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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-Paläontologisches Institut, Ruprecht-Karls-Uni ersität, Im Neuenheimer Feld 234 , 69120 Heidelberg , Germanyb Egyptian Geological Surey 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–N

bian shield (ANS). It trends broadly N–S from northern Sudan into southeastern Egypt and meets the present Re

Sea coast at 23°N. The HZ has been the subject of controversy with regard to its importance for the Pan-Africa

structural evolution. Interpretations range from a suture zone, a regional shear zone, or a large-scale transpression

wrench fault system. In this study, we characterize the nature of the high strain deformation by applying t

anisotropy of magnetic susceptibility method along with field and microstructural investigations. These investigatio

demonstrate that deformation in the HZ is dominated by pure shear under upper greenschist/amphibolite gra

metamorphic conditions, producing E–W shortening, but with a strong N–S-extensional component. This deformtion 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 wea

subsequent overprint was mostly non-coaxial. It took place under considerably lower temperature and led to a mino

NE–SW-trending, dextral wrench fault. Although it is of only local importance this fault may be itself a conjuga

relative to the prominent NW–SE-trending sinistral Najd faults in the northern ANS. Therefore, the HZ is dominate

by late orogenic compressional deformation and cannot be related to either large-scale transpressional orogeny o

major escape tectonics. © 2001 Elsevier Science B.V. All rights reserved.

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

www.elsevier.com/locate/precamr

1. Introduction

The Pan-African orogenic cycle has long been

recognized as a period of major crustal accretion,

where continental, island-arc, and oceanic teranes were brought together to form the cry

talline basement of the African continent as pa

of a late Neoproterozoic supercontinent (Unru

1997, for review). Whereas some parts of th

Pan-African orogen are characterized by cont

nental collisional tectonics (Burke and Sengö

* 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 0 3 0 1 - 9 2 6 8 ( 0 0 ) 0 0 1 4 1 - 8

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H . de Wall et al . / Precambrian Research 107 (2001) 179–194 180

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 of

Africa–Arabia, has been established as a typical

example of an accretionary orogen with lateral

accretion and suturing (Gass, 1981; Kröner, 1985;

Kröner et al., 1987; Stern, 1994). In the course of orogenic activity, numerous sutures and other

major deformational zones originated all across

this shield. Subsequent to the assembly of ter-

ranes, late deformation zones developed as the

expression of lateral escape tectonics or other

transtensional or transpressional episodes during

the final stages of continental crust formation

(Burke and Sengör, 1986; Stern, 1985, 1994;

Johnson and Kattan, 1999). Contrasting models

of 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, it

is essential to re-examine the evolution of the

orogenic structures in the ANS. Towards that

aim, the present authors studied a major Pan-

African deformational zone, the Hamisana zone

(HZ), which is exposed in the Red Sea Hills of

southeastern Egypt and northeastern Sudan (Fig.

1). The N–S-trending HZ is spectacularly visible

on satellite images, where it can be traced for

more than 300 km (Miller and Dixon, 1992). It is

parallel 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 is

located 100 km to the east/southeast of the HZ

(Abdelsalam, 1994, Fig. 1).

Well-defined ophiolites and ophiolite belts have

been 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 was

called the Onib-Sol Hamed suture (Fitches et al.,

1983; Dixon et al., 1987; Kröner et al., 1987;

Stern et al., 1989, 1990, Fig. 1). The HZ, cuts the

Onib-Sol Hamed suture and is, consequently,

younger. Although this structural relationship has

been well documented (Stern et al., 1990; Mille

and Dixon, 1992), a contrasting model of the H

as a major transpressional zone during terran

accretion has been proposed recently, comprisin

Fig. 1. Major deformation zones in the Pan-African baseme

of SE Egypt and NE Sudan and location of the study area

the northern HZ. Compiled from Kröner et al. (1987); Stern

al. (1990); Greiling et al. (1994); Greiling et al. (198

EGSMA (1996); EGSMA (1999); O’Connor (1996); Abd

salam et al. (1998); Rashwan and Greiling (1999) and

Makhlouf (unpublished data).

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Fig. 2. Geological sketch map of the study area. For locationsee Fig. 1. The right-lateral strike-slip fault in the SE corner of

the map is taken from Stern et al. (1990).

