Problems concerning the evolution of oceanic lithosphere in the northern Red Sea

Tectonophysics,

109

116 (1985) 109-122

Elsevier Science Publishers

PROBLEMS

B.V., Amsterdam

CONCERNING

IN THE NORTHERN

- Printed

in The Netherlands

THE EVOLUTION

OF OCEANIC LITHOSPHERE

RED SEA

R.W. GIRDLER School of Physics, The University, Newcastle upon Tyne ‘NE1 7RlJ (Unired Kingdom) (Received

by publisher

December

19, 1984)

ABSTRACT

Girdler,

R.W., 1985. Problems

In: G.F. Sharman The pole of rotation the fastest spreading spreading

concerning

and J. Francheteau

the evolution (Editors),

for the opening

lithosphere

in the northern

beneath

of the Red Sea at 36.S0N, 18.O”E (Arabia

the northernmost

over the last 4-5 Ma. Problems

Red Sea. The problem

from Africa)

is further

corresponds

Oligocene-early

to

Miocene)

shear (Plio-Pleistocene). The northern lineations.

centres,

first

(62

km)

and the third

These motions

Red Sea differs

Only a few isolated

(1) there is extremely volcanic

the

to the second

anomalies

continental

arise as to the nature

complicated

lithosphere

It is suggested

that the first explanation

anomalies tures

is present

to the two later (Aqaba-Dead may be due to a combination

and low sea floor spreading

cooling,

inhibiting

the

Aqaba-Dead

(45 km) movement

and southern are observed.

lithosphere

Sea

along

for the northernmost

shear

the

(?latest Sea

Red Sea are assessed.

Red Sea in having pierced

Oligocene),

the Aqaba-Dead

no clear magnetic

There are two possible

occasionally

of

by the fact that

explanations:

by strongly

magnetised

or,

(2) the oceanic explanation

along

and their implications

from the central

circular

thinned

movement

implies

studies show the

the Red Sea may have evolved in three phases. The first is the Gulf of Suez phase (mainly second

Red Sea.

Tectonophysics, 116: 109-122.

Lithosphere.

rates are in the south and are about 50% less in the north. Various

rate in the south to be about 1 cm/yr

the lithosphere

of oceanic

Oceanic

the acquisition

but without applies

magnetic

anomalies.

to the first (Gulf

Sea) phases.

It is further

of large thicknesses

of unstable

rates. These lead to the evolution of strong magnetisation

of Suez) phase

suggested

sediments of oceanic

and giving subdued

and the second

that the lack of magnetic (salt), high temperalithosphere

magnetic

with slow

anomalies.

INTRODUCTION

If the interpretation in the preceding paper that the oceanic lithosphere extends to the coast and beyond, is correct, it raises problems for the interpretation of geophysical data in the northern Red Sea. In particular, recent surveys have confirmed the absence of magnetic lineations north of about 24”N. Previously, the contrast in the magnetic anomaly patterns north and south of 24”N lead Girdler (1970) to propose that the northern Red Sea is totally underlain by stretched and

110

thinned

continental

lithosphere

with the extension

(Fig. 1). With more and more stretching broke

leading

magnetic

to the development

anomalies.

At about

of oceanic

The model has recently

the same time, McKenzie

Red Sea coastlines concept

towards

lithosphere been adopted

as noted

from north

finally large

( 1983).

by Cochran

the computer

by Alfred

that the excellent

to south

the lithosphere

with its associated

et al. (1970) studied

which is so impressive

of rigid plates they argued

increasing the south

fit of the

Wegener.

Using

fit of the coastlines

the

indicated

that the Red Sea should be floored by oceanic lithosphere. As more data became available in the 1970s support grew for the coast to coast fit. In particular, the release of aeromagnetic data over the southern Red Sea shelves where it is impossible to do ship surveys because of the reefs revealed the presence of most impressive magnetic lineations up to and beyond the coast on the western side (Girdler

0

Km

and Styles, 1974). Subsequent

compilation

of all the available

magnetic

data

E

50

It

II

I

11

11

11

0

200

1

“1’

”

Km Fig. 1. The formation

of the Red Sea by stretching and the evolution

and thinning

of oceanic

of the continental

lithosphere.

from north to south. In the north,

volcanic

to large local magnetic

In the south, the oceanic lithosphere

anomalies.

lineations

200

E;m

the break up of the continent

rise to magnetic

“1

(after Girdler,

centres occasionally 1970).

