post-collisional shortening in the late pan-african ...rjstern/egypt/pdfs...ne–sw-trending,...

Precambrian Research 107 (2001) 179–194

Post-collisional shortening in the late Pan-African Hamisanahigh strain zone, SE Egypt: field and magnetic fabric

evidence

Helga de Wall a,*, Reinhard O. Greiling a, Mohamed Fouad Sadek b

a Geologisch-Palaontologisches Institut, Ruprecht-Karls-Uni6ersitat, Im Neuenheimer Feld 234, 69120 Heidelberg, Germanyb Egyptian Geological Sur6ey and Mining Authority, 3 Salah Salem Street, Abbassiyah, Cairo, Egypt

Received 7 April 2000; accepted 8 September 2000

Abstract

The Hamisana zone (HZ) is one of the major high strain zones of the Pan-African (Neoproterozoic) Arabian–Nu-bian shield (ANS). It trends broadly N–S from northern Sudan into southeastern Egypt and meets the present RedSea coast at :23°N. The HZ has been the subject of controversy with regard to its importance for the Pan-Africanstructural evolution. Interpretations range from a suture zone, a regional shear zone, or a large-scale transpressionalwrench fault system. In this study, we characterize the nature of the high strain deformation by applying theanisotropy of magnetic susceptibility method along with field and microstructural investigations. These investigationsdemonstrate that deformation in the HZ is dominated by pure shear under upper greenschist/amphibolite grademetamorphic conditions, producing E–W shortening, but with a strong N–S-extensional component. This deforma-tion also led to folding of regional-scale thrusts (including the base of ophiolite nappes such as Gabal Gerf and Onib).Consequently, the high strain deformation is younger than ophiolite emplacement and suturing of terranes. A weaksubsequent overprint was mostly non-coaxial. It took place under considerably lower temperature and led to a minorNE–SW-trending, dextral wrench fault. Although it is of only local importance this fault may be itself a conjugaterelative to the prominent NW–SE-trending sinistral Najd faults in the northern ANS. Therefore, the HZ is dominatedby late orogenic compressional deformation and cannot be related to either large-scale transpressional orogeny ormajor escape tectonics. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Hamisana zone; Arabian–Nubian shield; Late-orogenic compression; Magnetic susceptibility

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1. Introduction

The Pan-African orogenic cycle has long beenrecognized as a period of major crustal accretion,

where continental, island-arc, and oceanic ter-ranes were brought together to form the crys-talline basement of the African continent as partof a late Neoproterozoic supercontinent (Unrug,1997, for review). Whereas some parts of thePan-African orogen are characterized by conti-nental collisional tectonics (Burke and Sengor,

* Corresponding author. Tel.: +49-6221-544843; fax: +49-6221-545503.

E-mail address: [email protected] (H. de Wall).

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00141 -8

H. de Wall et al. / Precambrian Research 107 (2001) 179–194180

1986; Stern, 1994) others are typical for accre-tionary orogens (Windley, 1992, for definition).During the last two decades, the Arabian–Nu-bian shield (ANS), the northeastern part ofAfrica–Arabia, has been established as a typicalexample of an accretionary orogen with lateralaccretion and suturing (Gass, 1981; Kroner, 1985;Kroner et al., 1987; Stern, 1994). In the course oforogenic activity, numerous sutures and othermajor deformational zones originated all acrossthis shield. Subsequent to the assembly of ter-ranes, late deformation zones developed as theexpression of lateral escape tectonics or othertranstensional or transpressional episodes duringthe final stages of continental crust formation(Burke and Sengor, 1986; Stern, 1985, 1994;Johnson and Kattan, 1999). Contrasting modelsof transpressional orogeny (Sanderson and Mar-chini, 1984) have been proposed for the ANS(O’Connor et al., 1994; Abdelsalam et al., 1998),some of which relate terrane assembly to a single,major event of transpression.

In order to distinguish between these models, itis essential to re-examine the evolution of theorogenic structures in the ANS. Towards thataim, the present authors studied a major Pan-African deformational zone, the Hamisana zone(HZ), which is exposed in the Red Sea Hills ofsoutheastern Egypt and northeastern Sudan (Fig.1). The N–S-trending HZ is spectacularly visibleon satellite images, where it can be traced formore than 300 km (Miller and Dixon, 1992). It isparallel with a zone of similar size on the Sudan-Ethiopia border, the Baraka zone (Dixon et al.,1987) and the less prominent Oko zone, which islocated 100 km to the east/southeast of the HZ(Abdelsalam, 1994, Fig. 1).

Well-defined ophiolites and ophiolite belts havebeen mapped on both sides of the HZ and inter-preted as a suture zone between major Pan-African terranes. East of the HZ this belt wascalled the Onib-Sol Hamed suture (Fitches et al.,1983; Dixon et al., 1987; Kroner et al., 1987;Stern et al., 1989, 1990, Fig. 1). The HZ, cuts theOnib-Sol Hamed suture and is, consequently,younger. Although this structural relationship has

been well documented (Stern et al., 1990; Millerand Dixon, 1992), a contrasting model of the HZas a major transpressional zone during terraneaccretion has been proposed recently, comprising

Fig. 1. Major deformation zones in the Pan-African basementof SE Egypt and NE Sudan and location of the study area inthe northern HZ. Compiled from Kroner et al. (1987); Stern etal. (1990); Greiling et al. (1994); Greiling et al. (1988);EGSMA (1996); EGSMA (1999); O’Connor (1996); Abdel-salam et al. (1998); Rashwan and Greiling (1999) and A.Makhlouf (unpublished data).

