Tectonophysics, 153 (1988) 221-233Elsevier Science Publishers B.V. Amsterdam - Printed in The Netherlands

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Longitudinal evolution of the Suez rift structure (Egypt)

B COLLETTA1, P. LE QUELL EC 2, L LETOUZEY 1 and L MORETTI *

1 Institut Français du Pétrole, Rueil Malmaison (France)2 Total-CFP, Paris la Défense (France)

(Revised version accepted October 12.1987)

Abstract

Coûetta, P , Le Quellec, P., Letouzey, J and Moretti, 1,1988. Longitudinal evolution of the Suez rift structure (Egypt).In: X Le Pichón and JR. Cochran (Editors), The Gulf of Suez and Red Sea Rifting. Tectonophysics 153: 221-233.

A three-dimensional study of the structure of the Suez rift has been carried out using field and subsurface data, inan attempt to determine the role of transverse faults and the longitudinal evolution of the rift.

As in most intracontinental rifts, the structure of the Gulf of Suez area is governed by normal faults and tiltedblocks whose crests constitute the main target of exploratory wells. The fault pattern consists of two major sets oftrends: (1) longitudinal faults parallel to the lift axis and created in an extensional regime where o3 was trendingENE-WSW; and (2) transverse faults with a N-S to NNE-SSW dominant trend. Ihe tiansverse faults are inheritedpassive discontinuities, while most of the longitudinal faults were created during Neogene times in a purely extensionalregime Both sets were simultaneously active, producing a zigzag pattern and rhombic-shaped blocks. The transversefaults can show horizontal strike-slip components and act as relays between major normal faults

Although the Suez rift appears as a simple narrow elongated trough dominated by two almost symmetricalshoulders, its internal structure is asymmetrical Cross-profiles show that all the major blocks are tilted in the samedirection However, the tilt direction changes twice along the rift To the north and to the south of the rift, the blocksare tilted eastward, while in the central paît they are tilled westward. To the north the change of dip is accompanied bya graben-type "twist zone" without transverse faulting, at least in the Neogene series. To the south the change of dip isaccompanied by a more complex structure involving both a major transverse fault and a horst-type "twist zone" Inthis latter case the transverse fault does not cut through the entire rift

Balanced cross-sections established from subsurface data show that the tilt angle and the amount of extensionincrease from north to south, while the width of the blocks decreases, indicating a pole of opening close to the northernend of the Gulf Minimum values for the amount of opening range from 5 km in the north to about 20 km in the south..

Introduction

The Red Sea and its two noithem exten-sions—the Gulf of Suez and the Gulf ofAqaba—are generally considered to be the resultof the divergence of lithospheric plates. The RedSea and the Gulf of Suez represent two rift sys-tems created by extensional movement more orless perpendicular to their border faults, whereasthe Gulf of Aqaba mainly results from strike-slipmovements with few extensional components(leaky transform zone, Ben Avraham et al., 1979;Eyal et al., 1981; Garfunkel et al., 1981)..

These two lifts and the Aqaba transform zonedelineate three continental blocks: Africa, Arabiaand Sinai (Fig. 1). The Red Sea represents amature stage of continental drift, with currentaccretion of oceanic crust along its axial troughbetween 15 ° and 16° N (Drake and Giidler, 1964;Lowell and Genik, 1972; Girdler and Styles, 1974;Young and Ross, 1978). On the contrary, the Suezrift and the Aqaba transform do not show anyevidence of oceanic material and the first onecould be viewed as a failed arm of the Red Sea(Garfunkel and Bartov, 1977). These two riftsseem to be contemporaneous and initiated during

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MEDITERRANEAN SEA

Fig 1 Tectonic framework of the Suez rift. Dotted areascorrespond to the onshore rilt zone Average present widths ofthe Suez and northern Red Sea rifts are shown. Left lateraldisplacement along the Aqaba Dead Sea is 105 km Thick linesshow the axial trough of the Red Sea. Thick arrows indicatethe present divergent movement in the Red Sea as deducedfrom overall kinematic reconstructions (Garfunket, 1981)

Oligocene-Lower Miocene times, but if tectonicextension has nearly stopped in the Suez lift it isstill active in the Red Sea. Decoupling of the twolifts has been accompanied by about 105 km ofleft-lateral displacement along the Aqaba trans-form system since the Upper Cretaceous (Quen-neU, 1958; Freund et al., 1970) or Miocene (Baitovet al, 1980).. A 40 km offset could have occurredin the Plio-Pleistocene (Gaifunkel, 1981). Mean-while, the opening of the Suez lift had almostceased.

