Geophysical Study of the Easternmost Walvis Ridge, South Atlantic:Deep

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Art. No 305 Contribution COB No254

Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure

J. GOSLIN Institut de Physique du Globe de Paris, Laboratoire de Géophysique Marine, 4 Avenue de Neptune, 941 00, St. Maur-des-Fossés, France J. C. SIBUET Centre Océanologique de Bretagne, B.P. 337, 29273, Brest, France

ABSTRACT proposed that the landward termination of ously been described (for example, Houtz the Walvis Ridge consists of two basaltic and others, 1968). The seismic source A seismic-refraction study of the basement ridges enclosiner a 2-km-thick aboard RiV Jean Charcot was the Flexotir sedimentary structure of the South West sedimentary basin and probYably continuing (Grau, 1969). All profiles were shot with a African continental shelf was carried out eastward beneath the sedimentam cover of shooting interval of 30 sec (that is, about between lat 17"s and 24"s using expendable the continental shelf. We suggésted that every 100 m). None of the profiles was re- sonobuoys. Striking differences exist both sediment coming from the south had par- versed. The analog signal from the in the topography and sedimentary struc- tially filled the central basin and that the sonobuoys was recorded on a variable-area ture between the shelf north and south of ridge had acted as a dam to any further film recorder and on magnetic tape, which the Walvis Ridge. South of the ridge, as far northward transport of sediment, thus ex- allowed replays of tape through appro- as lat 23"S, the shelf consists of a prograded plaining the contrast in sediment cover be- priate filters when necessary (Figs. 3a, 3b). series, whereas north of the ridge, at least as tween the northern and southern flanks of Although some sedimentary layers arc far as lat 17"S, east-trending canyons cut the ridge. The last major tectonic phase that thick, the sonobuoy time-distance graphs the shelf sedimentary cover. The steep affected the Walvis Ridge area was consid- show no curved segments to justify use of northem scarp of Walvis Ridge can be ered to have coincided with the change of the Herglotz-Wiechert method (for exam- traced eastward under the sediment of the the pole of opening of the South Atlantic ple, Jobert, 1973). Segments from the dif- continental margin. The southem Bank of Ocean during Late Cretaceous time. ferent interfaces were therefore considered the ridge is buried under a thicker sedimen- In this paper, we first present a seismic- as straight-line arrivals. Approximate cor- tary cover and could only be traced east- refraction study of the continuation of the rections for sloping interfaces were made ward to long. tOOE on seismic-reflection Walvis Ridge beneath the continental shelf with the help of continuous-reflection rec- records. This flank probably parallels the of South West Africa. The second part of ords. The velocity corrections are small, be- northem scarp under the continental mar- the paper is devoted to a gravity study of cause the slopes of the interfaces are seldom

gin. Two-dimensional structural models, the deep structure of the eastemmost seg- more than 2" (Fie.I 51., built with the help of seismic-retlection and ment of the Walvis Ridge. Lastly, we review Using oblique seismic-reflection records, seismic-refraction results and based on the ideas on the opening of the South Atlantic Houtz and others (1968) determined seis- hypothesis of local isostatic equilibrium, in the light of Our results and propose a rnic velocities ranging from 1.6 kmisec to account for the obsewed gravity profiles. A hypothesis for the creation of the aseismic 2.2 kmisec in recent sediment in the deep compensating root consists of light material ridges of the South Atlantic. basins of the Atlantic. Direct measurements (density 2.95 g/cm? and reaches a depth of of compressional velocities of JOIDES about 25 km. Gravity results also suggest PROLONGATION OF THE samples from abyssal plains and from the that the Walvis Ridge does not constitute a WALVIS RIDGE UNDER Rio Grande Rise area show that the P-wave superimposed load on the lithosphere; THE CONTINENTAL SHELF velocities in the upper sediments are lower rather, the ridge and its underlying com- than 1.64 kmisec, sometimes reaching val- pensating mass were created at approxi- Results of seismic-refraaion profiles over ues as low as 1.5 kmlsec (Maxwell and mately the same time as the adjacent ocean the South West African continental shelf others, 1970b). On the continental shelf, basins. (Fig. 1) enable us to determine the topog- however, higher values have been measured The creation of the two aseismic ridges of raphy of the acoustic basement, the in the upper sedimentary layer. For exam- . the South Atlantic - the Rio Grande Rise sedimentary structure of the shelf, and the ple, Leyden and others (1972) assumed a and Walvis Ridge - by a mantle hot spot possible eastward continuation of the struc- velocity of 1.8 kmisec for the topmost and plume is accepted; this theory seems to tural units of the Walvis Ridge under the sedimentary layer of the West African con- explain most of the peculiar features of the continental margin. Considerable differ- tinental shelf from lat 2"N to 22"s. Walvis Ridge. However, it is probable that ences between the continental shelf north The sonobuoy results presented in this the surface expression of the mantle hot and south of the Walvis Ridge are suggested paper were computed using an assumed spot was controlled by the presence of weak by the bathymetry and gravity contour map velocity of 1.8 kmlsec for the topmost layer. zones in the lithnsphere such as transform (Figs. 1, 2). The northem continental shelf This velocity was found to give the best fit faults. Key words: South West Africa, seis- is narrow and thinly covered with sediment, of depths of refracting horizons with those rnic refraction, geophysics, gravity, mantle and its slope is very steep. The southern estimated for recognizable horizons on the plume, transform faults. continental shelf has a great extension and seismic-reflection records made over the a more gentle slope (Fig. l), and it is cov- continental shelf (Fig. 5). The same veloc- ered with thick sedimentary deposits. ity was also used for the surface sediment INTRODUCTION over the central basin of the Walvis Ridge, Seismic-Refraction Recording described in a previous paper (Goslin and A study of the easternmost segment of Techniques and Data Processing others, 1974; sonobuoys 13, 14, 15, Fig. 1); the Walvis Ridge, based mainly on this is justified on the basis of the sedimen- seismic-reflection data, was presented in an Seismic-refraction profiling techniques tary transport and deposition mechanism earlier paper (Goslin and others, 1974). We using expendable sonobuoys have previ- proposed for this area (Goslin and others,

