Subsidence and origin of the Northern Red Sea and Gulf of Suez

0899-5362/89 $3.00 + 0.00 (~) 1989 Pergamon Press pie

Journal of African Earth Sciences, Vol. 8, Nos. 2/3/4, pp. 617-629, 1989 Printed in Great Britain

Subsidence and origin of the Northern Red Sea and Gulf of Suez JOHN J. W. ROGERS, MOHAMED E. DABBAGH, BRIAN M. WHITING, and SALLY A. WIDMAN Departmentof Geology3315 University of North Carolina Chapel Hill, North Carolina27599-3315, USA Abstract - Sedimentary accumulation and subsidence rates in the northern Red Sea and Gulf of Suez can be explainedby extremeseparationin central areas of rift basins and onlyminorthinningand tiltingof faultblocks around the margins.Duringthe Oligocene,localreliefpemaittedred clasticsedimentsto be shedfromthepresent area of the Red Sea. Veryhighrates of crustal thinningin LateOligoceneor earliestMiocenein the centralpart of the Gulfof Suez andpart of the Red Sea troughpermittedrapid subsidenceof an earlyMioceneerosionsurface. Miocene andyoungersedimentsare thickonlyin thesecentralareas. Movement on the DeadSea shearzonebegan at the sametimeas subsidence,effectivelyisolatingthe Gulfof Suezfromfurthertectonismand allowingthermal subsidence to occur to the present. Extensionin the northernRed Sea, southof the shear zone, has eitherbeen so extremeas to leavea crustof less than 10 km thicknessor has beenaccomplishedby injectionsof largequantities of mariemagma, forminga crust intermediatebetweencontinentaland oceanic.All observationsare consistent with models of rifting by pure stretchingor by detachmentfaulting. INTRODUCTION

The Red Sea a r e a is a classic example of recent d e v e l o p m e n t of a n o c e a n by rifting of a continent. This p a p e r s u p p l e m e n t s n u m e r o u s o t h e r s t u d i e s b y providing n e w i n f o r m a t i o n on s u b s i d e n c e r a t e s of u n l o a d e d b a s e m e n t in the Gulf of Suez p l u s o t h e r i n f o r m a t i o n from the n o r t h e r n Red Sea a n d n o r t h e m coastline of S a u d i Arabia. Generalized m a p s of t h e Red Sea (Fig. 1) a n d t h e n o r t h e r n part of t h e Red Sea a n d Gulf of Suez (Figs. 2 a n d 3) s h o w t h e a r e a s of investigation.

GEOLOGIC

SETTING

Red Sea

The Red Sea is t h e n o r t h w e s t e r n a r m of a riftrift-rift triple j u n c t i o n located in t h e Afar a r e a (Fig. 1; A l m o n d 1986). Initial s e p a r a t i o n of the A r a b i a n a n d African plates b e g a n a b o u t 25 to 30 m.y. ago. Since t h a t time, the A r a b i a n plate h a s r o t a t e d relatively t o w a r d t h e n o r t h e a s t a n d h a s also slid a p p r o x i m a t e l y 100 k m n o r t h w a r d along t h e D e a d Sea (Dead S e a - G u l f of Aqaba) s h e a r zone (Fig. i). The p r e s e n t geology of the Red Sea u n d e r g o e s a c h a n g e at a b o u t l a t i t u d e s 23 ° to 25 ° N. S o u t h of t h a t area, the b a s i n c o n t a i n s a n axial t r o u g h showing s y m m e t r i c m a g n e t i c stripes a n d floored b y typical MORB (review b y C o c h r a n 1983). The b a s e of t h e t r o u g h is a b o u t 2,000 m below sea level, c h a r a c t e r i s t i c of t h e h e i g h t of y o u n g oceanic ridges (Sclater et 02. 1980), a n d t h e area a p p e a r s as a

t r o u g h only b e c a u s e the s u r r o u n d i n g shelves are at a h i g h e r elevation t h a n the c r u s t of typical oceans. The axial t r o u g h formed b y sea floor spreading w i t h i n t h e p a s t 10 m.y. (Izzeldin 1987). The m a r g i n s of the s o u t h e r n a n d central Red Sea are controversial. T h e y are relatively shallow shelves, covered b y t h i c k s e q u e n c e s of Miocene a n d y o u n g e r s e d i m e n t s , a n d have seismic velocities c o n s i s t e n t with identification of t h e shelves as c o n t i n e n t a l c r u s t (review b y C o c h r a n 1983). Symmetric m a g n e t i c a n o m a l i e s characteristic of sea floor s p r e a d i n g are not p r e s e n t in the m a r g i n s , b u t long-wavelength m a g n e t i c l i n e a m e n t s o c c u r at v a r i o u s locations (review b y Gettings et aL 1986). Izzeldin (1987) proposed t h a t m a g n e t i c a n o m a l i e s in the m a r g i n s could be c a u s e d partly by lava flows covering s t e p - f a u l t e d c o n t i n e n t a l crust. The c o a s t a l plains are s o m e w h a t different on opposite sides of the Red Sea. Along the EgyptianS u d a n e s e coast, the plains are n a r r o w a n d are b o r d e r e d w e s t w a r d by t h e Red Sea Hills, uplifted P r e c a m b r i a n b a s e m e n t with elevations m o s t l y less t h a n 1,000 m. Along t h e A r a b i a n coast, the coastal p l a i n is g e n e r a l l y w i d e r a n d c o n s i s t s of P r e c a m b r i a n rock with local Cenozoic basins. The Arabian-Yemen coastal plain is bordered on the east by a n a b r u p t scarp u p to 3,000 m high, decreasing in elevation toward the north. Theories of the evolution of t h e s o u t h e r n a n d central part of t h e Red Sea (south of 23 ° to 25 ° N) generally fall into two e n d - m e m b e r categories ( B o h a n n o n 1986a). One t h e o r y is t h a t the shelves a n d coastal plains r e p r e s e n t t h i n n e d c o n t i n e n t a l

617

618

JOHN J. W. ROGERS et al.

