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Recent studies of the crustal structure in the Gulf of Aden
Tectonophysics - Elsevier Printed in The Netherlands
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Company,
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RECENT STUDIES OF THE CRUSTAL STRUCTURE IN THE GULF OF ADEN
A.S. LAUGHTON
and C. TRAMONTINI
National
of Oceanography,
Institute
Department
of Geodesy
Wormley,
and Geophysics,
Godalming,
Madingley
Rise,
Surrey
(Great
Cambridge
Britain)
(Great
Britain)
(Received August, 1968) (Resubmitted February, 1969) SUMMARY Cruise 16 of R.R.S. “Discovery” in 1967 has provided new data on the structure of the Gulf of Aden. The existence has now been demonstrated of a median valley, with associated magnetic anomaly, reaching from the Gulf of Tadjura in the west to the Owen fracture zone in the east. The valley is severely fractured and offset in the central region. New seismic refraction data have shown that throughout the Gulf of Aden, the crustal structure is similar to that found under the oceans and that at the western end, the axial zone is underlain by an anomalously low mantle velocity. A minimum separation of 260 km between the continental blocks of Arabia and Africa is indicated.
INTRODUCTION
During the Spring of 1967, an expedition in R.R.S. “Discovery” to the Gulf of Aden was planned(Cruise 16) with a number of specific objectives to elaborate on and to extend the data relating to its origin. Previous data (discussed by Laughton, 1966a,b) had been obtained largely from ships on passage through the Gulf of Aden and hence a predominance of tracks were oriented east-west. In many cases navigation was poor and the correlation of data on adjacent tracks was difficult even though the track density was high. However, an analysis of these data suggested many problems which could be solved by a more controlled investigation. The earlier crustal structure studies by R.V. “Atlantis” and R.V. “Vema” in 1958 were not regarded by their authors (Nafe et al., 1959) as being very reliable in respect of the deeper high velocity layers, and it was clearly desirable to increase the coverage with longer seismic refraction lines. No rocks had been dredged from any of the mountainous regions of the Gulf of Aden or from the Alula-Fartak Trench. The cruise therefore set out with the following specific objectives: (1) To establish whether there was, associated with the central rough zone, a median valley as has been commonly found on mid-oceanic ridges. (2) To map in more detail the axial magnetic anomaly and to establish &he trend of the other anomalies north and south of it. (3) To survey and sample rocks in three areas selected as being Tectonophysics,
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typical of: (a) the central rough zone; (h) the sediment filled main troughs north and south of the central rough zone; and (c) the Alula-Fartak Trench. (4) To determine the crustal structure by the seismic refraction method in areas critical to the theory of continental separation. In this paper the results of objectives (1) and (4) will be presented together with some preliminary results of the other studies.
THE MEDIAK VALl.EY
In Laughton (1966a) the topography of the Gulf of Aden was divided into three physiographic zones: the continental margins, the main troughs and the central rough zone. The central rough zone was believed to be the natural extension of the mid-ocean ridge system into the Gulf of Aden, the Carlsberg Ridge having suffered a displacement along the Gwen fracture zone (Matthews, 1966). The association of the central rough zone with the epicentre belt, large and linear magnetic anomalies and high heat flow showed that it had many of the same characteristics as a typical mid-ocean ridge, although in some respects of its topography it is different. In particular the topography differs in being dominated in places by strong northeast-southwest lineations giving rise to pronounced ridges and troughs. This echelon pattern has long been recognized in the Gulf of Aden (e.g., Farquharson, 1935). However, the evidence now points strongly to the generic connection between the central rough zone and the Carlsberg Ridge and it is proposed that it should be called the “Sheba Ridge”. A natural division into the east and west Sheba Ridge is provided by the Alula-Fartak Trench (Fig.1). It must, however, be recognized that in parts of the west Sheba Ridge, the valleys lie deeper than the sediment filled troughs to the north and south. The east Sheba Ridge was delineated by a survey during this cruise (Fig.2) and which has shown that from 55OE to the Gwen fracture zone there is a well developed median valley associated, in most places, with a negative magnetic anomaly (Matthews et al., 1967). This valley cuts through the ridgesof the Gwen fracture zone and terminates in the Wheatley Deep, a trough on the edge of the Indus Abyssal Plain and lying 600 fathoms (1,100 m) below it. The median valley is apparently curved (although it is possible that this is the result of a series of small transform faults). To the west it is offset at 54“E by a transform fault and joins at ‘right angles the Alula-Fartak Trench at its northern end. At the southern end of the trench, the valley emerges westward at a point 11.2 miles (206 km) further south and is continuous only for a length of some 50 miles (92 km). (See Fig. 1). Between this point and 46OE, the evidence for the existence of a median valley is very complex. It is a region of predominant northeast-soutwest ridges and valleys, and on the data presented in Laughton, 1966a (Fig.l), a median valley was suggested only on the basis of local and usually disconnected deeps. In 1967, a detailed survey was made of a sixty-mile square defined by the limits 12O-l3ON, 47O-48’E. Navigation was controlled by anchored buoys and radar transponders, and north-south survey lines were about 5 miles (9 km) apart. In addition there were numerous cross lines 360
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362
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and some more closely spaced and limited surveys. The area was crossed by two northeast-southwest ridges and contained the intervening valley. It was clear from the survey that a well defined median valley did exist, characterized by a negative magnetic anomaly, but that it consisted of a series of short east-west, or eastsoutheast-westnorthwest, sections disrupted by a series of left lateral offsets. Along these disruptions the magnetic anomaly was substantially reduced. By analogy with this area, it seems likely that similar disruptions to the median valley occur throughout the region between 46O and 51°E, and that the valley itself would show as an extra deep along the axis of a northeast-southwest valley or as a co1 on the northeast-southwest ridges. Associated with the median valley were minor east-west ridges both to the north and south, most of which showed an asymmetry, the steep slope facing the median valley. West of 46OE, the median valley continues as a distinct and continuous feature to the Gulf of Tadjura, paralleled to the north and south by linear ridges rising 200 fathoms (360 m) or so above the general level of the flat sedimented areas. No fracture zones cross it and the rough topography associated with the Sheba Ridge as a whole is very subdued. The axis of the median valley east of Djibouti, here called the Tadjura Trench, in fact runs into the coastline north of Djibouti and the Gulf of Tadjura itself appears to be offset to the south (Roberts and Whitmarsh, 1969). It is possible therefore, that another small transform fault runs along a line through Djibouti. The western end of the Gulf of Tadjura narrows to a half kilometre wide strait linking it with the Ghubbat Kharab, thought to be a breached caldera. In this region topographic trends on land, as seen in a study of maps, swing northwards to parallel that of the Red Sea.
Fig.3. Magnetic anomalies in the Gulf of Aden. Tectonophysics,
8 (1969) 359-375
363
Throughout its length, from the Owen fracture zone to the Gulf of Tadjura, the median valley is associated with a negative magnetic anomaly (Fig.3). In regions where the valley is continuous the amplitude of thr anomaly is higher than where it is fractured. The highest values of the anomaly were found between 45O and 46’E where it was over 2.000 1 from peak to trough. The negative anomaly associated with the Tadjura Trench is not continuous with that in the Gulf of Tadjura. giving further evidence for a fracture in the region of Djibouti. It is clear from the above description that the Sheba Ridge, in common with some other mid-ocean ridges, is typified by a median valley, which has, at both its eastern and western ends, rifted comparatively young sediments and hence must postdate them. The suggestion was made by Laughton (1966a) that this valley represented the latest phase of three stages of opening of the Gulf of Aden. Fresh basal& and pillow lavas dredged from the median valley from within the area of detailed survey confirm the theory that this is a region of young crustal spreading. Details of the rocks dredged from here and in the Alula-Fartak Trench are reported on by Cann (1970).
