data along these lines) but may be as- anomalies quite well. sociated with the edge of the plateau. The Koenigsberger ratios for fresh marine Fracture Zone anomaly near lat 41°S is interesting be- basalts are usually high (Vacquier, 1972), cause apparently it has no expression in the which means that although the anomalies The predominant topographic charac- basement relief. If it is a remanent reversal are obviously due to a mixture of induced teristics of fracture zones are scarps, anomaly it implies north-south spreading. and remanent magnetization, the mag- separating regions of different depth, single Another possible explanation for the nitude of the remanent vector is likely to very elongate troughs and high linear ridges anomalies, not due to relief, is injection of dominate. The cores of Saito and others (Menard and Chase, 1970). It is difficult to choose representative cross sections of fracture-zone topography TABLE 3. DETAILS OF GEOPHYSICAL PARAMETERS FROM because the morphology is so variable TRAVERSES OVER AGULHAS PLATEAU along the length of any one of them, but a few examples are shown in Figure 8. For Track Ship Navigation Bathymetry Reflection Magnetics Gravity profiling comparison, a drawing showing the basement relief along profile 16 on the same 1 to 3 Frank Harvey Omega No No Yes No scale is included. Tlie scale of the fracture- 4 to 16 Frank Harvey Omega No Yes Yes No zone topography compares superficially A188 Afrikaner II Celestial Yes No Yes No with that of the northern Agulhas Plateau, A189 Afrikaner II Celestial Yes No Yes No which more closely resembles the complex- AT10 Atlantis II Satellite Yes Yes Yes Yes trough type of relief typified by the Vema GA21 Gallieni Celestial Yes No Yes No Fracture Zone. If the rough northern R701 R.S.A. Omega Yes No Yes No physiography is a fracture zone, it is un- R711 R.S.A. Omega No No Yes No R712 R.S.A. Omega No No Yes No usual because it intersects an uplifted block R722 R.S.A. Omega Yes No Yes No of oceanic crust and its direction is quite R731 RS.A. Omega No No Yes No different from other fracture zones in the R732 R.S.A. Omega No No Yes No region — for example, the Agulhas and R702 RS.A. Omega Yes Yes Yes No Mozambique Fracture Zones (Fig. 9). It R733 R.S.A. Omega Yes Yes Yes No would imply an earlier regime of east-west T218 Thomas B. Davie Celestial Yes Yes Yes No spreading for which there is no geophysical T219 Thomas B. Davie Celestial Yes No Yes No evidence at present. T220 Thomas B. Davie Celestial Yes Yes Yes No T373 Thomas B. Davie Decca Yes Yes Yes No T375 Thomas B. Davie Decca Yes Yes Yes No Faulting Due to Uplift V83 Vema No Yes No No The hypothesis of faulting due to uplift Note: Traverses shown in Figure 1. requires an upthrust origin for the Agulhas

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 Figure 4. A. Line drawings of profiler records on western flank of Agulhas Plateau. Heavy stip- ple indicates prominent reflector P shown in B. B. Original profiler record showing outcrop of layer P on eastern section of profile 5.

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Plateau. Tensional failure resulting from ocean-floor spreading cannot be identified relationship (Sclater and others, 1971) for the stretching of the crust during uplift may with certainty. The magnetic evidence is not oceanic crust generated in the normal way. possibly produce topography of the type conclusive because of the poorly developed If the northern topography is fault in- observed. It is not clear why the faulting remanent anomalies, which could possibly duced, it would be of great interest to de- should then be confined to only a limited be explained by spreading during a termine whether the faults continue into part of the plateau. The seismic refraction monopolarity interval. Another problem deep water to the east. A few north-south results show that the uplift would have to concerns the morphology of the plateau. tracks east of the Agulhas Plateau, with a be accompanied by crustal thickening. Relief characteristic of a fossil ridge (Hayes good profiler to penetrate the considerable and Ringis, 1973) is not evident from the sediment cover, should settle this question. Extinct Spreading Center available bathymetric data. The most seri- ous criticism of this hypothesis concerns the SUGGESTED HISTORY OF Scrutton (1973) has suggested that the deep structure of the ridge. We may expect AGULHAS PLATEAU AREA Agulhas Plateau may be a remnant of the that when a ridge ceases to be active and Mid-Atlantic Ridge which subsequently begins to cool, it slowly reverts to a struc- Tectonic setting "jumped" southwestward after mid- ture similar to that prevailing below its Cretaceous time, during the separation of flanks. The thicknesses of the layers con- Although reconstructions such as those South America and Africa. Although stituting the Agulhas Plateau are greater of Smith and Hallam (1970) may be correct geometrically attractive, consideration of than those in normal oceanic crust. Isostatic in gross terms, they do little to clarify the the Agulhas Plateau as a straightforward forces acting upon its larger crustal thick- sequence of events or initial directions dur- abandoned spreading center involves some ness explain why it has not subsided ac- ing the fragmentation of Gondwanaland. difficulties. Magnetic anomalies caused by cording to the empirical depth versus age The problem of dating the initiation of