First interpretations of magnetic anomalies inthe Red Sea indicated two major stages of opening(Girdler and Styler, 1974). The oldest one wassupposed to occur during Late Eocene and EarlyOligocène times and the second, after a 30 millionyear gap, during Pliocene times. On the contrary,most recent papers consider a continuous openingof the Red Sea without major kinematic change

since the Oligo-Miocene (Le Pichón andFrancheteau, 1978; Cochran 1983; Mart and Hall,1984). The dying out of the Suez rift opening andthe activation of the Aqaba transform occurred atabout 15-12 Ma (Berthelot, 1985; Courtillot et al.,1986). This could be explained by the presence ofthe stronger Mediterranean oceanic lithosphèrejust north of the Gulf of Suez, which precludesnorthward propagation and leads to the transferof motion along the Aqaba fault zone (Stecklerand Ten Brink, 1986). Assuming a motion of theArabian plate almost parallel to the Aqaba trans-form, the pole of opening of the Red Sea could belocated near 32 °N 323 °E (Girdler and Darracott,1972; Freund, 1970; Garfunkel, 1981); the corre-sponding azimuth of the present-day openingwould be N25° in the northern Red Sea. A N35°azimuth is piedicted using the poles calculated byMcKenzie et al. (1970) and Le Pichón andFrancheteau (1978).

Three-dimensional structure of the Suez lift

Structural analysis of the Suez rift was carriedout using seismic profiles and well data. Becauseof the thick evaporitic sequence in the UpperMiocene series, the quality of seismic records isgenerally poor, and interpretation below the saltbottom is somewhat problematic Fortunately, thetwo sides of the rift offer excellent outcrops, andseismic interpretation was largely helped bystructural models built from field observations onboth sides of the Gulf.

As in most extensional systems, the structure ofthe Suez rift is governed by normal faults andtilted blocks. However, the structure of the Suezrift is not cylindrical, and the longitudinal exten-sion of normal faults and tilted blocks is con-trolled by transverse structures.

Transverse faults

The schematic structural map (Fig. 2) as well asfield measurements show two major fault trends: atrend N140-150°E, parallel to the lift axis, and atransverse oblique trend N170°E to N30°E, moreor less parallel to the Aqaba transform (Henry,1985; Jarrige et al., 1986).. This Aqaba trend is

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DEPOCENTER

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Fig. 2. Schematic structural map of the Suez rift, on which major faults and half-graben depocentres are shown.

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Fig. 3 Histograms of fault azimuths measured on the Sinaiside and of their average dips versus azimuths, showing thattransverse faults have a steeper dip than longitudinal ones..

mostly developed on the Sinai side. Two othertransverse fault sets are also present: N90°E andN110-130°E. These latter two trends are morecommon on the western coast, in the Mellaha andGharamul area (Ott d'Estevou et al,, 1986). Puretransverse cross-faults, perpendicular to the riftaxis, (between N30°E and N60°E) ate scarce andgenerally display minor throw.

Diagrams of the average fault dip versus faultazimuths show that transverse faults have a gener-ally steeper attitude than longitudinal ones (Fig3) This difference is the result of two complemen-tary phenomena: (1) the effect of tilting along anaxis parallel to the Gulf is more marked for longi-tudinal faults than for transverse faults; (2) tians-verse faults are probably old strike-slip faults orjoints inherited from an older tectonic event(Garfunkel and Bartov, 1977), and thus displayoriginal dips close to the vertical, whereas majornormal longitudinal faults have original dips be-tween 60 and 80 °.

As shown in Fig. 4, both transverse inheritedand longitudinal neoformed faults were activesimultaneously during the extensional phase ofrifting, but their roles were quite distinct. Duringthe extensional faulting phase, normal faults per-pendicular1 to the minimum stress o3 propagatedall along the entire rift zone, but their longitudinalpropagation could be stopped and laterally shiftedalong transverse faults, producing an oblique relaybetween normal faults In that case, the transversefault acts as a transfer fault (Gibbs, 1984) betweentwo longitudinal normal faults (Fig. 4).

As shown by the structural map of the AbuDurba area (Fig. 2 and Chénet and Letouzey,1983; Hemy, 1985), these transfer faults have alimited extension and affect only one or twoblocks. Seismic profiles and well data show thatthese transfer faults never cut through the entirerift zone as large transform faults.