CRUSTAL
STRUCTURE
TWO reversed and four unreversed seismic refraction stations were shot using the Hill technique of sono-radio buoys and single ship shooting. Shots were fired during the night between 23.00 and 04.00 because experience showed that this time was most free from radio interference. Arrivals were obtained out to a maximum range of 52 set (80 km). Four buoys were usually laid covering a range of 1.5-3 miles (2.8-5.5 km). At each station a profile of sound velocity in the sea to 820 fathoms (1,500 m) was obtained using a velocimeter. The surface horizontal velocity and the mean vertical velocity could therefore be calculated for use in the reduction of the seismic data. The results of the refraction stations are tabulated in Table I and illustrated in Fig.1, together with the 1958 “Vema”-“Atlantis” data.
These two stations (see Fig.4) were shot parallel to and just south of the Tadjura Trench in order to establish the crustal structure associated with high magnetic anomalies typical of the median line of the Gulf of Aden. The lines form a reversed pair. The depth below the stations increased steadily from 550 to 900 m from west to east. Arrival? have been corrected to a horizontal surface assuming all topography to be due to sediment with a velocity of 2.0 km/set. This procedure has been used in the reduction of all stations except where it is stated otherwise. In station 6235, a layer of 3.98 km/set was observed which was not seen in station 6236. This may, however, have resulted from the increased water depth under the buoys at station 6236 which did not allow this layer to appear as a first arrival. Assuming that this layer is present, then it is overlain by a sediment layer of between 0.65 km at the western end and 364
Tectonophysics,
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(1969)
359-375
-._I__
-
11048’ lo”56 12QCl’ 13OO2’ lln38’ 12012’ 12O53’ 14017’ 14O46’ 14”29’ 13029’
43057’ 44039’ 45*29’ 46001’ 46048’ 48~01 48035’ 50”W 51”32’ 51V43? 52”45’
.__ _~
N
E
N ___~_~. 11044’ 10050’ 11054 12047 11”Yti’ 12003’ 12053’ 13058’ 14“ll’ 13033’ 13”54’
44C40’ 45O22’ 46’04’ 46235 41y16’ 48037’ 49~10’** 51000’ 51’33’ 52e19’ 55’23’
E
1.52 1.52 1.52 1.51 1.50 1.51 1.50 1.50 1.51 1.51 1.50
water
(1.83) (2.0) (2.08) 1.83 (2.0) (2.0) I.85
1.90
(W (2.0) (2.0)
3.98 4.33 3.99 4.25 3.94 5.22 4.07 4.60 5.3 4.81 4.11
-2 6.40 6.54 6.13 6.52 5.79 6.96 6.91 6.61 6.90 6.65 6.44
layer 3
8.45 7.94 7.55
7.061 8.16 7.14* 7.82
__0.52-0.69 1.39 1.10 1.31-1.48 1.97-2.00 2.17 2.22-2.45 2.48-2.00 1.61 2,22 1.96-2.67
water
-~--
mantle 0.6H.28 1.16 0.42 1.33-1.12 0.44-0.61 0.41 0.4c-o.28 0.74,1.54 1.52 0.95--0.35 0.23-0.55
sediment
2.03 2.88 1.71 1.85-1.57 2.13-1.53 2.81 2.36-l-64 1.22-1.95 1.51 2.30-1.33 U.85-1.36
layer 2
---
6.62 2.4S-4.93 6.84-4.44
6.39-3.42 4.33 2.05 6.25-5.90
layer 3
16”. 1967 (stations 6215-6239)
Thickness (km) 1
1958 (stations 165-169) and from “Discovery
sediment21ayer
lRange of thicknesses from west to east. 2Anomaious mantle velocities. *Values in parentheses are assumed values. **Position of station 167 given in Laughton (19+X@ is m error.
___
6235/6236 A233 6239 169 168 6228 167 166 6218 62X/6213 165
--..
east
west
Crustal structures in the Gulf of Aden from “Vema 14”--“Atlantis 242” -__ _--) Station no. End points of profiles Velocities (km/se@
TABLE I
11 11.26 1.96-8.83 9.90-9.12
>
10.74?-10.07
9.96
Depth to Moho (km)
E
OW
...................................................................................................... .....................................................................................
fms
...............
..................
Fig.4. Time-distance plot and topography (V.E. = 8/l) for stations 6235 and 6236. (Distance is measured in set of water-wave travel at 1.52 km/set.
).
0.28 km at the eastern end, in which a velocity of 2.0 km/set is assumed. In station 6235, higher velocity layers of 6.40 and 7.27 km/set were observed, whereas in station 6236, the velocities were 5.86 km/set and 6.89 km/set. There is some ambiguity whether the two lower and the two higher velocities should be paired to give a solution with two dipping interfaces, or whether the 6.40 and 6.89 km/set velocities represent the same layer. At the reversal distance of 40.0 set, the intercepts are: (a) 11.7 set for 6.40 km/set; (b) 11.4 set for 7.27 km/set; and (c) 11.5 set for 6.89 km/set. It seems likely therefore that the 7.27 and 6.89 km/set-velocities belong to the same layer. If this is so, then the 6.40 and 5.86 km/secvelocities also belong to the same layer. Since both intercepts at zero range are the same, and the 6.40 km/set-velocity is established over a range of 25 set, it is assumed that the 5.86 km/set-velocity has been anomalously lowered by a local dip of the top surface of the layer over the range of 3-12 set in station 6236, and that the true velocity is 6.40 km/set. A mean velocity of 7.06 km/set is obtained for the deeper layer with its top surface inclined up to the east at 3O50’ from a depth of 9.59 km below station 6235. The arrivals at 50 set are good, and the absence of earlier arrivals indicates that if the 7.96 km/set-layer is underlain by an 8.1 km;’ set-layer, it cannot be shallower than 15 km below sea level. If the 6.40 and 6.89 km/set-velocities are considered to belong to the same layer, then the 5.66 km/set-velocity must belong to an extra block underlying station 6236 and which is not to be found under station 6235. On this assumption, the depth to the 7.27 km/set-layer under station 6235 is 366
Tectonophysics,
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9.8 km., not substantially different from that calculated under the first assumption. However, this interpretation is not considered as likely as the first.
Station
6239
Station 6239 (see Fig.5) lay some 80 km to the east of the reversed pair 6235 and 6236, and on the same line. The line was shot to the east and was not reversed. It lay just south of the median valley and ran obliquely onto the ridge associated with the south side of the valley. Corrections for the topography were based on the assumption that all layers were parallel to the bottom. The lowest velocity observed (3.99 km/set) agreed well with that in stations 6235 and 6236 and lay under 0.42 km of sediment with an assumed velocity of 2.0 kmjsec. A layer with velocity 6.15 km/set was established over a range of 6-12 set, but thereafter to 52 set all points lay on a line giving a velocity of 7.14 km/set. Since the line is not reversed, evidence for a uniform dipping basement was sought by examining the points on the line for “staggers”. Since the buoys are spread over a range of 4 set, in the presence of uniform dip, the dip can be established by combining the
ov 0
MC 10
20
30
40
50
W
E
Fig.5. Time-distance plot and topography (V.E. = 8/l) for station 6239. (Distance is measured in set of water-wave travel at 1.52 km/set.). Tectonophysics, 8 (1969) 359-375
367
apparent velocity obtained by one shot to tour buoys, with that of all shots to one buoy. No uniform dip was seen. The uniformity of dip, if it exists, is established by the small standard deviation of points from the line over the considerable range of 12-52 set!. The maximum dip up to the east that can reasonably be fitted to the section is 2 km in 80 km, or about 1.5“. Correcting for such an assumed dip, the high velocity is reduced to 7.03 km’sec. The absence of first arrivals from a higher velocity layer in the range lo 52 set suggests that a layer with velocity 8.1 km/set cannot be shallower than 14 km.
