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Opening of the Red Sea with two poles of rotation — Some comments
Earth and Planetary Science Letters, 33 (1976) 169-172
169
(9 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [41
OPENING OF THE RED SEA WITH TWO POLES OF ROTATION - SOME COMMENTS
R.W. GIRDLER and P. STYLES School of Physics, The University, Newcastle upon Tyne, NE1 7R U (Great Britain)
Received May 19, 1976 Revised version received June 25, 1976 The proposal of Richardson and Harrison that the last phase of Red Sea spreading took place in an east-west direction, completely different from the earlier phase and about a pole of rotation at 15.2°S, 32.8°E (for Arabia-Nubia) is examined and found to be inconsistent with various observational data. It is shown that it is very difficult to ascertain two Red Sea poles of rotation with confidence as the difference in their positions is small.
1. Introduction
In a recent paper, Richardson and Harrison [1] follow the suggestion of Girdler and Styles [2] that there have been two stages of sea-floor spreading in the Red Sea. They then proceed to a discussion of the two stages claiming that they took place in remarkably different directions, i.e., there are two very different poles of rotation. Their two main conclusions are (1) the direction of motion of the "present day" phase is almost east-west and (2) the pole of rotation for this phase is at 15.2°S, 32.8°E implying that the Red Sea has been widening from south to north! In their discussion, Richardson and Harrison [1 } state that "Girdler and Darracott [3] have also suggested that the later opening may be about a pole different from that describing the early opening, but without going into details concerning what the difference might be. However, in a more recent paper, Girdler and Styles [2] make no mention o f different poles of rotation". The reason for this is that this very interesting problem is very difficult. We feel that so far, there are insufficient data to enable a good solution to be reached. However, there are sufficient data to show that the two main conclusions of Richardson and Harrison [1 ] are far from the truth. In recent years, much detailed exploration work has been carried out in the deep axial trough, especially by German and American scientists and it is easy to see from an overall look at the data and plate tectonic setting that
the conclusions o f Richardson and Harrison [1] are not supported by the observations. We comment on the conclusions in the reverse order, as the second requires a more general background discussion.
2. Opening of the Red Sea
Richardson and Harrison [1 ] claim that the last phase of opening of the Red Sea is a consequence of a clockwise rotation of Arabia relative to Nubia whereas previous works (reviewed in Girdler and Darracott [3]) suggest an anticlockwise rotation. Richardson and Harrison reach their conclusion by fitting the 500-fro (914 m) bathymetric contours. It is, of course, important to examine the bathymetry in a little more detail. Examination of a series o f profiles [4,5] shows that the Red Sea has a main trough and an axial trough both of which widen from north to south. The 300-fm (549 m) contout roughly encloses the main trough and the 700-fm (1280 m) contour the axial trough. South of about 16.5°N the situation is complicated by the presence of the Danakil plate causing the Red Sea to nearly close near the Straits of Babel-Mandeb. It is also complicated by the presence of evaporites and the build up of coral reefs, i.e., the Farisan Bank in the southeast (south of latitude 20°N) and the Dahlak Bank in the southwest (south of latitude 17°N). Even so, the increasing width from north
170 I
t o south of the axial trough (which is the most closely related t o the last phase o f spreading) is still clearly
seen from the profiles as far south as 16°N [4,5]. Next, the seismic reflexion work by scientists of the Woods Hole Oceanographic Institution [ 6 - 8 ] cannot be ignored. They found a strong reflector (designated reflector S). The reflector seems to be present everywhere except in the axial trough. It is typically 2 0 0 - 3 0 0 m beneath the sea floor. The reflector is known from Deep Sea Drilling Project Leg 23 to be due to evaporite of late Miocene age [9]. In the northernmost Red Sea, the reflector is continuous but gently folded. A little further south (between 23 and 27°N) narrow crevasses about 100 m deep appear, then still further south there is a gap in the reflector which widens towards the south with the axial trough. It seems reasonable to assume that S was b o t h horizontal and continuous in the late Miocene before the recent phase of spreading. The configuration o f S is then disturbed by the onset o f the recent phase o f spreading, the disturbance from north to south corresponding to an increase in spreading rate from north to south. This is also consistent with the nature o f the observed magnetic anomalies which are well known to increase in complexity from north to south [5] consistent with a north-south increase in spreading for the recent spreading phase. A consequence o f proposing a pole o f rotation at
32
32
~
34
~
,
36I
l
3832
DEAO SEA
I
! I
/
CONRAD
9
500[ nT. ~.