Gcological Society of America Bulletin, v. 86, p. 1713-1724, 11 figs., December 1975, Doc. no. 51209. 53 Figure 1. Bathyrnetry in corrected metres (Matthews, 1939). Heavy lines: continuous reflection profiles used in the paper. Circles: sonobuoy launching points.

1974). Table 1 lists the results of the cor- West Africa is shown in Figure 4, which tary horizons, probably in the area of the responding sonobuoy refraction profiles. also includes information derived from eastward continuation of the northern sonobuoy data from areas that cannot be scarp of the Walvis Ridge (Figs. 2, 5). considered typical of the shelf sedimentary Buoys 13 and 14 were monitored over the Relationship between Seismic structure. Buoy 4 was recorded over a complex region where the central basin of Velocity and Depth basement high off Walvis Bay. This high the Walvis Ridge meets the African margin. also appears on the free-air anomaly map The velocity-depth distribution on al1 The compressional wave velocity versus (Fig. 2) and on the seismic profiles of Du other sonobuoys records obtained over the depth relationship for the sonobuoys re- Plessis and others (1972). Buoy 12 was continental shelf is close to a linear rela- corded over the continental shrlf of South monitored over a sharp rise of al1 sedimen- tionship with very little deviation. The ve- GEOPHYSICAL STUDY OF THE EASTERNMOST WALVIS RIDGE, SOUTH ATLANTIC

Figure 2. Gravity-anomal~map. Contour interval, 10 mgal. Free-air anomal~at sea and Bouguer anomal~on land (from the Geological Map of South West Africa [South Africa Geol. Survey, 19631). Dotted lines, surface ships' gravity control; triangles, pendulum gravity stations (Wonel, 1965); squares, RN Ahademiù Kurtchatov measurements. locity, ranging from 3.5 to 5 kdsec, shows velocity-depth law is linear, no curved re- We have chosen, rather arbitrarily, to a greater deviation, which we attribute to fraction arrivals were detected. We con- separate the velocities of the various hori- the slopes of the deeper interfaces; these are clude that the rectilinear refraaion curves zons into five groups: (1)approximately 2.2 difficult to evaluate precisely on the indicate the presence of horizons with kdsec (2.28 kmlsec [12], 2.24 kdsec [Il], reflection records (Fig. 5). The seismic noticeable velocity contrast, and we suggest 2.24 kmlsec [IO], 2.13 kdsec [8], 2.27 velocities lower than 3.5 kmlsec seem to be that these horizons separate sedimentary kdsec [2], and 2.28 kdsec [3]); (2) ap- mainly a function of the depth of the refrac- layers of different natures or different de- proximately 2.9 kdsec (2.98 kmlsec [12], tive horizons. This suggests that the in- positional histories. These velocity con- 3.0 kdsec [IO], 2.65 kdsec [8], 3.09 crease in velocity with depth is mostly due trasts would have been preserved through kmlsec [2], 2.74 kmlsec [3]); (3) approxi- to compaction. Although the mean burial and compaction. mately 3.65 kdsec (3.67 km/sec [Il], 3.68 GOSLiN AND SIBUET