Dead Sea shear zone

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Fig. I Index m a p of Red Sea area. The d a s h e d line labeled ,scarp, is the western edge of the Arabian plateau. The axial trough is an a r e a of symmetric magnetic anomalies a n d development of MORB.

Subsidence and origin of the northern Red Sea and Gulf of Suez c r u s t formed during stretching and rifting (e.g., Cochran 1983). This concept permits sea floor spreading (generation of new oceanic crust) only in the axial trough and explains as m u c h as 75% of the separation of Africa and Arabia by stretching (ductile deformation, listric faulting, etc.) and by dike injection. The other theory interprets seismic data in s o u t h e r n Saudi Arabia to show abrupt thinning of the c r u s t from continental thickness in the m a i n l a n d to oceanic thickness and seismic velocities west of a line about midway between the scarp a n d the Red Sea coastline (e.g., Mooney 1984; Mooney et a/. 1985; B o h a n n o n 1986b; Gettings et al. 1986). This interpretation indicates that the entire Red Sea a n d even some parts of the coastal plains are the result of sea floor spreading; the line of separation between continental and oceanic crust coincides with a zone of massive development of mafic intrusive rocks. Oceanic crust h a s not been proven to exist in the Red Sea north of latitude 25 ° N. The axial trough to the south disappears northward, its extension seen only in irregular sea floor topography, including several enclosed deeps (Mart and Hall 1984; Cochran et al. 1986; Uchupi and Ross 1986). Small-scale seismic activity shows that this extension is a plate b o u n d a r y (Daggett et al. 1986). The n o r t h e r n part of the ocean does not contain symmetric magnetic stripes, a fact that supports the widespread a s s u m p t i o n that it consists of thinned continental crust. Girdler (1985), however, proposed t h a t the c r u s t is oceanic but that sea-floor spreading did not cause the formation of magnetic stripes because the basaltic m a g m a s were emplaced beneath thick salt deposits. This mode of emplacement c a u s e d slow cooling and inhibited magnetization. Seismic velocities in the b a s e m e n t underlying the Red Sea sediments have been regarded as characterisUc of continental crust (Drake and Girdler 1964: Mart and Hall 1984), but these velocities are also consistent with crust thoroughly invaded by mafic m a g m a s (e.g., Bonatti and Seyler 1987, discussed below). Some of the conflict between interpretations of the crust in the Red Sea margins and n o r t h e r n Red Sea m a y result from the fact that the crust is intermediate between continental and oceanic. A recent s t u d y by Bonatti and Seyler (1987) showed that two islands outside of the spreading center consist of intrusive mafic/ultramafic complexes emplaced in typical continental gneisses. Crustal extension by injection of mafic m a g m a s would permit subsidence of the heavy crust and separation of the sides of the trough without the extreme crustal thinning required ffthe Red Sea had opened solely by crustal extension, a point discussed later. The Red Sea h a s commonly b e e n considered to

619

have formed by some type of pure s h e a r mechanism involving symmetric ductile stretching of the lower crust and sub-crustal lithosphere combined with brittle stretching (e.g., listric faulting) of the u p p e r crust. The apparent a s y m m e t r y of the Red Sea margins, however, plus recent proposals that continental rifting is controlled largely by lowangle d e t a c h m e n t faults, have led some workers to propose a similar origin for the whole Red Sea trough (e.g., papers in this volume by B o h a n n o n and by Voggenreiter and Hotzl). The proposed d e t a c h m e n t dips eastward. This orientation causes: I) the Sudan-Egypt margin to consist largely of lower crust overlain by thin fault slivers of upper crust; and 2) the Arabian margin to consist largely of u p p e r crust that h a s undergone block faulting above the d e t a c h m e n t zone. Structures exposed at the surface do not permit easy distinction between upper crustal and lower crustal margins (Lister et a/. 1986). Gulf of Sue=

The Gulf of Suez extends northwestward from the m a i n body of the Red Sea (Figs. 2 and 3). Its s o u t h e r n termination is a n extension of the Dead Sea s h e a r zone along a scarp raising the floor of the Gulf about 800 m above the floor of the Red Sea. The Dead sea s h e a r zone extends along the Gulf of Aqaba (Fig. 1), which m a y be a series of pull-apart

X

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MIOCENE AND PLIOCENE

I [

100 KM

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- - • SUEZ BOUNDARY FAULTS

Fig. 2 Index map of northern Red Sea, including Gulf of Suez. The Suez boundary faults are the farthest faults from the Gulf, but significant crustal extension may not have occurred outside of the water-filled central trough of the Gulf.

620

JOHN J. W. ROGERS et al.

RED SEA HILLS

SINAI

L LEVEL

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100

KM

Fig. 4 Generalized cross section of Gulf of Suez. Thls section represents the southern part of the Gulf, where the Sinai peninsula is most uplifted. The section shows sedimentary rocks in down-dropped fault blocks between Precambrian massffs. A possible eastward-dipping, low-angle detachment beneath the Gulf is not easily recognized at the surface and is not shown here.