0
~
d : 6Km.
z,o Km.
d = 4Km.
0"5 cm/¥
E5
5
0
5 My
Fig. 1. A. Map showing the northern Red Sea and the AqabaArava-Dead Sea transform fault; the 300- and 700-fm contours from Laughton's bathymetric map of the Red Sea [24] are shown together with the deep water axes. The post-Miocene opening of the Red Sea deduced from the "Conrad" (C9) total intensity magnetic anomaly prof'fle is seen to be consistant with the amount of transform motion along the Dead Sea rift [12,13]. B. The total intensity magnetic anomaly profile (location shown in Fig. 1A) across the northern Red Sea approximately in the direction of spreading. The observed profile is compared with two synthetic profiles for depths to the magnetic layer of 4 and 6 km, respectively, with spreading rate of 0,5 cm/yr as required for the Dead Sea transform motion. The causative bodies are assumed to be striking N30°W with total magnetic intensity equivalent to 10 A/m except for the central body which is assumed to have intensity 20 A/m. The magnetic declination used is + l 3 ° and magnetic inclination is +38 ° . The'profile is seen to be very simple compared with the more complex prorfles further south illustrated in references 2, 4, 5, 18, 19, 20 and 21. This is mainly due to the slower spreading rate of about 0.5 cm/yr compared with about 0.9 cm/yr further south at 18°N.
~//~ 40-45 Km
~! / pOSt Miocene
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3o
!
~GULF OF
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26 38
15.2°S, 32.8°E for the last,phase o f opening is that normal faulting is predicted for the Dead Sea rift. Richardson and Harrison [1 ] note that this is inconsistent with the work of some middle eastern geologists but they dismiss their evidence for recent transform motion along the Dead Sea shear. We feel that the evidence o f Quennell [10], Freund [11 ], Freund et al. [12,13] and Zak and Freund [14] is particularly impressive. We leave the geologists concerned to answer these criticisms for it is sufficient to illustrate that
171 their work is consistent with the opening o f the northern Red Sea. Freund et al. [12] propose a post-Miocene left lateral shear along the Dead Sea rift o f 4 0 45 km. This estimate has to be increased a little for the northern Red Sea due to a small amount o f opening of the Gulf o f Suez. Allowing for this and assuming the Miocene/Pliocene boundary to be at 5 m.y., a half spreading rate of about 0 . 5 - 0 . 6 cm/yr can be expected in the northern Red Sea. This can be checked against the magnetic anomalies over the northern Red Sea. Fig. 1 shows a northeast-southwest profile of the R.V. "Conrad" (1965), together with two computed profiles for depths o f 4 and 6 km to the top o f the magnetized volcanic layer with spreading rate o f 0.5 cm/ yr. It is seen that there is reasonably good agreement with the observed and computed curves for the last few million years illustrating that the post-Miocene shear along the Dead Sea rift is consistent with the slower opening of the northernmost Red Sea. Many lines of evidence therefore indicate that the Red Sea widens from north to south for the last phase o f sea-floor spreading and not from south to north as proposed by Richardson and Harrison [1 ].
