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GondwanaResearch

Gondwana Research, V. 8, No. 4, pp. 457-471.© 2005 International Association for Gondwana Research, Japan.ISSN: 1342-937X

A Quantitative Structural Study of Late Pan-AfricanCompressional Deformation in the Central Eastern Desert(Egypt) During Gondwana Assembly

M.M. Abdeen1 and R.O. Greiling2

1 National Authority for Remote Sensing and Space Sciences, 1564 Alf Maskan, Joseph Broz Tito St., El-Nozha El-Gedida,Cairo, AR Egypt

2 Geologisch-Paläontologisches Institut, Ruprecht-Karls-Universität Heidelberg, Germany, Im Neuenheimer Feld 234, 69120Heidelberg, FR Germany

(Manuscript received December 1, 2004; accepted June 10, 2005)

Abstract

The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceaniclithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently,they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolitenappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak.Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins andmetamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Groupwas deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Groupsediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailedafter the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds andSE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balancedsection across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the NajdFault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structuresthat comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates15�17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases producedan interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetailstructure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct spaceand time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages ofNajd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episodebeing related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonicfeature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compressionand transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficientlyeffective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable offurther gravitational deformation.

Key words: Pan-African, Arabian-Nubian Shield, section restoration, late-orogenic structure, transpression.

Introduction

Whereas the Arabian-Nubian Shield is generally acceptedas an example of an accretionary orogen, which developedfrom an assemblage of mostly intra-oceanic magmatic arcterranes with subordinate continental terranes (e.g., Gass,1981; Kröner, 1985; Stern, 1994), there is still no generalconsensus on the evolution from the accretion to thesubsequent cratonization during the final collisionbetween East and West Gondwana. An early review byStern (1985) stressed the extensional component of

transform faulting at this stage, whereas later models(O�Connor, 1996; Fritz et al., 1996; Smith et al., 1999)viewed the orogeny as being dominated by transform ortranspressional processes. This is in contrast to othermodels of early convergence and collision, followed bylate orogenic extension or escape (Burke and Sengör,1986; Stern, 1994; Blasband et al., 2000). Recently, Fowlerand El Kalioubi (2004) developed a further model thatstresses nappe transport during gravitational collapseaccompanied by tectonic squeezing as the dominantprocess to explain the observed deformational sequence.

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Gondwana Research, V. 8, No. 4, 2005

M.M. ABDEEN AND R.O. GREILING

In general, the processes involved in convergenceinclude components of pure shear compression and simpleshear wrenching, i.e., transpression, which can lead tolithospheric thickening and subsequent re-adjustment oflithospheric thickness to normal and potentially stableconditions. Apart from erosion and associated isostaticre-equilibration, such re-adjustment may be achievedby tectonic processes, in particular extensional collapse(e.g., Dewey, 1988). This late orogenic stage ofgravitational collapse is generally regarded as the finalepisode of orogenic deformation (Dewey, 1988; Blasbandet al., 2000). However, during the last decade, there isgrowing evidence from the northwestern part of theArabian-Nubian Shield, along the suture between East andWest Gondwana, that gravitational collapse is followedby further orogenic deformation, which is mostlycompressional or transpressional. This late-orogenicdeformational sequence has become well-establishedqualitatively (e.g., references in Table 1). However, the bulkstrains associated with these deformation events has notyet been properly quantified. Therefore, the present workfirst establishes the late orogenic structural evolution ofa key region in detail and then estimates the bulk strains.The example presented here is from a Hammamat Basinnear Wadi Queih, near the termination of a major,regional transform zone in Egypt, the Najd Fault System(Stern, 1985; see Fig. 1).

Regional Geology

The Arabian-Nubian-Shield (ANS) is composed of anumber of island-arc terranes together with fragments ofoceanic lithospere and minor continental terranes. Accretionand collision established two tiers of orogenic rocks(Bennett and Mosley, 1987). The upper tier is characterizedby sequences of low metamorphic grade with their primarytextures still discernible. The lower tier is composed ofgneissic rocks with relatively higher metamorphic gradebut with similar geochemical characteristics to those ofthe upper tier. Primary contact relationships within andbetween the tiers are rarely preserved due to intensefaulting and shearing. Hermina et al. (1989) and Said(1990) provided comprehensive descriptions andreferences, which are briefly summarized here.

The ANS and the Eastern Desert in particular are knownfor numerous well-preserved ophiolite fragments withsequences from layered gabbros, sheeted dykes, pillowlavas, to black cherts (Shackleton et al., 1980; Kröner,1985; Kröner et al., 1987; Johnson and Woldehaimot,2003). Even more ubiquitous are bodies of ultramaficrocks. These ophiolite fragments represent remnantsof oceanic lithosphere, which are around 800 Ma old.

They are presently preserved as parts of tectonic mélangesor as kilometre-scale nappes. Stratigraphically above theseophiolites occur andesitic volcanic sequences, often aspillow lavas, and related volcaniclastic rocks. Eventually,the volcanic rocks evolve with time towards more felsic,sometimes even rhyolitic compositions. Sedimentary rocksare subordinate, with mostly clastic sequences and rareBIF, and carbonates. All of these upper tier rocks wereweakly metamorphosed (greenschist, locally amphibolitegrade), prior to the deposition of a further volcanicsequence, the Dokhan volcanics of mostly andesitic andrhyolitic composition. The Dokhan volcanics are overlainby molasse-type clastic sedimentary sequences of theHammamat Group and felsic volcanic rocks, which maybe both extrusive (ignimbrites) or intrusive at a shallowlevel. The post-ophiolite evolution of volcanic sequencesis accompanied by compositionally related plutonic rocks,which evolved from calc-alkaline compositions (gabbro,diorite, granitoids) to alkaline granites. Frequent post-tectonic, alkaline granites and dyke swarms, which intrudedc. 600 to 550 Ma ago, represent the latest rocks of theorogen.