PRE-RIFT STAGE

Transfer fault

Normal faults created duringextensional phase

Fig. 4. Sketch showing the role of pre-existing discontinuitiesand the relation between transverse and longitudinal faults..

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The overall structural pattern is also com-plicated by numerous transverse hinge faultslimited to one single block, which accompanylongitudinal tilt variation. Throw along these hingefaults is generally higher at the crest of the blockthan in the adjacent half-graben. This means thatthe highest paît of a block crest is often located atthe intersection of a transverse and a longitudinalfault.

In some cases, such as north of Gebel Zeit, achange of dip direction can occur on each side ofa transfer fault (see below), but this is not ageneral rule, and in the aiea of the GebelAraba-Abu Dmba transfer' faults are never asso-ciated with tilt direction change.

Transfer faults that aie relatively oblique withrespect to the a minimum stress, such as the N-Sto N30° set, can have large throws, as large as theadjacent longitudinal faults, producing locally deeprhombic-shaped troughs, as in the Baba plain (Fig.2)

The present movement along a transverse faultis essentially passive and guided by movementsalong active adjacent normal faults. The result isthat transverse faults generally display strike-slipcomponents (Chénet and Letouzey, 1983; Chenetet al., 1987), depending on the dip of adjacentnormal faults.

Twist zones

Asymmetry seems to be the rule for most of theintracontinental extensional systems such as theEast African rift, the Rhine graben, the North Seaand the Oslo graben (Bally, 1981; Ramberg andMorgan, 1984; Rosendahl et al, 1986). As shownby the cross-sections (Fig. 5), the inner structureof the Suez rift is clearly asymmetric. This meansthat along the same cross-line all the major blocksare tilted in the same direction. According to thepolarity of block tilting, the dip direction changestwice along the Suez rift (Moustafa, 1976; Abdine,1981). Consequently, three major tectonic pro-vinces 50-100 km long can be identified (Fig. 6):

(1) the northern Darag basin with southwestdips;

(2) the central Belayim province with northeastdips;

(3) the southern Amal-Zeit province withsouthwest dips.

In some previous structural studies (Moustafa,1976; Abdine, 1981; Angelier and Bergeiat, 1981)the change of dip was interpreted as the result oflarge transverse faults more or less perpendicularto the Gulfs axis, against which all longitudinalfaults abut, Bosworth (1985) and Rosendahl et al(1986) proposed models of progressive dip changeswithout major transverse faulting. Such transitionzones are well documented in the Gulf of Suez,Between the northern and the central provinces,seismic interpretation shows that the transition isprogiessive, at least in the syn-rift deposits (Fig.7).. The throw along the western North Galalafault decreases progressively southward. Mean-while, the throw along the opposite HammanFaraun-Abu Zenima fault progressively increases.The change of dip direction occurs without trans-verse faulting, producing a relatively flat warpedsurface between the two major normal faults. Weproposed to call this accommodation zone a"twist-zone" because of the shape of the beddingsurface between the two faults In this case thechange of dip is accompanied by a "graben-typetwist zone".

The transition zone between the central Be-layim province and the southern Amal-Zeit prov-ince is more complicated. Over there, the changeof dip is accompanied both by a transveise faulttrending N-S to NNE-SSW and a "horst-typetwist zone" (Fig 8). The transverse fault boundsthe Zeit half-giaben to the north and merges west-ward with the master fault of the Mellaba range,where its throw is over 3 km It separates thenortheastward tilted blocks of the GharamulShukheir area from the southwestward tilted blockof the Gebel Zeit. Its throw decreases progres-sively to the east and becomes zero betweenShukheir Bay and South Ramadan, In the SouthRamadan area, the bedding of the pre-iift units isalmost horizontal, and in this zone the change ofdip is accompanied by a horst-type twist zone,where the major southwest dipping fault boundingthe Ramadan field progressively dies out and isrelayed by the northeast dipping fault borderingthe Morgan field-

In both the graben type or the horst type, the

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Fig. 5. Serial balanced cross-section in the Suez rift. The structure is based on well data and seismic profiles From sections D to /,the structure below the evaporites is partly interpreted. Crosses—Precambrian basement; hachured—pre-rift sediments;shaded—Lower Miocene; blank—evaporitic and post-evaporitic sediments Assumed hypotheses for the construction of cross-sec-tions are: (1) fault dips range from 60 to 80° and can flatten slightly at depth; (2) deformation is brittle to a depth of 10 1cm; (3)below 10 km deformation is plastic; (4) pre-rift topography was subhorizontal.