3. Direction of motion Examination o f the tracks o f research vessels shows that there has been considerable exploration o f the axial trough in contrast to the margins where the water shoals and navigation is difficult and sometimes hazardous. It should therefore be possible to check the conclusion o f west-east spreading for the axial trough. Several contour maps have been published (some o f them very detailed) o f b a t h y m e t r y , gravity and magnetic anomalies. All these show several northeast-southwest features. It is possible to measure the azimuths o f these features but because of the small width o f the axial trough these measurements are not very accurate. With these reservations, the azimuths o f various features have been measured. These include offsets o f bathymetric features from Ross et al. [15], Backer and Schoell [16], Backer et al. [17], magnetic features from Phillips et al. [18], Allan [19], Kabbani [20], Searle and Ross [21 ], free-air and Bouguer gravity from Allan [19], fault-plane solutions from Fairhead and Girdler [22]. In addition, the azimuths o f the
26
24
Girdler 8
\
~McKenzie
IRIchordson 8
Z
/
22 o .,.I
20
18
/
0
Fig. 2. The variation of azimuth with latitude of various features indicative of transform motion for the last phase of Red Sea spreading. The measurements have been averaged over 1° intervals of latitude and the means and standard errors are shown with the number of observations in parentheses. The curves show the azimuths to be expected along the axis of the Red Sea using the poles of rotation of Girdler and Darracott (1972) [3], McKenzie et al. (1970) [23] and Richardson and Harrison (1976) [I l- The McKenzie et al. (1970) pole is obtained from the fit of the coastlines. It is seen that it is difficult to demonstrate a significant difference between the direction of the last phas.e of spreading and the total movement, but the data are far from the curve for the pole of Richardson and Harrison (1976).
transforms shown on the interpretation map o f Backer et al. [17] have also been measured. In all 67 measurements were made. These were listed and averaged over 1° intervals of latitude. The means and standard errors are shown in Fig. 2. In addition, the theoretical curves for the azimuths to be expected along the axis o f the Red Sea from the poles o f rotation of Girdler and Darracott [3], McKenzie et al. [23] and Richardson and Harrison [1 ] are also shown. It is seen that the observations for the axial trough, although scattered, show that the direction o f motion can be represented by the poses of rotation o f Girdler and Darracott [3] and McKenzie et al. [23]. The observations are all far from the curve obtained using the Richardson and Harrison pole o f rotation.
172 4. Conclusions Careful studies of the bathymetry, seismic reflexion profiles, magnetic anomalies all indicate that north o f 16.5°N the Red Sea widened from north to south during the last phase of spreading. In addition, the estimates o f the amount o f post-Miocene shear along the Dead Sea rift are consistent with the interpretation o f magnetic anomalies over the northern Red Sea. It has been demonstrated that the direction of motion o f Arabia from Nubia for the last phase o f spreading is north easterly and not very different from that o f the early phase of spreading. This is not to say that there has not been some small change in the position of the pole o f rotation. Several features lead us to suspect this, for example, the axis o f the deep water in the northern Red Sea is not exactly parallel to the margins and the evidence from magnetic anomalies for the possible presence o f leaky transform faults further south. Unfortunately, the short distances involved give rise to large errors. However, it is possible to show beyond doubt that the proposals o f Richardson and Harrison [1 ] are incompatible with the observational data presently available.
7
8
9
10
11
12
13
14 15
16 Acknowledgements We are grateful to the Lamont-Doherty Geological Observatory for supplying us with the 1965 R.V. " C o n r a d " magnetometer profile and to the U.K. Natural Environment Research Council for financial support.