H. de Wall et al. / Precambrian Research 107 (2001) 179–194 181

Fig. 2. Geological sketch map of the study area. For locationsee Fig. 1. The right-lateral strike-slip fault in the SE corner ofthe map is taken from Stern et al. (1990).

2. Evolution of the Arabian–Nubian shield andthe Hamisana zone

2.1. Arabian–Nubian shield

The ANS evolved during the Neoproterozoic,Pan-African episode from a number of island-arcand minor continental terranes. They were ac-creted onto an East Sahara craton, which waspreviously affected in the Neoproterozoic and isnow mostly covered by Phanerozoic sedimentsand is presumed to extend westwards from theKeraf suture (Abdelsalam et al., 1998). Lithologi-cally, this tectonic evolution from oceanic litho-sphere to island-arcs and continental lithosphereis well documented by the rock sequences buildingup the ANS: ophiolite suites, ocean floor (cherts)and passive margin (quartzo-feldspathic and car-bonate) sedimentary sequences, island arc, calc-al-kaline igneous rocks, both intrusive andsupracrustal, together with ‘‘syn-orogenic’’ sedi-ments derived from these igneous rocks. Finally,syn-collisional and late- to post-tectonic mag-matic activity produced some gabbroic but mostlygranitoid intrusives and related volcanics. Late-orogenic, molasse-type sediments are only foundin some intramontane basins, mostly in the NWof the ANS (Akaad and El Ramly, 1958; Osmanet al., 1993). Structurally, at least parts of thetectonic evolution are also documented, for exam-ple subduction-related tectonic melange or fossilaccretionary prism – (Wadi Ghadir; El Sharkawyand El Bayoumi, 1979; Shackleton et al., 1980) orearly, ‘syn-island-arc’ deformation (Wadi Hafafit;El Ramly et al., 1984; Rashwan, 1991). Subse-quent structures comprise both extensional fea-tures such as fault-bounded (molasse) basins (Riceet al., 1993) and metamorphic core complexes(Sturchio, et al., 1983; Fritz et al., 1996), large-scale thrusts (Ries et al., 1983; El Ramly et al.,1984), and major wrench fault systems (Stern,1985; Sultan et al., 1988; Shimron, 1990; O’Con-nor et al., 1994; Abdelsalam et al., 1998; Greilinget al., 1998). Apparently, both extensional andcompressional deformation, as they are docu-mented in the ANS, represent post-collisionalstructures and are succeeded by and associatedwith wrench faulting (Abdeen et al., 1992; Greil-ing et al., 1994; Johnson and Kattan, 1999).

a relatively simple evolution from oblique conver-gence and suturing towards transpressional shearzones as a final stage of orogeny (O’Connor et al.,1994; Nasr et al., 1998; Smith et al., 1999).

As it is clear from this controversy, the timesequence and the general tectonic significance ofthese structures at a regional scale are not yetfully known. Based on recent mapping by theEgyptian Geological Survey, a part of the HZ(Figs. 1 and 2) was studied in the field andsampled for detailed structural studies. In addi-tion to the usual structural geological investiga-tions, the anisotropy of magnetic susceptibility(AMS) of the rocks in the HZ was studied. Thislatter method has been shown to be most sensitiveto document the type and style of deformationeven in places where no or hardly any deforma-tion is visible macroscopically (Tarling andHrouda, 1993). In that way, it is possible not onlyto characterize the deformation within the HZ butalso to determine a subsequent overprint, whichmay escape detection by other methods. Thisapplication of the AMS method is the first majorstudy of this kind in the ANS, apart from minorpilot studies (Greiling, 1997; de Wall et al., 1998,1999; El Eraki and Greiling, 1998).


The most important of such faults in the north-western ANS are the NW–SE-oriented Najd sys-tem, including Wadi Kharit-Wadi Hodein faultzones, the broadly N–S-oriented HZ and the OkoShear zone (Abdelsalam, 1994; Kroner et al.,1987; Stern, 1985, 1994; Greiling et al., 1994; Fig.1).

2.2. The Hamisana zone

The northern part of the HZ cuts across se-quences of gneisses, which are structurally over-lain by ophiolitic nappes and intruded by anumber of granitoids (EGSMA, 1999, Fig. 2).The probably oldest rocks exposed are a sequenceof variegated gneisses, containing both metasedi-mentary and igneous components. The latter com-prise both mafic, amphibolitic layers as well asfelsic, fine-grained and coarse-grained para- andortho-gneisses and, locally, close to younger intru-sives, also migmatites. The gneissic sequencesform a basal tier (tier 1, Bennet and Mosley, 1987;Greiling et al., 1994) and are structurally overlainby a number of major nappe units, which aredominated by ophiolite fragments (Gerf nappe,Kroner et al., 1987) and form a second tier (tier2). Both these major units are folded and intrudedby a number of syn- to late-tectonic granitoidplutons as well as some dyke swarms (Stern et al.,1990; Miller and Dixon, 1992).