Figure 5. Line drawings of profiler records obtained on north-south tracks in northern physiographic province. Right-hand part of profile 16 shows southern edge of Transkei Basin.

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breakup may be approached in several in- to southern Africa in its present position. present site of the Agulhas Plateau. This is dependent ways. The most direct and de- Sclater and McKenzie (1973) have shown true, even if the 500-fm isobath, as used by pendable methods require boreholes, or the that the present-day Mid-Atlantic Ridge Bullard and others (1965), is drawn instead identification of sea-floor magnetic earthquake epicenters, when rotated to the of the 3-km isobath. It is therefore clear anomalies, adjacent to the continental fit of Bullard and others (1965), accurately that the time of formation of the Agulhas margins. overlie the original break between the two Plateau postdates the time of initial rifting continents, except for the area on either and that it originated only after the Falk- Opening of Indian Ocean side of the Walvis and Rio Grande Ridges. land Plateau had slid toward the southwest Figure 9 shows the epicenters rotated ac- along the Agulhas Fracture Zone. Thus, The Deep Sea Drilling Project (DSDP) recording to this pole. They cut across just to although the Agulhas Plateau lies in the In- sults in the southwest Indian Ocean are of the northeast of the present Agulhas dian Ocean, it is probably structurally re- limited help and in the absence of published Plateau site. If the Falkland Plateau indeed lated to the formation of the Atlantic data on identifiable old magnetic anomalies reached as far as Durban, the Falkland Ocean. in this area, we must fall back on other Fracture Zone was the site of a very large means for estimating the time of breakup. A marginal offset in the early spreading his- SUMMARY firm date of 150 m.y. ago for the separation tory. A westward jump of a hypothetical of Australia comes from the DSDP leg 27 ridge segment running southeast of Durban The crust of the narrow channel of the boreholes (Heirtzler and others, 1973). The is required to achieve the present configura- Agulhas Basin between the Agulhas Bank paleomagnetic (McElhinny, 1970), volcanic tion of the Mid-Atlantic Ridge crest (Fran- and Agulhas Plateau is oceanic in structure. (Nicolaysen, 1962; McDougall, 1963; cheteau and Le Pichon, 1972). The crust of the Agulhas Plateau is thick- Manton, 1968; Fitch and Miller, 1971) and The exact nature of the Scotia Ridge is ened relative to normal oceanic crust, and stratigraphic (King, 1967) evidence is fully not clear, but this ridge and Burdwood its structure is similar to that of certain compatible with this date which suggests Bank may have suffered some distortion as other features in the oceans which are that the Indian Ocean may have begun to a result of disruption of the Andean—West thought to be overthickened volcanic piles. form in Late Jurassic time. Sclater and Antarctica Cordillera in Cenozoic time The plateau is believed to have been formed Fisher (1974) suggested that India began to (Dalziel and Elliott, 1971). Even so, the by supranormal, marine volcanism. separate from Antarctica near the boundary Falkland Plateau effectively covered the The Agulhas Plateau can be divided into of Early and Late Cretaceous time. The dating of the opening of the Indian Ocean is therefore still by no means certain but in any case is not crucial here.