17
18
19
References 1 E.S. Richardson and C.G.A. Harrison, Opening of the Red Sea with two poles of rotation, Earth Planet. Sci. Lett. 30 (1976) 135-142. 2 R.W. Girdler and P. Styles, Two-stage Red Sea floor spreading, Nature 247 (1974) 7-11. 3 R.W. Girdler and B. Darracott, African Poles of rotation, Comments Earth Sci., Geophys. 2 (1972) 131 - 138. 4 R.W. Girdler, Geophysical studies of rift valleys, Phys. Chem. Earth 5 (1964) 121-156. 5 C.L. Drake and R.W. Girdler, A geophysical study of the Red Sea, Geophys. J. R. Astron. Soc. 8 (1964) 473-495. 6 S.T. Knott, E.T. Bunce and R.L. Chase, Red Sea seismic
20
21
22
23
24
reflection studies, in: The World Rift System, Geol. Surv. Can. Paper 66-14 (1966) 33-61. J.D. Phillips and D.A. Ross, Continuous seismic reflexion profiles in the Red Sea, Philos. Trans. R. Soc. Lond., Ser. A, 267 (1970) 143-152. D.A. Ross and J. Schlee, Shallow structure and geologic development of the southern Red Sea, Bull. Geol. Soc. Am. 84 (1973) 3827-3848. R.B. Whitmarsh, O.E. Weser and D.A. Ross, Initial Reports of the Deep Sea Drilling Project, 23 (U.S. Government Printing Office, Washington, D.C., 1974). A.M. Quennell, The structural and geomorphic evolution of the Dead Sea Rift, Q. J. Geol. Soc. Lond. 114 (1958) 1-24. R. Freund, A model of the structural development of Israel and adjacent areas since Upper Cretaceous times, Geol. Mag. 102 (1965) 189-205. R. Freund, I. Zak and Z. Garfunkel, Age and rate of the sinistral movement along the Dead Sea rift, Nature 220 (1968) 253-255. R. Freund, Z. Garfunkel, I. Zak, M. Goldberg, T. Weissbrod and B. Derin, The shear along the Dead Sea rift, Philos. Trans. R. Soc. Lond., Ser. A, 267 (1970) 107-130. I. Zak and R. Freund, Recent strike slip movements along the Dead Sea Rift, Israel J. Earth Sci. 15 (1966) 33-37. D.A. Ross, E.E. Hays and F.C. Allstrom, Bathymetry and continuous seismic profiles of the hot brine region of the Red Sea, in: Hot Brines and Recent Heavy Metal Deposits in the Red Sea, E.T. Degens and D.A. Ross, eds. (SpringerVerlag, New York, N.Y., 1969) 82-97. H. B~icker and M. SchoeU, New deeps with brines and metalliferous sediments in the Red Sea, Nature Phys. Sci. 240 (1972) 153-158. H. B~icker, K. Lange and H. Richter, Morphology of the Red Sea Central Graben (Valdivia Enzschl~rnme A & B, Preussag). J.D. Phillips, J. Woodside and C.O. Bowin, Magnetic and gravity anomalies in the central Red Sea, in: Hot Brines and Recent Heavy Metal Deposits in the Red Sea, E.T. Degens and D.A. Ross, eds. (Springer-Verlag, New York, N.Y., 1969) 98-113. T.D. Allan, Magnetic and gravity fields over the Red Sea, Philos. Trans. R. Soc. Lond., Ser. A, 267 (1970) 153-180. F.K. Kabbani, Geophysical and structural aspects of the central Red Sea valley, Philos. Trans. R. Soc. Lond., Set. A, 267 (1970) 89-97. R.C. Searle and D.A. Ross, A geophysical study of the Red Sea axial trough between 20.5 ° and 22°N, Geophys. J. R. Astron. Soc. 43 (1975) 555-572. J.D. Fairhead and R.W. Girdler, The seismicity of the Red Sea, Gulf of Aden and Afar triangle, Philos Trans. R. Soc. Lond., Ser. A, 267 (1970) 49-74. D.P. McKenzie, D. Davies and P. Molnar, Plate tectonics of the Red Sea and East Africa, Nature 226 (1970) 243-248. A.S. Laughton, A new bathymetric chart of the Red Sea, Philos. Trans. R. Soc. Lond., Ser. A, 267 (1970) 21-22.