The station 6233 (see Fig.6) was chosen to be as near to the coast of Somalia as possible without being on the continental slope and to form, together with station 6239 and station 169 of “Vema”-“Atlantis” in 1958, a section across the Gulf of Aden west of the confused area of transform faults between 45’ and 50”E. The profile, which was not reversed, was shot to the west from a point 40 km off the Somalia coast. The topography was
61233
PC
a0
50
30
0
10
10 0
W
Fig.6. Time-distance plot and topography (V.E. = 8/l) for station 6233. (Distance is measured in set of water-wave travel at 1.52 km/see.). 368
Tectonophysics,
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extremely smooth rising gently to the west. A typical oceanic crustal structure was obtained. The depth to the Moho was 9.96 km. In view of the importance of this result in relation to the amount of lateral separation between the continental blocks of Arabia and Africa, the effect of a possible undetected dip in the 8.16/6.54 km/set boundary must be investigated. As in station 6239, evidence was not found for “staggers”, implying that no uniform dip existed between shots and buoys. If 8.16 km/set is attributed to a lower velocity layer dipping upwards to the west, then reasonable limits can be placed on the angle of dip, from the length of the line and the crustal thicknesses. A rise of 4 km over the line length of 73 km would essentially pinch out the 6.54 km/set-layer, and is equivalent to a dip of 3.5’. The true high velocity layer would then be 7.84 km/set, and the mean depth would be 8 km. An appreciably higher gradient than this over the line length and parallel to the continental margin seems unlikely: if anything, it is more probable that the crustal thickness would increase towards the west as the continent is approached, which would imply the high velocity layer to have a true value greater than 8.16 km/set. There can be little doubt either way, that the high velocity layer comprises upper mantle material.
0228 m a0
0
40 E
Fig.7. Time-distance plot and topography (V.E. = 8/l) for station 6228. (Distance is measured in set of water-wave travel at 1.51 km/see.). Tectonophysics, 8
(1969)
359-375
369
The station 6228 (see Fig.7) was planned to be typical of the flat main trough south of the Sheba Ridge and parallel to it, but in fact was too far north. The line shot to the east, crossed the southwest extensions of two of the northeast-southwest ridges which cross the Sheba Ridge, the first showing up only as a rough region of topography and the second lying under the last shot of the line, as a mountain 150 fathoms (0.27 km) high. The line lay at the northern edge of an area surveyed in detail and the trends of these ridges have been established. The lowest velocity layer observed (5.22 km/secJ was not well established and arrivals stopped beyond B range of 6.5 sec. However, the sediment thickness of 0.41 km calculated assuming a velocity of 2.0 km. sec. agrees very well with the depth of a strong reflector found on two siesmic reflection profiles crossin, STthe refraction line. Conversely, the refraction data indicate that this strong reflector is in fact the top of layer 2. The absence of arrivals from this layer beyond the range of 6.5 set may be due to poor transmission across a fault associated with crossing the first ridge. To a range of 31 set, a layer of velocity 6.96 km;‘sec was well established, although unreversed. The arrivals from the last shot, fired over the second northeast--southwest ridge, lay 0.6 set below the 6.96 km:‘secline, if no allowance is made for the topography. The topographic correction can be made on several different assumptions: (a) If all boundaries between the 6.96 kmjsec-layer and the water are raised by 0.27 km, then the correction is 0.18 sec. (b) If the exposed relief of the ridge is assumed to be layer 2, and there is no sediment cover, the correction is 0.31 sec. (c) If, in addition to (b), the top boundary of the 6.96 km/set-layer is raised by 1.2 km, the correction is 0.6 sec. On assumption (c), there is no evidence for a layer with velocity higher than 6.96 km,isec, and the ridge must be regarded as a horst uplifted by at least 1.2 km. On assumption (b) the uplift is less, but a break to a higher velocity line is required at about 31 sec. If a velocity of 8.1 km!‘sec is assumed for this layer, then the depth to the Moho is l! km. Such a line would lie between points corrected on assumptions (b) and (c). implying vertical displacement of the ridge by about 1 km.
Statio)r 0‘218 This unreversed station (see Fig.8) was shot towards the northeast along a line parallel to and northwest of the Alula-Fartak Trench. It lay over a flat sedimented region clear of the ridge that runs along the northwest margin of the trench. A few arrivals were observed from a 5.3 km/’ set-layer, but the velocity is poorly determined. The 6.90 km/set-layer is well determined, although the arrivals from one shot at the range of 19 set were all delayed by 0.25 sec. The fact that arrivals from the following shot. resumed the same line suggests that the late arrivals were due to a local increase in sediment thickness. Assuming this to be due to a local depres3iO
Tectonophysics,
8 (1969) 359-37b
6218
OV
10
0
,
20
set 30
40 NE
Osw
MQ-
k _~.~~~:::::::::::::::::::::::::::::::::::::::::::::::::::.............,
~~‘~~“.~.~.............,_.._..*..‘._f . . ..-.............................._.._.. . . . . ..~......I......._........~~. . . . . . . . . ..*......... . . . . . . . . . . . . .._...
fms.’
Fig.8. Time-distance plot and topography (V.E. = 8/l) for station 6218. (Distance is measured in set of water-wave travel at 1.51 km/set.)
sion in the 2.0/5.3 kmjsec-boundary, the depression would be about 0.8 km. The depression may be tectonic or may be an erosion channel in layer 2 associated, perhaps, with a seaward extension of the Wadi Hadramaut. The upper mantle velocity of 8.45 km/set is well established although unreversed. The unusually high velocity might result from shooting up dip, although one might expect the Moho to dip downwards toward the northeast, or from a significant thinning of sediments and layer 2 and consequent replacement by the 6.90 km/set-layer. In either case the mean depth to the Moho will not change significantly from the value quoted, of 11.3 km.
Stattol7s
6215 ad
6219
Stations 6215 and 6219 (see Fig.9) form a reversed pair of lines situated parallel to the Alula-Fartak Trench on its southeast side. Station 6215 was shot going northeast and in the final shots ran across some mountainous topography associated with the south side of the median valley where it meets the trench. The reversed line 6219 was therefore placed further south to avoid this complex area. In station 6215, a layer of velocity 4.93 km/set was seen. In station Tectonophysics,
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37 1
Fig.9. Time-distance plot and topography (V.E. = 8/l) for stations 6215 and 6219. (Distance is measured in set of water-wave travel at 1.51 kmjsec.).