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Fig 6. Schematic structural map of the Suez rift showing thethree asymmetrical provinces: the northern Darag Basin withSW dips; the central Belayim province with NE dips; thesouthern Amal-Zeit province with SW dips. Double circlesshow zones of polarity changes.

twist zones have no closure of their own sincetheir structure can be compared to a saddle. How-ever, they constitute local highs which could be-come interesting targets for petroleum explorationwith secondary closures or sedimentary traps.

Twist zones have been described in severalgrabens or continental lifts (Bosworth, 1985;Rosendahl et al, 1986) and it seems that in mostcases the tilt changes are progressive and occur insuch zones rather than along transverse faults.However, the reason for such changes remainsunsolved.. Why does the tilt of blocks changealong a rift?

Concerning the asymmetry of the lifts andgiabens, several models have been proposedAsymmetric giabens were first modelled by Cloos(1951) in experiments on clay cakes. The asymme-tiy created in the model, with a single master faultmerging at depth with a detachment surface, was

directly related to the shearing foices applied atthe bottom of the clay cake- Brun et al (1985)proposed, from sand box experiments, that thedirection of block tilting was governed by thedirection of shearing at the ductile fragile transi-tion. It can be noted that the directions of shear-ing proposed for a given direction of block tiltingare opposite in the Cloos and Brun et al. experi-ments. Therefore, it appeais that the origin of theasymmetry itself has not yet been clearly de-termined. However, if one admits that the direc-tion of shearing is the factor controlling the asym-metiy of a lift, it must be admitted in the case ofthe Suez rift that the shearing direction changestwice. This phenomenon seems rather unrealisticunless small-scale (less than 100 km long) convec-tive cells with opposite movement exist under-neath a rift

Another explanation of the rift asymmetiy isthe single low-angle ciustal shear proposed byWeinicke (1985). In the case of the Suez rift sucha single low-angle fault does not seem appiopriateto explain the asymmetry, since this asymmetiychanges twice over a 300 km distance and no largecrustal "transfoim" fault can be observed in theSuez rift area.

Infening a right lateral movement along theNW-SE North Gallala-Abu Zenima system(Courtillot et al, 1987) with the creation of twoopposite pull-apait basins (the Gallala-Daragbasin and the Abu Zenima-Baba plain basin) isalso an attractive way to explain the northerntwist zone which separates two asymmetric basins.In this model right lateral movement is supposedto be active from the beginning of rifting until thepresent time, since the Plio-Quaternary series areveiy thick in each opposite basin Such activedextial movements are not suppoited by fieldevidence, which indicates that most of the strike-slip movements ceased at the beginning (Chenet etal., 1987) or just after the beginning of rifting(Jarrige et al., 1986; Montenat et al, 1986) andwere replaced by an E-SW extensional regime.

Bosworth (1985) proposed the model that seemsthe best suited for the Suez rift graben-type twiststructure. The asymmetry is limited to the brittleciust and is not related to a deeper asymmetricalprocess At first "extension initiated as a broad

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Fig. 7. Serial drawing of lines across the northern twist zone between the SW dipping block of the Darag Basin and the NE dippingblocks of the Belayim province Transition is accompanied by a symmetrical graben zone with flat-lying beds

zone of diffuse faulting but quickly evolved to asystem composed of a few main faults". When afault is created it accommodates most of the ex-tension, producing an initial asymmetiic half-giaben. Because of the initial diffuse and ratherrandom first stage of extension faulting, the op-posite polaiity of half-grabens can occur. During

longitudinal propagation of faults theii throw in-creases and asymmetry is accentuated, However,when two opposite-dipping bounding faults mergeand cross at depth, one of them becomes locked.Such locking prevents the simultaneous propa-gation of opposite-dipping faults and thus pre-vents the formation of symmetrical grabens. If the

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Fig. 8. Schematic block diagram of the southern transition zone. Change of dip is accompanied by both a major transverse fault and ahorst-type twist zone

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Fig. 9. Sketch showing two stages of opposite normal fault propagations. Ihe graben-type twist zone can be explained by thepropagating of two opposite en échelon normal faults which lock when merging.