5 Km O BUOY 21 a Km 0 R RU3Y 21 O

Figure 3. A, variable-area film record of sonobuoy 8, with interpreted Mm on right. B, variable-area film record of sonobuoy 21, with interpreted film on right.

km~sec[IO], 3.48 kmlsec [8]); (4) approxi- A representation of the depths of equal- ity of sedimentary layers where compres- mately 4.35 kmisec (4.15 kmisec [Il],4.38 velocity horizons is shown on Figure 6 sional velocity is lower than 3.0 kmisec kmsec 1101, 4.21 kmisec 181); (5) approxi- (bottom); this gives an insight into the sed- suggests that there has been no uplift or mately 5.50 kmisec (5.57 kmisec [12], 5.45 imentary history of the South West African subsidence since the deposition of the kmisec [8], 5.46 kmisec 131). The fact that shelf. The lines thus determined do not 3.0-krnisec layer. South of the northern the seismic velocities can be grouped in represent any real refractive horizons. In flank of the Walvis Ridge, the bowl shape such a way sugpests that the sedimentary the central part of the profile. the equal- of the lines of equal velocity higher than 3.0 cover is fairly homogeneous over the whole velocity lines are horizontal down to the 3.0 kmsec indicates that this area !,ubsided, as- continental shelf of South West Africa. The km~secline, a result that could have been suming that the compaction limit is deviation of the compressional velocities foreseen with the help of Figure 4, in which reached, or that the compressional velocity around the five chosen mean values can be the velocity-depth distribution for the of the sediments is insensitive to compac- partly explained by the fact that the profiles upper horizons of sonobuoy records 1 1, 10, tion. If the compaction limit has not been were unreversed. 8, and 2 is markedly linear. Because com- reached, the deviation of the data rules out paction of sediments is a function of their any hypothesis about vertical motion in this Sedimentas, Structure and History age and depth (Faust, 195 l), the horizontal- area.

Two basement highs appear on the con- SEISMIC REFRACTION PROFILES SHOT IN THE AREA OF THE EASTERNMST WALVIS RIDGE tinental shelf, one in the prolongation of the TABLE 1. northern flank of the Walvis Ridge near lat Sonobuoy M) V1 Hl V2 HZ V3 H3 V4 H4 V5 H5 19"s and the other off Walvis Bay near lat no. 23"s (the northern and southern parts of profiles 7 and 4, Fig. 5). These basement highs are marked by high-amplitude magnetic anomalies. Between these two basement highs, profiles 4 and 7 cut obliquely the isobaths between 100 and 300 m (Fig. 1). The profiles that were run perpendic~larl~to the margin during the Walda and Atlantis 11-67 cruises show the 20 0.11 1.80 1.10 5.88 presence of a thick prograding sedimentary 21 0.11 1.80 0.30 3.34 0.30 5.75 cover, which appears between the two Note. Results obtained using expendable Sonobuoys. Positions of profiles are shown in Figure 1. H is thickness basement highs, on refraction profiles 2, 8, in kilometres; V ts compressional velocity in kilometres per second of the various layers. Mean water velocity was 10, and 11 (Fig. 6). set equal to 1.49 kmlsec (Matthews. 1939). GEOPHYSICAL STUDY OF THE EASTERNMOST WALVIS RIDGE, SOUTH ATLANTIC

Figure 4. Velocity-depth plot of seismic- refraction profiles shot over continental margin of South West Africa from lat 19% to 23"s (see text).