The ophiolites are c. 730 Ma old (Zimmer et al.,1995) and the tier 1 gneisses originated about c.660 Ma ago, during and after the formation andcollision of island-arcs (Stern et al., 1989). Activ-ity along the HZ may have begun as early as 660Ma ago, culminating in intense thermal activity550–580 Ma ago (Stern et al., 1989). Unde-formed, post-tectonic granites dated at 510 Maago define an upper age limit of the deformation(Stern et al., 1989). Whereas tiers 1 and 2 aregenerally flat-lying at a regional scale, they areoverprinted by the N–S-trending HZ, up to a fewtens of km wide, where foliations are generallysteep (Fig. 3) and strain appears to be higher anddeformation more complex than in the surround-ing domains (Stern et al., 1990; Miller and Dixon,1992).

An apparent offset of the Onib-Sol Hamedsuture zone to the east of the HZ against theAllaqi suture zone to the west led earlier authorsto presume a horizontal, strike-slip movementalong the HZ (Kroner et al., 1987; Stern et al.,1989). However, recent results from the Allaqisuture zone (Sadek, 1995; EGSMA, 1996; ourobservations) show that this suture is curvingfrom the Wadi Allaqi towards the southeast andthat related ophiolite fragments occur to the southof the Allaqi-Heiani Belt of ophiolite nappes,which was earlier thought to represent the suturezone. Therefore, an alternative location for theAllaqi suture is tentatively shown on Fig. 1.

3. Field observations

The gneissic rocks of the HZ and the surround-ing area are characterized by a distinct metamor-phic banding (s1), which is particularly clearbetween felsic or intermediate layers intercalatedwith amphibolite (Fig. 9(A)–(C)). On both sidesof the HZ this banding is flat-lying at a regionalscale with gentle folds at km-scale wavelengths(EGSMA, 1999).

Towards the HZ, the N–S-trending folds be-come tighter, so that the metamorphic banding

Fig. 3. Schematic section across the HZ (a) and structural fielddata from the study area, (b) orientation of foliation showinga girdle distribution. The best-fit great circle and its pole(p-pole) are shown, (c) orientation of mineral and stretchinglineations. Diagrams (b) and (c) are equal area lower hemi-sphere stereonets.


dips more steeply (Fig. 9(A)–(B)). At the westernmargin of the HZ, the orientation of the meta-morphic banding rotates successively from moder-ate to steep westerly dips to almost vertical at theHZ itself. There, a zone c. 5–10 km wide ischaracterized by steep to vertical dips, strikingbroadly N–S (Figs. 2 and 3(b)), with subhorizon-tal mineral and stretching lineations in N–S direc-tions (Fig. 3(c)). At lithologic contacts in the HZ,a penetrative foliation (s2) can be observed, whichcuts the metamorphic banding at a low angle.Towards east, the dip shallows again and shows atransition to flat-lying ophiolite nappes, includingthe Wadi Onib ophiolite fragment (Fitches et al.,1983), overlying granitoid and other gneissicsequences.

Consequently, the HZ appears as a major N–S-trending, upright antiformal structure, with a highstrain zone broadly along and parallel with itsaxial surface. In some parts, minor folds withamplitudes and wavelengths in a cm-scale aroundN–S-trending axes can be recognized (El Kady,unpublished Diploma thesis, and own observa-tions). Apparently, the foliation (s2) is the axialsurface cleavage of these folds.

Locally, a subhorizontal to moderately plung-ing N–S lineation is well developed, marked bystretched plagioclase and a hornblende minerallineation in the amphibolites (Fig. 9(D)) andmetavolcanic rocks and by a quartz and feldsparstretching lineation in the orthogneisses. The lin-eation orientation forms a well-defined cluster onthe stereonet with a mean of 354/05 (Fig. 3(c)). Itssignificance is discussed in the context of themicrostructural observations.

4. Magnetic fabric analyses

4.1. Anisotropy of magnetic susceptibility (AMS)methods

Anisotropic magnetic behaviour of low fieldsusceptibility has been established as a method forthe detection of fabric anisotropies (Tarling andHrouda, 1993; Borradaile and Henry, 1997). Theadvantage of this method, compared to conven-tional macroscopic and microscopic and X-ray

diffraction techniques, is its high sensitivity toweak differences in the crystallographic orienta-tion of paramagnetic minerals (such as phyllosili-cates and amphiboles) and in the grain shape anddistribution of ferrimagnetic minerals (magnetite).AMS studies have been found to be particularlyuseful in constraining very low strains both re-lated to regional deformation events (Borradaile,1988) and pluton emplacements (Bouchez et al.,1990, 1997). In deformation fabrics there is acorrelation between the rock foliation and themagnetic foliation and rock lineation and mag-netic lineation, respectively (Tarling and Hrouda,1993; Siegesmund et al., 1995).

For detailed magnetic fabric analyses by theAMS method (Tarling and Hrouda, 1993) variousrock types from 35 sites distributed over an areaof approximately 500 km2 were sampled (Fig. 2).Handspecimens were oriented in the field andfrom every sample up to 6 AMS standard cylin-ders (1 in. in diameter, 0.8 in. in height) weredrilled in the laboratory. A total of 109 specimenswere investigated. The measurements were per-formed in low fields (300 A/m) with a Kap-pabridge (KLY-2) manufactured by AGICO(Jelinek, 1980). Thermomagnetic investigationsfor magneto-mineralogy used a CS-2 furnace ap-paratus of AGICO (Hrouda, 1994).