NEOGENE (RIFT) SEDIMENTARY ROCKS

PALEOGENE AND OLDER SEDIMENTARY ROCKS

PRECAMBRIAN CRYSTALLINE ROCKS

, TRANSFER FAULT

I

100 KM

j

Fig. 3 Index map of Gulf of Suez. Boundary faults of the downdropped area are essentially the gulf-ward edges of major outcrops of Precambrian crystalline rocks. Location of major transfer faults is from Chorowicz (this volume). D is Abu Durba block; M is Esh El Melaha range. Locations of studied wells are shown by numbers: 1, Rahmi-1; 2, Bakr-7; 3, El Ayun-1; 4, Shagar-2; 5, Gebel El Zeit-1; 6, Ras El Bahar-1; 7, Belayim Marine and Land fields. b a s i n s along the fault (Ben A v r a h a m e t al. 1979; B e n A v r a h a m 1985) or a n embryonic rift (Mart a n d Rabinowitz 1986). Left-lateral m o v e m e n t h a s b e e n proposed along the s h e a r zone beginning a b o u t 18 m. y. ago a n d c o n t i n u i n g to the present, with a total d i s p l a c e m e n t of a b o u t 60 kin b y the end of the Miocene a n d 105 k m now (Quennell 1984). The intersection of t h e scarp at t h e s o u t h e r n e n d of the Gulf of Suez with the n o r t h e m e x t e n s i o n of the Red Sea axial t r o u g h is seismically active a n d m a y be a triple j u n c t i o n (Daggett e t al. 1986).

Fig. 4 s h o w s t h e G u l f of S u e z as a n axial g r a b e n s u r r o u n d e d b y c o a s t a l plains. To t h e east, in Sinai, the c o a s t a l plain c o n s i s t s of fault b l o c k s containing Middle a n d Late Cenozoic s e d i m e n t s on eroded pre-Tertiary b a s e m e n t ; t h e p l a i n s are b o r d e r e d on the e a s t by P r e c a m b r i a n r o c k s in a scarp t h a t is very a b r u p t in the s o u t h a n d d i m i n i s h e s to t h e north. The p l a i n s west of t h e axial t r o u g h of the Gulf of S u e z also c o n t a i n f a u l t blocks of Middle a n d Late Cenozoic s e d i m e n t s ; to t h e west, t h e s e p a r a t i o n of t h e p l a i n s from t h e pre-Miocene b a s e m e n t rocks is not as topographically a b r u p t as in s o u t h e m Sinai. Some of t h e l e s s - s u b s i d e d fault blocks in the c o a s t a l p l a i n s expose P r e c a m b r i a n rocks. The w i d t h of t h e Gulf b a s i n b e t w e e n b o u n d i n g s c a r p s c o n t a i n i n g c o n t i n u o u s o u t c r o p s of Pre c a m b r i a n rocks is a b o u t 80 k m (Fig. 3). The width, however, m i g h t be m o r e a c c u r a t e l y m e a s u r e d b e t w e e n a b o u n d i n g s c a r p on one side of t h e Gulf a n d P r e c a m b r i a n rocks in m a j o r u p w a r d - t i l t e d fault blocks on the opposite side of the Gulf. Using t h i s procedure: 1) a w i d t h of 60 k m is m e a s u r e d in the s o u t h e r n p a r t of t h e G u l f b e t w e e n t h e Sinai m a s s f f a n d a P r e c a m b r i a n fault slice (Esh El Melaha r a n g e ) in the E g y p t i a n c o a s t a l plain; a n d 2) a w i d t h of 50 k m is m e a s u r e d in the c e n t r a l p a r t of t h e Gulf b e t w e e n the Red Sea hills a n d a P r e c a m b r i a n block (Abu Durba) in Sinai. The value of 60 kin is u s e d in c a l c u l a t i o n s in t h i s paper. The c o a s t a l p l a i n s of t h e Gulf of S u e z c o n t a i n m i n o r o u t c r o p s of red, clastic, terrestrial Oligocene

Subsidence and origin of the northern Red Sea and Gulf of Suez sedimentary rocks (Robson 1971). The oldest marine strata in the Gulf are Lower Miocene, mostly limestones and fine-grained clastic rocks with local b a s a l conglomerates (SeUwood and Netherwood 1984}. They lle on an erosional surface that t r u n c a t e s Precambrian and Phanerozoic rocks and apparently h a d a low relief b u t irregular topography. Clearly, some parts of the Gull area were at sea level in the Early Miocene and have undergone a net subsidence since then. At least three e n d - m e m b e r models have b e e n proposed for the evolution of the Gulf of Suez. The s t a n d a r d stretching model regards the Gull as the down-dropped top ofa domal uplift, with the inner, ocean-filled axis as the m a i n central graben and the surrounding coastal areas occupied b y lesswell-developed fault blocks. Two principal lines of evidence s u p p o r t this model: 1) reconstruction of an unfaulted b a s e m e n t below the Miocene deposits b y Steckler (i 985) shows a typical domal uplift over a thermal anomaly, although Steckler did not propose early doming; and 2) the pre-Miocene unconformity indicates erosion, although low Oligocene sea levels m a y have b e e n responsible rather t h a n uplift. The second principal model for the inception of the Gulfiscrustal strectchingbypure shear without uplift, following the m e c h a n i s m proposed for rifting b y McKenzie (1978) and Jarvis and McKenzie (1980). Evidence for this model includes the scarcity of conglomeratic or other coarse clastic sedimentary rocks at the b a s e of the Miocene, as would be expected if surrounding rift flanks had b e e n high (Sellwood and Netherwood 1984; Scott and Govean 1985). By this model, the present elevation of the rift flanks would have been c a u s e d by elastic r e b o u n d (Hellinger and Sclater 1983), secondary convection cells (Steclder 1985; B u c k 1986), e n h a n c e d lateral heat flow (Jarvis 1984), or some other mechanism. The third model proposes crustal thinning in the Gulf b y separation along an eastward-dipping, low-angle d e t a c h m e n t zone (Perry and Schamel 1984). This model is virtually identical to the detachment model for the entire Red Sea d i s c u s s e d above. According to this proposal, fault blocks in the Egyptian coastal plain are formed b y synthetic listric faults, and the high-angle faults in Sinai are antithetic. Stretching across a detachm e n t could c a u s e the s a m e type of lithospheric thinning u n d e r the Gulf that is proposed b y the other two models.