6219, this line barely appeared as a first arrival but a few points could be fitted to a 4.7 km/set line. Seismic reflection profiles across the trench indicate that the sediment thickness near the northerly buoys is 0.35 km and near the southerly buoys is 0.95 km. Assuming a uniform gradient for the top layer 2, the dip angle is O”40’ up to the northeast and the true velocity is 4.81 km/set. The refraction and reflection data give a consistent result for the sediment thickness. Station 6219 gave a well determined 6.48 km/set whereas station 6215 gave only a few points below the 4.93 and 8.15 km/set lines. A line through these points and through the reversal point of&station 6219 gave a velocity of 6.85 km/set. Taken together these lines give a true velocity of 6.65 km set with a slope of 1’40’ up towards the northeast. The high velocity lines reversed to within 0.2 set at the reversal points and gave a mean velocity of 7.94 km/set with a slope of 34’ down to the northeast. However, this velocity and slope might be slightly too high since the 8.15 km/set velocity is influenced by the shots at long range on station 6215 where crustal rocks outcrop. The error is unlikely to be greater than about 0.1 km/set since the line is long and the final shot was in fact fired in the valley beyond the northeast hill. A mean depth to the Moho is 8.4 km. 372
Tectonophysics,
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DISCUSSIONOF
SEISMIC RESULTS
A summary of the crustal structures described above together with the 1958 “Vema”-“Atlantis” results is shown diagrammatically in Fig.1 where the placing of the sections relates roughly to their distribution in the Gulf of Aden. It will be seem from tabulated velocities of the 1958 and 1967 data in Table I, that the values can be grouped as follows: (a) 1.83-2.08 km/see. These are attributed to unconsolidated sediments. Many of the stations have assumed a value of 2.0 km/set in order to derive a thickness of sediment cover from the difference between the intercept of the underlying layer and the water depth. The thickness of the unconsolidated sediment layer varies from 0.23 to 1.52 km. It increases systematically with distance from the median valley, or conversely decreases with the distance from the coast. There is no appreciable trend in sediment thickness along the axis of the Gulf. (b) 3.94-5.3 km/set. This range of velocities is commonly grouped together as layer 2, and has been attributed variously to consolidated sediments, evaporites, pyroclastics or lava flows. At the south end of the Red Sea, velocities of 3.3-4.5 km/set (Drake and Girdler, 1964) are associated with several thousand metres of evaporite obtained in nearby boreholes. On the other hand, where layer 2 has been traced to outcrop on the walls of the Alula-Fartak Trench, basaltic rocks have been dredged. Elsewhere the flatness of the top of layer 2, identified in the seismic reflection profiles, suggests a sedimentary origin. Possibly flat bedded lava flows are interbedded with sedimentary sequences. The thickness of layer 2, which is found in all stations, varies from 1.85-2.89 km with no pronounced trend in relation to position. (c) 6.15-6.96 km/set. Apart from a rather low value of 6.15 km/set at station 6239, all velocities fall approximately within the range of layer 3 velocities as defined by Raitt (1963) (6.69 !: 0.26 km/set), and hence are typically oceanic. Thicknesses range from 2.05-6.84 km varying within somewhat wider limits than those quoted by Raitt (4.86 ? 1.42 km). (d) 7.55-8.45 km/set. This range is rather large for the normal subMoho velocities (8.13 2 0.24 km/set) but it includes at the low end a poorly determined velocity (station 165) and at the high end, an unreversed velocity that may be rather high (station 6218). Excluding these two, the velocities are normal for the upper mantle. Depths to the Moho, where determined, vary between 7.96 and 11.26 km. (e) 7.06-7.14 km/set. These two anomalous velocities(from stations 6235/6236 and 6239) have been omitted from the above scheme of a normal oceanic crust. They could be grouped with the layer 3-velocities were it not for the fact of another layer 3 velocity above them. Furthermore they are somewhat deeper and thicker than in the normal oceanic layer. On the other hand the layer 3 velocities are abnormally low, and the 7.1 km/set velocity is lower than the anomalous mantle velocity range of 7.2-7.6 km/set of Le Pichon et al. (1965) and Menard (1960). The attribution of the 7.06-7.14 km/set velocities to the generally classified crustal layers is therefore rather difficult. The position of stations 6235/6236 at the extreme western end of the Gulf of Aden might lead one to suppose the structure to be somewhat continental in character and that the 6.1-6.4 km/set layer which increases in thickness westwards is continental crust, albeit of Tectonophysics, 8 (1969)359-375
373
Fig.10. Section across Gulf of Aden through stations 169, indicating postulated low density mass (V.E. = 5/l).
6233, 6239 and
rather high velocity. Alternatively the similar structure of station 6239 lies between two more typically oceanic structures and is associated with the median line of the Sheba Ridge. It is proposed therefore, to classify the 7.1 kmi/sec layer as anomalous mantle. Station 167, also lying on the axial zone, does not show this velocity, but the line may not have been long enough. Gf the remaining nine profiles in the Gulf of Aden, all show a normal oceanic crustal structure in spite of the depth of the water being less then half the normal oceanic depth. As was pointed out by Laughton (1966b), the combination of a normal oceanic crust and a relatively shallow water depth in the Gulf of Aden implies a large positive gravity anomaly (about 300 mgal for a reduction in ‘ocean depth from 5 to 2 km) unless there is an undetected low density body in the upper mantle. Many gravity traverses in the Gulf have shown that it is in isostatic equilibrium and that no such large anomaly exists. Although the necessary detailed calculations have not yet been made, it is possible to state that a low density (3.15 gjcm3) body, of the order of 30 km thick is necessary underlying the entire Gulf of Aden to satisfy both gravity and seismic data. As in the case of the Mid-Atlantic Ridge, in the axial zone this body lies immediately below the lower velocity crustal layers, whereas on the flanks of the ridge, it must be below the 8.1 kmisec upper mantle, and there must be a density inversion. The Gulf of Aden structure differs from that under the Mid-Atlantic Ridge, in that the oceanic layer 3 is found all over the Gulf whereas it is absent under the Mid-Atlantic Ridge. In this respect, the Gulf is more similar to the East Pacific Rise. Fig.10 shows a crustal section between Africa and Arabia parallel to the northeast-southwest ridges and running through seismic stations 6233, 6239 and 169. It is along such a line that it has been proposed (Laughton, 1966a) that the separation of Arabia and Africa has taken place, due to a process of sea floor spreading. The existence of a thin oceanic crust closely adjacent to the continental margins is critical in evaluating the 374
Tectonophysics,
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distance moved since the initial split. This data indicates that the relative movement cannot have been less than 260 km along this section. Such an opening must be consistent with the mechanism of formation of the south end of the Red Sea and the Afar depression. A mechanism has been proposed (Laughton, 1966b) in which the opening of the Red Sea is far larger than that suggested by Drake and Girdler (1964) and in which a large part of the Afar depression has an origin similar to that of new oceanic crust. If such a hypothesis is disproved on the grounds of new geological or geophysical data, then it is necessary to assume that the continental blocks of Africa and Arabia are not behaving as single rigid units, but must have been fractured along transcurrent faults of considerable size. No evidence of such faults has yet been found on land. The resolution of this problem must await new data. ACKNOWLEDGEMENTS
The geophysical studies during Cruise 16 of R.R.S. “Discovery” were only possible through the co-operation of many scientists. The authors are particularly indebted to Dr. D.H. Matthews, Dr. D. Davies and Mrs. Charlotte Keen for their contribution to the success of the seismic work, and to Miss Carol Williams for her contribution to bathymetric and magnetic studies, all from the Department of Geodesy and Geophysics, Cambridge University. REFERENCES
Cann, J.R., 1970. Petrology of basalts dredged from the Gulf of Aden. Deep-Sea Res., in press. Drake, C.L. and Girdler, R.W., 1964. A geophysical study of the Red Sea. Geophys. J., 8: 473-495. Farquharson, W.I., 1935. Topography. Sci. Rept. John Murray Expedition, 1933-1934. 1: 43-61. Laughton, A.S., 1966a. The Gulf of Aden. Phil. Trans. Roy. Sot. London, Ser. A, 259: 150-171. Laughton, A.S., 196613. The Gulf of Aden, in relation to the Red Sea and the Afar depression of Ethiopia. In: The World Rift System. Geol. Surv. Canada, 66-14: 78-97. Le Pichon, X., Houtz, R.E., Drake, C.L. and Nafe, J.E., 1965. Crustal structure of the mid-ocean ridges, 1. Seismic refraction measurements. J. Geophys. Res., 70: 319-339. Matthews, D.H., 1966. The Owen fracture zone and the northern end of the Carlsberg Ridge. Phil. Trans. Roy. Sot. London, Ser. A, 259: 172-186. Matthews, D.H., Williams, C. and Laughton, A.S., 1967. Mid-ocean ridge in the mouth of the Gulf of Aden. Nature, 215: 1052-1053. Menard, H.W., 1960. The East Pacific Rise. Science, 132: 1737-1746. Nafe, J.E.. Hennion, J.F. and Peter, G., 1959. Geophysical measurements in the Gulf of Aden. Preprints Intern. Oceanog. Congr., New York, 1959: pp.42-43. Raitt, R.W., 1963. The crustal rocks. In: M.N. Hill (Editor), The Sea. Interscience, New York, N.Y., 3: 85-101. Roberts, D.G. and Whitmarsh, R.B., 1969. A bathymetric and magnetic survey of the Gulf of Tadjura, western Gulf of Aden. Earth Planetary Sci. Letters, 5: 253-258.