169
(9 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [41
OPENING OF THE RED SEA WITH TWO POLES OF ROTATION - SOME COMMENTS
R.W. GIRDLER and P. STYLES School of Physics, The University, Newcastle upon Tyne, NE1 7R U (Great Britain)
Received May 19, 1976 Revised version received June 25, 1976 The proposal of Richardson and Harrison that the last phase of Red Sea spreading took place in an east-west direction, completely different from the earlier phase and about a pole of rotation at 15.2°S, 32.8°E (for Arabia-Nubia) is examined and found to be inconsistent with various observational data. It is shown that it is very difficult to ascertain two Red Sea poles of rotation with confidence as the difference in their positions is small.
1. Introduction
In a recent paper, Richardson and Harrison [1] follow the suggestion of Girdler and Styles [2] that there have been two stages of sea-floor spreading in the Red Sea. They then proceed to a discussion of the two stages claiming that they took place in remarkably different directions, i.e., there are two very different poles of rotation. Their two main conclusions are (1) the direction of motion of the "present day" phase is almost east-west and (2) the pole of rotation for this phase is at 15.2°S, 32.8°E implying that the Red Sea has been widening from south to north! In their discussion, Richardson and Harrison [1 } state that "Girdler and Darracott [3] have also suggested that the later opening may be about a pole different from that describing the early opening, but without going into details concerning what the difference might be. However, in a more recent paper, Girdler and Styles [2] make no mention o f different poles of rotation". The reason for this is that this very interesting problem is very difficult. We feel that so far, there are insufficient data to enable a good solution to be reached. However, there are sufficient data to show that the two main conclusions of Richardson and Harrison [1 ] are far from the truth. In recent years, much detailed exploration work has been carried out in the deep axial trough, especially by German and American scientists and it is easy to see from an overall look at the data and plate tectonic setting that
the conclusions o f Richardson and Harrison [1] are not supported by the observations. We comment on the conclusions in the reverse order, as the second requires a more general background discussion.
2. Opening of the Red Sea
Richardson and Harrison [1 ] claim that the last phase of opening of the Red Sea is a consequence of a clockwise rotation of Arabia relative to Nubia whereas previous works (reviewed in Girdler and Darracott [3]) suggest an anticlockwise rotation. Richardson and Harrison reach their conclusion by fitting the 500-fro (914 m) bathymetric contours. It is, of course, important to examine the bathymetry in a little more detail. Examination of a series o f profiles [4,5] shows that the Red Sea has a main trough and an axial trough both of which widen from north to south. The 300-fm (549 m) contout roughly encloses the main trough and the 700-fm (1280 m) contour the axial trough. South of about 16.5°N the situation is complicated by the presence of the Danakil plate causing the Red Sea to nearly close near the Straits of Babel-Mandeb. It is also complicated by the presence of evaporites and the build up of coral reefs, i.e., the Farisan Bank in the southeast (south of latitude 20°N) and the Dahlak Bank in the southwest (south of latitude 17°N). Even so, the increasing width from north
170 I
t o south of the axial trough (which is the most closely related t o the last phase o f spreading) is still clearly
seen from the profiles as far south as 16°N [4,5]. Next, the seismic reflexion work by scientists of the Woods Hole Oceanographic Institution [ 6 - 8 ] cannot be ignored. They found a strong reflector (designated reflector S). The reflector seems to be present everywhere except in the axial trough. It is typically 2 0 0 - 3 0 0 m beneath the sea floor. The reflector is known from Deep Sea Drilling Project Leg 23 to be due to evaporite of late Miocene age [9]. In the northernmost Red Sea, the reflector is continuous but gently folded. A little further south (between 23 and 27°N) narrow crevasses about 100 m deep appear, then still further south there is a gap in the reflector which widens towards the south with the axial trough. It seems reasonable to assume that S was b o t h horizontal and continuous in the late Miocene before the recent phase of spreading. The configuration o f S is then disturbed by the onset o f the recent phase o f spreading, the disturbance from north to south corresponding to an increase in spreading rate from north to south. This is also consistent with the nature o f the observed magnetic anomalies which are well known to increase in complexity from north to south [5] consistent with a north-south increase in spreading for the recent spreading phase. A consequence o f proposing a pole o f rotation at
32
32
~
34
~
,
36I
l
3832
DEAO SEA
I
! I
/
CONRAD
9
500[ nT. ~.