We suggest the following for the sedimentary history of the South West Afri- Figure 5. Variable-density record of seismic- can continental shelf. Possible tectonic reflection profiles 4 and 7 (see location on movements could have influenced the area Fig. 1). Ship's speed was about 6 knots; verti- in Late Cretaceous time, when the pole of cal exaggeration is about 15 to 20. Depths of re- opening of the South Atlantic migrated fracting horizons are shown at the vertical of the northward (7.5 m.y. B.P.). There is a sugges- sonobuoys' launching points; velocity assumed tion that these movements occurred before for topmost layer varies from 1.7 kdsec to 2.2 or at the time of the deposition of the kmlsec. 3.5-kmlsec layer (Fig. 6). Such a dating fits in with that of Bryan and Simpson (1971), south of lat 18"301S, where the northern who considered the 3.0-kmisec sediments to scarp of the Walvis Ridge meets the conti- be of early Tertiary age. The sediment de- nental margin, are different. To the north, position would have then taken place in the shelf is narrow, and the sedimentary successive and distinct phases, thus creating cover is thin; less than 0.5 sec of sediment velocity contrasts between different layers. appears on sonobuoy profiles 20 and 21 Two distinct areas of the continental (Table 1). The structure is also apparently shelf south of the Walvis Ridge stand out simple - no sedimentary reflectors can be from the free-air anomaly map (Fig. 1). seen on the recording from sonobuoy 20. South of lat 21.5"s the anomalies are Free-air gravity anomalies of medium am- oriented southwest-northeast over the plitude closely follow the topography. To basement rise. which appears south of re- the south, the continental shelf is wide, and fraction profile 2 (Fig. 6). North of lat the shelf break is around 400 m (Goslin and 21°5'S the anomalies roughly parallel the others, 1974). shelf and slope isobaths. The continuation of the northern scarp of The high amplitude (a few hundred the Walvis Ridge can be followed under the gammas) and small wave length (around 20 continental shelf on seismic refraction and central basin of the ridge (Goslin and km) magnetic anomalies are consistent with reflection records, as well as on magnetic- others, 1974). Nevertheless, the plot of the the rise of the basement layer to the south- anomaly profiles (Fip. 5, 6). The con- magnetic anomalies along the ships' tracks east of sonobuoy 2 and to the northwest tinuation of the southern Bank of the Wal- (Goslin and others, 1974, Fig. 5) allows us toward sonobuoy 12 (Fig. 6). The rapid vis Ridge does not appear on seismic- to rule out the first of the two possibilities flattening of the magnetic profile north- reflection and seismic-refraaion records. mentioned above. Consequently, the south- westward of buoy 2 and southeastward of However, two possibilities for the eastward ern crest of the Walvis Ridge seems to buoy 12 indicates a steep descent of the continuation of the southern Bank beneath parallel the northern flank beneath the con- magnetic basement layer. the shelf stand out from the free-air ano- tinental shelf. maly contour map (Fig. 2): either toward Seismic-Refraction Study of the the gravity high situated near lat 20°2'S, DEEP STRUCTURE OF THE Prolongation of the Walvis Ridge long 12O2'E or toward the high situated EASTERNMOST WALVIS RIDGE near iat 18"8'S, long 1 1°6'E. In both cases, The surface expression and subsurface a northward associated gravity low would Bathymetric and free-air gravity anomaly structure of the continental shelf north and correspond to the eastward extension of the data (Figs. 1, 2) were presented and dis- GOSLIN AND SIBUET