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375
Publishing
Company,
Amsterdam
RECENT STUDIES OF THE CRUSTAL STRUCTURE IN THE GULF OF ADEN
A.S. LAUGHTON
and C. TRAMONTINI
National
of Oceanography,
Institute
Department
of Geodesy
Wormley,
and Geophysics,
Godalming,
Madingley
Rise,
Surrey
(Great
Cambridge
Britain)
(Great
Britain)
(Received August, 1968) (Resubmitted February, 1969) SUMMARY Cruise 16 of R.R.S. “Discovery” in 1967 has provided new data on the structure of the Gulf of Aden. The existence has now been demonstrated of a median valley, with associated magnetic anomaly, reaching from the Gulf of Tadjura in the west to the Owen fracture zone in the east. The valley is severely fractured and offset in the central region. New seismic refraction data have shown that throughout the Gulf of Aden, the crustal structure is similar to that found under the oceans and that at the western end, the axial zone is underlain by an anomalously low mantle velocity. A minimum separation of 260 km between the continental blocks of Arabia and Africa is indicated.
INTRODUCTION
During the Spring of 1967, an expedition in R.R.S. “Discovery” to the Gulf of Aden was planned(Cruise 16) with a number of specific objectives to elaborate on and to extend the data relating to its origin. Previous data (discussed by Laughton, 1966a,b) had been obtained largely from ships on passage through the Gulf of Aden and hence a predominance of tracks were oriented east-west. In many cases navigation was poor and the correlation of data on adjacent tracks was difficult even though the track density was high. However, an analysis of these data suggested many problems which could be solved by a more controlled investigation. The earlier crustal structure studies by R.V. “Atlantis” and R.V. “Vema” in 1958 were not regarded by their authors (Nafe et al., 1959) as being very reliable in respect of the deeper high velocity layers, and it was clearly desirable to increase the coverage with longer seismic refraction lines. No rocks had been dredged from any of the mountainous regions of the Gulf of Aden or from the Alula-Fartak Trench. The cruise therefore set out with the following specific objectives: (1) To establish whether there was, associated with the central rough zone, a median valley as has been commonly found on mid-oceanic ridges. (2) To map in more detail the axial magnetic anomaly and to establish &he trend of the other anomalies north and south of it. (3) To survey and sample rocks in three areas selected as being Tectonophysics,
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typical of: (a) the central rough zone; (h) the sediment filled main troughs north and south of the central rough zone; and (c) the Alula-Fartak Trench. (4) To determine the crustal structure by the seismic refraction method in areas critical to the theory of continental separation. In this paper the results of objectives (1) and (4) will be presented together with some preliminary results of the other studies.
THE MEDIAK VALl.EY
In Laughton (1966a) the topography of the Gulf of Aden was divided into three physiographic zones: the continental margins, the main troughs and the central rough zone. The central rough zone was believed to be the natural extension of the mid-ocean ridge system into the Gulf of Aden, the Carlsberg Ridge having suffered a displacement along the Gwen fracture zone (Matthews, 1966). The association of the central rough zone with the epicentre belt, large and linear magnetic anomalies and high heat flow showed that it had many of the same characteristics as a typical mid-ocean ridge, although in some respects of its topography it is different. In particular the topography differs in being dominated in places by strong northeast-southwest lineations giving rise to pronounced ridges and troughs. This echelon pattern has long been recognized in the Gulf of Aden (e.g., Farquharson, 1935). However, the evidence now points strongly to the generic connection between the central rough zone and the Carlsberg Ridge and it is proposed that it should be called the “Sheba Ridge”. A natural division into the east and west Sheba Ridge is provided by the Alula-Fartak Trench (Fig.1). It must, however, be recognized that in parts of the west Sheba Ridge, the valleys lie deeper than the sediment filled troughs to the north and south. The east Sheba Ridge was delineated by a survey during this cruise (Fig.2) and which has shown that from 55OE to the Gwen fracture zone there is a well developed median valley associated, in most places, with a negative magnetic anomaly (Matthews et al., 1967). This valley cuts through the ridgesof the Gwen fracture zone and terminates in the Wheatley Deep, a trough on the edge of the Indus Abyssal Plain and lying 600 fathoms (1,100 m) below it. The median valley is apparently curved (although it is possible that this is the result of a series of small transform faults). To the west it is offset at 54“E by a transform fault and joins at ‘right angles the Alula-Fartak Trench at its northern end. At the southern end of the trench, the valley emerges westward at a point 11.2 miles (206 km) further south and is continuous only for a length of some 50 miles (92 km). (See Fig. 1). Between this point and 46OE, the evidence for the existence of a median valley is very complex. It is a region of predominant northeast-soutwest ridges and valleys, and on the data presented in Laughton, 1966a (Fig.l), a median valley was suggested only on the basis of local and usually disconnected deeps. In 1967, a detailed survey was made of a sixty-mile square defined by the limits 12O-l3ON, 47O-48’E. Navigation was controlled by anchored buoys and radar transponders, and north-south survey lines were about 5 miles (9 km) apart. In addition there were numerous cross lines 360
Tectonophysics,
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361
362
Tectonophysics,
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and some more closely spaced and limited surveys. The area was crossed by two northeast-southwest ridges and contained the intervening valley. It was clear from the survey that a well defined median valley did exist, characterized by a negative magnetic anomaly, but that it consisted of a series of short east-west, or eastsoutheast-westnorthwest, sections disrupted by a series of left lateral offsets. Along these disruptions the magnetic anomaly was substantially reduced. By analogy with this area, it seems likely that similar disruptions to the median valley occur throughout the region between 46O and 51°E, and that the valley itself would show as an extra deep along the axis of a northeast-southwest valley or as a co1 on the northeast-southwest ridges. Associated with the median valley were minor east-west ridges both to the north and south, most of which showed an asymmetry, the steep slope facing the median valley. West of 46OE, the median valley continues as a distinct and continuous feature to the Gulf of Tadjura, paralleled to the north and south by linear ridges rising 200 fathoms (360 m) or so above the general level of the flat sedimented areas. No fracture zones cross it and the rough topography associated with the Sheba Ridge as a whole is very subdued. The axis of the median valley east of Djibouti, here called the Tadjura Trench, in fact runs into the coastline north of Djibouti and the Gulf of Tadjura itself appears to be offset to the south (Roberts and Whitmarsh, 1969). It is possible therefore, that another small transform fault runs along a line through Djibouti. The western end of the Gulf of Tadjura narrows to a half kilometre wide strait linking it with the Ghubbat Kharab, thought to be a breached caldera. In this region topographic trends on land, as seen in a study of maps, swing northwards to parallel that of the Red Sea.
Fig.3. Magnetic anomalies in the Gulf of Aden. Tectonophysics,
8 (1969) 359-375
363
Throughout its length, from the Owen fracture zone to the Gulf of Tadjura, the median valley is associated with a negative magnetic anomaly (Fig.3). In regions where the valley is continuous the amplitude of thr anomaly is higher than where it is fractured. The highest values of the anomaly were found between 45O and 46’E where it was over 2.000 1 from peak to trough. The negative anomaly associated with the Tadjura Trench is not continuous with that in the Gulf of Tadjura. giving further evidence for a fracture in the region of Djibouti. It is clear from the above description that the Sheba Ridge, in common with some other mid-ocean ridges, is typified by a median valley, which has, at both its eastern and western ends, rifted comparatively young sediments and hence must postdate them. The suggestion was made by Laughton (1966a) that this valley represented the latest phase of three stages of opening of the Gulf of Aden. Fresh basal& and pillow lavas dredged from the median valley from within the area of detailed survey confirm the theory that this is a region of young crustal spreading. Details of the rocks dredged from here and in the Alula-Fartak Trench are reported on by Cann (1970).
