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

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Geophysical Study of the Easternmost Walvis Ridge, South Atlantic:Deep More free publications from Archimer 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.
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