The magnetic susceptibility (k) is a materialproperty which describes the ability to acquire amagnetization (M) in an applied field (H). Asboth M and H expressed in SI units (SystemeInternationale), are measured in A/m, the con-stant k is dimensionless. The AMS is a secondrank tensor which is expressed by the AMS-ellip-soid with principal axes kmax; kint; kmin. The vol-ume susceptibility (k= (kmax+kint+kmin)/3),shape and anisotropy factors are calculated fromdirectional measurements taken in different sam-ple positions using the standard software for theKLY-2 equipment (Hrouda et al., 1990). Theshape of the AMS-ellipsoid is described by theparameter T: (2(ln kint− ln kmin)/(ln kmax− lnkint)−1) which is \0 to +1 for oblate, 0 forneutral and B0 to −1 for prolate geometry(Jelinek, 1981). The anisotropy of the ellipsoid isexpressed by the P% value, the so-called correctedanisotropy factor: exp (2(ln kmax− ln k)2+2(ln


Table 1Magnetic data for the individual samples

Rock typeSample N ka Linb a95 Folc a95 Td P %e

P 1 orthogneiss 1 850 181/11 – 094/76 – 0.22 1.312 447 223/70 4.3orthogneiss 281/79P 2a 4.4 0.88 1.132 395 011/01P 2b 2.2orthogneiss 101/84 5.2 0.84 1.112 395 011/01 2.2orthogneiss 101/84P 3 5.2 0.84 1.11

orthogneissP 4 4 6849 213/16 3.8 133/59 3.7 −0.14 1.27amphiboliteP 5 3 377 041/16 1.2 315/75 1.3 0.70 1.12

4 9900 351/04 3.2orthogneiss 079/62P 6 6.1 −0.56 1.502 2120 248/52P 7 6.8orthogneiss 264/51 4.8 0.34 1.244 4743 169/24 1.5biotite-schist 252/75P 8 1.3 0.33 1.60

amphiboliteP 9 4 733 015/30 4.7 295/22 4.5 0.17 1.07orthogneissP 10 3 324 028/45 1.7 360/45 1.2 0.41 1.11

3 4297 353/06 1.9orthogneiss 065/19P 11 2.5 0.12 1.234 20 026/47 18.0P 12 049/43leucogranite 6.5 0.38 1.064 405 020/39 4.5amphibolite 293/87P 13 2.3 0.64 1.04

amphiboliteP 14 4 740 341/53 3.1 261/84 4.3 −0.51 1.03amphiboliteP 15 3 717 347/11 4.6 262/65 4.7 0.83 1.11

4 229 008/29 4.1orthogneiss 076/57P 16 10.8 0.15 1.025 737 120/38 3.0P 17 054/63amphibolite 2.4 0.88 1.073 290 168/13 2.0biotite-schist 078/89P 18 1.9 0.46 1.11

leucograniteP 19 4 7208 179/05 7.5 263/40 2.7 0.73 1.40orthogneissP 20 4 303 170/10 2.6 089/47 3 0.25 1.17

4 6398 346/13 5.5orthogneiss 067/55P 21 4.1 0.27 1.514 61 359/20 2.9P 22 270/88orthogneiss 2.6 0.70 1.233 1112 060/04 3.0orthogneiss 138/15P 23 2.8 0.34 1.59

amphiboliteP 25 3 3159 198/03 2.6 289/76 10.1 −0.11 1.19amphiboliteP 26 2 14370 172/04 3.3 106/10 2.7 −0.18 1.34

3 805 018/15 6.7amphibolite 092/44P 27 5.7 0.68 1.103 9027 190/06 4.6 279/77P 29 5.1orthogneiss −0.26 1.283 55 198/21 2.6orthogneiss 246/29P 30 16.6 −0.54 1.12

amphiboliteP 34 2 3605 049/37 11.2 109/56 1.6 0.20 1.20orthogneissP 36 4 2406 354/21 1.9 314/27 1.1 −0.10 1.23

3 663 175/34 5.1amphibolite 264/86P 37 3.5 0.46 1.111 977 006/37 – 281/84P 38 –amphibolite −0.17 1.101 427 357/42 –amphibolite 298/61P 39 – 0.83 1.15

paragneissP 40 4 82 171/10 1.9 221/17 2.2 0.69 1.10

a k=volume susceptibility in 10−6 SI.b Lin – trend and plunge of the magnetic lineation and a95 confidence angle.c Fol – dip direction and angle of dip of the magnetic foliation and a95 confidence angle.d T=mean shape factor of the AMS-ellipsoid.e P %=mean corrected anisotropy factor for the AMS-ellipsoid.

kint− ln k)2+2(ln kmin− ln k)2)1/2 (Jelinek,1981). From the determined geographic orienta-tion for the principal axes of the susceptibilityellipsoids, the mean values for the magnetic lin-eation and the magnetic foliation are calculatedbased on Fisher distribution analyses. The mag-

netic lineation refers to the direction of maximumsusceptibility (kmax), the direction of minimumsusceptibility (kmin) is the pole to the magneticfoliation plane. Directional data are presented instereonets (Figs. 5 and 6(a) and (b)). All results ofthe AMS analyses are compiled in Table 1.