2. Evolution of the Arabian–Nubian shield and

the Hamisana zone

2 .1. Arabian – Nubian shield

The ANS evolved during the NeoproterozoiPan-African episode from a number of island-a

and minor continental terranes. They were acreted onto an East Sahara craton, which w

previously affected in the Neoproterozoic and now mostly covered by Phanerozoic sedimenand is presumed to extend westwards from th

Keraf suture (Abdelsalam et al., 1998). Lithologcally, this tectonic evolution from oceanic lith

sphere to island-arcs and continental lithospheis well documented by the rock sequences buildin

up the ANS: ophiolite suites, ocean floor (chertand passive margin (quartzo-feldspathic and ca

bonate) sedimentary sequences, island arc, calc-a

kaline igneous rocks, both intrusive ansupracrustal, together with ‘‘syn-orogenic’’ sedments derived from these igneous rocks. Finall

syn-collisional and late- to post-tectonic mamatic activity produced some gabbroic but most

granitoid intrusives and related volcanics. Latorogenic, molasse-type sediments are only foun

in some intramontane basins, mostly in the NWof the ANS (Akaad and El Ramly, 1958; Osmaet al., 1993). Structurally, at least parts of th

tectonic evolution are also documented, for example subduction-related tectonic mélange or fos

accretionary prism – (Wadi Ghadir; El Sharkawand El Bayoumi, 1979; Shackleton et al., 1980) o

early, ‘syn-island-arc’ deformation (Wadi HafafiEl Ramly et al., 1984; Rashwan, 1991). Subs

quent structures comprise both extensional fetures such as fault-bounded (molasse) basins (Ric

et al., 1993) and metamorphic core complex(Sturchio, et al., 1983; Fritz et al., 1996), larg

scale thrusts (Ries et al., 1983; El Ramly et al1984), and major wrench fault systems (Ster

1985; Sultan et al., 1988; Shimron, 1990; O’Con

nor et al., 1994; Abdelsalam et al., 1998; Greilinet al., 1998). Apparently, both extensional ancompressional deformation, as they are documented in the ANS, represent post-collision

structures and are succeeded by and associatewith wrench faulting (Abdeen et al., 1992; Grei

ing et al., 1994; Johnson and Kattan, 1999).

a relatively simple evolution from oblique conver-

gence and suturing towards transpressional shear

zones 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 time

sequence and the general tectonic significance of

these structures at a regional scale are not yet

fully known. Based on recent mapping by the

Egyptian Geological Survey, a part of the HZ(Figs. 1 and 2) was studied in the field and

sampled 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. This

latter method has been shown to be most sensitive

to document the type and style of deformation

even in places where no or hardly any deforma-

tion is visible macroscopically (Tarling and

Hrouda, 1993). In that way, it is possible not only

to characterize the deformation within the HZ butalso to determine a subsequent overprint, which

may escape detection by other methods. This

application of the AMS method is the first major

study of this kind in the ANS, apart from minor

pilot studies (Greiling, 1997; de Wall et al., 1998,

1999; El Eraki and Greiling, 1998).

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The most important of such faults in the north-

western ANS are the NW–SE-oriented Najd sys-

tem, including Wadi Kharit-Wadi Hodein fault

zones, the broadly N–S-oriented HZ and the Oko

Shear zone (Abdelsalam, 1994; Kröner 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 a

number of granitoids (EGSMA, 1999, Fig. 2).

The probably oldest rocks exposed are a sequence

of variegated gneisses, containing both metasedi-

mentary and igneous components. The latter com-

prise both mafic, amphibolitic layers as well as

felsic, fine-grained and coarse-grained para- andortho-gneisses and, locally, close to younger intru-

sives, also migmatites. The gneissic sequences

form a basal tier (tier 1, Bennet and Mosley, 1987;

Greiling et al., 1994) and are structurally overlain

by a number of major nappe units, which are

dominated by ophiolite fragments (Gerf nappe,

Kröner et al., 1987) and form a second tier (tier

2). Both these major units are folded and intruded

by a number of syn- to late-tectonic granitoid

plutons 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 and

collision of island-arcs (Stern et al., 1989). Activ-

ity along the HZ may have begun as early as 660

Ma ago, culminating in intense thermal activity

550– 580 Ma ago (Stern et al., 1989). Unde-

formed, post-tectonic granites dated at 510 Ma

ago define an upper age limit of the deformation

(Stern et al., 1989). Whereas tiers 1 and 2 are

generally flat-lying at a regional scale, they areoverprinted by the N–S-trending HZ, up to a few

tens of km wide, where foliations are generally

steep (Fig. 3) and strain appears to be higher and

deformation more complex than in the surround-

ing domains (Stern et al., 1990; Miller and Dixon,

1992).