I N V E S T I G A T I O N S IN GULF O F SUEZ

Subsidence analyses to determine the rate of downward movement of unloaded b a s e m e n t have AES 8 - 2 / ~ F F

621

b e e n performed using the formulations of Heller et aL (1982) on various weUs in the Gulf of Suez area (Figs. 2, 3, and 5; stratigraphic information summarized byWhiting 1984, Widman 1985). All wells bottom near the b a s e of the Miocene section, above or near the pre-Miocene unconformity that lies primarily on faulted Cretaceous and Eocene rocks. The Miocene section generally consists of limestones and shales n e a r the b a s e and increasing a m o u n t s of anhydrite upward, with massive an hydrite in the Upper Middle and Upper Miocene. Plio-Pleistocene strata mainly include coarse clastic rocks, indicating uplift of rift flanks. The lowest part of the Miocene Section (Gharandal Group) probably h a s an age of a b o u t 18 m.y. (summary by Widman 1985), identical to the age of inception of movement on the Dead Sea shear zone (Steckler and ten Brink 1986). Table 1 shows estimates of subsidence rates of unloaded b a s e m e n t u n d e r various wells in the western coastal plain (Widman 1985) and in the Belayim Marine and Land Fields (Whiting 1984). The Belayim Marine wells (Fig. 6) are in the main trough of the Gulf, whereas the other wells are in the less-subsided margins. The a m o u n t s of accumulation in the different areas are highly variable, and b e c a u s e all sections represent the same period of time (starting a b o u t 18 m.y. ago), the subsidence rates are also highlyvariable. The subsidence rates of unloaded basement, however, do not correspond well with stratigraphic thicknesses b e c a u s e of the different densities of the rock types involved. In particular, the high density of anhydrite permits extensive accumulation of evaporites with very minimal tectonic subsidence of unloaded basement. The subsidence and stratigraphic data are comparable to those obtained in the Gulf area by Moretti and Colletta (1987) and lead to several conclusions. One is that all parts of the Gulf of Suez and surrounding coastal plains appear to have reached sea level at a b o u t the same time during their subsidence following Oligocene erosion. The basal strata are roughly the same age in all wells regardless of their location, including the Belayim Marine area in the central trough and both thick and thin s e q u e n c e s on the coastal plains. Furthermore, most basal sedimentary rocks are not coarse clastics derived from flanking highlands. T h u s the topography of the present Gulf region m u s t have b e e n essentially fiat near the beginning of Miocene time, a b o u t 18 to 20 m.y. ago. A second conclusion is that subsidence in the central trough of the Gulf of Suez h a s apparently been a simple and c o n t i n u o u s process. A plot of averaged subsidence curves for the Belayim Marine wells (Fig. 6) shows that elevations of unloaded b a s e m e n t are proportional to the square root of

622

JOHN J. W. ROGERS et al.

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Subsidence and origin of the northern Red Sea and Gulf of Suez

623

Table I. Subsidence rates in Gulf of Suez TIME IN M.Y. BEFORE PRESENT (rates in m/m.y.)

LOCATION 17--15

15-~i0

I0~0

Western Coast Rahni I Bakr 7

240

55

55

i0

El Ayun I

i00

25

Shaga r 2

ii0

5

E1 Zeit I

320

30

Ras el Bahar

300

15

112-12

125

40

30

113-45

80

20

40

Central Gulf (Belayim Marine)

I00

60

30

Ocean Ridge

250

80

55

Continenta 1 Crust Thinned to 10-15 km

i00

40

20

Eastern Coast (Belayim Land)

Subsidence rates are shown for specific wells in the eastern and western coastal areas and as an average for wells in the Belayim Marine field. Rates for ocean ridge and thin continental crust are shown for comparison (based on Sclater and Christie 1980).

time, a relationship characteristic of onedimensional heat loss and resultant contraction. In this situation, the absolute rate of subsidence is proportional to the a m o u n t of thinning that the crust h a s undergone prior to subsidence because t h i n n e r c r u s t s cool and subside faster (McKenzie 1978). Using the curves of McKenzie (1978) and Sclater a n d Christie (1980), assuming t h a t subsidence started 18 m.y ago on a continental c r u s t thinned from a n original thickness of about 30 km, the present c r u s t u n d e r the central trough of the Gulf would have thickness of about 10 to 15 k m (stretching factor of 2 to 3). This estimate of stretching is greater t h a n that of Chenet and Letouzey (1983), who a s s u m e d subsidence began nearly 25 m.y. ago. This observation of thermal subsidence is consistent with the possibility that the central trough h a s been detached by normal

faults (summary by Steckler 1985) from the surrounding areas of the Gulf and has undergone no stretching or other tectonic, activity since the early Miocene. A third conclusion is t h a t the coastal plains of the Gulf of Suez show exceptional variations in subsidence rates and total sediment accumulation from place to place (Table II. If all of these areas started accumulating sediments at sea level at the same time, the best explanation for the variation is differential rotation of fault blocks along listric faults, with thick and rapid accumulation over areas that dropped rapidly along the downthrown sides of normal faults and less subsidence in areas of relative upward rotation. After initial pulses of rapid subsidence, most of these marginal areas have shown little tectonic movement, and subsidence h a s been driven primarily by sediment

624

JOHN J. W. ROGERS et al.