The amount

pierce the thinned

lithosphere

resulting

of extension lithosphere

forms by seafloor

in

increases giving rise

spreading

giving

111

(Hall et al., 1978) supported the shore to shore fit for all of the southern two thirds of the Red Sea, with reservations concerning the northern third. The interpretation of a long gravity profile (Brown and Girdler, 1982) at 20’N using borehole and seismic refraction profiles for control also support the coast to coast fit at this latitude. Now, the integrated interpretation of all the geological and geophysical data for the Gizan area suggests that a large part of the coastal plain in this region is most likely underlain by oceanic lithosphere (Girdler and Underwood, 1985, this volume). It is clearly necessary to re-examine the nature of the lithosphere beneath the northern Red Sea. Figure 2 shows the computer fit of the coasts by moving the Arabian coast into juxtaposition with the African coast. As can be seen, the fit is truely remarkable. The pole of rotation is at 36S”N, 18.O”E implying that the spreading rate increases from north to south. The spreading rate in the southern Red Sea is about 1 cm/yr over the last 4-5 Ma and the expected spreading rate in the northern Red Sea is thus about 0.5 cm/yr. It therefore seems worthwhile to investigate an alternative possibility that the northern Red Sea may be wholly or partly underlain by oceanic lithosphere even though magnetic lineations are absent.

Fig. 2. The computer fit of the Red Sea coasts (Africa and Arabia) gives the pole of rotation at 36.5’N, 18”E and a rotation angle of 6.1”. The Arabian coast is fitted on to the African coast. The fit is so good that it is difficult to distinguish the two coasts but note Arabia overlaps Sinai by about 40 km.

112

It is possible that the lack of magnetic lineations is in some way related to the slow spreading rate. To do this, it is necessary to consider the tectonic setting and history of the northern Red Sea including the Gulfs of Suez and Ayaba. Figure 2 shows that for the coast to coast fit, part of Arabia overlaps Sinai indicating that it is necessary to consider Sinai as a separate plate. THE GULFS OF SUEZ AND AQABA

Figure 3 shows a tracing of the northern Red Sea-Gulf of Suez-Gulf of Aqaba region from LANDSAT imagery. The Precambrian shield outcrops are shown shaded. It is seen that the Gulf of Suez is very much wider than the Gulf of Aqaba. It is generally considered that this is because the Gulf of Suez is a tensional graben structure while the Gulf of Aqaba is a shear zone. It is also seen that the western fault scarp of the Gulf of Suez is collinear and in continuity with the western fault

Fig. 3. Map of the Africa (Nubia), Sinai and Arabia plates bordering the Gulfs of Suez and Aqaba constructed from ERTS-LANDSAT imagery. The shaded regions represent the Precambrian. The 107 km of dispkement of the Precambrian is seen to affect the northern Red Sea.

113

scarp of the Red Sea. Hence, the Gulf of Suez and early Red Sea must have been formed at the same time.

Gulfof Suez The Gulf of Suez occupies about one third of an 80 km wide rift trough (Fig. 3) sometimes referred to as the “Clysmic rift”. In addition to the major normal faults forming the margins of the 80 km wide Clysmic rift, there are numerous fault blocks within the graben. The Precambrian is occasionally exposed (Fig. 3) and has been reached in several boreholes on the central eastern side and in the southwest. There are also several dykes and sills, the dykes being mostly parallel to the main trend of the graben. The rift movements began at the end of the Eocene (Robson, 1971) and a series of tilt blocks developed within the main graben. Activity was most pronounced in the Oligocene as indicated by the deep erosion of the tilted blocks. The faulting continued through the Oligocene leading to a major unconformity at the base of the Miocene. The igneous activity was also in the Oligocene; the dykes and sills intrude the Mesozoic and Eocene and terminate abruptly at the base of the Miocene. After the Oligocene, there is no evidence of igneous activity apart from the hot springs which occur along the lines of the major faults today. It is difficult to obtain good estimates for the amount of extension. Estimates range from 9 km (Robson, 1970) to 25 to 30 km (Freund, 1970). It is also difficult to ascertain whether there has been a significant component of shear. Riad (1977) suggests there are a number of shear zones striking in a no~hwest-southeast direction with right-lateral movement; they probably started developing in the Oligocene and are presently reactivated. Riad considers the Gulf of Suez to be mainly due to transform motion along these faults but this seems controversial (Robson, pers. cormnun., 1983). Gurf of Aqaba-Dead