FREE-AIR ANOMALY t50 '-' +50 g/cm3density, emplaced in the Angola Basin, .*. accounts for the difference between the Y>1 \xd-/\ f depth of this basin and the Cape Basin. The ?O \ ,/-, /- ----'. 1, 05 fit between the observed anomaly and the -J 2 f anomaly computed for this very simple model is quite good (Fig. 8). -50 -50 Free-air and Bouguer anomaly observa- NW SE tions indicate with some precision the ex- MAGNETIC ANOMALY tent to which a structure is isostatically +500 compensated; however, they do not allow C us to choose between the different hypoth- --v- O 3 eses of compensation. As far as we know, 0 $ -. no long refraction profiles have yet been -5W -500 shot in the Walvis Ridge area. Conse- quently, the only way to choose between compensation hypotheses and to infer the wAL,BAY SE -22 3 c" approximate shape and density of layers situated below the acoustic basement is 18 ...... ------2 O gravity modeling. We assume, for each ...... 22 ...... model, that the Walvis Ridge is everywhere 2 5 .. 1 in local isostatic equilibrium; that is, the / weight of any column of unit surface is set / equal at any point of the structure...... - Construction of Models . . I The basic hypothesis of al1 the models - h -3: h local isostatic equilibrium everywhere - W has an important consequence. Slight varia- . 5 . . tions of the chosen densities or of the to- r4k O 50 IOOK~ -4" pography of the layers will only change the l ---- 5 5 - mass distribution of each vertical seczion. The resulting changes in the computed Figure 6. Crustal section through continental shelf of South West Africa from lat 19"s to 23"S, as deduced from setsmic-refraction profiles. Top and center: free-air and magnetic gravity-anomaly anomaly w"' remai' than profiles. Dotted part of free-air anomaly profile is not considered reliable. Bottom. equal-velocity lines by shallOw sources and will help are drawn every 0.5 kmisec from 2.0 kmlsec to 4.5 km,sec and do not represent actual observed re- much in choosing among different models. fracting horizons. Dotted lines represent mean velocities of probable refracting horizons. Such a choice will be possible only between models that differ radically, such as the fol- cussed in an earlier paper (Goslin and tence of a thick sedimentary basin (Fig. 7). lowing: others, 1974). They indicate that the eas- The width of the Walvis Ridge (as much as 1. Models in which the crustal structure ternmost segment of the Walvis Ridge has a 200 km) is great compared to the estimated under the Walvis Ridge differs from a typi- cylindrical structure; this structure has been isostatic compensation depth (about 30 cal oceanic crustal structure. This would be followed down to the acoustic basement on km). It thus seems probable that the Walvis a consequence, for example, of the presence srisiriic-retlection records (Fig. 7). Ridge is in local isostatii eqiiilibriuni over of a mass of inateri~llightrr than laver 3 Profiles 2, 9, and 11 (Fig. 1) will be used its entire extension (Bott, 1971). Further- under the ridge, such as a body of serpen- to investigate the deeper structure. These more, high-amplitude negative anomalies tinite or serpentinized peridotite. The iso- profiles have been projected on a line of are found near the base of the flanks of the static compensation would then be mostly constant azimuth (330"). The acoustic ridge. They reach -60 mgal and -30 mgal of the Pratt type. basement can be traced continuously along in the Angola and Cape Basins, respec- 2. Models in which the distribution of the these three profiles (Fig. 7). We will thus be tively. These negative anomalies do not densities is similar to the typical oceanic able to correct the observed free-air anom- seem to coincide with any thick sedimen- crust. In this case. however, the nature of aly for the attraction of the sedimentary tary basins in these regions and could there- the layers under the Walvis Ridge could dif- layers and focus our interest on the deep fore be the result of an edge effect due CO a fer notably from that of the ones composing layers situated below the acoustic base- deep compensating mass below the Walvis the oceanic crust, even though they have the ment. Ridge. same densities. Two-dimensional Bouguer anomalies were computed along profiles 2, 9, and 11, Test of Models Basic Hypothesis for Gravity Models assuming a crustal density of 2.7 dçm! The bowl shape of the calculated anomaly (Fig. Densities are assigned to the sediment The gravity anomaly rnap (Fig. 2) and 8) suggests that the Walvis Ridge is com- layers from seismic velocities, making use gravity profiles 2, 9, and 11 (Fig. 7) show pensated by rather deep-seated masses of the Nafe and Drake curve (Talwani and that the free-air anomalies remain close to (Bott, 1971). A very simple model is tested others. 1959). A density of 2.65 gcm3 is the zero level in the Angola and Cape along profile 2 in order to confirm such a taken for layer 2 (Nafe and Drake in Tal- Abyssal Plains, apart from local highs due hypothesis (Fig. 8). The compensating mass wani and others, 19.59) and of 2.95 gcm3 to seamounts. These abyssal plains are is arbitrarily taken to consist of a rectangu- for layer 3 (for example, Le Pichon, 1969; therefore in general isostatic equilibrium. lar prism below the Walvis Ridge with a Goslin and others, 1972). Models were ex- The amplitude of the free-air anomalies is density chosen to be 3.0 g/çm3(-0.4 gcm3 tended to great distances in both directions also quite small along the axial part of the density contrast with the adjacent upper in order to avoid edge effects, and the com- Walvis Ridge (Fig. 2), in spite of the exis- mantle). A cylindrical body with a 2.7- putation was carried out with a two- 58 ANOOLA NORTHERN SOUTHERN CAPE MSSIL,PLAIN FLfNK FLFNK AOVSSAL , PLAIN I 1 1 1 1 INTEINAL SN ! 1 1 1- 1 1 ! MAGNETIC GRAVITY aw . . . .." -...... +m. .,...'YAONETIC .cm : :...... June 26, 1971 - 400 C -LW