Steckler (1985). The r e s u l t of t h e c a l c u l a t i o n s of e x t e n s i o n in t h e 0 Gulf of S u e z is t h a t a n a r e a t h a t is n o w a b o u t 60 k m ._1 wide is p r e s u m e d to have b e e n a b o u t 40 k m wide uJ > before extension. The e x t e n s i o n m u s t have b e e n LU 2_. .-.I a c c o m p l i s h e d b y s o m e p r o c e s s of c r u s t a l thinning a n d lateral s e p a r a t i o n in addition to (or i n s t e a d ol) UJ 09 simple d e t a c h m e n t faulting, b u t t h e exact n a t u r e 4 of the p r o c e s s is u n c e r t a i n . R e g a r d l e s s of t h e o process, a l m o s t all of t h e s e p a r a t i o n w a s acLU m m c o m p l i s h e d during the Oligocene a n d Early Miocene, a n d ,active, s e p a r a t i o n since t h e n pro0 b a b l y a c c o u n t s for no m o r e t h a n 10% to 20% of the 0 8total expension. The thinning m u s t have b e e n m u c h g r e a t e r in t h e central t r o u g h t h a n a r o u n d the margins. W h e t h e r the a r e a w a s uplifted during ]0 ] I I I I thinning is u n k n o w n . T h e r m a l s u b s i d e n c e h a s 0 I 2 5 4 5 t a k e n place since the Early Miocene, with f u r t h e r lateral s e p a r a t i o n being v e r y small (< 10 km). I/2 (TIME IN M.Y.) All s u b s i d e n c e in t h e Gulf of S u e z area o c c u r r e d during the period of time w h e n t h e D e a d S e a s h e a r zone h a s b e e n active, a n d it is likely t h a t t h e Gulf Fig. 6 Rate of subsidence of unloaded basement in w a s shielded from m o r e active stretching proBelayim Marine field (Whiting 1984; location in Fig. 3). c e s s e s in the m a i n p a r t of the Red S e a b y t h a t s h e a r Subsidence beginning 18 m.y. ago would indicate a zone. Steckler a n d t e n B r i n k (1986) p r o p o s e d t h a t stretching factor of about 3. the n o r t h e r n p a r t of the s h e a r zone follows t h e loading (Widman 1985). The lack of relative Mesozoic b o u n d a r y of t h e North African contectonic m o v e m e n t is s u p p o r t e d b y the observa- tlnental m a r g i n a n d t h a t s t r o n g e r r o c k (more tion t h a t s e d i m e n t a r y s e c t i o n s t h r o u g h o u t the difficult to rift) n o r t h of the b o u n d a r y p r e v e n t e d Gulf record n u m e r o u s eustatic f l u c t u a t i o n s in s e a extension of the Gulf of S u e z f a r t h e r north. This proposal is c o n s i s t e n t with o u r conclusion. level (Perry a n d S c h a m e l 1985). The a m o u n t of extension in the Gulf of Suez is Certainly the Gulf of S u e z h a s b e e n a p u r e l y difficult to evaluate. The central t r o u g h is a b o u t 30 passive tectonic feature for the p a s t 18 m.y. k m across, a n d if it w a s c r e a t e d b y extension with I N V E S T I G A T I O N S ALONG S A U D I a stretching factor of 3, t h e n it m u s t have b e e n ARABIAN COAST a b o u t 10 k m wide before rifting, for a n extension of 20 km. Creation of a central trough 30 k m a c r o s s with a c r u s t a l t h i c k n e s s of 10 k m c o u l d have b e e n Miocene formations have b e e n investigated in three a c c o m p l i s h e d b y m o v e m e n t on a d e t a c h m e n t fault a r e a s of the n o r t h e m S a u d i A r a b i a n c o a s t a l plain with a dip of 18 ° (angle w h o s e t a n g e n t is 10/30), (Yanbu, Dhaylan, Aznam; Figs. 1 a n d 7). In adb u t the lateral stretching b y this m e c h a n i s m w o u l d dition, Miocene s t r a t a crop o u t in the M a g n a area only have b e e n 1.7. If t h e Gulf w a s formed b y (Fig. 2) a n d at v a r i o u s p l a c e s n e a r J i d d a h a n d d e t a c h m e n t , t h e n additional thinning a n d s e p a r a - farther s o u t h . All Miocene r o c k s along the S a u d i tion of the m a r g i n s w a s p r o b a b l y c a u s e d b y t h e A r a b i a n coastal plain are c o m m o n l y t e r m e d t h e p r o c e s s t h a t c a u s e d isostatic uplift of the rift flanks R a g h a m a Formation (U. S. Geol. Surv. 1963; Powers and associated magmatism. e t al. 1966). In addition to extension in t h e central trough of At Y a n b u (Fig. 7) t h e b a s a l Miocene section of the Gulf, the coastal plains m u s t also have u n d e r - conglomerates a n d s a n d s t o n e s is overlain b y marls. gone s o m e extension. The e x p o s u r e of Pre- The s e q u e n c e developed o n a n irregular topograc a m b r i a n rock on t h e u p t h r o w n sides of s o m e fault p h y that w a s later flooded d u r i n g a period of blocks, however, indicates t h a t the extension w a s evaporite deposition in t h e Middle to Late Miocene. minor. The a p p e a r e n c e of e x t e n s i o n could have The total t h i c k n e s s of the s e q u e n c e is only a few b e e n c a u s e d b y m o v e m e n t of b l o c k s along listric h u n d r e d meters, a n d ages h a v e n o t b e e n well faults d o w n into the central trough. A total established. extension in the coastal plains of less t h a n 10 k m Miocene s t r a t a at J a b e l D h a y l a n (Fig. 7) lie s e e m s appropriate a n d is c o n s i s t e n t with t h e u n c o n f o r m a b l y on eroded Oligocene red s a n d estimate of 25 to 27 k m total extension b e t w e e n s t o n e s a n d siltstones. The b a s e of t h e Oligocene is b o u n d i n g s c a r p s on either side of the Gulf m a d e b y not exposed, b u t a m i n i m u m t h i c k n e s s is a b o u t