0
~
d : 6Km.
z,o Km.
d = 4Km.
0"5 cm/¥
E5
5
0
5 My
Fig. 1. A. Map showing the northern Red Sea and the AqabaArava-Dead Sea transform fault; the 300- and 700-fm contours from Laughton's bathymetric map of the Red Sea [24] are shown together with the deep water axes. The post-Miocene opening of the Red Sea deduced from the "Conrad" (C9) total intensity magnetic anomaly prof'fle is seen to be consistant with the amount of transform motion along the Dead Sea rift [12,13]. B. The total intensity magnetic anomaly profile (location shown in Fig. 1A) across the northern Red Sea approximately in the direction of spreading. The observed profile is compared with two synthetic profiles for depths to the magnetic layer of 4 and 6 km, respectively, with spreading rate of 0,5 cm/yr as required for the Dead Sea transform motion. The causative bodies are assumed to be striking N30°W with total magnetic intensity equivalent to 10 A/m except for the central body which is assumed to have intensity 20 A/m. The magnetic declination used is + l 3 ° and magnetic inclination is +38 ° . The'profile is seen to be very simple compared with the more complex prorfles further south illustrated in references 2, 4, 5, 18, 19, 20 and 21. This is mainly due to the slower spreading rate of about 0.5 cm/yr compared with about 0.9 cm/yr further south at 18°N.
~//~ 40-45 Km
~! / pOSt Miocene
3(
3o
!
~GULF OF
\\
27 (:L./ •
0t
26 ~"
.50 ~
32
"~
I00 i Z)
RED SEA
~ ~'~. 'j~
34
/
:~ "~'
36
26 38
15.2°S, 32.8°E for the last,phase o f opening is that normal faulting is predicted for the Dead Sea rift. Richardson and Harrison [1 ] note that this is inconsistent with the work of some middle eastern geologists but they dismiss their evidence for recent transform motion along the Dead Sea shear. We feel that the evidence o f Quennell [10], Freund [11 ], Freund et al. [12,13] and Zak and Freund [14] is particularly impressive. We leave the geologists concerned to answer these criticisms for it is sufficient to illustrate that
171 their work is consistent with the opening o f the northern Red Sea. Freund et al. [12] propose a post-Miocene left lateral shear along the Dead Sea rift o f 4 0 45 km. This estimate has to be increased a little for the northern Red Sea due to a small amount o f opening of the Gulf o f Suez. Allowing for this and assuming the Miocene/Pliocene boundary to be at 5 m.y., a half spreading rate of about 0 . 5 - 0 . 6 cm/yr can be expected in the northern Red Sea. This can be checked against the magnetic anomalies over the northern Red Sea. Fig. 1 shows a northeast-southwest profile of the R.V. "Conrad" (1965), together with two computed profiles for depths o f 4 and 6 km to the top o f the magnetized volcanic layer with spreading rate o f 0.5 cm/ yr. It is seen that there is reasonably good agreement with the observed and computed curves for the last few million years illustrating that the post-Miocene shear along the Dead Sea rift is consistent with the slower opening of the northernmost Red Sea. Many lines of evidence therefore indicate that the Red Sea widens from north to south for the last phase o f sea-floor spreading and not from south to north as proposed by Richardson and Harrison [1 ].