Sea rift

In contrast to the Gulf of Suez, the Aqaba-Dead Sea rift is a major shear zone with the amount of shear known with considerable accuracy. It has been recognised as a major shear zone since the pioneering works of Lartet (1869) and Quennell (1958, 1959). Quennell estimates the total displacement as 107 km and considers this to have an accuracy of &l km (Quennell, pers. commun., 1982). Quennell (1958, 1959) describes this as a 6” rotation of Arabia with respect to Sinai about a “centre of rotation at appro~mately 33”N, 24°F’ which seems to be the first use of poles of rotation and rotation angles. Quennell(l958) also notes that the “angle of rotation is of the same amount (approximately 6”) as the departure from parallelism of the continental margins of Arabia and Africa which form the coasts of the Red Sea”. He then adds “the movement of Arabia in relation to Sinai-Palestine is believed to have

t 14

taken place intermittently during two principal phases between which there was a prolonged pause. During the first, the horizontal movement was 62 km and the rotation angle more than 3”. During the second the displacement was 45 km and the rotation angle more than 2i”” (Quennell, 1958). The last phase of movement (45 km) is considered to be in the Plio-Pleistocene and still continuing. The earlier phase (62 km} has been more difficult to date but is considered to be early Miocene and/or latest Oligocene (Quennell, pers. commun., 1982). The evidence for these Lnovements is most impressive. Quennell (1959) lists ten geological features which come into juxtaposition when Arabia is restored with respect to Sinai by the 107 km total movement. The evidence for the two stages of movement include an elegant argument using the sedimentary history of the Dead Sea. The Dead Sea has a very shallow southern part and a deep northern part (depths greater than 400 m). Quennell argues that the last 45 km movement produced the deep northern part which is little affected by modern sedimentation. The mouths of the very impressive Wadis Zarqa Ma’in, Mujib, fbin Hainmand and El Karak have deep water where large deltas are to be expected. The deltaic sediments are of course located up to 45 km to the south, offset from the Wadis by the latest movement northwards of the Arabian plate. The evidence suggests that the Gulf of Suez formed first and then the crack propagated at a different angle, to the NNE forming the Aqaba-Dead Sea shear zones, the shear movements taking place in two major stages. We now examine the consequences of these movements for the northern Red Sea. THE NORTHERN

RED SEA

The relevance of the 107 km total displacement along the Aqaba-Dead Sea shear zone to the Red Sea can be checked by examining the ERTS-LANDSAT imagery (Fig. 3). It is seen that the main Precambrian outcrop for western Sinai is several kilometres from the coast of the Gulf of Suez, but for Arabia it comes very close to the coast towards the south. The distance between the two measured along the Aqaba transform direction (N14”) is remarkably close to 107 km. Figure 3 also shows that if the Precambrian outcrops are brought into alignment by moving Arabia clockwise by 107 km with respect to Sinai there is still a large gap in the Red Sea of about 80 km measured at right angles to the coast of Africa. This is related to the opening of the Gulf of Suez. Clearly, the motions and their timings along the Suez and Aqaba rifts are very relevant to the tectonic evolution of the northern Red Sea. in Fig. 4, we reconstruct the Red Sea using the three stages as seen in the Gulfs of Suez and Aqaba. We call these GSZ (Oligocene) for the Gulf of Suez and GAQl (?latest Oligocene early Miocene) for the first (62 km) movement in the Gulf of Aqaba and GAQ2 (Plio-Pleistocene) for the second (45 km) movement in the Gulf of Aqaba. The r~nst~ction is done starting with the latest movement first

115

(GAQ2).

The mean

work of Quennell and for GAQl

radial

distance

to the Sinai-Arabia

is 1096 km. The rotation

pole of rotation

from the

angle for GAQ2 (45 km) is therefore

(62 km) 3.2’. The total rotation

corresponding

2.4”

to 107 km is 5.6”.