An apparent offset of the Onib-Sol Hame

suture zone to the east of the HZ against th

Allaqi suture zone to the west led earlier autho

to presume a horizontal, strike-slip movemen

along the HZ (Kröner et al., 1987; Stern et a

1989). However, recent results from the Alla

suture zone (Sadek, 1995; EGSMA, 1996; ou

observations) show that this suture is curvin

from the Wadi Allaqi towards the southeast an

that related ophiolite fragments occur to the sout

of the Allaqi-Heiani Belt of ophiolite nappe

which was earlier thought to represent the sutu

zone. Therefore, an alternative location for th

Allaqi 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 metamophic banding (s1), which is particularly cle

between felsic or intermediate layers intercalate

with amphibolite (Fig. 9(A)–(C)). On both side

of the HZ this banding is flat-lying at a region

scale with gentle folds at km-scale wavelength

(EGSMA, 1999).

Towards the HZ, the N– S-trending folds b

come tighter, so that the metamorphic bandin

Fig. 3. Schematic section across the HZ (a) and structural fie

data from the study area, (b) orientation of foliation showi

a girdle distribution. The best-fit great circle and its po

(-pole) are shown, (c) orientation of mineral and stretchin

lineations. Diagrams (b) and (c) are equal area lower hem

sphere stereonets.

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dips more steeply (Fig. 9(A)–(B)). At the western

margin of the HZ, the orientation of the meta-

morphic banding rotates successively from moder-

ate to steep westerly dips to almost vertical at the

HZ itself. There, a zone c. 5– 10 km wide is

characterized by steep to vertical dips, striking

broadly 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, which

cuts the metamorphic banding at a low angle.

Towards east, the dip shallows again and shows a

transition to flat-lying ophiolite nappes, including

the Wadi Onib ophiolite fragment (Fitches et al.,

1983), overlying granitoid and other gneissic

sequences.

Consequently, the HZ appears as a major N–S-

trending, upright antiformal structure, with a high

strain zone broadly along and parallel with its

axial surface. In some parts, minor folds withamplitudes and wavelengths in a cm-scale around

N – S-trending axes can be recognized (El Kady,

unpublished Diploma thesis, and own observa-

tions). Apparently, the foliation (s2) is the axial

surface cleavage of these folds.

Locally, a subhorizontal to moderately plung-

ing N–S lineation is well developed, marked by

stretched plagioclase and a hornblende mineral

lineation in the amphibolites (Fig. 9(D)) and

metavolcanic rocks and by a quartz and feldspar

stretching lineation in the orthogneisses. The lin-eation orientation forms a well-defined cluster on

the stereonet with a mean of 354/05 (Fig. 3(c)). Its

significance is discussed in the context of the

microstructural observations.

4. Magnetic fabric analyses

4 .1. Anisotropy of magnetic susceptibility ( AMS )

methods

Anisotropic magnetic behaviour of low field

susceptibility has been established as a method for

the detection of fabric anisotropies (Tarling and

Hrouda, 1993; Borradaile and Henry, 1997). The

advantage of this method, compared to conven-

tional macroscopic and microscopic and X-ray

diffraction techniques, is its high sensitivity

weak differences in the crystallographic orient

tion of paramagnetic minerals (such as phyllosil

cates and amphiboles) and in the grain shape an

distribution of ferrimagnetic minerals (magnetite

AMS studies have been found to be particular

useful in constraining very low strains both r

lated to regional deformation events (Borradail

1988) and pluton emplacements (Bouchez et a

1990, 1997). In deformation fabrics there is

correlation between the rock foliation and th

magnetic foliation and rock lineation and ma

netic lineation, respectively (Tarling and Hroud

1993; Siegesmund et al., 1995).