Although the plutonic rocks are relatively more frequentin the lower tier, other rock types like ophiolitic gabbroand ultramafic rocks, calc-alkaline gabbros, granitoids,and psammitic sedimentary rocks have also been identifiedas protoliths within the gneissic sequence. Since these rocktypes are similar in composition to the upper tier rocks, ithas been argued that they are time-equivalent with theupper tier sequences. Recent radiometric work providedevidence for such a view in some places (e.g., Gabal Sibai,Bregar et al., 2002; location 7 on Fig. 2). However, inother places, gneisses contain fragments of a pre-ophiolite(continental) crust, for example at the Meatiq dome(Loizenbauer et al., 2001, location 5 on Fig. 2).

Tectonic Evolution

The terranes collided with each other and weresubsequently accreted onto West Gondwana, or the EastSahara Craton, west of the present River Nile, untilc. 600 Ma ago (see Stern, 1994; Unrug, 1997; Johnsonand Woldehaimot, 2003, and references therein). Apartfrom widespread ophiolite nappe emplacement(e.g., Shackleton et al., 1980), collisional deformationand related lithospheric thickening appear to be ratherweak (e.g., Shackleton, 1986; Loizenbauer et al., 1999;Fritz et al., 2002). For example, in the whole of the northernpart of the ANS no traces of high-pressure metamorphicassemblages have yet been found (e.g., Stern, 1994).Early post-collisional structures comprise extensionalfeatures such as fault-bounded (molasse) basins

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LATE PAN-AFRICAN COMPRESSIONAL DEFORMATION DURING GONDWANA ASSEMBLY, EGYPT

(Grothaus et al., 1979; Osman et al., 1993; Rice et al.,1993; Warr et al., 1999) and metamorphic core complexes(Sturchio et al., 1983; Fritz et al., 1996; Blasband et al.,2000), but also major wrench fault systems (Stern, 1985;Sultan et al., 1988; O� Connor, 1996; Abdelsalam et al.,1998) and thrusts and folds (Greiling et al., 1994; Fowlerand El Kalioubi, 2004). Hammamat Group sediments areintruded by late orogenic granites, dated as c. 595 Ma old(e.g., Ries et al., 1983; Willis et al., 1988). Figure 1 showsthe regional tectonic features of the ANS as the northernpart of the East African Orogen between East and WestGondwana, respectively. It also includes some of theimportant structures of the ANS, which are relevant forthe understanding of the late Pan-African orogenic history.In particular, the complex Najd Fault System of NW-SE-trending, sinistral wrench faults dominates the northernANS (Stern 1985). Detailed studies at the NW tip of thiswrench system in Egypt showed considerable NE-SWcompression and NW-SE extension after the depositionof terrestrial, coarse- to fine-grained clastic sequences,resembling molasse-type deposits (see Table 1).

The territory from Quseir in the east to Wadi Hammamatin the west (see Fig. 2) has been the topic of numerousstudies and is recognized as a classic section through thePan-African basement (e.g., Hume 1934, 1937; Ries et al.,1983; Sturchio et al., 1983; Stern, 1985; El-Gaby et al.,1988; Sultan et al., 1988; Fritz et al., 1996). While Stern(1985) pointed out interrelationships of transform faultingand extension, Fritz et al. (1996) and Fritz and Messner(1999) interpreted the structures to have originated duringorogenic compression or convergence. Detailed structuralstudies in the Wadi Queih Basin (Abdeen and Warr, 1998)demonstrated a time sequence from basin formation byextension to (orthogonal) NNW-SSE compression andsubsequent transpression. Based on these qualitative anddirectional data, detailed cross-sections parallel with theprincipal compression directions have been constructedand are graphically restored here in order to help quantifythe deformation. The results, together with published andunpublished information from the adjacent areas are thenused to discuss the late Pan-African tectonic evolution inthe wider context of orogenic evolution and Gondwana

Fig. 1. Tectonic map of the northern part of the East African Orogen (Arabian-Nubian Shield) between continental terranes of East and WestGondwana, respectively. Black aeras show spatial extent of ophiolite remnants, Phanerozoic cover is white; compiled from BRGM (2004),and Johnson and Woldehaimot (2003). Thin lines represent general faults (BRGM 2004), thick lines represent Pan-African faults, potentialsutures (barbed) and strike-slip faults (arrows; from Johnson and Woldemainot 2003). Selected major elements of the NW-SE-trendingNajd Fault System (Stern 1985; Sultan et al., 1988) are marked N. The box shows the study area at the NW end of the Najd Fault System(Fig. 2). The zone of Najd Faults is now being separated by Red Sea extension, approximately along the dashed line in the northern Red Sea.

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assembly. Accordingly, the geology of the Wadi Queih areais presented first and then the evolution of the surroundingbasins of comparable tectonic setting is discussed.