These successive rotations

are carried out in Fig. 4. It is seen that the coast of Arabia

aligns

the west coast

impressively

corresponding

with

points

on the African

of Sinai

and Arabian

(eastern

Gulf

of Suez).

coasts are now joined

If the

we obtain

the GSZ motion. The direction is seen to be slightly different, the difference being about 20” in the extreme northern Red Sea and becoming progressively less towards the south. This is because

of the closeness

and

difference

Arabia-Sinai.

The

of the poles of rotation

in the two pole

positions

for Arabia-Africa is only

about

6”,

implying that the circles of rotation between Africa and Arabia become closer with increasing radial distance. The effect of the difference is seen in the extreme north but gets less and less towards

the south.

The change in direction of movement is important Red Sea. This can be seen in Fig. 5 where an attempt day plate boundaries from considerations is seen that the present plate boundary

for the northern third of the is made to locate the present

of greatest water depths and seismicity. It in the northern part is not parallel to the

coasts but makes an angle of 10” to 15”, in agreement with the geometrical based on the geology of the Suez and Aqaba-Dead Sea rifts.

analyses

Fig. 4. The restoration of Arabia with respect to Sinai using the pole of rotation and two stages of movement of Quennell (1958, 1959). (Azimuthal great circle projection with distances and angles correct from the centre of rotation.)

Fig. 5. Sketch map of the northern Red Sea showing the possible location of the present day pk boundaries. The Red Sea plate boundary is obtained by joining the regions of deepest water.

Northern Red Sea: oceanic or continental

lithosphere?

The northern Red Sea differs from the central and southern Red Sea in that no clear magnetic lineations are observed. There are two possible explanations for this: (1) the oceanic lithosphere is absent; instead there is extremely thinned continental lithosphere (Fig. 1) which is occasionally pierced by strongly magnetised volcanic centres, or (2) the oceanic lithosphere is present without the source of the magnetic lineations. The former can satisfy other geophysical data such as the positive gravity anomalies but is hard to reconcile with the shore to shore extent of the magnetic lineations and the inferred extent of oceanic lithosphere to the south (Girdler and Underwood, 1985, this volume). It is also hard to reconcile with the 107 km shear

117

along the Aqaba-Dead Sea rift to the north. It is especially hard to envisage what happens at the corner of the Arabian plate where the Red Sea and Gulf of Aqaba join, Here, 107 km clean shear has to give way to extreme stretching and attenuation of the continental lithosphere which seems mechanically difficult. The second possibility seems more likely, at least for the last two phases GAQl and GAQZ. This raises the question, does the absence of magnetic anomalies necessarily imply the absence of oceanic lithosphere? This seems worth investigating in view of all the other compelling evidence and the expected slow spreading rate. It is suggested that the absence of magnetic lineations may be due to a combination of the presence of large thicknesses of salt and the slow spreading rate. If the flowage of the salt exceeds the slow spreading rate, the environment will be such that there will be no extrusive rocks but only intrusive rocks. This implies that the oceanic crust is intruded beneath the salt where the temperatures are high. The slow cooling may then inhibit the acquisition of strong magnetisation. It is the extrusive rocks which are strongly magnetized often due to quench cooling at the sea floor. Where the spreading rate begins to exceed the flowage of salt towards the south, the extrusive rocks are able to form giving rise to the large magnetic anomalies. This also explains why the small regions of large anomalies in the north are closely related to the bathymetric deeps, these being local regions not filled with salt. Salt is known to be present from boreholes and its thickness on the margins is often several kilometres (Girdler, 1970). Further, in 1972, the Deep Sea Drilling Project drilled in the central Red Sea and found late Miocene evaporites (older than 5 Ma) where the magnetic anomalies indicate an age of less than 2.5 Ma for the underlying oceanic crust! Girdler and Whitmarsh (1974) explained this by the flowage of salt on to the younger oceanic crust subsequent to the recommencement of seafloor spreading in the early Pliocene (Fig. 6). They cited as evidence for salt flowage, the disturbed nature of the sediments in the cores and the flow structures seen on seismic reflexion profiles. The flowage of salt is enhanced by the high heat flow in the Red Sea (Girdler and Evans, 1977) the strength of salt decreasing rapidly with increasing temperature. Heard (1972) showed that for geologically representative strain rates, steady state flow may exist at temperatures as low as 50°C. Temperatures much higher than this are known to exist in the Red Sea: indeed, temperatures in excess of 100°C are possible near the base of the salt, a few kilometres from the centre. It is suggested that the presence of large thicknesses of evaporites and high temperatures are contributory causes to the lack of large magnetic lineations in the northern Red Sea. The northern Red Sea opened in accord with the movements observed along the Gulf of Aqaba-Dead Sea transform but the movements caused the salt to be unstable and to flow faster than the evolution of oceanic lithosphere thus preventing the extrusion of strongly magnetised basaltic rocks, i.e. the process might lead to the formation of coarser grained less strongly magnetised gabbros rather than fine-grained pillow basalts.