CRUSTAL
STRUCTURE
TWO reversed and four unreversed seismic refraction stations were shot using the Hill technique of sono-radio buoys and single ship shooting. Shots were fired during the night between 23.00 and 04.00 because experience showed that this time was most free from radio interference. Arrivals were obtained out to a maximum range of 52 set (80 km). Four buoys were usually laid covering a range of 1.5-3 miles (2.8-5.5 km). At each station a profile of sound velocity in the sea to 820 fathoms (1,500 m) was obtained using a velocimeter. The surface horizontal velocity and the mean vertical velocity could therefore be calculated for use in the reduction of the seismic data. The results of the refraction stations are tabulated in Table I and illustrated in Fig.1, together with the 1958 “Vema”-“Atlantis” data.
These two stations (see Fig.4) were shot parallel to and just south of the Tadjura Trench in order to establish the crustal structure associated with high magnetic anomalies typical of the median line of the Gulf of Aden. The lines form a reversed pair. The depth below the stations increased steadily from 550 to 900 m from west to east. Arrival? have been corrected to a horizontal surface assuming all topography to be due to sediment with a velocity of 2.0 km/set. This procedure has been used in the reduction of all stations except where it is stated otherwise. In station 6235, a layer of 3.98 km/set was observed which was not seen in station 6236. This may, however, have resulted from the increased water depth under the buoys at station 6236 which did not allow this layer to appear as a first arrival. Assuming that this layer is present, then it is overlain by a sediment layer of between 0.65 km at the western end and 364
Tectonophysics,
8
(1969)
359-375
-._I__
-
11048’ lo”56 12QCl’ 13OO2’ lln38’ 12012’ 12O53’ 14017’ 14O46’ 14”29’ 13029’
43057’ 44039’ 45*29’ 46001’ 46048’ 48~01 48035’ 50”W 51”32’ 51V43? 52”45’
.__ _~
N
E
N ___~_~. 11044’ 10050’ 11054 12047 11”Yti’ 12003’ 12053’ 13058’ 14“ll’ 13033’ 13”54’
44C40’ 45O22’ 46’04’ 46235 41y16’ 48037’ 49~10’** 51000’ 51’33’ 52e19’ 55’23’
E
1.52 1.52 1.52 1.51 1.50 1.51 1.50 1.50 1.51 1.51 1.50
water
(1.83) (2.0) (2.08) 1.83 (2.0) (2.0) I.85
1.90
(W (2.0) (2.0)
3.98 4.33 3.99 4.25 3.94 5.22 4.07 4.60 5.3 4.81 4.11
-2 6.40 6.54 6.13 6.52 5.79 6.96 6.91 6.61 6.90 6.65 6.44
layer 3
8.45 7.94 7.55
7.061 8.16 7.14* 7.82
__0.52-0.69 1.39 1.10 1.31-1.48 1.97-2.00 2.17 2.22-2.45 2.48-2.00 1.61 2,22 1.96-2.67
water
-~--
mantle 0.6H.28 1.16 0.42 1.33-1.12 0.44-0.61 0.41 0.4c-o.28 0.74,1.54 1.52 0.95--0.35 0.23-0.55
sediment
2.03 2.88 1.71 1.85-1.57 2.13-1.53 2.81 2.36-l-64 1.22-1.95 1.51 2.30-1.33 U.85-1.36
layer 2
---
6.62 2.4S-4.93 6.84-4.44
6.39-3.42 4.33 2.05 6.25-5.90
layer 3
16”. 1967 (stations 6215-6239)
Thickness (km) 1
1958 (stations 165-169) and from “Discovery
sediment21ayer
lRange of thicknesses from west to east. 2Anomaious mantle velocities. *Values in parentheses are assumed values. **Position of station 167 given in Laughton (19+X@ is m error.
___
6235/6236 A233 6239 169 168 6228 167 166 6218 62X/6213 165
--..
east
west
Crustal structures in the Gulf of Aden from “Vema 14”--“Atlantis 242” -__ _--) Station no. End points of profiles Velocities (km/se@
TABLE I
11 11.26 1.96-8.83 9.90-9.12
>
10.74?-10.07
9.96
Depth to Moho (km)
E
OW
...................................................................................................... .....................................................................................
fms
...............
..................
Fig.4. Time-distance plot and topography (V.E. = 8/l) for stations 6235 and 6236. (Distance is measured in set of water-wave travel at 1.52 km/set.
).
0.28 km at the eastern end, in which a velocity of 2.0 km/set is assumed. In station 6235, higher velocity layers of 6.40 and 7.27 km/set were observed, whereas in station 6236, the velocities were 5.86 km/set and 6.89 km/set. There is some ambiguity whether the two lower and the two higher velocities should be paired to give a solution with two dipping interfaces, or whether the 6.40 and 6.89 km/set velocities represent the same layer. At the reversal distance of 40.0 set, the intercepts are: (a) 11.7 set for 6.40 km/set; (b) 11.4 set for 7.27 km/set; and (c) 11.5 set for 6.89 km/set. It seems likely therefore that the 7.27 and 6.89 km/set-velocities belong to the same layer. If this is so, then the 6.40 and 5.86 km/secvelocities also belong to the same layer. Since both intercepts at zero range are the same, and the 6.40 km/set-velocity is established over a range of 25 set, it is assumed that the 5.86 km/set-velocity has been anomalously lowered by a local dip of the top surface of the layer over the range of 3-12 set in station 6236, and that the true velocity is 6.40 km/set. A mean velocity of 7.06 km/set is obtained for the deeper layer with its top surface inclined up to the east at 3O50’ from a depth of 9.59 km below station 6235. The arrivals at 50 set are good, and the absence of earlier arrivals indicates that if the 7.96 km/set-layer is underlain by an 8.1 km;’ set-layer, it cannot be shallower than 15 km below sea level. If the 6.40 and 6.89 km/set-velocities are considered to belong to the same layer, then the 5.66 km/set-velocity must belong to an extra block underlying station 6236 and which is not to be found under station 6235. On this assumption, the depth to the 7.27 km/set-layer under station 6235 is 366
Tectonophysics,
8 (1969) 359-375
9.8 km., not substantially different from that calculated under the first assumption. However, this interpretation is not considered as likely as the first.
Station
6239
Station 6239 (see Fig.5) lay some 80 km to the east of the reversed pair 6235 and 6236, and on the same line. The line was shot to the east and was not reversed. It lay just south of the median valley and ran obliquely onto the ridge associated with the south side of the valley. Corrections for the topography were based on the assumption that all layers were parallel to the bottom. The lowest velocity observed (3.99 km/set) agreed well with that in stations 6235 and 6236 and lay under 0.42 km of sediment with an assumed velocity of 2.0 kmjsec. A layer with velocity 6.15 km/set was established over a range of 6-12 set, but thereafter to 52 set all points lay on a line giving a velocity of 7.14 km/set. Since the line is not reversed, evidence for a uniform dipping basement was sought by examining the points on the line for “staggers”. Since the buoys are spread over a range of 4 set, in the presence of uniform dip, the dip can be established by combining the
ov 0
MC 10
20
30
40
50
W
E
Fig.5. Time-distance plot and topography (V.E. = 8/l) for station 6239. (Distance is measured in set of water-wave travel at 1.52 km/set.). Tectonophysics, 8 (1969) 359-375
367
apparent velocity obtained by one shot to tour buoys, with that of all shots to one buoy. No uniform dip was seen. The uniformity of dip, if it exists, is established by the small standard deviation of points from the line over the considerable range of 12-52 set!. The maximum dip up to the east that can reasonably be fitted to the section is 2 km in 80 km, or about 1.5“. Correcting for such an assumed dip, the high velocity is reduced to 7.03 km’sec. The absence of first arrivals from a higher velocity layer in the range lo 52 set suggests that a layer with velocity 8.1 km/set cannot be shallower than 14 km.