4.2. Volume susceptibility

In polycrystalline rocks as they are characteris-tic for the HZ the volume susceptibility is the sumof susceptibility and volume content of the vari-ous mineralogical components. Since differentrock types occur in the HZ a strong variation involume susceptibility due to their variations inmineral composition is observed. Lowest valuesare in the order of 10−5 SI for leucogranites, thehighest susceptibilities exceed 5×10−2 SI in mag-netite-bearing amphibolites (Table 1). In the sam-ples with low susceptibility, paramagneticminerals such as biotite in the paragneisses andbiotite schists and hornblende in the amphibolitesare dominating the magnetic fabric. In the sam-ples with higher susceptibilities (k\10−3) themagnetic behaviour is determined by the ferri-magnetic mineral fabric. Thermomagnetic curvesreveal a strong decrease in susceptibility at tem-peratures of 590°C indicating that the ferrimag-netic fabric is represented by pure magnetite (Fig.4).

4.3. Orientation and interpretation of the AMSdirectional data

A compilation of the orientation of the AMSdata from all samples measured is given in thestereographic projection in Fig. 5. The magnetic

Fig. 5. Orientation of magnetic fabric elements in geographicorientation. Compilation of all field measurements instereonets (lower hemisphere). kmin represents the poles of themagnetic foliation, which show a girdle distribution. A best-fitgreat circle and its pole (p-pole) are indicated. kmax representsthe orientation of the magnetic lineation. The mean directionis shown.

foliation poles (kmin) are distributed along a greatcircle, documenting a rotation of the foliationsurfaces around a horizontal N–S trending axis(p-pole). This geometry is in agreement with thegeometry of a regional-scale concentric fold struc-ture with a horizontal, N–S trending b-axis (Fig.3). The p-pole (002°/08°) constructed from themagnetic foliation poles is subparallel with themagnetic lineation (Fig. 5(b)) with a mean orien-tation of 179°/01°.

The general pattern of magnetic fabric corre-lates well with the field-measured foliation andlineation data, as presented in Fig. 3(b) and (c).However, whilst the lineations measured in thefield appear to have a clear unimodal distributionwith a single maximum around a horizontalmean, the magnetic lineations tend to scatter to-wards moderate to steep inclinations. Thestronger scatter in the magnetic lineation datarelative to the field data reveals that the fabric inthe HZ is more complex than expected from theanalysis of the field data alone. Therefore, weanalyzed the AMS data in more detail: the direc-tional data are presented in foliation and lineationmaps, respectively, with added stereographic pro-jections of the structural elements (Fig. 6(A) and(B)). In both maps, a subdivision into two struc-tural domains is evident. In domain I, the mag-netic foliation trends generally N–S. However,dip angles vary between 10° and 89°. The stereo

Fig. 4. Examples for thermomagnetic curves, k(T), for aparamagnetic sample (HAMS 8) and a ferrimagnetic sample(HAMS 10). The curves show the evolution of the magneticsusceptibility during heating. See the text for further discus-sion.


graphic projection exhibits a distribution of themagnetic foliation poles along a great circle with anearly horizontal p-pole, which is trending N–S,and subparallel with the strike of the HZ. Thispattern reflects the general geometry in the HZand corresponds to the field observations (Fig.3(b)). The magnetic lineations trend N–S withgentle to moderate plunges towards the S and N,respectively, but the lineations are generallysteeper in the western part of the HZ. This orien-tation accords with the field data presented in Fig.3(c), where the magnetic lineation is parallel withthe best-fit p-pole for the magnetic foliation. So,generally, the magnetic fabric in structural do-main I is symmetrical with a girdle distribution ofthe magnetic foliation around a N–S-trendingaxis, which is parallel with the observed lineation.

A second structural domain (domain II) formsa SW–NE trending sigmoidal trace across the HZwhere the magnetic foliations and lineations arereoriented towards a NE–SW direction. The an-gle of plunge for the magnetic lineations is vari-able and ranges from low (03°) to high (70°)values.

4.4. Shape of AMS-ellipsoids

The AMS measurements provide detailed infor-mation on fabric anisotropies and give strain andmineralogical information in addition to conven-tional structural data (Borradaile and Henry,1997). However, for a reliable interpretation ofthe magnetic fabric a detailed knowledge of thecomponents of the rock fabric and their respectivecontributions to the final AMS values is necessary(Borradaile, 1988; Rochette, 1987). In the presentstudy various rock types were investigated. As aconsequence, a strong variation in shape (T) rang-ing between +0.88 and −0.56, and anisotropy(P %) reaching from 1.02 up to 1.60 (Table 1) isobserved. In the Jelinek-diagram (P % vs. T) (Je-linek, 1981) this variation is obvious for bothstructural domains (Fig. 7(a)). This may indicatea general fabric inhomogeneity for the samplesfrom the HZ with both prolate (stretched: kmax\\kint=kmin) and oblate (flattened: kmax=kint\\kmin) subfabrics. However, it could also be aconsequence of single crystal magnetic anisotropyof the paramagnetic mineral components. AMS in

Fig. 6. (A) Magnetic foliation map and (B) magnetic lineation map, covering the same area as Fig. 2. Inset stereonets show theorientation means as calculated for each sampling site (data in Table 1). Note two domains (I and II, respectively), which are distinctby their fabric orientation.


Fig. 7. Anisotropy data for structural domains I (squares) andII (triangles), respectively. Plotted are the mean values for eachsampling site (data in Table 1). (A) Shape factor (T) andanisotropy (P %) of the AMS-ellipsoids, presented in a Jelinek-diagram (Jelinek, 1981). (B) Shape factor (T) versus bulksusceptibility (kmean).