Zone

salt

of

Km

Legend

:

(b)

20

0 I

I Km 2

‘.‘_‘.~.‘.’

t::::-_1:!

3

4 .-.-.-. .El-.-

Fig. 6. The effect of salt flowage represents

the middle

levels due to evaporation; forming

by seafloor

represents I = oceanic

(b) represents

spreading.

the Quatemary crust:

in relation

and late Miocene

2 = sandstones;

situation

become unstable

with widening 3 = evaporites;

formation

with the deposition

the Pliocene

The evaporites

situation

to ocean lithosphere

situation

Red Sea. (a)

and fluctuating

sea

with the last stage of ocean lithosphere and begin to flow towards

of the axial trough 4 = nanno

in the central

of evaporites

oozes.

by continuing (After

Girdler

the centre;

seafloor and

(c)

spreading. Whitmarsh,

1974.)

The explanation assumes that the bulk of the magnetisation of the oceanic lithosphere is in the basaltic Layer 2 as envisaged in the original Vine-Matthews hypothesis. Recent studies of the magnetic properties of submarine rocks including those from the deep sea drilling project (Dunlop and Prevot, 1982; Day, 1983; Day et al., 1983) as well as studies of the magnetic properties of ophiolite complexes (Luyendyk et al., 1982; Luyendyk and Day, 1982) have all tended to confirm this. For example, Bleil and Petersen (1983) demonstrate that the variations in intensities of magnetization of ocean floor basalts of ages from 0 to 150 Ma correlates well with the variations in amplitudes of marine maenetic anomalies. Their two diagrams are

119

1o"TJ

0.80.60.4z $ 0.2z 7 1o-3: 0.8 1 0.60.4 r

a

I

I

I

0

20

40

1

r

I.

60

*

80 Age

I

’

1

100

-

120

II

1

140

160

(MO)

;200 C ; 100 ?! %

0

E -100 r[d15 9 IllI

-200

b

1,500

2,500

2.w Distance

15 I

4g5”p t

5761 A!&Ma) 1 1 3PQO

(km)

Fig. 7. Comparison of natural remanent intensities for ocean basalts with the amplitudes of seafloor spreading anomalies. a. Geometric mean values (*) and standard deviations for intensities of natural remanent magnetizations of DSDP ocean-floor basalts for sites from the Atlantic, Indian, Antarctic and Pacific Oceans. b. Magnetic profile across the North Atlantic between Puerto Rico and Canary Islands (DSDP Leg 46). (After Bleil and Petersen, 1983.)

shown together in Figs. 7(a) and (b) for comparison. They conclude from this correlation that the predominant source of the magnetic anomalies is confined to the uppermost portion of the oceanic crust (i.e. Layer 2A). Clearly if Layer 2A is absent in the northern Red Sea due to the blanket of salt, the large magnetic lineations will also be absent. E~~~uti~n,structure and nature of oceanic I~th~spherein the northern Red Sea We have seen that studies of the Suez and Aqaba-Dead Sea rifts suggest the northern Red Sea evolved in three major phases, i.e. the Gulf of Suez phase in the

120

late Eocene-Oligocene, the Aqaba--Dead Sea phase 1 in the early Miocene and the Aqaba-Dead Sea phase 2 in the Plio-Pleistocene. Girdler et al. (1980) used these as boundary

conditions

for the interpretation

Gulf of Aden where the spreading They obtained

dates of 43-35.5

and nature

anomalies

111the western much clearer.