The Wadi Queih Area

Geology and structural evolution

The structural geological map (Fig. 3) shows theexposed part of the Hammamat Group in the Wadi Queiharea to be juxtaposed to the NE and SW against Cainozoicsedimentary successions along normal faults (Fig. 3).It is only the northern basin margin, where a primary,unconformable contact between the underlying Dokhanvolcanics and the Hammamat sequence is preserved. Thebasal part of the Hammamat sedimentary succession

consists of several tens of metres of a coarse, pebble tocobble basal conglomerate, which contains fragments ofPan-African rocks such as metavolcanic, relatedmetasedimentary, and plutonic rocks, includingundeformed, pink, younger granites, which are regardedas belonging to the early post-collisional, alkaline granites(e.g., Hassan and Hashad, 1990). The basal conglomerateis overlain by a sequence, which generally fines upwardsand contains major depositional cycles in the order of ahundred metres thick and subcycles with several metresin thickness (El-Taky, 1988). The finer-grained parts ofthese cycles contain rain-drop prints and mud crackpolygons (Abdeen et al., 1997), which indicate subaerialconditions during deposition. Also the upper, primarycontact against �felsite� may suggest subaerial deposition

Table 1. Compilation of late Pan-African structural episodes, starting from syn-Hammamat Group extension and basin formation to subsequentcompressional and transpressional episodes. The time sequence is from left to right (latest). Location numbers are also shown on thegeological map of figure 2.

Area Extension direction Early Strike-slip late shortening References(see Fig. 2) during basin formation shortening (sinistral) and (transpression)

Wadi Meesar 1 NNW-SSE - - - Osman (1996)Wadi Zeidun

Wadi Hammamat 2 unknown NNW-SSE subordinate NE-SW de Wall et al. (1998),Um Had (NNW-thrusts) (SW, NE-thrusts) Fowler (2001),Um Effein Fowler and Osman

(2001), our data

Atalla zone 3 �- ? subordinate NE-SW de Wall et al. (1998)Fowler and Osman(2001)

Um Esh- 4 �- NW-SE NNW-SSE NE-SW Mostafa and BakhitUm Seleimat (1988),Fawakhir Fowler (2001),

Loizenbauer andNeumayr (1996),Fowler and ElKalioubi (2004)

Gabal Meatiq 5 �- NW-SE NW-SE ENE-WSW Sturchio et al. (1983)(NNW-thrusts) (NNW-SSE folds) Fritz et al. (1996)

NE-SW Loizenbauer et al.(2001)

Wadi Kareim 6 NW-SE NNW-SSE NW-SE NE-SW Fritz and Messner''local thrusts'' NW ENE-WSW (1999)WSW-ENE synform

Gabal El Sibai 7 NW-SE WNW-ESE NW-SE NE-SW Kamal El Din et al.El Shush-Umm WNW-thrusts WNW-ESE NNE-SSW (1992)Gheig (WNW-ESE folds) Kamal El Din (1993)

Ibrahim andCosgrove (2001)Bregar et al. (2002)Abdeen (2003)

Wadi Umm Gheig 8 unknown ? N-S NE-SW Hamad (2003)Folds, thrusts

Wadi Queih 9 N-S? NNW-SSE NW-SE NE-SW El-Taky (1988)(NNW-thrusts) (SE-NW folds, thrusts) Abdeen and Warr

(1998) our data

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(no. 1 on Fig. 5) represents also the oldest thrust, assuminga forward-propagating (towards NNW) thrust system.At a later stage, this thrust system also developed an out-of-sequence thrust (no. 3 on Fig. 5) in the SE of the area.This out-of-sequence thrust cut across the hangingwall ofthrust number 1 and transported the Hammamat Groupsequence over the pre-Hammamat metamorphic sequence,thus creating a window of Hammamat rocks. The primarycontact, at the top of the Hammamat sequence is preservedas a weak erosional unconformity in the eastern part ofthe area (Fig. 3). There, thrust (no. 1) which bounds theWadi Queih Basin sequence at its top is cutting across thefelsite, up to 300 m stratigraphically above the base ofthe felsite. As can be seen from the section and the map(Figs. 3, 5), the thickness of the preserved felsite isdecreasing towards south so that no felsite is preserved inthe very south of the section, at the Hammamat window.As a consequence, thrust number 1 is drawn as a flat inthe south, where it is probably following the top surfaceof the Hammamat sequence. This is consistent with theobservation that the thrust is generally subparallel with the

of an ignimbrite, which contains fragments of Hammamatsediments at its base (Abdeen et al., 1991). The structuralgeological map also shows the trend of (early) folds,as E-W to ENE-WSW (Fig. 3). These folds are associatedwith early thrusts (Abdeen et al., 1992; Abdeen andWarr, 1998), and both these elements represent the effectsof a common, compressive stress field (Fig. 4) that actedafter basin formation and deposition of Hammamat Groupsediments (see Abdeen and Warr, 1998, for furtherdetails).