fault plane suiface is larger at depth than close tothe surface, there will be no visible oveilap of thetwo fault traces since intei section (and locking)will occur at depth. On the contiary, if the faultplane surfaces are larger close to the suiface (inother words, the fault plane propagates fasterclose to the suiface) than at depth, there will beoveilapping of the two blanches of the faults andthe cential paît of the twist zone will appear as asymmetrical giaben (Fig. 9)

In this model, the asymmetric structure of therifts and its change of polarity is limited to thebrittle ciust and is supposed to be random. In thecase of the Gulf of Suez, we think that the changeof polarity is not completely random but is largelyconstrained by pre-existing discontinuities. In-deed, the two twist zones are geographically re-lated to two major pre-rift structures: the WadiAraba flexure to the north and the Dara uplift tothe south.

Like most of the Syrian Arc structures, theN65° Wadi Araba flexure and the Ayun Musaanticline (see location in Fig. 2) probably resultfrom the inversion of normal faults (Chorowicz etal., 1987) related to the Tethysian margin subsi-

dence. Those normal faults have created majordiscontinuities which produce segmentation of theSuez rift zone. Such inherited segmentation seemsto have paitially controlled the tilt polarity, atleast in the northern paît of the Gulf, where theAyun Musa anticline is associated with the majorN35° transverse fault which bounds the piesentGulf in the north and where the Wadi Arabaflexure corresponds to a twist zone.

To the south, the tectonic origin of the Darauplift has not been clearly established, but it cor-responds to the only basement outcrop inside thelift which is not related to a block crest. Thisstructurally high zone already existed before rift-ing, as shown by the thinning of the Nubian andUpper Cretaceous seiies (Buiollet et al-, 1982).During extensional movements the Dai a upliftcould have acted as a stronger zone which inducedtilt change in the central part of the rift.

Longitudinal changes of the rift structure

Balanced stiuctuial cross-sections compiledfrom well and seismic data have been used toemphasize the longitudinal changes of the Suez

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Fig. 10. longitudinal variations of the average tilt angle in theSuez rift as deduced from the serial cross-section in Fig. Sn — number of block-dip; 9" average tilt angle; a""1-»constant deviation.

stones of the Gebel Zeit (Colletta et al., 1986).These veiy high values are generally associatedwith closely spaced normal faults, and could indi-cate large internal deformation of blocks, at leastin the southernmost part of the rift.

The tilt angle decreases progressively in thesyn-iift units, and the base of the evaporites ex-ceptionally exceeds 10°. Well coiTelations in theZeit basin clearly indicate the synsedimentary tiltof the block, producing the pinch-out of the syn-lift deposits towaid the crest of the block (Evansand Moxon, 1986).

Block size

If we consider only the faults with kilometricthrows, the average width of the resulting blocksdecieases from north to south. From a 50 kmwidth in the Darag basin wheie a single monocli-nal block occupies the whole Gulf, it rapidly de-cieases to 20 km in the central Belayim piovinceand is only 15 km in the southern piovince. In thecentral piovince, the length of the blocks variesbetween 50 and 80 km, which is 2-5 times theiiwidth.

lift. Several parameters have been quantified usingthese cross-sections.

Tilt angle

Although the cross-sections have been paitlyinterpreted, the tilt angle of the pre-rift units isprobably one of the best constraining parameteis.It was deduced by dip-meter measurements,seismic interpretation and well con dations. Asshown in Fig. 5, there is a clear increase of tiltangle towaid the south. We tentatively quantifiedthis change by using the tilt angle of the mainblocks and by calculating the average value foreach cross-section and its standard deviation (Fig.10). The tilt angle is between 5° and 10° in thenorthern Darag basin and reaches 20-25° in theAmal Zeit piovince (Fig. 10) lust south of theZeit block, dip-meter iesults indicate block tiltingof about 40°. Local higher tilt angles of about70° have been measured in the Nubian sand-

Amount of superficial extension

Foi each of the nine cross-sections shown inFig. 5, the amount of extension, E, has beenestimated by calculating Lx — LQ (Fig. 11), whereLo is the original length measured at the top ofthe Eocene at the restored undefoimed stage andLx is the present extended length The ends of theextended line Lx have been extiapolated fromborder fault geometiy where pie-iift units havebeen eroded. The resulting values vaiy fiom 10%in the north to 26% in the south. Since it was notpossible to take into account secondary faults andinternal defoimation of blocks, these values mustbe considered as minima. Similar values have beenproposed by Chenet and Letouzey (1983), Ange-lier (1985) and Beithelot (1986), and the supeifi-cial extension ratio seems to be fairly well de-teimined.