-7-

GRA VITY

MAGNETIC GRAVITY 7m r

Figure 7. Interpreted seismic-reflection profiles 2,9, and 11 across Walvis Ridge with free-air gravity and magnetic anomalies. Lowest reflector corresponds to top of layer 2. 59 GOSLIN AND SIBUET dimensional approximation (Talwani and computed gravity anomaly is good along small local attraction (less than 5 mgal). Heirtzler, 1964). most of the length of the profiles when the The depth reached by the light root is com- Al1 models have been tested against ob- thickness of layer 2 is taken to be 1 km, parable to that of constant-ratio models. served profiles 2, 9, and 11 (Fig. 1). In the which is in the range of thicknesses gener- following discussion, emphasis will be on ally proposed for oceanic layer 2 (Bott and lnterpretation of the Models profile 9, which is located in a central posi- others, 1971; Goslin and others, 1972; Bot- tion and close enough to seismic-refraction tinga, 1973). In this case, the 2.95-g/cm3 The shape of the Bouguer anomaly and profiles 13, 14, and 15 (Table 1). density root reaches depths around 28 km the attraction computed for a simple model under the Walvis Ridge on profiles 2 and 9. (Fig. 8) suggest that the Walvis Ridge is Models with a Low-Density 2. Models in which the ratio between the compensated at a depth of 25 to 30 km. Body under the Walvis Ridge thicknesses of layers 2 and 3 is kept con- The computed Bouguer anomaly is not very stant. In this type of model, the deep struc- sensitive to the shape of the deep compen- The computed two-dimensional Bouguer ture of the Walvis Ridge results from a sating body. The attraction caused by a anomaly (Fig. 8) seems to favor compensa- thickening of layers 2 and 3, the ratio be- double-rooted prism also fit the observed tion by deep-seated bodies of small density tween thicknesses remaining constant and anomaly. contrast. We nevertheless tested some mod- equal to 0.25, which is typical for oceanic The models of the three profiles, spaced els in which the compensation is due to a crust (Le Pichon, 1969; Bottinga, 1973). over the eastern segment of the Walvis shallow body of very light material and in The computed attraction fits well with the Ridge and locally in equilibrium, confirm which isostatic compensation is achieved observed free-air anomaly along the the compensation of the Walvis Ridge by a according to Pratt's hypothesis. F. Vine profiles; discrepancies are smaller than 10 light root reaching depths of around 25 km, ( 1973, persona1 commun.) suggested a mgal. The base of layer 3 reaches a depth of whereas the Moho in adjacent basins model in which the compensating mass is 25 km under the ridge in this model. The at- reaches depths of 10 to 1.5 km. The shape of serpentinite or serpentinized peridotite, fre- traction due to this model is not strikingly the root is quite similar in al1 three profiles quently recovered by dredging in fracture different from the one due to the model (Fig. 10). zone areas (Bonatti, 1968). If the density where the thickness of layer 2 is kept con- The lack of deep refraaion lines in the chosen for serpentinite is 2.4 glcm3 (F. Vine, stant. area studied does not allow us to exclude 1973, persona1 commun.), that is, at the 3. Models with smoothed interfaces. the possibility that the light root is made up lower end of the density range for serpen- Local isostatic equilibrium is achieved in of some abnormal layer 3. Palmason (1971) tinite (Hatherton, 1967), the computed at- the two models discussed above, in Airy's suggested the existence of an unusually traction does not fit the observed anomaly hypothesis, by variation of the thickness of thick layer 3 of high density (3.00 @cm3 highs and produces no significant edge ef- layers 2 and 3. This hypothesis produces under Iceland. Bowin (1973) proposed a fect on either side of the Walvis Ridge. jagged and unrealistic shapes for the deep similar structure under the Ninetyeast interfaces. Models were consequently con- Ridge. Unfortunately, no data are available Models with Variable Thickness of the stmcted with smoothed interfaces (Figs. 9, in the Walvis Ridge area to determine the Crustal Layers 10). In this case, the isostatic equilibrium nature and chemical composition of such a becomes "regional" at a scale of a few tens of root. The composition of layer 3 under the 1. Models in which layer 2 is of constant kilometres instead of "local," the depar- ridge could be different from that of the thickness. The fit between the observed and tures from local equilibrium causing only typical oceanic layer created at mid-oceanic