The section subparallel with the major principalstress direction (Fig. 5a) shows the geometry of the thrustsand their relation with the early folds. The highest thrust

Fig. 2. Geological and structural sketch map of the Pan-African basement rocks of the Eastern Desert of Egypt in the Quseir area. The loweststructural level is occupied by gneissic rocks, the highest stratigraphic level is represented by the Hammamat Group sedimentary sequence.Compiled from the geological maps of the Geological Survey of Egypt (1992 a, b) and Klitzsch et al. (1987), and sources listed in table 1.For location see figure 1. Numbers in circles refer to locations in table 1. The Wadi Queih area is shown in detail on figure 3. Fold tracestrending broadly E-W are shown with continuous lines, NW-SE-oriented ones with dotted lines. The major Hammamat Group occurrencesoccupy structural basins and the gneiss-cored domes are interpreted as structural domes, suggesting a dome-and-basin interference pattern.This interference pattern is spatially related to and partly overprinted by the major strike-slip faults.

Table 2. Numerical values on the section restorations of figures 5 and 8.

Late-stage transpression Early shortening,NE-SW shortening (Fig. 8) NNW-SSE (Fig. 5)

section A - A� section B-B� section C-C�

Deformed length 4,25 km 5,95 km 8,05 kmRestored length 5,10 km 7,00 km 10,73 kmShortening 17% 15% 25%

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bedding in the Hammamat sediments. Towards the north,however, the thrust is ramping up into the felsite.

The early structures are overprinted by sinistral strike-slip faults (e.g., Um Kujura Wrench Fault, UKWF, Fig. 3)and related folds in NW-SE direction. The system of strike-slip faults in the western part of the area is clearly theNW tip of the UKWF, where displacement is distributedbetween minor faults (Fig. 3), similar to a horse-tailstructure (e.g., Woodcock and Schubert, 1994). Togetherwith the Eastern Wrench Fault (EWF, Fig. 3), the UKWFrepresents part of the NW tip of the orogen scale NajdFault System (Stern, 1985; Sultan et al., 1988). The UKWFand EWF together with the associated folds indicate atranspressional stress field which is distinctly differentfrom the earlier, NW-ward thrust-related stresses (Fig. 4).Fault data on the strike-slip faults may not be sufficient todetermine the stress field related to this episode ofdeformation, since a reactivation of the strike-slip faultsduring extension cannot be excluded. As can be seen onthe map (Fig. 3), some of the strike-slip faults, for examplethe EWF, are subparallel with younger normal faults

(e.g., EQBBF on Fig. 3), and, therefore, likely to show bothtraces of strike-slip and normal faulting. As a consequence,additional information on the associated folds is used here.These folds show a consistent NW-SE trend on the map(Fig. 3), parallel with the folds and related linear data(fold axes, bedding-axial surface cleavage intersections)documented in the field (Fig. 6). In combination with faultdata published by Abdeen and Warr (1998), the foldsindicate a stress field with the major principal axis in aNE direction (Fig. 7). This direction, approximately normalto the fold axis and fault trends, is then used for thestructural cross sections (Fig. 8).

Subsequent overprint by normal faults (e.g., faultsnos. 4, 5 on Fig. 5a) can be related to Cainozoic extension.Although their displacement has been restored (seebelow), they are not considered or quantified further here.

Section balancing and restoration

The detailed sections on the thrust units of Wadi QueihHammamat sediments and the surrounding rock units byAbdeen and Warr (1998) provide an opportunity to

Fig. 3. Structural geological map of the Wadi Queih area, redrawn from Abdeen and Warr (1998). For location see figure 2. Sections A�A� and B�B� areshown on figure 8, section C�C� on figure 5.

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quantify the deformation in this area (Figs. 5a, and 8unrestored). Together with the data on strain/stressgeometry discussed above and compiled in figures 4 and 7,they form the basis for the section balancing andrestoration presented here. Original sections andrestorations were drawn at a scale of 1:25.000. Figures 5and 8 show a reduced, slightly simplified version. Sincethe topographic relief is low, mostly less than a hundredmetres, the surface is drawn as flat. Earlier straindeterminations by Abdeen and Warr (1998) showedconsiderable values of internal strain. However, theydetermined strain only from rare, fine-grained, incompetentbeds (mudstones), which are not representative of theHammamat sequence as a whole. Even in these incompetentbeds a pervasive foliation is developed only at the axialsurfaces of some tight folds. Pebble strains in coarse clastic,competent rocks are restricted to the vicinity of faults,where pebbles are faulted. Elsewhere, for example in thebasal conglomerate at the northern margin of the WadiQueih Basin and away from the faults, pebbles do notshow any strain effects at all (our observation). Therefore,the strain data of Abdeen and Warr (1998) are taken aslocal, small-scale features, which are not relevant at themap and section scales considered here. Following thegeneral procedures outlined by Woodward et al. (1989),the youngest structural elements were restored first, whichare the Cainozoic normal faults in figures 5 and 8.Subsequently, the elements of transpressional deformation

were restored (Fig. 8) and, finally, those of the first, thrust-and-fold episode (Fig. 5).

Sections A�A� and B�B� (Fig. 5) were selected in orderto restore the transpressional deformation, since they areapproximately normal to the major principal stressdirection (compare Fig. 7). In the first step, the reversefault elements in ENE-WSW section, together with relatedfolds, (which are associated with later wrench faulting),were restored. Late normal faults are related to Red Searifting (e.g., Khalil and McClay, 2002) and are notimportant here. The EWF, however, is related to thePan-African wrenching. Since no compression effects acrossthis fault have been observed in the field, no restorationwas necessary. The restoration, therefore, representsexclusively the shortening across what is interpreted as apositive (half-)flower structure, which branches from theUKWF. For the fault block between the UKWF and theEWF a shortening of 17% and 15% can be shown forsections A�A� and B�B�, respectively, using the line lengthat the base of the Hammamat Group sediments (Fig. 8,Table 2). The eroded and buried parts, respectively, canbe reliably reconstructed by down-plunge projection,which is possible due to the gentle plunge of the structurestowards the SE (Figs. 3, 5a).