Subsidence and origin of the northern Red Sea and Gulf of Suez

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JOHN J. W. ROGERSet al.

1,400 m. Paleocurrent directions are toward the northeast, indicating derivation of the Oligocene debris from a source region to the west, in the area now occupied by the Red Sea. Miocene rocks, however, become conglomeratic toward the east and were derived from an eastern source. The Miocene sequence is a b o u t 460 m thick. Sparse fossil evidence indicates t h a t the lower limestone units are lower Miocene, and the evaporites are p r e s u m a b l y Middle to Upper Miocene. Thus, the thinness of the sequence indicates that total Miocene deposition and subsidence was small. The Miocene strata are compressed into a series of open folds with axes roughly perpendicular to the Red Sea coast. At Aznam (Azlam), Miocene sedimentary rocks occur along the w e s t e r n edge of a graben (Aznam trough) and in the area where the graben intersects the Red Sea coast. The basal unit is a Lower Miocene limestone that rests on eroded Oligocene red s a n d s t o n e s and on crystalline rocks. This unit dips a b o u t 15 ° to the west, on the flank of the graben. Miocene units higher in the section inelude evaporites, and t h u s the thickness of about 200 m apparently represents deposition throughout Miocene time. The Aznam graben is a b o u t 100 k m long and 8 to 15 km wide (Fig. 2). its eastern margin is crystalline rocks oftheArabian shield, and Miocene and younger strata occur only to the west. The trough contains a thin sequence of Oligocene sedimentary rocks resting on crystalline b a s e m e n t (Vial et al, 1983); p r e s u m a b l y the Oligocene is a relic of preservation rather t h a n a suite formed within an active graben. Undated mafic volcanic rocks crop out at two places in the northern part of the graben. These rocks contain olivine and plagioclase quench textures indicative of very high t e m p e r a t u r e s of extrusion. The bounding faults of the Aznam trough clearly underwent some vertical movements, b u t they are also part of the series of northwesterly-trending strlke-slip faults conjugate to the Dead Sea shear zone (Fig. 2). Abdel-Gawad (1970) referred to the major fault at Aznam as the Abu Masarib shear zone and correlated it with the Gebel Duwi shear zone in Egypt. The high-temperature volcanism in the Aznam graben, plus the tilting of Lower Miocene strata along one side, probably indicates that the trough formed from high-level, postMiocene intrusion of mafic magma and later subsidence. If vertical movement and m a g m a inJection occurred along other faults in the conjugate set, the problem of Plio-Pleistocene extension of the n o r t h e m Red Sea would be greatly reduced (discussed below). In general, the Miocene rocks of the Saudi Arabian coastal plain apparently cover the entire

s p a n of Miocene time in sections not more t h a n a few h u n d r e d m e t e r s thick. These observations are consistent with a m a x i m u m t h i c k n e s s of 600 m in the Maqna area (Motti e t al. 1982; Zakir 1982; Duno e t a / . 1983). Clearly, there h a s b e e n very little subsidence, and t h u s very little crustal thinning, in the coastal plain. The b r e a k b e t w e e n deep water of the Red Sea trough and the shallow water of the marginal shelf is a b o u t 10 to 20 k m s e a w a r d of the coastline along the n o r t h e m A r a b i a n coast (Drake and Girdler 1964; Laughton 1970). Thus, crustal thinning and extension in the n o r t h e m Red Sea w a s apparently confined to the m a i n trough, with a width of a b o u t 150 km; little tectonic activity occurred in the marginal areas. Very little is k n o w n a b o u t the sedimentary sequence within the northern Red Sea trough itself. Three wells s o u t h of the Maqna area reached granitic b a s e m e n t at d e p t h s of 2,000 to 3,000 m, b u t no data were available on the ages of the covering s e d i m e n t s (Ahmed 1972). Shallow seismic work (Mart and Hall 1984; Uchupi and Ross 1986) showed vertical m o v e m e n t s at least since the Late Miocene. Normal faults and salt diapirs have b e e n active at various times and places t h r o u g h o u t the area, and there is a significant unconformity at the top of Upper Miocene evaporites. The only information about strata below the u p p e r layer of Miocene evaporites w a s provided b y a refraction profile of Drake a n d Girdler (1964), who showed a b o u t 5 k m of sedimentary rocks lying on b a s e m e n t that they identified as granitic b y its P-wave velocity of 5.8 k m / s e c . Apparently, several kilometers of Miocene sediment a c c u m u l a t e d in the trough of the n o r t h e m Red Sea. CONCLUSIONS