3. Direction of motion Examination o f the tracks o f research vessels shows that there has been considerable exploration o f the axial trough in contrast to the margins where the water shoals and navigation is difficult and sometimes hazardous. It should therefore be possible to check the conclusion o f west-east spreading for the axial trough. Several contour maps have been published (some o f them very detailed) o f b a t h y m e t r y , gravity and magnetic anomalies. All these show several northeast-southwest features. It is possible to measure the azimuths o f these features but because of the small width o f the axial trough these measurements are not very accurate. With these reservations, the azimuths o f various features have been measured. These include offsets o f bathymetric features from Ross et al. [15], Backer and Schoell [16], Backer et al. [17], magnetic features from Phillips et al. [18], Allan [19], Kabbani [20], Searle and Ross [21 ], free-air and Bouguer gravity from Allan [19], fault-plane solutions from Fairhead and Girdler [22]. In addition, the azimuths o f the
26
24
Girdler 8
\
~McKenzie
IRIchordson 8
Z
/
22 o .,.I
20
18
/
0
Fig. 2. The variation of azimuth with latitude of various features indicative of transform motion for the last phase of Red Sea spreading. The measurements have been averaged over 1° intervals of latitude and the means and standard errors are shown with the number of observations in parentheses. The curves show the azimuths to be expected along the axis of the Red Sea using the poles of rotation of Girdler and Darracott (1972) [3], McKenzie et al. (1970) [23] and Richardson and Harrison (1976) [I l- The McKenzie et al. (1970) pole is obtained from the fit of the coastlines. It is seen that it is difficult to demonstrate a significant difference between the direction of the last phas.e of spreading and the total movement, but the data are far from the curve for the pole of Richardson and Harrison (1976).
transforms shown on the interpretation map o f Backer et al. [17] have also been measured. In all 67 measurements were made. These were listed and averaged over 1° intervals of latitude. The means and standard errors are shown in Fig. 2. In addition, the theoretical curves for the azimuths to be expected along the axis o f the Red Sea from the poles o f rotation of Girdler and Darracott [3], McKenzie et al. [23] and Richardson and Harrison [1 ] are also shown. It is seen that the observations for the axial trough, although scattered, show that the direction o f motion can be represented by the poses of rotation o f Girdler and Darracott [3] and McKenzie et al. [23]. The observations are all far from the curve obtained using the Richardson and Harrison pole o f rotation.
172 4. Conclusions Careful studies of the bathymetry, seismic reflexion profiles, magnetic anomalies all indicate that north o f 16.5°N the Red Sea widened from north to south during the last phase of spreading. In addition, the estimates o f the amount o f post-Miocene shear along the Dead Sea rift are consistent with the interpretation o f magnetic anomalies over the northern Red Sea. It has been demonstrated that the direction of motion o f Arabia from Nubia for the last phase o f spreading is north easterly and not very different from that o f the early phase of spreading. This is not to say that there has not been some small change in the position of the pole o f rotation. Several features lead us to suspect this, for example, the axis o f the deep water in the northern Red Sea is not exactly parallel to the margins and the evidence from magnetic anomalies for the possible presence o f leaky transform faults further south. Unfortunately, the short distances involved give rise to large errors. However, it is possible to show beyond doubt that the proposals o f Richardson and Harrison [1 ] are incompatible with the observational data presently available.
7
8
9
10
11
12
13
14 15
16 Acknowledgements We are grateful to the Lamont-Doherty Geological Observatory for supplying us with the 1965 R.V. " C o n r a d " magnetometer profile and to the U.K. Natural Environment Research Council for financial support.
17
18
19
References 1 E.S. Richardson and C.G.A. Harrison, Opening of the Red Sea with two poles of rotation, Earth Planet. Sci. Lett. 30 (1976) 135-142. 2 R.W. Girdler and P. Styles, Two-stage Red Sea floor spreading, Nature 247 (1974) 7-11. 3 R.W. Girdler and B. Darracott, African Poles of rotation, Comments Earth Sci., Geophys. 2 (1972) 131 - 138. 4 R.W. Girdler, Geophysical studies of rift valleys, Phys. Chem. Earth 5 (1964) 121-156. 5 C.L. Drake and R.W. Girdler, A geophysical study of the Red Sea, Geophys. J. R. Astron. Soc. 8 (1964) 473-495. 6 S.T. Knott, E.T. Bunce and R.L. Chase, Red Sea seismic
20
21
22
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