5. Microstructures

Rocks in the HZ are characterized by a meta-morphic fabric developed under amphibolite fa-cies conditions. Amphibolites and hornblende-bearing gneisses, derived from volcanic rocks andgabbros contain coarse-grained green hornblende,biotite, feldspar and quartz. In the amphibolites,pronounced linear as well as distinct planar fab-rics occur, which are defined by elongatedfeldspar aggregates and the preferred orientationof hornblende (Fig. 9(D)). Aspect ratios for pro-late feldspar porphyroclasts in amphibolite, with awell-developed linear fabric, reach up to 4.

Coarse-grained biotite and muscovite producethe metamorphic foliation in the orthogneissesand metagranitoids. Occasionally, a stretching lin-eation is marked by elongated quartz grains andribbon quartz. Inclusions of a continuous micafoliation within large anhedral quartz grains and

Fig. 8. Map showing magnetic lineation trajectories in theinvestigated part of the HZ. The area is the same as for Fig. 2and Fig. 6. Domain II is representing a late-stage dextral shearzone (NE–SW) that is overprinting the N–S oriented struc-tural grain of the HZ. This late dextral shear zone may berelated to another dextral fault mapped by Stern et al. (1990),part of which is shown in the SE corner of the present map.

paramagnetic minerals is a function of single crys-tal anisotropy and degree of preferred orientation(Siegesmund et al., 1995; Hrouda et al., 1997).However, from Fig. 7 it is obvious that scatteringof the shape factor values is indicated both insamples with low (mainly paramagnetic minerals)and with high (ferrimagnetic minerals) susceptibil-ities. Therefore, we conclude that beside miner-alogical influences the observed variations inellipsoid shape are related to differences in fabricgeometry across the HZ. However, a significantrelationship of the shape factors to a particularstructural domain or a systematic variation in theshape along and across the HZ was not detected.In general, the AMS results agree with the fieldobservations, where also variations in the rockfabrics have been detected.


irregular phase boundaries between quartz andfeldspar indicate secondary grain growth (Fig.9(E)). The quartz grains are overprinted by asubsequent deformation, which produced undula-tory extinction and deformation bands.

In samples displaying a gneissic fabric, S–Cgeometries are observed, which are defined byelongated grains of quartz, mica or hornblende.In structural domain I, this fabric is not verypronounced and only occasionally a shear sensedetermination is possible. The angle between theoblique fabric (S) and the shear foliation (C)ranges between 30 and 45° and indicates a com-ponent of left-lateral shear for the ductile defor-mation (Fig. 9(E)).

In structural domain II the strain is concen-trated within discrete micro-shear zones that canbe observed locally between quartz and feldsparphase boundaries. In these zones, quartz grainsshow dynamic recrystallisation and anisometricsubgrains with long axes oriented oblique (40°)to the shear zone boundaries (Fig. 9(F)). In thefeldspar, sets of intracrystalline microfractureswere observed and deformation twinning anddiscontinuous undulatory extinction is abundant.Occasionally, subgrain development and recrys-tallisation is also observed within narrow zonesin the feldspar grains. Biotite recrystallized dur-ing this deformation episode. Apart from thedistinct shear zones, the fabric was only weaklyaffected by deformation, resulting in undulatoryextinction in quartz and twinning in feldspar.These observations indicate that in the domain IIdeformation was distinctly localized. The hightemperature deformation fabric observed instructural domain I was overprinted by lowertemperature deformation in structural domain II.The microstructures in samples P 23 (Fig. 9(F))and P 6 from this domain indicate a dextralmovement for this low temperature shear defor-mation which is opposite to the left-lateral move-ment under amphibolite facies metamorphicconditions at the HZ as a whole (Fig. 8). Astrong indication of this deformation episode isalso observed in the samples P 9, P 11 and P 12at the western part of the study area, where themagnetic lineations are rotated from N–S (struc-tural domain I) towards NW–SE (structural do-

main II). Here, quartz is strongly affected byundulosity and subgrain development and infeldspar densely spaced deformation twinningand fracturing is observed.

Almost all samples experienced a minor over-print under lower greenschist facies metamorphicconditions. This caused fracturing of feldsparand hornblende and, occasionally, was accompa-nied by retrograde mineral reactions such as for-mation of chlorite along fractures in hornblende,as well as the sericitization of feldspar.

6. Interpretation of AMS data andmicrostructures

The axes of the AMS-ellipsoids are generallysubparallel with the fabric axes with kmax parallelto the mineral or stretching lineation and kmin

normal to the metamorphic foliation (Figs. 3 and5). In samples with high susceptibility this an-isotropy is due to anisometric magnetite grainselongated in the direction of lineation (Gregoireet al., 1995), and to an anisotropy in distributionof magnetite (Hargraves et al., 1991). In samplesdominated by a paramagnetic fabric the coinci-dence is caused by the preferred orientation ofbiotite and hornblende, which are minerals withhigh single crystal susceptibility anisotropies(Friedrich, 1995; Siegesmund et al., 1995;Hrouda et al., 1997). The magnetic fabric, there-fore, can provide information about the orienta-tion of the finite strain in the HZ. However, ithas to be taken into account, that the AMSmethod integrates across the whole fabric and,therefore, may reflect resultants of interferingsubfabrics (Rochette and Vialon, 1984; Hroudaand Potfaj, 1993). For example in a S–C geome-try (sensu Berthe et al., 1979), deflections be-tween the magnetic foliation (S) and the shearfoliation (C-plane) may occur (Aranguren et al.,1995). However, in the samples investigated here,such a shear fabric is only weakly developed andthe resulting deflections are considered to be neg-ligible. Another problem interpreting AMS datacan be caused by mineralogical components thatproduce an inverse magnetic fabric as it is occa-sionally observed for hornblende with kmax per-