Opening and Early History of South Atlantic Ocean

The paper of Larson and Ladd (1973) has largely dispelled doubt about the time of opening of the south Atlantic Ocean. They have identified Mesozoic anomalies in the Cape Basin to the edge of the continental margin and have placed the initiation of rifting in Valanginian time (125 to 130 m.y. B.P.). Thus, it is possible that the opening of the Indian Ocean predates the separation of South America and Africa. I accept the South America-Africa fit of 100 200 300 Bullard and others (1965) because it is sup- k m ported by a wealth of data. There is also strong evidence (Le Pichon and Hayes, 1971; Francheteau and Le Pichon, 1972; Scrutton and du Plessis, 1973; Larson and Ladd, 1973) that the southeast continental margin of South Africa represents a line of shear with the northern scarp of the Falk- land Plateau. This prominent fracture zone appears to be marked by a line of sea- mounts, the northeasternmost being the Davie Seamount (Fig. 1). DSDP hole 330 (Barker and others, 1974) as well as a considerable number of seismic stations (Ewing and others, 1971) indicate that the Falkland Plateau is a submerged extension of continental southern South America. Moreover, the refraction lines CD, EF, and EG of Ewing and others suggest that the 3-km 100 200 300 400 isobath may demarcate the limit of the con- km tinental material reasonably well. Figure 7. Observed magnetic anomalies and anomalies induced in present-day Earth's field by Figure 9 shows the 3-km isobath relative two-dimensional models (shaded) along two selected tracks (1 nT = 1 y).

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two physiographic provinces separated ap- possibly nondeposition have occurred. The probably basalt. Several magnetic trends proximately by lat 38.5°S. The southern origin of the northern topography is not occur that cannot be related to basement province is characterized by a smooth clear. topography, but remanent sea-floor magne- basement overlain by relatively undisturbed Large magnetic anomalies have been tic anomalies cannot be identified with cer- sediment. The basement of the northern found over the Agulhas Plateau. Most of tainty. province is rough. The sediments are more the anomalies can be related to topography The Agulhas Plateau formed during or disturbed in this region, and erosion and in the basement. The basement material is after the separation of the Falkland Plateau from South Africa. The work reported here provides strong evidence that the Agulhas Plateau has an essentially oceanic constitution.

ACKNOWLEDGMENTS

The refraction work is part of a joint program involving the Southwest Center for Advanced Studies (now the University of Texas at Dallas), supported by a grant from the Office of Naval Research, and the Bernard Price Institute, University of the Witwatersrand, supported by a grant from the South African National Committee for Ocenographic Research. I thank the of- ficials of the Union Whaling Company and the officers and crew of the Frank Harvey for their enthusiastic help during two cruises. The refraction scientific party was led by Dr. A. L. Hales and included D. L. Spence, L. Bacon, D. Edmondson, J. Fincher, and E. de Ridder. Leading Seaman J. Cooke of the South African Navy assisted with the shooting. On the 1972 Frank Har- vey cruise were J. Hope, A. H. Shand, J. Hemp, and P. T. Heidstra. The 1969 T. B. Davie cruise was made Figure 8. Comparison of fracture zone and Agulhas Plateau topography. Sources of data are Bergh possible through the co-operation of Dr. J. (1971) for southwest Indian Fracture Zone, Matthews (1963) for Owen Fracture Zone, Fleming and F. Enslin and Prof. E.S.W. Simpson. The others (1970) for Gibbs Fracture Zone, and Heezen and others (1964) for Vema Fracture Zone. scientific party included A. du Plessis, J. Engelbrecht, and D. L. Spence. I thank the following persons for kindly allowing me to use unpublished data: H. Figure 9. Fit of Falkland W. Bergh, A. du Plessis, R. A. Scrutton, K. Plateau (shaded) to southern Af- O. Emery, H. Hoskins, R. Schlich, and F. rica, using pole and rotation angle Saito. of Bullard and others (1965). I thank Prof. L. O. Nicolaysen for his Dashed line = 3-km isobath guidance and support during the project around Falkland Plateau; thin and colleagues at the Bernard Price Insti- solid line = 3-km isobath around tute, in particular R.W.E. Green and D. L. southern Africa. Earthquake Spence, for discussions of the work. P. epicenters (dots) are from Barazangi and Doiman (1969) and Chetty helped in the reduction of some of have been rotated about same pole the data. but through half the angle. This research formed part of a Ph.D. the- Agulhas Fracture Zone has been sis submitted to the University of the Wit- approximated by 68° small circle watersrand, Johannesburg. about early pole of opening of South Atlantic (Francheteau and Le Pichon, 1972). Other fracture APPENDIX 1 zones are from Heezen and Tharp (1965) and Bergh (1971). Recent Navigation work by H. W. Bergh and I. D. Norton (in prep.) give fracture Navigation made use of the Omega VLF sys- zone trends in agreement with tem, operating in the range-range-range mode those in Heezen and Tharp's dia- with a shipboard rubidium frequency standard as gram. A. P. = Agulhas Plateau, reference, utilizing the stations of Aldra, M. R. = Mozambique Ridge, and Forestport, and Trinidad. In this geographic lo- Mad. R. = Madagascar Ridge. cation absolute positional accuracy was ±5 km, with a better relative accuracy.