The station 6233 (see Fig.6) was chosen to be as near to the coast of Somalia as possible without being on the continental slope and to form, together with station 6239 and station 169 of “Vema”-“Atlantis” in 1958, a section across the Gulf of Aden west of the confused area of transform faults between 45’ and 50”E. The profile, which was not reversed, was shot to the west from a point 40 km off the Somalia coast. The topography was
61233
PC
a0
50
30
0
10
10 0
W
Fig.6. Time-distance plot and topography (V.E. = 8/l) for station 6233. (Distance is measured in set of water-wave travel at 1.52 km/see.). 368
Tectonophysics,
8 (1969) 359-375
extremely smooth rising gently to the west. A typical oceanic crustal structure was obtained. The depth to the Moho was 9.96 km. In view of the importance of this result in relation to the amount of lateral separation between the continental blocks of Arabia and Africa, the effect of a possible undetected dip in the 8.16/6.54 km/set boundary must be investigated. As in station 6239, evidence was not found for “staggers”, implying that no uniform dip existed between shots and buoys. If 8.16 km/set is attributed to a lower velocity layer dipping upwards to the west, then reasonable limits can be placed on the angle of dip, from the length of the line and the crustal thicknesses. A rise of 4 km over the line length of 73 km would essentially pinch out the 6.54 km/set-layer, and is equivalent to a dip of 3.5’. The true high velocity layer would then be 7.84 km/set, and the mean depth would be 8 km. An appreciably higher gradient than this over the line length and parallel to the continental margin seems unlikely: if anything, it is more probable that the crustal thickness would increase towards the west as the continent is approached, which would imply the high velocity layer to have a true value greater than 8.16 km/set. There can be little doubt either way, that the high velocity layer comprises upper mantle material.
0228 m a0
0
40 E
Fig.7. Time-distance plot and topography (V.E. = 8/l) for station 6228. (Distance is measured in set of water-wave travel at 1.51 km/see.). Tectonophysics, 8
(1969)
359-375
369
The station 6228 (see Fig.7) was planned to be typical of the flat main trough south of the Sheba Ridge and parallel to it, but in fact was too far north. The line shot to the east, crossed the southwest extensions of two of the northeast-southwest ridges which cross the Sheba Ridge, the first showing up only as a rough region of topography and the second lying under the last shot of the line, as a mountain 150 fathoms (0.27 km) high. The line lay at the northern edge of an area surveyed in detail and the trends of these ridges have been established. The lowest velocity layer observed (5.22 km/secJ was not well established and arrivals stopped beyond B range of 6.5 sec. However, the sediment thickness of 0.41 km calculated assuming a velocity of 2.0 km. sec. agrees very well with the depth of a strong reflector found on two siesmic reflection profiles crossin, STthe refraction line. Conversely, the refraction data indicate that this strong reflector is in fact the top of layer 2. The absence of arrivals from this layer beyond the range of 6.5 set may be due to poor transmission across a fault associated with crossing the first ridge. To a range of 31 set, a layer of velocity 6.96 km;‘sec was well established, although unreversed. The arrivals from the last shot, fired over the second northeast--southwest ridge, lay 0.6 set below the 6.96 km:‘secline, if no allowance is made for the topography. The topographic correction can be made on several different assumptions: (a) If all boundaries between the 6.96 kmjsec-layer and the water are raised by 0.27 km, then the correction is 0.18 sec. (b) If the exposed relief of the ridge is assumed to be layer 2, and there is no sediment cover, the correction is 0.31 sec. (c) If, in addition to (b), the top boundary of the 6.96 km/set-layer is raised by 1.2 km, the correction is 0.6 sec. On assumption (c), there is no evidence for a layer with velocity higher than 6.96 km,isec, and the ridge must be regarded as a horst uplifted by at least 1.2 km. On assumption (b) the uplift is less, but a break to a higher velocity line is required at about 31 sec. If a velocity of 8.1 km!‘sec is assumed for this layer, then the depth to the Moho is l! km. Such a line would lie between points corrected on assumptions (b) and (c). implying vertical displacement of the ridge by about 1 km.
Statio)r 0‘218 This unreversed station (see Fig.8) was shot towards the northeast along a line parallel to and northwest of the Alula-Fartak Trench. It lay over a flat sedimented region clear of the ridge that runs along the northwest margin of the trench. A few arrivals were observed from a 5.3 km/’ set-layer, but the velocity is poorly determined. The 6.90 km/set-layer is well determined, although the arrivals from one shot at the range of 19 set were all delayed by 0.25 sec. The fact that arrivals from the following shot. resumed the same line suggests that the late arrivals were due to a local increase in sediment thickness. Assuming this to be due to a local depres3iO
Tectonophysics,
8 (1969) 359-37b
6218
OV
10
0
,
20
set 30
40 NE
Osw
MQ-
k _~.~~~:::::::::::::::::::::::::::::::::::::::::::::::::::.............,
~~‘~~“.~.~.............,_.._..*..‘._f . . ..-.............................._.._.. . . . . ..~......I......._........~~. . . . . . . . . ..*......... . . . . . . . . . . . . .._...
fms.’
Fig.8. Time-distance plot and topography (V.E. = 8/l) for station 6218. (Distance is measured in set of water-wave travel at 1.51 km/set.)
sion in the 2.0/5.3 kmjsec-boundary, the depression would be about 0.8 km. The depression may be tectonic or may be an erosion channel in layer 2 associated, perhaps, with a seaward extension of the Wadi Hadramaut. The upper mantle velocity of 8.45 km/set is well established although unreversed. The unusually high velocity might result from shooting up dip, although one might expect the Moho to dip downwards toward the northeast, or from a significant thinning of sediments and layer 2 and consequent replacement by the 6.90 km/set-layer. In either case the mean depth to the Moho will not change significantly from the value quoted, of 11.3 km.
Stattol7s
6215 ad
6219
Stations 6215 and 6219 (see Fig.9) form a reversed pair of lines situated parallel to the Alula-Fartak Trench on its southeast side. Station 6215 was shot going northeast and in the final shots ran across some mountainous topography associated with the south side of the median valley where it meets the trench. The reversed line 6219 was therefore placed further south to avoid this complex area. In station 6215, a layer of velocity 4.93 km/set was seen. In station Tectonophysics,
8 (1969) 359475
37 1
Fig.9. Time-distance plot and topography (V.E. = 8/l) for stations 6215 and 6219. (Distance is measured in set of water-wave travel at 1.51 kmjsec.).