Fig. 9. Structural features of the HZ: A, B Field photographs from the western margin of the HZ, looking S (A) and SW (B),respectively, from a point at the southern end of Gabal Um Rasein (Fig. 2 for location). The photographs are arranged so as toshow the steep dip of the foliation within the HZ (cliffs in A) and the shallowing dips away from the HZ (towards W, B). The centralpart of the slope in B is build up of the lbib gneiss, the top of the Gabal Gerf ophiolite nappe (compare map, Fig. 2). C Exampleof banded gneiss with amphibolite layers (black), showing (broadly symmetric) boudinage, which indicates N–S extension and E–Wshortening. East side of Um Rasein, Fig. 2 for location; hammer for scale (above central boudin). D Handspecimen of amphibolitefrom the HZ (P 26) cut parallel with a mineral lineation marked by elongated feldspar porphyroclasts (structural X direction; XYsection) and normal to the mineral lineation (YZ section), respectively. The amphibolite shows a pronounced stretching fabric. E,F Photomicrographs of orthogneisses from the HZ, showing examples from domain I (E) and II (F), respectively: (E) coarse-grained,elongated quartz with lobate boundaries but little internal deformation defines an early foliation (S). Younger foliation surfaces (C),defined by mica minerals, cut the early foliation and are interpreted here as shear surfaces. This S–C fabric indicates a sinistral senseof shear (sample P 21, X polarizers). (F) Dynamically recrystallized quartz. Quartz grains and subgrains are oriented oblique to theearlier foliation and indicate a dextral shear deformation (sample P 23, X polarizers).

pendicular to the long mineral axis (Rochette etal., 1992; Friedrich, 1995). For the investigatedamphibolites and hornblende-bearing gneisses, aparallel orientation of the magnetic lineationwith the macroscopically visible mineral lin-eation was deduced so that a contribution ofinverse fabric elements can be excluded.

Microstructural analyses and AMS-directional

data and shape parameters indicate a variationin deformation fabric along the HZ rangingfrom strongly oblate to strongly prolate ge-ometries. Simple shear deformation is concen-trated in discrete zones. Between these zones thefabric shows mostly a pronounced stretching lin-eation but locally also flattening fabrics with nomacroscopically visible mineral lineations.


As a consequence of strong simple shear defor-mation a divergence between the high strain andlow strain geometries should be expected, sincethe angle between the shear zone boundary andthe X-axis of the strain ellipsoid decreases from45° towards a subparallel orientation with pro-gressive simple shear deformation. Such obliquemagnetic fabrics have been observed in regionswere granite emplacement is related to strike slipfault zones (Djouadi et al., 1997) or to transformgeometry (Bouillin et al., 1993).

However, in the present study, the lineations inhigh strain and low strain areas are parallel, forexample 352/06 for the nearly undeformed meta-granitoid (P 10) and 359/20 for a strongly de-formed orthogneiss (P 21). This supports theresults of microstructural observations that non-coaxial deformation is a minor component of thebulk deformation in the HZ.

The pronounced sigmoidal deflection of themagnetic lineation in structural domain II (Fig. 6)is clear on the map in Fig. 9, where the trend ofthe traces of the magnetic lineation trajectories ispresented. This deflection and progressive rota-tion of the magnetic lineation in the SW-part ofthe study area are accompanied by low tempera-ture deformation as described in the chapterabove (fractures in feldspar, strong undulose ex-tinction and localized recrystallization in quartz).For some samples a sense of shear could bedetermined and is indicated on the map (Fig. 9).

7. Discussion

From the regional tectonic context and fromgeologic maps, a strike-slip movement along theHZ has been inferred (Vail, 1985, 1988; Kroner etal., 1987; Greiling et al., 1994; Nasr et al., 1998).However, at least some field-based structuralstudies (Stern et al., 1990; Miller and Dixon,1992) failed to detect traces of such a large-scaledeformation and in particular those of a strike-slip episode along the north-central part of theHZ. These latter results are partly in accordancewith our observations in the field that the defor-mation appears to be relatively weak in places.But detailed studies revealed a more complex

story: the amount and geometry of strain showstrong variations across the HZ.

Macroscopic fabric elements and the magneticfabric in the study area show a good correlation.For example, magnetic lineations are subparallelwith the stretching or mineral lineations and mag-netic foliations are subparallel with the metamor-phic banding. AMS and field data can thus becombined for an interpretation of the deformationregime in the zone of concentrated deformation.In structural domain I (Figs. 6 and 9), the majorpart of the HZ, the foliation poles form a girdledistribution around the orientation of a well-defined subhorizontal N–S trending lineation thatis developed locally as a stretching lineation. Thisgeometry is attributable to a constrictionalregime, in contrast to a flattening regime, where awell-clustered foliation and a more scattered lin-eation should be expected. These results are inconflict with the interpretation of Miller andDixon (1992) who described a down dip directionfor the lineation and inferred a predominance ofcrustal shortening by folding and thrusting. How-ever, we agree with these authors that the ductiledeformation in the HZ is mainly coaxial andstrike slip deformation is limited.