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Shooting and Recording Technique Determination of Velocities method has the same limitations as other standard methods, in that plane dipping layers are as- A single-ship method employed the whale The sedimentary velocity structure was deter- sumed. In other words, the scale of the topog- catcher Frank Harvey, which was specially mined from variable-angle reflections, using a raphy on a layer must be small compared to the equipped for the operation. method similar to that described by Ewing and distance over which arrivals from it are recorded. The free-floating, recoverable buoys have been Nafe (1963). Models having both linear and The use of overlapping split profiles, rather than described by Hales and Nation (1973). The sur- square-root velocity gradients were tried. Linear true reversed profiles, introduces the further as- vey area, lying on the fringe of the "Roaring For- velocity gradient models gave the best fit, and a sumption that the interfaces continue with the ties," is often plagued by heavy seas and currents, reasonable average for the area is V = 1.52 + 1.3 same average dip through the profile. and in such conditions the hydrophone suspen- Z, where V is the velocity in kilometres per sec- All the record sections and a more detailed in- sion did not always decouple the wave motion ond and Z is sediment thickness in kilometres. terpretation are found in Barrett (1974). sufficiently; as a result, some of the recorded This equation was used for computing the tapes were too noisy for use. A "safety first" thicknesses of all the low-velocity sediment in the Profile 4 strategy was adopted in which four buoys were refraction interpretations. launched in the target area with about 4 km be- Before determining apparent velocities from Three buoys operated successfully. The plot of tween them, and shooting was carried out on the refracted arrivals, topographic corrections composite reduced travel time is shown in Ap- either side of the buoys. Under these conditions, were made after the manner of Sutton and pendix Figure 1. Refracted arrivals from the at least two buoys operated sufficiently quietly Bentley (1953). The buoys and shots were con- 4.54-km/s layer are all second arrivals and are during the shooting of each line. The result was sidered to be sufficiently colinear to use the weak, particularly on the northeast side. essentially a number of overlapping, split profiles method of overlapping profiles (Ewing and There is a suggestion, mainly from the data of at each station. others, 1940) to determine layer velocities. This the southwest, that there are two crustal layers

Q. HI 5 3 Q5-3

UJ H< 5.4 5

<0 -10 CD N X I 9

60 50 40 30 20 10 0 10 20 30 40 50 60 DISTANCE, km Appendix Figure 1. Composite travel-time plot for profile 4. Solid lines refer to model B. Dashed line refers to model A (Tables 1, 2).

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below the layer with velocity 4.54 km/s, having from either the 6.69-km/s layer of model A, or are very clear on three buoys and yield an appar- velocities of 6.69 and 7.17 km/s, respectively. the 6.77-km/s layer of model B. ent velocity of 7.99 km/s. Converted shear waves This interpretation is called model B in Table 2. were recorded by buoys in this profile also. On Scatter of the arrival times for different buoys Profile 5 the southwest side, there is evidence for two dis- makes it less clear that a two-segment interpreta- tinct segments corresponding to the crustal shear tion is tenable for shots to the northeast. Utilizing Three buoys operated successfully. The com- phase (velocity 3.69 km/s) and the mantle shear only the data from the southwest, the apparent posite travel-time plot is shown in Appendix Fig- phase (velocity 4.82 km/s). The arrival times on velocity of the upper layer is closer to 6.4 km/s. A ure 2. The arrivals having a velocity of 4.54 km/s the record section agree with predicted times as- phase having a velocity greater than 7 km/s could are very weak. suming the structure determined from the com- not be identified on profile 5, which overlaps the The 6.9-km/s layer is represented by good first pressional wave data and a Poisson ratio of 0.25 profile 4 region with an offset of 10 to 15 km. arrivals on all three buoys. To the northeast, for layer B. The profile 5 data is generally of poorer quality structural complications are present to the extent and may lack the necessary resolution. that it is impossible to identify phases on the Profiles on Agulhas Plateau Model A (Table 2) assumes that there is only basis of linear travel time segments. The south- one deep crustal layer, having a velocity of 6.77 west intercept times on the buoys agree closely Powerful currents and fickle sea conditions km/s. In this model it is assumed that what ap- with those from the northeast segment up to forced us to shoot the plateau line in three sec- pear to be two travel-time segments are caused about 25 km; thereafter the complicated be- tions. For convenience, the resulting profiles have by undetermined departures from plane havior begins. Northeast arrivals up to 25 km been labelled 7, 8, and 9, but for purposes of in- geometry or lateral velocity variations. were used in the velocity determination that gives terpretation they were considered as a single 6.9 km/s. Beyond 25 km, the arrival times are profile. The choice of the survey area was based Shear Waves scattered and systematically different on different upon limited bathymetric data and proved to be buoys. It is difficult to say whether the arrivals unfortunate because the northern part is compli- A conspicuous phase, having a velocity of 3.69 are crustal or mantle. This behavior is thought to cated by rough topography. km/s, was recorded on all three buoys, and the be due to buoy drift over structure that violates Four buoys operated successfully or partly combined velocity determination is good. The ar- plane layer geometry. successfully. The travel-time plot is shown in rivals are identified with a converted shear phase First arrivals beyond 25 km to the southwest Appendix Figure 3 and a sample record section in