For detailed magnetic fabric analyses by th

AMS method (Tarling and Hrouda, 1993) variou

rock types from 35 sites distributed over an are

of approximately 500 km2 were sampled (Fig. 2

Handspecimens were oriented in the field an

from every sample up to 6 AMS standard cylinders (1 in. in diameter, 0.8 in. in height) we

drilled in the laboratory. A total of 109 specimen

were investigated. The measurements were pe

formed in low fields (300 A/m) with a Kap

pabridge (KLY-2) manufactured by AGIC

(Jelinek, 1980). Thermomagnetic investigation

for magneto-mineralogy used a CS-2 furnace ap

paratus of AGICO (Hrouda, 1994).

The magnetic susceptibility () is a materi

property which describes the ability to acquire

magnetization (M) in an applied field (H). Aboth M and H expressed in SI units (Systèm

Internationale), are measured in A/m, the con

stant is dimensionless. The AMS is a secon

rank tensor which is expressed by the AMS-ellip

soid with principal axes max; int; min. The vo

ume susceptibility (=(max+int+min)/3

shape and anisotropy factors are calculated fro

directional measurements taken in different sam

ple positions using the standard software for th

KLY-2 equipment (Hrouda et al., 1990). Th

shape of the AMS-ellipsoid is described by thparameter T: (2(ln int− ln min)/(ln max−

int)−1) which is 0 to +1 for oblate, 0 fo

neutral and 0 to −1 for prolate geometr

(Jelinek, 1981). The anisotropy of the ellipsoid

expressed by the P value, the so-called correcte

anisotropy factor: exp (2(ln max− ln )2+2(

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Table 1

Magnetic data for the individual samples

Rock typeSample N a Linb a95 Folc a95 T

d P e

P 1 orthogneiss 1 850 181/11 – 094/76 – 0.22 1.31

2 447 223/70 4.3orthogneiss 281/79P 2a 4.4 0.88 1.13

2 395 011/01P 2b 2.2orthogneiss 101/84 5.2 0.84 1.11

2 395 011/01 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 1.50

2 2120 248/52P 7 6.8orthogneiss 264/51 4.8 0.34 1.24

4 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 1.07

orthogneissP 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 1.23

4 20 026/47 18.0P 12 049/43leucogranite 6.5 0.38 1.06

4 405 020/39 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 1.03

amphiboliteP 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 1.02

5 737 120/38 3.0P 17 054/63amphibolite 2.4 0.88 1.07

3 290 168/13 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 1.40

orthogneissP 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 1.51

4 61 359/20 2.9P 22 270/88orthogneiss 2.6 0.70 1.23

3 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 1.19

amphiboliteP 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 1.10

3 9027 190/06 4.6 279/77P 29 5.1orthogneiss −0.26 1.28

3 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 1.20

orthogneissP 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 1.10

1 427 357/42 – 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 =volume susceptibility in 10−6 SI.b Lin – trend and plunge of the magnetic lineation and 95 confidence angle.c Fol – dip direction and angle of dip of the magnetic foliation and 95 confidence angle.d T =mean shape factor of the AMS-ellipsoid.e P =mean corrected anisotropy factor for the AMS-ellipsoid.

int− ln )2+2(ln min− ln )

2)1/2 (Jelinek,

1981). From the determined geographic orienta-

tion for the principal axes of the susceptibility

ellipsoids, the mean values for the magnetic lin-

eation and the magnetic foliation are calculated

based on Fisher distribution analyses. The mag-

netic lineation refers to the direction of maximu

susceptibility (max), the direction of minimu

susceptibility (min) is the pole to the magnet

foliation plane. Directional data are presented i

stereonets (Figs. 5 and 6(a) and (b)). All results o

the AMS analyses are compiled in Table 1.