The curvature and bending of pre-existingstructural units towards the HZ and into paral-lelism with the HZ-trend (Fig. 1) may be sugges-tive of a ‘drag’, as it is produced by (strike-slip)fault movement. However, this ‘drag’ is broadlysymmetrical on both sides of the HZ and thus notindicative of any significant relative movementacross the HZ. Instead, our data imply that thisoutcrop pattern is that of an antiform, whichbecomes progressively tighter at the HZ. Al-though this antiform is almost horizontal at alocal scale (Fig. 3), it plunges gently towards thesouth at a regional scale, thus producing theobserved outcrop pattern of fold limbs bendingaway from the tight fold closure at the HSZtowards the NW and NE (Fig. 1).

No deviation of magnetic lineation orientationsbetween the different lithologies is observed. Thisholds also for the deformed granitoids. Therefore,the intrusion of the granitoids is considered tohave taken place before or during the deformationalong the HZ. Consequently, such high strain


zones as the HZ are not only important late- topost-tectonic elements but may have played amajor role also during and after the accretionaryorogenic evolution, probably also during the as-cent of granitoid plutons (Hutton and Reavy,1992; D’Lemos et al., 1992; Castro et al., 1999).

The observed lower temperature overprint inthe NE–SW-trending structural domain II bydextral shear deformation is geometrically relatedto the NE–SW-trending dextral shear zone, de-scribed by Stern et al. (1990) (Figs. 1 and 9) aslate structures truncating the N–S-trending fabricof the HZ. These authors interpreted such struc-tures as shear components of a transpressive de-formation with shortening normal to the HZ,which developed contemporaneously or at a latestage of HZ deformation. However, our studyshows that the structures in the HZ are related totwo different episodes of deformation. An early,mainly coaxial phase, with E–W shortening andpronounced N–S extension under amphibolite fa-cies metamorphic conditions (structural domain I)and a subsequent, mostly non-coaxial overprintunder considerably lower temperature, greenschistfacies metamorphic conditions (structural domainII).

The late overprint could be related to stressfields active during the break-up of the Red Sea,where continental high strain zones may havebeen reactivated (Dixon et al., 1987; Talbot andGhebreab, 1997). However, earlier work on post-Pan-African basement deformation showed amore brittle faulting during Red Sea tectonics(Greiling et al., 1988). Therefore, it appears to bemore likely that also the domain II deformation isrelated to a latest Pan-African deformationalepisode. Consequently, the NE–SW-trending,dextral strike-slip zone at domain II is interpretedas a conjugate zone relative to the NW–SE-trend-ing, sinistral fault zones of Wadi Hodein or Najd(Stern, 1985, 1994; Greiling et al., 1994). In addi-tion to Stern et al. (1990) our results show thatsuch dextral, conjugate strike-slip zones may bemore widespread than it was known previously,although their regional importance is limited.

In conclusion, the present results show the HZas a zone of high strain but with only minorcomponents of wrench faulting. Deformation is

dominated by pure shear producing E–W-short-ening, with a strong N–S-extensional component,resulting in constrictional strain. It is only a sub-sequent overprint, which produced minor dextralwrench movement along a discrete, NE–SW-trending zone (domain II on Fig. 9).

Since these structures overprint the ophiolitenappes of the Allaqi-Sol Hamed suture, it can beinferred that the E–W-shortening which producedthe HZ took place after suturing of the Gabgabaterrane with the terrane adjacent towards north.

In a wider regional context, the present resultsprovide an important complement to the tectonichistory of the ANS: the Allaqi-Sol Hamed suture,trending broadly E–W, is overprinted by E–Wshortening during the formation of the HZ. Suchan E–W shortening has been proposed during thesuturing of the Gabgaba terrane with older conti-nental crust towards W (Keraf suture, Abdel-salam et al., 1998, Fig. 1). As a consequence, theaccretion of the ANS in northern Sudan–south-ern Egypt, can be confirmed as polyphase or atleast a two-phase event with early suturing byN–S convergence (formation of Allaqi-SolHamed suture) and a later stage with c. E–Wconvergence leading to collision of the earlierassembled terranes with a craton towards westand the formation of the Keraf suture (Abdel-salam et al., 1998). These results preclude earlierviews of the Pan-African evolution as a broadlycontinuous episode of transpressional orogeny(O’Connor et al., 1994). Neither are there tracesof large-scale escape movements in the presentarea to the south of the Najd Faults (Stern, 1985,1994) in a wider sense.

Acknowledgements

This work is a part of a cooperation betweenthe Geologisch-Palaontologisches Institut, Heidel-berg, and the Egyptian Geological Survey andMining Authority (EGSMA), Cairo, supported bythe BMBW through the ForschungszentrumJulich, International Bureau. The financial assis-tance is gratefully acknowledged. We thank theChairmen of EGSMA, G.M. Naim and M. ElHinnawy for their generous support. Many


thanks to L. Nano for his assistance in laboratorywork and L.N. Warr, A. Kontny and A. Maklouffor helpful discussions. M. El Eraki providedsome of the samples. We thank reviewers J. Meertand R.J. Stern for detailed comments and sugges-tions that considerably improved the quality ofthe paper.

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