Any synthesis of the evolution of the Gulf of Suez, northern Red Sea, and surrounding areas m u s t account for a n u m b e r of observations. The m o s t pertinent evidence is as follows: (1) The central trough of the Gulf of Suez w a s thinned by a factor of 2 to 3 at the end of the Oligocene or earliest Miocene, leaving a c r u s t less t h a n 15 k m thicl~ The sides of the Gulf were thinned m u c h less t h a n the m a i n trough, and the overall stretching across the Gulf area was probably less t h a n 50%. (2) The central trough of the Gulf of Suez h a s undergone simple thermal subsidence since its formation a b o u t 18 m.y ago. Although blocks on the sides of the trough underwent differential s u b s i d e n c e a n d rotation in the Early to Middle Miocene, they have merely subsided since then. Not more t h a n 10% to 20% of the separation across the Gulf h a s t a k e n place since the Early to Middle Miocene. (3) Approximately 60 k m of left-lateral movement probably occurred on

Subsidence and origin of the northern Red Sea and Gulf of Suez t h e D e a d S e a s h e a r zone in t h e Miocene, a n d 45 k m during t h e Plio-P1eistocene. The s h e a r zone a c t e d a s a t r a n s f o r m , isolating t h e G u l f of Suez from f u r t h e r s t r e t c h i n g in t h e n o r t h e r n Red Sea. (4) The m a r g i n s of t h e n o r t h e m R e d S e a c o n t a i n t h i n Miocene s e c t i o n s llthologically similar to t h o s e in t h e G u l f of Suez. A p p a r e n t l y a shallow s e a adv a n c e d over t h e entire a r e a during Early Miocene time, covering Oligocene to P r e c a m b r i a n r o c k s with a n u n c o n f o r m i t y of a p p r o x l m a t e l y e q u a l age t h r o u g h o u t t h e area. M o s t of t h e early d e p o s i t s are l i m e s t o n e s or fine-grained clastic rocks, a n d conglomeraUc m a t e r i a l o c c u r s only locally. Oligocene d e p o s i t s in S a u d i A r a b i a s h o w derivation from a

627

s o u r c e a r e a in t h e p r e s e n t R e d Sea, b u t t h e y do not prove t h e existence of a m a j o r upliR in t h e a r e a at t h a t time. (5) Thick s e d i m e n t s a c c u m u l a t e d in t h e m a i n t r o u g h of t h e n o r t h e r n Red S e a during Miocene a n d y o u n g e r tlme. (6) S h e a r s c o n j u g a t e to t h e D e a d S e a s h e a r zone o c c u r at n u m e r o u s locations in n o r t h w e s t e r n S a u d i Arabia. Where t h e s e faults b o u n d t h e g r a b e n at Aznam, t h e y also s h o w dip-slip m o v e m e n t a n d m a y b e t h e sites of c r u s t a l e x t e n s i o n b y high-level i n t r u s i o n of mafic m a g m a . An a t t e m p t to provide a c o h e r e n t view of t h e evolution of t h e area a r o u n d t h e n o r t h e r n R e d Sea is s h o w n in Figure 8. T o w a r d t h e e n d of t h e Oligocene, the a r e a w a s one of local relief, c a p a b l e

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Fig. 8 Diagram showing evolution of northern Red Sea area consistent with measured movement rates and amount of crustal thinning estimated from subsidence studies. All distances are in kilometers. Further explanation and assumptions are given in the text.

628

JOHN J. W. ROGERS et al.

of f o r m i n g m i n o r c o n g l o m e r a t e s a n d s h e d d i n g s e d i m e n t s e a s t w a r d . B y t h e s t a r t of t h e Miocene, however, t h e a r e a h a d r e a c h e d s e a level over a l m o s t its e n t i r e extent. E x t r e m e c r u s t a l t h i n n i n g o c c u r r e d in a n a r r o w t r o u g h n o w p r e s e r v e d in t h e G u l f of Suez. C o u p l e d with less t h i n n i n g a r o u n d t h e s id es of t h e Gulf, t h i s s t r e t c h i n g i n c r e a s e d t h e w i d t h o f t h e G u l f a r e a f r o m a b o u t 4 0 k m to n e a r l y its p r e s e n t width. At a p p r o x i m a t e l y t h e s a m e time, or p e r h a p s slightly later, m o v e m e n t w a s ini t i at ed along t h e D e a d S e a s h e a r zone, w h i c h isolated t h e G u l f of S u e z f r o m f u r t h e r s t r e t c h i n g a n d p e r m i t t e d t h e r m a l s u b s i d e n c e to c o n t i n u e to t h e p r e s e n t . T h e 6 0 k m of m o v e m e n t along t h e D e a d S e a s h e a r z o n e in t h e Miocene w o u l d r e q u i r e a b o u t 50 k m of e x t e n s i o n in t h e n o r t h e r n Red Sea, s o u t h e as t of t h e s h e a r (Fig. 8). T h u s , t h e initial 40 k m w i d t h of t h e c r u s t h a d to b e c o m e 110 k m (60 + 50), for a s t r e t c h i n g f a c t o r of a b o u t 3. If t h e e x t e n s i o n w a s a c c o m p l i s h e d solely b y l i t h o s p h e r i c s t r e t c h ing, t h e r e s u l t i n g c r u s t c o u l d only be 10 to 15 k m thick, w h i c h is c o n s i s t e n t with a c c u m u l a t i o n of u p to 5 k m of Miocene s e d i m e n t s . O p e n i n g of t h e n o r t h e m Red S e a f r o m 110 k m to its p r e s e n t 150 to 160 k m b e t w e e n s h e l v e s w a s p r e s u m a b l y relat e d to t h e Plio-Pleistocene offset along t h e D e a d S ea s h e a r zone. T h e s h e a r z one i n t e r s e c t s t h e Red S ea at a n angle of 50 degrees, a n d 45 k m m ovem e n t w o u l d r e q u i r e only a b o u t 35 k m of f u r t h e r o p e n i n g of th e o c e a n ba s i n. C o n s i d e r i n g t h e e r r o r s of t h e v a r i o u s e s t i m a t e s , t h i s 35 k m is approxim a t e l y e q u a l to t h e a ddi t i ona l o p e n i n g n e e d e d to f o r m t h e p r e s e n t Red Sea. T h e m e c h a n i s m of o p e n i n g of t h e Red S ea is a m a j o r p r o b l e m . If t h e c r u s t in t h e n o r t h e r n Red S e a t r o u g h f o r m e d b y s t r e t c h i n g a n d t h i n n i n g , t h e n it is p r o b a b l y less t h a n 10 k m thick. It s e e m s u n l i k e l y t h a t t h i s degree of t h i n n i n g c o u l d ha ve b e e n achiev e d w i t h o u t b r e a k i n g t h e c r u s t into s e p a r a t e fragm e n t s . T h e e x t e n s i o n , however, dould h a v e b e e n a c c o m p l i s h e d largely b y injection o f m a f i c m a g m a s (e.g., B o n a t t i a n d Se yl e r 1987), f o r m i n g a c r u s t i n t e r m e d i a t e b e t w e e n c o n t i n e n t a l a n d oceanic. S u c h a c r u s t w o u l d h a v e b e e n d e n s e e n o u g h to s u b s i d e b u t t h i c k e n o u g h to r e m a i n c o h e r e n t .