100 FM-AIR ANOMALr

-1 3 ;15- -15 I Figure 8. Left part of figure shows simplest isostatic compensation hypothesis with a low-density body (3.0 g/cmq under the Walvis Ridge between depths of 13 and 22 km. Right pan of figure shows bathymetnc 20 - -20 profile and free-air gravity-anomaly profile. A 2.7-glcm~densebody is in- troduced at the nonhwestern end of profile 2 in order to account for difference of water depth between Angola and Cape Basins. Continuous line A shows computed attraction of model; doned line B shows two-dimensional 25 -es Bouguer anomaiy computed with density of 2.7 @cm3foc filling material. GEOPHYSICAL STUDY OF THE EASTERNMOST WALVIS RIDGE, SOUTH ATLANTIC ridge axes. Abnormal mechanisms that could be due to a mantle hot spot now situated under lceland (Morgan, 1971, 1972a, 1972b) were proposed by Bott and others (1971) to explain the structure and formation of the Iceland-Faeroes Ridge. Two free-air anomaly profiles (Dehlinger, 1969; Lucas, 1971) recorded over the Hawaiian chain are shown in Figure 10 for comparison with those recorded over the Walvis Ridge. The free-air anomaly shows a high-amplitude peak (150 mgal) over the Hawaiian chain and no minimum over the center of the structure (Vening Meinesz, 1941; Gunn, 1943). A negative anomaly of large wave length (several hundred kilometres) is present at the base of the chain and is followed seaward by a weak positive anomaly (Dehlinger, 1969; Lucas, 1971; Watts and Cochran, 1974). It seems that the mass of the Hawaii-Emperor chain, which was superimposed over a cold and rigid lithosphere, caused a depression of this lithosphere on both sides of the chain and an upward flexure seaward of this depression (Vening Meinesz, 1941; Gunn, 1943; Walcott, 1969; Lucas, 1971; Watts and Cochran, 1974). Along profiles over the Walvis Ridge (Fig. Il), on the other hand, the free-air anomaly returns to a low amplitude (less than 15 mgal) as near as 50 km away from the ridge. Consequently, the Walvis Ridge does not appear to be a load superimposed over the lithosphere. Because the Walvis Ridge is locally in isostatic equilibrium, it seems to have been created, together with its compensating root of light material. contemporaneously with the ad- Figure 9. Gravity model of the Walvis Ridge along profile 9. Continuous line, observed free-air jacent lithosphere. The presence of middle anomaly. Doned line, computed attraction. Interfaces between layers 2 and 3 and layers 3 and 4 were Cretaceous sediments, dredged on the top smoothed visually. Middle curve of figure indicates variations of weight of 10-km-spaced columns of the Walvis Ridge (Pastouret and Goslin, from weight of a standard column of oceanic crust. 1974), favors a progressive creation of the ridge together with the opening of the opening itself. We review here various the South Atlantic during a Cenozoic tec- South Atlantic Ocean. Moreover, the total hypotheses that have been proposed, and tonic phase as inferred by Ewing and others subsidence of the ridge (2.5 km) was similar we outline a model based on the present (1966)can be rejected in light of recent data to the subsidence of adjacent basins (Pas- study which incorporates evidence from on micropaleontology, sedimentology, and touret and Goslin, 1974; Sclater and De- previous studies. gravity. The analysis of fossils dredged on trick, 1973). This suggests that no decoup- the northern scarp of the Walvis Ridge ling existed between the ridge and the adja- Review of Previous Hypotheses dates the central part of the easternmost cent lithosphere and that isostatic equilib- segment as being Albian-Cenomanian rium was probably preserved since the for- Uplift Hypothesis (about 100 m.y. B.P.; Pastouret and Goslin, mation of the Walvis Ridge. 1974). These authors also suggested that Ewing and others (1966) argued that the the Walvis Ridge may have undergone a EVOLUTION OF THE Walvis Ridge and the Rio Grande Rise may large-amplitude subsidence from its posi- WALVIS RIDGE have been emplaced by an uplift of the tion of formation in shallow water. Results oceanic crust quite a long time after forma- of deep-sea drilling led Maxwell and others Aseismic ridges are of great interest as tion of the crust. Pliocene vertical motions (1970a, 1970b) to a similar conclusion for possible guides for tracing plate motions of great amplitude would have occurred the Rio Grande Rise. Conclusions drawn back in time (Morgan, 1972b; Wilson, along Eocene lines of weakness in the from gravity data presented in this paper 1973). The two major aseismic ridges of the oceanic crust, and the movements would also favor formation of the Walvis Ridge South Atlantic, the Rio Grande Rise and the not have been accompanied by widespread over a nonrigid and therefore young Walvis Ridge, are particularly well suited to volcanism. Dickson and others (1968) re- oceanic lithosphere, rather than by uplift of investigation because, for example, the vived the uplift hypothesis within the older oceanic crust. original fit between South America and Af- framework of a nonpermanent Atlantic rica is well controlled. Any hypothesis on Ocean. According to them, the uplift would Transform Fault Hypothesis the opening of the South Atlantic must take have occurred between 75 and 50 m.y. B.P., account of the two ridges both relative to relieving the stresses that had been built up According to Le Pichon (1968), Fran- their origin and to the geometric constraints during a previous period of slow spreading. cheteau (1970), Le Pichon and Hayes that they enforce on the kinematics of the The formation of the aseismic ridges in (1971), and Francheteau and Le Pichon

GEOPHYSICAL STUDY OF THE EASTERNMOST WALVIS RIDGE, SOU- ATLANTIC South America - Amaral and others, 1966).The motion of the African plate over the mantle would have had a northeast- ward direction during Cretaceous time. A sinistral rotation would have accompanied this motion until the closure of the Tethys 1s Sea in early Cenozoic time (65 m.y. B.P.; b""""'"IOOKM ""'l 1 Dietz and Holden, 1970).