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4 .2 . Volume susceptibility

In polycrystalline rocks as they are characteris-

tic for the HZ the volume susceptibility is the sum

of susceptibility and volume content of the vari-

ous mineralogical components. Since different

rock types occur in the HZ a strong variation in

volume susceptibility due to their variations in

mineral composition is observed. Lowest values

are in the order of 10−5 SI for leucogranites, the

highest susceptibilities exceed 5×10−2 SI in mag-

netite-bearing amphibolites (Table 1). In the sam-

ples with low susceptibility, paramagnetic

minerals such as biotite in the paragneisses and

biotite schists and hornblende in the amphibolites

are dominating the magnetic fabric. In the sam-

ples with higher susceptibilities (10−3) the

magnetic behaviour is determined by the ferri-

magnetic mineral fabric. Thermomagnetic curves

reveal 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 AMS

directional data

A compilation of the orientation of the AMS

data from all samples measured is given in the

stereographic projection in Fig. 5. The magnetic

Fig. 5. Orientation of magnetic fabric elements in geograph

orientation. Compilation of all field measurements

stereonets (lower hemisphere). min represents the poles of t

magnetic foliation, which show a girdle distribution. A best-

great circle and its pole (-pole) are indicated. max represen

the orientation of the magnetic lineation. The mean directio

is shown.

foliation poles (min) are distributed along a gre

circle, documenting a rotation of the foliatiosurfaces around a horizontal N – S trending ax

(-pole). This geometry is in agreement with th

geometry of a regional-scale concentric fold stru

ture with a horizontal, N–S trending -axis (Fi

3). The -pole (002°/08°) constructed from th

magnetic foliation poles is subparallel with th

magnetic lineation (Fig. 5(b)) with a mean orien

tation of 179°/01°.

The general pattern of magnetic fabric corr

lates well with the field-measured foliation an

lineation data, as presented in Fig. 3(b) and (cHowever, whilst the lineations measured in th

field appear to have a clear unimodal distributio

with a single maximum around a horizont

mean, the magnetic lineations tend to scatter to

wards moderate to steep inclinations. Th

stronger scatter in the magnetic lineation da

relative to the field data reveals that the fabric

the HZ is more complex than expected from th

analysis of the field data alone. Therefore, w

analyzed the AMS data in more detail: the dire

tional data are presented in foliation and lineatiomaps, respectively, with added stereographic pr

jections of the structural elements (Fig. 6(A) an

(B)). In both maps, a subdivision into two stru

tural domains is evident. In domain I, the ma

netic foliation trends generally N– S. Howeve

dip angles vary between 10° and 89°. The stere

Fig. 4. Examples for thermomagnetic curves, (T ), for a

paramagnetic sample (HAMS 8) and a ferrimagnetic sample

(HAMS 10). The curves show the evolution of the magnetic

susceptibility during heating. See the text for further discus-

sion.

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graphic projection exhibits a distribution of the

magnetic foliation poles along a great circle with a

nearly horizontal -pole, which is trending N–S,

and subparallel with the strike of the HZ. This

pattern reflects the general geometry in the HZ

and corresponds to the field observations (Fig.

3(b)). The magnetic lineations trend N –S with

gentle to moderate plunges towards the S and N,

respectively, but the lineations are generally

steeper 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 with

the best-fit -pole for the magnetic foliation. So,

generally, the magnetic fabric in structural do-

main I is symmetrical with a girdle distribution of

the magnetic foliation around a N –S-trending

axis, which is parallel with the observed lineation.

A second structural domain (domain II) forms

a SW–NE trending sigmoidal trace across the HZ

where 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 info

mation on fabric anisotropies and give strain an

mineralogical information in addition to conven

tional structural data (Borradaile and Henr

1997). However, for a reliable interpretation

the magnetic fabric a detailed knowledge of th

components of the rock fabric and their respectiv

contributions to the final AMS values is necessar

(Borradaile, 1988; Rochette, 1987). In the presen

study various rock types were investigated. As

consequence, a strong variation in shape (T ) ran

ing between +0.88 and −0.56, and anisotrop

(P ) reaching from 1.02 up to 1.60 (Table 1)

observed. In the Jelinek-diagram (P vs. T ) (J

linek, 1981) this variation is obvious for bo

structural domains (Fig. 7(a)). This may indica

a general fabric inhomogeneity for the sampl

from the HZ with both prolate (stretched: max

int=min) and oblate (flattened: max=int

min) subfabrics. However, it could also be

consequence of single crystal magnetic anisotrop

of the paramagnetic mineral components. AMS

Fig. 6. (A) Magnetic foliation map and (B) magnetic lineation map, covering the same area as Fig. 2. Inset stereonets show th

orientation means as calculated for each sampling site (data in Table 1). Note two domains (I and II, respectively), which are distin

by their fabric orientation.