Ma, 23.5 -16 Ma and 4.5-O Ma for the three phases

using the La Breque et al. (1977) magnetic structure

of magnetic

rates are faster and the anomalies

of the oceanic

polarity

lithosphere

time-scale.

We now look at the

for the three phases in the northern

Red Sea. (I) Gulf of Suez-Proto-Red Seu phase (?43--34 Mu) The continuity of the western scarp from the Gulf of Suez through Egypt, Sudan and Ethiopia to the southern Red Sea is most impressive making it very likely there was a proto-Red Sea forming coincident with the Gulf of Suez. Downfaulted continental lithosphere margins of the northern

is known to be present in the Gulf of Suez and on the Red Sea (geological map of Coleman, 1974) so it is likely

there is some stretched and thinned continental lithosphere near the coasts. Because of this, it is envisaged that the Gulf of Suez-proto Red Sea was very much like the Kenya-Ethiopia rift today, i.e. a continental lithosphere along its axis (Searle, 1970) leading

rift with extreme thinning to the evolution of volcanic

of the centres

such as Suswa, Longonot and Menengai. The main difference is in age, the Gulf of Suez volcanism being Oligocene whereas the Kenya volcanic centres are recent and active today. The Suez-proto-Red Sea was covered by the incursion of the Mediterranean Sea leading to the deposition of marine sediments. The continental lithosphere finally broke along the line of volcanic centres and new oceanic lithosphere began

to form.

(2) Aqaba-Dead Sea phase I (?24-16 Mu) It seems generally agreed that there was a major stage of evolution

of the Red Sea

in the early Miocene possibly commencing in the latest Oligocene. This was accompanied by a change in direction of relative movement of the plates and the propogation of the new Aqaba-Dead Sea rift resulting in the formation of the Sinai microplate. becoming evolution most part Red Sea hence it evaporite

The major movement

transferred

to the Gulf of Aqaba,

the Gulf of Suez

relatively inactive. The 62 km shear was accompanied by the opening and of oceanic lithosphere in the Red Sea, the spreading rate in the northernbeing of the order of 0.5 cm/yr and the direction as shown in Fig. 5. The was closed in the south (due to blocking by the Danakil microplate) and became an evaporite basin throughout the Miocene, vast thicknesses of being deposited.

(3) Gulf of Aqaba-Dead Sea phase 2 (4.5 Ma-O Ma) After a quiet period throughout the middle and late Miocene, there was major reactivation of the plate movements in the early Pliocene. The sea broke through the

121

Straits

of Bab-el-Mandeb,

deposition

the

of nanno-oozes.

deposition

A further

of evaporites

being

45 km shear occurred

Sea rift accompanied

by further

opening

and

the centre

of the Red Sea. In the north

flowed

spreading

towards rate

lithosphere

is slow, no axial

has remained

of the Red Sea. The salt became

deep

blanketed

replaced

trough

resulting

by

the

along the Aqaba-Dead

has formed

where

and

unstable

the seafloor

the new oceanic

in a lack of extrusive

rocks to cause

large magnetic anomalies. Hence it is likely that a considerable amount of oceanic lithosphere is present but constituted mainly of coarse grained gabbroic intrusive which cooled slowly preventing acquisition of strong magnetization. In conclusion, stages

it is seen that the nature

of the break

of oceanic lithosphere

up of the continents

is related

forming

in the early

to the spreading

rate

and

sedimentary environment. In the Red Sea, as the spreading rate increases towards the south, true oceanic lithosphere forms giving rise to the accustomed magnetic anomaly pattern but in the north where the spreading rate is slow, the ocean lithosphere is blanketed and the magnetic anomalies are subdued due to the high heat flow and slow cooling of the intruding rocks. ACKNOWLEDGEMENTS

I am greatly indebted to D.A. Robson for helpful discussions of Suez and to A.M. Quennell concerning the Aqaba-Dead

concerning the Gulf Sea rift. I am also

indebted to Herbert Blodget of NASA GSFC for providing the LANDSAT imagery and John Young of the Procurement Executive, MOD, Blacknest for help with the map projections. REFERENCES

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