Proposed Hypothesis

Gravity data for the easternmost Walvis Ridge, presented above, show that the ridge (-1100 LJ~'~I(III4I is compensated at depth by a body with a O IOOKM density of almost 2.95 @cm3.We have suggested that this body probably represents a PROFILE 2 thick, abnormally dense layer 3. Palmason fi (1971) measured the presence of a 10-km-thick layer 3 under Iceland, that is, about twice the thickness of the typical SE oceanic layer; the density, deduced from seismic-refraction profiles, is 3.0 @cm3.Bott O IOOKM and others (1971) suggested that a 15- to PROFILE 8 Id IO0 20-km-thick layer with a density of 2.90 g/cm3lies under the Iceland-Faeroes Ridge. Lastly, Bowin (1973) inferred a density of 1 3.05 @cm3for a 25-km-deep "root" under SE ,111'1111141 the Ninetyeast Ridge. The existence of a IOOKM thick layer with a density close to 3.0 @cm3 thus seems a feature common to several PROFILE II aseismic ridges. The formation of this layer could be the result of a rise of deep material from abnormal renions- of the mantle accompanied by peculiar magmatic processes, which account for the creation of structures with large relief such as the Walvis Ridge Figure Il. Observed free-air anornaly dong profiles 2,9. and 11 (continuous lines) and cornputed (W. J. Morgan, 1973, persona1 commun.). attractions causcd by models of Figure 10 (dotted lines). Free-air anornaly profiles over the Hawaiian The results of geochemical analyses of seamount chain (A, Dehlinger, 1969; B, Lucas, 1971) are shown for comparison. basalt dredged on the northern scarp of the Walvis Ridge (Hekinian, 1972; H. for the absence of any recently created seg- mantle plume are fairly large (more than Bougault, 1974, oral commun.) show that ment extending from the Rio Grande Rise 150 km in diameter, according to Morgan this basalt has a different rare-earth com- to the Mid-Atlantic Ridge on the South [1972a]), and magma rising from the man- position than both mid-oceanic-ridge American plate (W. J. Morgan, 1973, per- tle extrudes through the most convenient basalt and "island-type" basalt. The rare- sonal commun.). zone of weakness. As the plate motion con- earth distribution of the Walvis Ridge The coincidence of the trends of the tinues, this zone of weakness goes away basalt is, on the other hand, close to that of northern scarp of the Walvis Ridge and of from the plume until it is too far away from other aseismic ridges (Ninetyeast Ridge, the small circles centered about the initial its center for extrusion to continue through Cocos Ridge, Iceland-Faeroes Ridge) that pole of opening for the South Atlantic (Le it. Some magma then issues from the next are believed by some authors to have been Pichon and Hayes, 1971; Mascle and zone of weakness, eventually creating a created by hot spots and mantle plumes Sibuet, 1974) implies that the direction of transverse segment. The geometric and (Bowin, 1973; Hekinian, 1975). relative motion between the African and physical parameters of the mantle hot spot The creation of the Walvis Ridge by a South American plates and the motion of bear directly on the dimensions of the suc- mantle hot spot and plume provides an ap- these same plates over the mantle were cessive segments that are formed by such a pealing explanation for the peculiar petrol- identical throughout the first phase of open- process. ogy of the basalt, the deep structure of the ing. Such a coincidence between the direc- eastemmost segment as deduced from grav- tions would be more probable if the hot ACKNOWLEDGMENTS ity results, and the large relief of the ridge. spots had initiated the rifting in the South The mantle hot spot has had noticeable ef- Atlantic, as proposed by Morgan (1972b). We gratefully acknowledge the help of fects on land when rifting began in the The great change in direction of motion of scientists and crew aboard the RIV Jean South Atlantic (Kaoko and Seral Geral vol- the African plate over the mantle, as indi- Charcot. J. Bonnin wrote the computer canism, for example). The mid-oceanic cated by the presence of three main Walvis programs for projecting geophysical data ridge was situated over the plume through- Ridge segments of different strike, is on a given azimuth. Discussions with out the initial phase of opening (Burke and difficult to believe, especially if the hot J. Francheteau, X. Le Pichon, W. J. Morgan, Wilson, 1972; Wilson, 1973). The older spots provide the energy for plate motion and F. Vine were very useful. C. O. Bowin, segments, oriented roughly east-west, were (Morgan 197213). H. Hoskins, X. Le Pichon, J. Mascle, and created during this first phase (before 75 The following mechanism is proposed to H. D. Needham critically reviewed the m.y. B.P.). The hot spot has remained under account for the en echelon structure of the manuscript and made helpful suggestions. the African plate since early Cenozoic time. Walvis Ridge (X. Le Pichon, 1973, oral We thank D. Carré, N. Guillo, and Y. This location of the hot spot is responsible commun.): the horizontal dimensions of the Potard for assistance. GOSLIN AND SIBUET

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