E ¿C

P 5.0 Q. tu Q DC 5.5 UJ H I

CO s X I 8

60 50 40 30 20 10 0 10 20 30 40 50 60 DISTANCE, km Appendix Figure 2. Composite travel-time plot for profile 5.

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Appendix Figure 4. At about 50 km on profile 8, serve this phase, but large-amplitude second arri- tion. Better fits were obtained if a velocity gra- the northern physiographic province was en- vals are observed over roughly the same distance. dient was assumed in the layer having a tered. Beyond this distance, the travel times were Amplitude calculations indicate that this can- refraction-determined velocity of 6.72 km/s affected by large structure and could not be in- not be a refracted wave but is, rather, a supercrit- (layer C). Appendix Figure 4 shows one of the terpreted satisfactorily. ical reflection. This interpretation is supported by possible models, in which layer C has a thickness The 4.84-km/s and 6.72-km/s segments are the curvature of the arrival-time curve. Theoreti- of 8 km and a velocity varying linearly from 6.6 very clear. Beyond 45 km on profile 9, a phase cal arrival times were fitted to the data, using km/s at the top of this layer to 7.0 km/s at the that begins as a second arrival, but with large models having varying velocities and depths for bottom. amplitude, was recorded. The structural compli- the 6.72-km/s layer. The upper structure for the Refracted arrivals from layer D could not be cations on profile 8 make it more difficult to ob- models was based on the refraction interpreta- identified with confidence. This is not surprising,

DISTANCE, km

PROFILE 9 MODEL LAYER VEL THICK

-50 -40 DISTANCE (km) Appendix Figure 4. Sample record section for profile 9, buoy 36. Large amplitudes following about 0.755 after first arrival (very clear on shots 20 to 24), are first bubble-pulse oscillation. Low bubble-pulse frequency was caused by detonating shots at a shallow depth.

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since the expected amplitudes are small and the Africa): Am. Assoc. Petroleum Geologists equatorial Atlantic: Jour. Geophys. Re- signal to noise ratio for the distant shots was not Bull., v. 59, p. 3-59. search, v. 69, p. 733-739. good. Without such refractions, it is difficult to Ewing, J. I., and Nafe, J. E., 1963, The uncon- Heirtzler, J. R., and others, 1973, Age of the address the important questions of the velocity solidated sediments, in Hill, M. N., ed., The floor of the eastern Indian Ocean: Science, and hence the nature of layer D. The position of sea, Vol. 3: New York, Interscience, v. 180, p. 952-954. the amplitude maximum in a reflection is a func- p. 73-84. IAGA Commission 2, Working Group 4, tion of a number of variables, including the vel- Ewing, M., Woollard, G. P., and Vine, A. C., Analysis of the geomagnetic field, 1969, In- ocity ratio across the reflecting boundary and the 1940, Geophysical investigations in the ternational geomagnetic reference field frequency of the arrival. Using the method of emerged and submerged Atlantic coastal 1965.0. Jour. Geophys. Research, v. 74, Cerveny (1963), we estimated that the amplitude plain: Geol. Soc. America Bull., v. 51, p. 4407-4409. maximum should lie about 7 km beyond the crit- p. 1821-1840. King, L. C., 1967, The morphology of the Earth: ical point. For the model used in Appendix Fig- Ewing, M., Eittreim, S., Truchan, M., and Ew- London, Oliver and Boyd, 699 p. ure 4, the distance of the amplitude maximum ing, J. I., 1969, Sediment distribution in the Larson, R. L., and Ladd, J. W., 1973, Evidence should therefore be 40 km. 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