Finally, section C�C� (Fig. 5) was used to restore theearlier compressional strain. In order to exclude the effectsof strike-slip deformation, without deviating too muchfrom the principal strain direction, the section was drawnin between the EWF and the UKWF, respectively. Thesection is the most complete one for the thrusts in thearea, since it includes the fault-bounded window ofHammamat Sediments in the SE of the area and a felsiteinlier farther NW. In addition, it is only mildly overprintedby subsequent transpression and extension, respectively.For the restoration the primary, unconformable contactbetween the Dokhan volcanics and the overlyingHammamat Sediments at the NNW end of the section(C�C�, Fig. 5a) provided a reference, and the thrust contactbetween Hammamat Sediments and the overlyingmetavolcanics a datum. The results are shown in figure 5with sequential restorations, starting from the presentsection (C�C�, Fig. 5a). The faults are numbered accordingto their relative age with 1 oldest and 5 youngest. Theyoungest faults (4, 5) are extensional and oblique to thesection line (see map, Fig. 3). They are probably relatedto Cainozoic, Red Sea extension and their displacementis limited to a few tens of metres. Therefore, these faultsare simply restored within the section line (Fig. 5b).

Shortening is restored, starting with the out-of-sequencethrust number 3 (Fig. 5c). Restoration of the folds proceedsalong the sole thrust number 2 between the DokhanVolcanics and the overlying Hammamat Sediments(Fig. 5d). The folds have earlier been mapped in detail and

Fig. 4. Stress regime as interpreted from the early structural elementsshown on figure 3. See Abdeen and Warr (1998) for details.

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documented in a number of field photographs, sketches,and cross-sections (Abdeen and Warr, 1998). Therefore,their geometry can be reliably reconstructed, as shownby a form line in the sections in figure 5 (a�c). Similar tothe procedure used in restoring the sections on figure 8(above), the length of the form line at or closest to thebottom of the Hammamat sequence was determined.However, the felsite within the Hammamat Sediments hasnot been restored since the displacement along itsboundaries cannot be quantified reliably. The contact maybe a thrust as is shown on the map (Fig. 3), but it may alsobe an intrusive contact, which is only slightly overprinted.Since the lateral extent of the fault-bounded felsite bodyis limited, the displacement distance is regarded here assmall. As the last step, the thrust fault number 1 betweenthe Hammamat Sediments and the overlying metavolcanicrocks is restored (Fig. 5e). The section restoration indicates25% of shortening in the NNW-SSE direction (Fig. 5,Table 2). It should be noted, however, that this value isonly a minimum figure. As is pointed out on the restored

section (Fig. 5e), there are two areas, where section is missing.One area is that of the Hammamat window, where materialwas removed by erosion in the hangingwall of the out-of-sequence thrust number 3. The other area at the SSE marginof the section is unexposed so that the primary basinmargin is buried and the primary basin width is unknown.Therefore, the transport distance of the metavolcanic thrustunit over the Hammamat Group sediments cannot bequantified any further. However, considering the differencein metamorphic grade from greenschist grade in themetavolcanic rocks to sub-greenschist-grade in theHammamat Sediments (Abdeen and Warr, 1998), thedisplacement along the low angle thrust would be substantial.

Other Hammamat Group Basins and RegionalStructure

Following up on the fundamental studies by Akaad andEl Ramly (1958), Akaad and Noweir (1969), andGrothaus et al. (1979) on the distribution and sedimentary

Fig. 5. Section C�C� showing geometry of structures related to early compression. a: present section, b�e: sequentially restored sections. See figure 3for location. Thrusts are numbered according to their relative age, with 1 oldest and 5 youngest. See text for further discussions.

B–B’ A–A’

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environment of the Hammamat Group basins, modernstudies cover now many of the late-orogenic sedimentarydeposits (see reviews by Garfunkel, 1999; Blasband et al.,2000, and references in Table 1). Although most of thesestudies discuss some regional aspects, there is not yet anycomprehensive study on the late Pan-African deformationalpattern at the scale of the orogen. Therefore, it isattempted here to relate the structural results of the WadiQueih area to the adjacent areas of the Eastern Desert, theCentral Eastern Desert (CED) in order to investigate theregional-scale structural pattern and to test whether thepost-Hammamat structural evolution seen at Wadi Queihis representative at the regional scale.