REFERENCES

Abdel-Gawad, M. 1970. Interpretation ofsateUite photographs of the Red Sea and Gulf of Aden. Phil. Trans. Roy. Soc. London 267, 23-40. Ahmed, S. S. 1972. Geology and petroleum prospects in eastern Red Sea. Am. Assoc. Petrol Geol. Bull. 58, 707719. Almond, D. C. 1986. Geological evolution of the AfroArabian dome. Tectonophyslcs 131, 301-332. Ben-Avraham, Z. 1985. Structural framework of the Gulf of Elat (Aqaba), northern Red Sea. J. Geophys. Res. 90, 703-726. Ben-Avraham, Z., Almagor, G. and Garfunkel, Z. 1979. Sediments and structure of the Gulf of Elat (Aqaba) nort hem Red Sea. Sed. GeoL 23, 239-267. Bohannon, R. G. 1986a. How much divergence has occurred between Africa and Arabia as a result of the opening of the Red Sea? Geology 14, 510-513. Bohannon, R.G. 1986b. Tectonic configuration of the western Arabian continental margin, southern Red Sea. Tectonics 5, 477-498. Bonatti, E. and Seyler, M. 1987. Crustal underplating and evolution in the Red Sea rift: uplifted gabbro/ gneiss crustal complexes on Zabargad and Brothers Islands. J. Geophys. Res. 92,12, 803-12, 821. Buck, W. R. 1986. Small-scale convection induced by passive rifting: the case for uplift of rift shoulders. Earth Planet. Sci. Lett. 77, 362-372. Chenet, P.-Y. and Letouzey, J. 1983. Tectonique de la zone comprise entre Abu Durba et Gebel Mezzezat (Sinai, Egypte) dans le contexte de l'6volution du rift de Suez. BulL Centre Recherches Explor. -Prod. Elf Aqultalne, 201-215. Cochran, J. R. 1983. A model for development of the Red Sea. Am. Assoc. Petrol. Geol. BulL 67, 41-69. Cochran, J. Ih, Martinez, F., Steckler, M. S. and Hobart, M. A. 1986. Conrad deep: a new northern Red Sea deep: origin and implications for continental rifting. Earth Planet. Sc/. Lett. 78, 18-32. Daggett, P. H., Morgan, P., Boulos, F.K., Hennen, S. F. EI-Sherif, A. A. , EI-Sayed, A . / u , Basta, N. Z. and Melek, Y. S. 1986. Seismicity and active tectonics of the Egyptian Red Sea margin and the northern Red Sea. Tectonophyslcs 125, 313-324. Drake, C. L. and Girdler, I~ W. 1964. A geophysical study of the Red Sea. Geophys. J. Roy. Astroru Soc. 8, 473-495. Dullo, W.-C., Hotzl, H. and Jado, A. R. 1983. New stratigraphical results from the Tertiary sequence of the Midyan area, NW SaudiArabia. Newslett. Strat. 12, 75Acknowledgements-Work in the Gulf of Suez by B. 83. Whiting and S. A. Widman was made possible with funds Gettings, M. E . , Blank, H. R., J r . , Mooney, W.D. and provided by National Science Foundation Grant INTHealey, J. H. 1986. Crustal structure of southwestern 01469 to the Earth Sciences and Resources Institute, Saudi Arabia. J. Geophys. Res. 91, 6491-6512. University of South Carolina. Support for S. A. Widman Girdler, R. W. 1985. Problems concerning the evolution was provided by a grant from the Research Council of the of oceanic lithosphere in the northern Red Sea. University of North Carolina. Work in Saudi Arabia by M. Tectonophysics 116, 109-122. E. Dabbagh was supported by King Saud University at Riyad. In addition to persons responsible for administra- Heller, P. L., Wentworth, C. M. and Poag, C. W. 1982. Episodic post-rlft subsidence of the United States tion of these opportunites, we wish thank R. Laws, C. Atlantic continental margin. Geol. Soc. Am. Bull. 93, Powell, R. Ressetar, R. Strelitz, and J. St Jean for their 379-390. assistance.

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