For comparison, the amount of extension hasbeen computed with the simplified method(Thompson, 1960) using the average tilt angle 6

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Fig 11. Longitudinal variations of the amount of extensiondeduced from geometric reconstruction of serial balancedcross-sections (open circle) and from a simplified method usingthe average tilt angle S and the bedding-fault angle a (cross,dot and triangle).

and the bedding fault angle a, consideiing threedistinct values of 60, 65 and 70°. These indepen-dently computed amounts of extension are inrather good agreement with previous results,showing that this veiy simple method can be usedto give a quick estimate of the amount of exten-sion.

According to these estimates, the overall open-ing of the rift varies from 4 to 5 km in the noith toabout 20 km in the Gebel Zeit area, suggesting ascission-like movement. The southward wideningof the rift mainly results from the counterclock-wise rotation of the Sinai sub-plate in relation tothe African plate, with a pole close to Cairo(Angelier, 1985).

The restored original width between the twoborder faults of the rift was almost unifoim fromnorth to south and averages 60-65 km, except inthe Wadi Araba zone where it was about 50 km.The initial break-up of the Red Sea was probablycomparable to that of the Suez rift, and a 60-65km original distance between the two bolder faultsbefore lifting could be used in order to make akinematic reconstruction.

Discussion and conclusions

The asymmetric structure and change in dipdirection have been described in several grabenand rift systems In the Suez lift this change isaccompanied by two kinds of structures: (1) "twistzones" without any transverse fault; (2) steeplydipping transverse faults inherited from old weak-ness zones.

Twist zones can be explained as the intersectionof two piopagating normal faults with oppositedip. In such a model it is clear that fault propa-gation will be dependent on structural heterogene-ities, and the old structural grain will play a largepart in the rift geometry (Janige et al, 1986). Thevarious models of extension proposed to explainrifting processes do not give any mechanical ordynamic explanations of such changes along a liftsystem These changes seem to be induced byupper ctustal heterogeneities rather than by deep-seated upper mantle phenomena.

A quantitative approach to extension is basedon interpreted cross-sections, and thus the ab-solute values obtained must be consider ed withcaie. However, the main block crests and their tiltangles aie well described by subsurface data, andthe overall variations in the vaiious parameters ofthe extension along the Suez rift are undeniable.The Gulf of Suez opened in a scissor-like manner,producing an increasing extension in the south-ward direction

Because of the differential amount of exten-sion, the structure of the Suez lift varies fromnorth to south. There is a clear relation betweenthe tilt angle, the width of the blocks and theamount of extension. The higher the extension, thegreater the tilt angle and the smaller the blocksize

The stretching of the upper crust in the south-ern part of the rift is greater than 26%, and isprobably about 35-40%, taking into account a10-20% internal extension of blocks. On the otherhand, thinning of the crust deduced from seismicrefraction (Gaulier et al, this volume) is estimatedto be more than 100% Such a discrepancy be-tween superficial extension and crustal thinningsuggests several remarks. As mentioned above, thesuperficial stretching is probably underestimated

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since all the deformation cannot be taken intoaccount in the geometrical reconstruction. In ourreconstruction the average fault-bedding angle hasbeen considered to be 60-80°. It is clear that asmaller fault angle (45° or less) would inducemuch higher values for the extension. Thefault-bedding angle is probably one of the lessconstrained parameters at depth, since fault tracescannot be precisely followed Nevertheless, a lowfault-bedding angle cannot be documented in theSuez rift, whereas 60-80° is a general value en-countered for most fault planes, even when verti-cal displacement has caused deep zones to out-crop.

Thus we believe that an underestimation ofstretching cannot explain the disaepancy with thethinning and that thinning is much greater thanstretching Such a discrepancy can be observed inseveral basins (Pinet et al., 1987) and can beexplained by a thermomechanical model with heattransfer by convective movements (Steckler, 1986;Moretti and Chénet, 1987; Moretti and Pinet,1987).

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Berthelot, F , 1986 Etude thermique du Golfe de Suez dansson contexte géodynamique (in French) Thesis, Universitéde Paris VI, 190 pp.

Boswonh, W., 1985 Geometry of propagating continentalrifts Nature, 316: 625-627

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