Regional post-Hammamat structures

As a first step, the structures are summarized in thischapter, whilst their genetic implications and tectonicsignificance are discussed in the following chapters.Table 1 gives a compilation of the most important post-depositional structural events and related featuresaffecting the Hammamat Basins in the region. The basinsand their structures are broadly grouped from west to east(Fig. 2). Although younger structures overprinted theregional pattern, careful inspection shows that the generalstrike of the Hammamat Group sequences in the Quseir-

Hammamat region is broadly ENE-WSW. This is clear fromWadi Queih, which was discussed above, and is also clearfrom the maps of Wadi Kareim by Fritz and Messner(1999). Towards W, at Wadi Zeidoun and Wadi Meesar,the beds are, in general, gently dipping towards northerlydirections (Osman, 1996). The situation in Wadi Hammamatis more complex, due to local, NW-SE trending folds andlate tectonic granite intrusions (Fowler, 2001; Fowler andOsman, 2001, and references therein). However, at aregional scale, the strike of the northern margin of theHammamat Group sediments is broadly E�W, with minorfold structures plunging SE (Fowler and Osman, 2001).As a consequence, the major Hammamat exposures canbe seen as representing the cores of a set of broadlyENE-WSW oriented synclinal structures separated byanticlinal areas. This set of folds was subsequentlyoverprinted by NW-SE trending strike-slip faults andassociated transpression that produced some NW-SE toNNW-SE oriented folds (Fig. 2). The resulting interferencepattern can be traced across the whole area between WadiHammamat in the west and Quseir in the east. Althoughthere are minor differences in the orientation of therelevant structures, all the published data and the newdata presented here are consistent and point to a regionalsignificance of these structures. Based on these spatialcorrelations of structural elements, it is now possible torelate the structural evolution of the different areas withsome confidence.

Regional structural evolution

Extension during basin formation is not well constrainedbut appears to have been in a general N-S to NW-SEdirection (see Table 1). Subsequently, there is a compressionalstructural event with σ1 broadly NNW-SSE, similar to thesituation in Wadi Qheih (Fig. 4). Thrusts and folds of thisearly compressional stage are overprinted by complexstructural systems associated with a second compressionalevent. These second generation structures include large-scale wrench faults (broadly NW-SE) with associatedpositive (half-)flower structures, and folds with NW-SEto NNW-SSE-oriented fold traces. As in the Wadi Queiharea a change in the direction of compression fromNNW-SSE (Fig. 4) to NE-SW (Fig. 7) can be interpreted.This change in σ1 direction may also be accompanied bya change in σ3 direction from subvertical to subhorizontal,with a NNW-SSE direction, on the basis of the formationof strike slip faults (σ2 vertical; compare Fig. 7). This σ3

direction is broadly parallel with the late extensiondirection as observed, for example, at Um Esh (Fowlerand El Kalioubi, 2004), the Meatiq and El Sibai domes,and the Fawakhir granite (Loizenbauer and Neumayr,1996). As a whole, the local structural studies across theCED show a remarkably consistent orientation of early

Fig. 6. Stereonet (lower hemisphere, equal area projection) showingelements of a fold related to the late stage of transpressionaldeformation. Bedding data, shown as poles (dots) show a greatcircle distribution, defining a fold with an axial trend of c. 130°,consistent with related lineation data (contoured, N = 8). Forlocation see figure 3.

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post-Hammamat folds and thrusts indicating NNW-SSEcompression, and subsequent folds oriented NNW-SSE,which are associated with extension parallel to their foldaxial direction and with NW-SE-oriented sinistral wrenchfaults (Table 1). However, whilst the geometry of thesestructures and their relative time-sequence appears to bevery similar, their absolute timing may be less consistent,as is discussed below.

Discussion

The Wadi Queih area

The Hammamat Group sediments exposed in the WadiQueih area were deposited in a basin with a broadly E-W-trending northern margin. Basal conglomerates occur onlyin the NE part of the basin, along the exposed primarymargin. This supports the model of NE-SW to E-Welongation directions of the Hammamat Basins proposedby Stern et al. (1988). The sediments, together with theother rock units in the Wadi Queih area were subjected tolate Pan-African folding and thrusting processes asindicated by the presence of S and SE dipping low anglethrusts and NNW verging folds (Abdeen et al., 1992; Abdeenand Warr, 1998; Greiling et al., 1994). Subsequently, boththe Hammamat sediments and the Pan-African thrust unitswere deformed by NW-SE oriented sinistral strike-slipfaults that belong to the Najd Fault System in the CED

(Stern, 1985). These faults are associated with sub-parallelhigh angle reverse faults that form a positive (half-)flowerstructure. They are interpreted to be a result of transpressionfollowing the Pan-African thrusting (Abdeen et al., 1992;Abdeen and Warr, 1998). Both of these structural epidsodesclearly post-date basin formation of Hammamat Groupsediments, which until recently were presumed to representthe latest features of Pan-African orogeny. Sectionrestorations show a modest amount of compression duringboth the first and second post-Hammamat deformationphases of >25% and 15�17%, respectively (Table 2,Figs. 5, 8). The restorations quantify the latest stages of ashortening deformation and do not take into considerationany earlier extension, which may have been associatedwith basin formation. However, they confirm that thePan-African orogeny at Wadi Queih did not end with late-tectonic extension and collapse but with overall shortening.