6219, this line barely appeared as a first arrival but a few points could be fitted to a 4.7 km/set line. Seismic reflection profiles across the trench indicate that the sediment thickness near the northerly buoys is 0.35 km and near the southerly buoys is 0.95 km. Assuming a uniform gradient for the top layer 2, the dip angle is O”40’ up to the northeast and the true velocity is 4.81 km/set. The refraction and reflection data give a consistent result for the sediment thickness. Station 6219 gave a well determined 6.48 km/set whereas station 6215 gave only a few points below the 4.93 and 8.15 km/set lines. A line through these points and through the reversal point of&station 6219 gave a velocity of 6.85 km/set. Taken together these lines give a true velocity of 6.65 km set with a slope of 1’40’ up towards the northeast. The high velocity lines reversed to within 0.2 set at the reversal points and gave a mean velocity of 7.94 km/set with a slope of 34’ down to the northeast. However, this velocity and slope might be slightly too high since the 8.15 km/set velocity is influenced by the shots at long range on station 6215 where crustal rocks outcrop. The error is unlikely to be greater than about 0.1 km/set since the line is long and the final shot was in fact fired in the valley beyond the northeast hill. A mean depth to the Moho is 8.4 km. 372
Tectonophysics,
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DISCUSSIONOF
SEISMIC RESULTS
A summary of the crustal structures described above together with the 1958 “Vema”-“Atlantis” results is shown diagrammatically in Fig.1 where the placing of the sections relates roughly to their distribution in the Gulf of Aden. It will be seem from tabulated velocities of the 1958 and 1967 data in Table I, that the values can be grouped as follows: (a) 1.83-2.08 km/see. These are attributed to unconsolidated sediments. Many of the stations have assumed a value of 2.0 km/set in order to derive a thickness of sediment cover from the difference between the intercept of the underlying layer and the water depth. The thickness of the unconsolidated sediment layer varies from 0.23 to 1.52 km. It increases systematically with distance from the median valley, or conversely decreases with the distance from the coast. There is no appreciable trend in sediment thickness along the axis of the Gulf. (b) 3.94-5.3 km/set. This range of velocities is commonly grouped together as layer 2, and has been attributed variously to consolidated sediments, evaporites, pyroclastics or lava flows. At the south end of the Red Sea, velocities of 3.3-4.5 km/set (Drake and Girdler, 1964) are associated with several thousand metres of evaporite obtained in nearby boreholes. On the other hand, where layer 2 has been traced to outcrop on the walls of the Alula-Fartak Trench, basaltic rocks have been dredged. Elsewhere the flatness of the top of layer 2, identified in the seismic reflection profiles, suggests a sedimentary origin. Possibly flat bedded lava flows are interbedded with sedimentary sequences. The thickness of layer 2, which is found in all stations, varies from 1.85-2.89 km with no pronounced trend in relation to position. (c) 6.15-6.96 km/set. Apart from a rather low value of 6.15 km/set at station 6239, all velocities fall approximately within the range of layer 3 velocities as defined by Raitt (1963) (6.69 !: 0.26 km/set), and hence are typically oceanic. Thicknesses range from 2.05-6.84 km varying within somewhat wider limits than those quoted by Raitt (4.86 ? 1.42 km). (d) 7.55-8.45 km/set. This range is rather large for the normal subMoho velocities (8.13 2 0.24 km/set) but it includes at the low end a poorly determined velocity (station 165) and at the high end, an unreversed velocity that may be rather high (station 6218). Excluding these two, the velocities are normal for the upper mantle. Depths to the Moho, where determined, vary between 7.96 and 11.26 km. (e) 7.06-7.14 km/set. These two anomalous velocities(from stations 6235/6236 and 6239) have been omitted from the above scheme of a normal oceanic crust. They could be grouped with the layer 3-velocities were it not for the fact of another layer 3 velocity above them. Furthermore they are somewhat deeper and thicker than in the normal oceanic layer. On the other hand the layer 3 velocities are abnormally low, and the 7.1 km/set velocity is lower than the anomalous mantle velocity range of 7.2-7.6 km/set of Le Pichon et al. (1965) and Menard (1960). The attribution of the 7.06-7.14 km/set velocities to the generally classified crustal layers is therefore rather difficult. The position of stations 6235/6236 at the extreme western end of the Gulf of Aden might lead one to suppose the structure to be somewhat continental in character and that the 6.1-6.4 km/set layer which increases in thickness westwards is continental crust, albeit of Tectonophysics, 8 (1969)359-375
373
Fig.10. Section across Gulf of Aden through stations 169, indicating postulated low density mass (V.E. = 5/l).
6233, 6239 and
rather high velocity. Alternatively the similar structure of station 6239 lies between two more typically oceanic structures and is associated with the median line of the Sheba Ridge. It is proposed therefore, to classify the 7.1 kmi/sec layer as anomalous mantle. Station 167, also lying on the axial zone, does not show this velocity, but the line may not have been long enough. Gf the remaining nine profiles in the Gulf of Aden, all show a normal oceanic crustal structure in spite of the depth of the water being less then half the normal oceanic depth. As was pointed out by Laughton (1966b), the combination of a normal oceanic crust and a relatively shallow water depth in the Gulf of Aden implies a large positive gravity anomaly (about 300 mgal for a reduction in ‘ocean depth from 5 to 2 km) unless there is an undetected low density body in the upper mantle. Many gravity traverses in the Gulf have shown that it is in isostatic equilibrium and that no such large anomaly exists. Although the necessary detailed calculations have not yet been made, it is possible to state that a low density (3.15 gjcm3) body, of the order of 30 km thick is necessary underlying the entire Gulf of Aden to satisfy both gravity and seismic data. As in the case of the Mid-Atlantic Ridge, in the axial zone this body lies immediately below the lower velocity crustal layers, whereas on the flanks of the ridge, it must be below the 8.1 kmisec upper mantle, and there must be a density inversion. The Gulf of Aden structure differs from that under the Mid-Atlantic Ridge, in that the oceanic layer 3 is found all over the Gulf whereas it is absent under the Mid-Atlantic Ridge. In this respect, the Gulf is more similar to the East Pacific Rise. Fig.10 shows a crustal section between Africa and Arabia parallel to the northeast-southwest ridges and running through seismic stations 6233, 6239 and 169. It is along such a line that it has been proposed (Laughton, 1966a) that the separation of Arabia and Africa has taken place, due to a process of sea floor spreading. The existence of a thin oceanic crust closely adjacent to the continental margins is critical in evaluating the 374
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distance moved since the initial split. This data indicates that the relative movement cannot have been less than 260 km along this section. Such an opening must be consistent with the mechanism of formation of the south end of the Red Sea and the Afar depression. A mechanism has been proposed (Laughton, 1966b) in which the opening of the Red Sea is far larger than that suggested by Drake and Girdler (1964) and in which a large part of the Afar depression has an origin similar to that of new oceanic crust. If such a hypothesis is disproved on the grounds of new geological or geophysical data, then it is necessary to assume that the continental blocks of Africa and Arabia are not behaving as single rigid units, but must have been fractured along transcurrent faults of considerable size. No evidence of such faults has yet been found on land. The resolution of this problem must await new data. ACKNOWLEDGEMENTS
The geophysical studies during Cruise 16 of R.R.S. “Discovery” were only possible through the co-operation of many scientists. The authors are particularly indebted to Dr. D.H. Matthews, Dr. D. Davies and Mrs. Charlotte Keen for their contribution to the success of the seismic work, and to Miss Carol Williams for her contribution to bathymetric and magnetic studies, all from the Department of Geodesy and Geophysics, Cambridge University. REFERENCES
Cann, J.R., 1970. Petrology of basalts dredged from the Gulf of Aden. Deep-Sea Res., in press. Drake, C.L. and Girdler, R.W., 1964. A geophysical study of the Red Sea. Geophys. J., 8: 473-495. Farquharson, W.I., 1935. Topography. Sci. Rept. John Murray Expedition, 1933-1934. 1: 43-61. Laughton, A.S., 1966a. The Gulf of Aden. Phil. Trans. Roy. Sot. London, Ser. A, 259: 150-171. Laughton, A.S., 196613. The Gulf of Aden, in relation to the Red Sea and the Afar depression of Ethiopia. In: The World Rift System. Geol. Surv. Canada, 66-14: 78-97. Le Pichon, X., Houtz, R.E., Drake, C.L. and Nafe, J.E., 1965. Crustal structure of the mid-ocean ridges, 1. Seismic refraction measurements. J. Geophys. Res., 70: 319-339. Matthews, D.H., 1966. The Owen fracture zone and the northern end of the Carlsberg Ridge. Phil. Trans. Roy. Sot. London, Ser. A, 259: 172-186. Matthews, D.H., Williams, C. and Laughton, A.S., 1967. Mid-ocean ridge in the mouth of the Gulf of Aden. Nature, 215: 1052-1053. Menard, H.W., 1960. The East Pacific Rise. Science, 132: 1737-1746. Nafe, J.E.. Hennion, J.F. and Peter, G., 1959. Geophysical measurements in the Gulf of Aden. Preprints Intern. Oceanog. Congr., New York, 1959: pp.42-43. Raitt, R.W., 1963. The crustal rocks. In: M.N. Hill (Editor), The Sea. Interscience, New York, N.Y., 3: 85-101. Roberts, D.G. and Whitmarsh, R.B., 1969. A bathymetric and magnetic survey of the Gulf of Tadjura, western Gulf of Aden. Earth Planetary Sci. Letters, 5: 253-258.
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