Late orogenic basin formation and deformation in theCentral Eastern Desert (CED)

The molasse type, Hammamat Group sedimentscomprise thick, epiclastic successions deposited by alluvialfan-braided stream systems in intermontane basins(Grothaus et al., 1979), and in down-faulted grabens thatwere trending NE-SW to E-W (Stern et al., 1988). Tectonicoverprint of the basins strarted with granite intrusionsat c. 595 Ma ago (Ries et al., 1983) and lasted untilc. 580 Ma ago (Loizenbauer et al., 2001, and referencestherein). As is evident from the traces of major structures,broadly E-W-trending, post Hammamat folds cover thewhole of the area, albeit with varying intensity (Fig. 2).This is also the case for NW-SE-trending folds and spatiallyrelated strike-slip faults. In addition, a number of studiesshowed that the time sequence may also be the same overthe region (Table 2). In particular, the structural sequencein the Wadi Hammamat type area and adjacent areas(Fig. 2, locations 1�4) has been worked out by severalindependent groups, which all agree on a post-Hammamat-sediment structural sequence of ENE-WSWfolds and thrusts overprinted by NNW-SSE-oriented folds,which are mostly associated with sinistral strike-slip faultsin NW-SE direction (e.g., Mostafa and Bakhit, 1988;de Wall et al., 1998; Fowler and Osman, 2001; Fowlerand El Kalioubi, 2004). However, there may also be localexamples of a more complex or a diachronous evolution.A deformation phase with NW-SE extension at Fawakhir(Loizenbauer and Neumayr, 1996) may be related withthe early extension during basin formation or,alternatively, with the late phase of NE-SW compression.The recorded extensional joints strike ENE-WSW.This strike direction is consistent with the regionalσ1 direction interpreted here (Fig. 7). Therefore, theseextensional joints do not necessarily indicate a dominant

Fig. 7. Stress regime as interpreted from the late, strike-slip andtranspressional structural elements shown on figures 3 and 6,and fault data from Abdeen and Warr (1998).

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component of gravitational deformation, i.e., verticalσ1 as envisaged by Loizenbauer and Neumayr (1996), butare interpreted here as related to the late stage of compressionwith a subhorizontal σ1. This late phase of compressionhas been shown to be associated with NW-SE extension,normal to the shortening direction, by Fowler andEl Kalioubi (2004).

In the Um Esh area (location 4, Fig. 2), subhorizontalfolds with NW-SE stretching lineations (Fowler andEl Kalioubi, 2004) have been correlated here with the lateshortening episode (Table 1). However, this is in contrastwith the interpretation of Fowler and El Kalioubi (2004)that this episode is associated with gravitational collapse.Further work will have to show, whether the simplecorrelation in table 1 is correct, or whether an additionalepisode of post-Hammamat deformation can be verified.This may also be true for the adjacent Gabal Meatiq area(location 5), where Loizenbauer et al. (2001) documenteda tectonic history lasting for more than 200 Ma. Theseauthors show only two deformational events after 620 Maago. Whilst the early one of N-S extension may becorrelated with the basin formation, the second one issimilar to the late stage of transpression. However, thereappears to be no trace of post 620 Ma (post-Hammamat?)age NNW-SSE compression. Furthermore, there is not yetconclusive evidence, whether the Meatiq dome originatedexclusively by gravitative rise of buoyant gneissic materialas suggested by Fritz et al. (1996) and Loizenbauer et al.(2001), or exclusively as a dome in a compressionalstructural interference pattern (Fig. 2). Since these twoextremes are not mutually exclusive, it may well be thatthese two mechanisms acted together.

In the area towards south, Fritz and Messner (1999)argued that the Wadi Kareim Basin (location 6, Fig. 2,Table 1) evolved in two steps. The early step was basinsubsidence that was magmatically-induced when extractionof magmas from the lower crust led to compensatingdownward movement of cold, dense material. The secondstep was the modification of the basin geometry by strike-slip deformation (belonging to the Najd Fault System).The structural overprint appears to be more complex,when maps and sections from the Wadi Kareim Basin ofFritz and Messner (1999) are considered. There, anapproximately E�W oriented syncline is obvious. Thissyncline is interpreted here as belonging to the system ofearly post-Hammamat folds which developed mainly afterdeposition by horizontal compressional forces, and priorto the strike-slip Najd Fault-related deformation.

A comparable structural sequence has also beenobserved SE of Wadi Kareim in the Sibai Gneissic Complexand at Umm Gheig (locations 7; 8; Ibrahim and Cosgrove,2001). However, there, the directions of thrusts (WNWstrike) and refolding folds (WNW-ESE) are 20° different,

appearing to be rotated from the normal NW-SE directionsin the other areas by anti-clockwise rotation. Such arotation is consistent with the sinistral movement alongthe Najd Fault System (Stern, 1985). Therefore, thedifference in the orientation of the structures in the UmGheig area is interpreted here to be an effect of a latestincrement of strike-slip along the Najd Faults. In that case,the wrench faulting has been substantial, in contrast toareas in the north, where the tips of wrench faults can beobserved (Wadi Queih), or no wrenching at all (WadiHammamat, Atalla Zone). As a consequence, thestrike-slip component appears to be subordinate in theNW (e.g., Wadi Hammamat, Atalla zone, de Wall et al.,1998), and more prominent towards SE (e.g., Um Esh,Fawakhir, Fowler and El Kalioubi, 2004; Loizenbauer andNeumayr 1996), and increasing even more farther towardsSE, at El Sibai and Um Gheig (Kamal El Din, 1993; Ibrahimand Cosgrove, 2001; Bregar et al., 2002). A variation instrain intensity is also supported by the availablequantitative data. 15�17% NE-SW directed shortening aredocumented in the Wadi Queih area as accompanying thetranspression (Fig. 5, Table 2). A section across the Meatiq-Um Esh area shows c. 20% of shortening in the samedirection (Fowler and El Kalioubi, 2004; Fowler, pers.comm. 2005).

Such strain vatiations and apparent disagreement asto the importance of strike-slip faulting can be resolvedby the observation that the CED as a whole covers theNW-tip of the regional scale strike-slip faults (see Fig. 2).This information also solves the volume problem at thenorthern end of the Najd Fault System as discussed byStern (1985). Displacement is simply dying outnorthwards with the large-scale faults fanning out intominor faults and associated folds. Together with the earliercompressional event, the Najd Fault episode representsthe latest stages of Pan-African orogenic deformation.At a regional scale (Fig. 2), these two post-Hammamatcompressional events produced an interference patternwith domes and basins. The structural basins are invariablycored by Hammamat sediments. It has to be noted thatsome of the basinal sediments are eroded and the basinsdescribed here may once have been contiguous, forexample the Wadi Hammamat and Wadi Zeidun and WadiMeesar Basins (see Fig. 2).

Late orogenic structural evolution at a regional scale

Earlier, Greiling et al. (1994) and Blasband et al. (2000)referred the basin formation to the phase 4 of the 5 phasetectonic evolution of mountain belts proposed by Dewey(1988). This phase comprises orogenic collapse andformation of extensional basins within the orogen andalong its margins and leads to cooling and a stabilizationof the newly formed lithosphere (see discussion by

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Garfunkel, 1999). In spite of wide-spread magmatism, theillite crystallinity data from the Hammamat Basinsshow that there was no pervasive heating throughout thecrust. As can be seen from the available P and T data(Osman et al., 1993; Warr et al., 1999; Abdeen and Warr,1998; Loizenbauer et al., 2001), subsequent erosion ofthe orogen prior to early Palaeozoic cover sedimentationwas limited to a few kilometers. Apparently, earlierthinning, which ended with the formation of theHammamat Basins had produced a crust of approximately��normal�� continental thickness, which was relatively stablegravitationally, cooled relatively quickly and becamestrong. However, until the crust was completelyconsolidated, it suffered at least two episodes of (limited)compressional deformation with σ1 in (sub-) horizontalorientations.

Conclusions

The Hammamat Group was deposited in intramontanebasins formed due to N-S to NW-SE directed extension,differential uplift, and erosion of the Pan-African orogen,which provided clastic sediments to the basins.A comparison of post-Hammamat Group structuralfeatures in the basement areas west of Quseir andincluding the type area at Wadi Hammamat, shows anumber of similarities. A NNW-SSE-oriented compression

prevailed after the deposition of the Hammamat sediments.This is documented by the presence of NW-verging foldsand SE-dipping thrusts that were refolded and thrustedin the same direction. Restoration of a NNW-SSE orientedbalanced section across Wadi Queih indicates more than25% shortening. Transpressional wrenching related to theNajd Fault System followed this stage. The wrenchingproduced NW-SE sinistral faults associated with positive(half-)flower structures that comprise NE verging foldsand SW-dipping thrusts. Section restoration across theselate structures in the Wadi Queih area indicates 15�17%shortening in the NE-SW direction. The Najd Fault Systemsplays into a horsetail structure in the Wadi Queih areaand loses displacement towards N and NW. At the WadiHammamat type area and at the Atalla Zone, wrenchdeformation becomes insignificant and shows that theNajd Fault System dies out also towards west. These strainvariations are also supported by the available quantitativedata. At a regional scale, the two post-Hammamatcompressional phases produced an interference patternwith domes and basins.

The present study shows a distinct space and timerelationship between deposition of Hammamat Group late-Pan-African clastic sediments and late stages of NajdSystem wrench faulting: Hammamat deposition isfollowed by two episodes of compression, the later of thetwo being related to Najd Fault transpression. Therefore,

Fig. 8. Sections A�A� and B�B� across late strike-slip faults, related reverse faults and folds, which, together, form a positive flower structure.The sinistral sense of strike-slip faults is indicated by conventional, round symbols. See figure 3 for location. Below each section a restorationof the NE part is shown.

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the Hammamat Sediments do not represent the latesttectonic feature of the Pan-African orogeny in the ANS. Itis speculated that extension and (Hammamat) basinformation was sufficiently effective so that it reduced thethickness of the orogenic lithosphere until it becamegravitationally stable. Instead of further gravitationaldeformation, the latest orogenic episodes were the twosuccessive phases of compression and transpression,respectively.

Acknowledgments

This work is the result of the cooperation between theNational Authority for Remote Sensing and SpaceSciences, Cairo, Egypt, and Geologisch-PaläontologischesInstitut (GPI), Ruprecht-Karls-Universität Heidelberg. Thefirst author visited GPI as a visiting scientist and lecturer,funded successively by Deutsche Forschungsgemeinschaft(DFG) and Ruprecht-Karls-University. The authors thankthese institutions, as well as Nahed Abdelsalam Farag forher general support, and G. Froelich and E. Hofmann forcomputer assistance. Field data for figure 6 were collectedduring the 2004 Marsa Um Gheig field workshop byEGSMA, Cairo, and GPI. We thank Kathrin Abraham,Evelyn Böhm, Frank Dietze, René Grobe, Maren Kahl,Friedemann Maaß, Tina Pelzer, Marc Petras, SebastianPfeil, Lena Schilling, Manuel Sehrt, and Henrik Wild, fortheir support. Reviewers Profs. H. de Wall and A.-R. Fowlerare thanked for their careful reviews and detailedcomments and suggestions for improvement.

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