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Fig. 7. Anisotropy data for structural domains I (squares) and

II (triangles), respectively. Plotted are the mean values for each

sampling site (data in Table 1). (A) Shape factor (T ) and

anisotropy (P ) of the AMS-ellipsoids, presented in a Jelinek-

diagram (Jelinek, 1981). (B) Shape factor (T ) versus bulk

susceptibility (mean).

5. Microstructures

Rocks in the HZ are characterized by a meta

morphic fabric developed under amphibolite f

cies conditions. Amphibolites and hornblend

bearing gneisses, derived from volcanic rocks an

gabbros contain coarse-grained green hornblend

biotite, feldspar and quartz. In the amphibolitepronounced linear as well as distinct planar fab

rics occur, which are defined by elongate

feldspar aggregates and the preferred orientatio

of hornblende (Fig. 9(D)). Aspect ratios for pr

late feldspar porphyroclasts in amphibolite, with

well-developed linear fabric, reach up to 4.

Coarse-grained biotite and muscovite produ

the metamorphic foliation in the orthogneiss

and metagranitoids. Occasionally, a stretching li

eation is marked by elongated quartz grains an

ribbon quartz. Inclusions of a continuous micfoliation within large anhedral quartz grains an

Fig. 8. Map showing magnetic lineation trajectories in t

investigated part of the HZ. The area is the same as for Fig

and Fig. 6. Domain II is representing a late-stage dextral she

zone (NE–SW) that is overprinting the N–S oriented stru

tural grain of the HZ. This late dextral shear zone may

related to another dextral fault mapped by Stern et al. (199

part of which is shown in the SE corner of the present ma

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 scattering

of the shape factor values is indicated both in

samples with low (mainly paramagnetic minerals)

and with high (ferrimagnetic minerals) susceptibil-

ities. Therefore, we conclude that beside miner-

alogical influences the observed variations in

ellipsoid shape are related to differences in fabric

geometry across the HZ. However, a significantrelationship of the shape factors to a particular

structural domain or a systematic variation in the

shape along and across the HZ was not detected.

In general, the AMS results agree with the field

observations, where also variations in the rock

fabrics have been detected.

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irregular phase boundaries between quartz and

feldspar indicate secondary grain growth (Fig.

9(E)). The quartz grains are overprinted by a

subsequent deformation, which produced undula-

tory extinction and deformation bands.

In samples displaying a gneissic fabric, S –C

geometries are observed, which are defined by

elongated grains of quartz, mica or hornblende.

In structural domain I, this fabric is not very

pronounced and only occasionally a shear sense

determination is possible. The angle between the

oblique 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 can

be observed locally between quartz and feldspar

phase boundaries. In these zones, quartz grains

show dynamic recrystallisation and anisometricsubgrains with long axes oriented oblique (40°)

to the shear zone boundaries (Fig. 9(F)). In the

feldspar, sets of intracrystalline microfractures

were observed and deformation twinning and

discontinuous undulatory extinction is abundant.

Occasionally, subgrain development and recrys-

tallisation is also observed within narrow zones

in the feldspar grains. Biotite recrystallized dur-

ing this deformation episode. Apart from the

distinct shear zones, the fabric was only weakly

affected by deformation, resulting in undulatoryextinction in quartz and twinning in feldspar.

These observations indicate that in the domain II

deformation was distinctly localized. The high

temperature deformation fabric observed in

structural domain I was overprinted by lower

temperature deformation in structural domain II.

The microstructures in samples P 23 (Fig. 9(F))

and P 6 from this domain indicate a dextral

movement 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). A

strong indication of this deformation episode is

also observed in the samples P 9, P 11 and P 12

at the western part of the study area, where the

magnetic lineations are rotated from N–S (struc-

tural domain I) towards NW–SE (structural do-

main II). Here, quartz is strongly affected b

undulosity and subgrain development and

feldspar densely spaced deformation twinnin

and fracturing is observed.

Almost all samples experienced a minor ove

print under lower greenschist facies metamorph

conditions. This caused fracturing of feldsp

and hornblende and, occasionally, was accomp

nied by retrograde mineral reactions such as fo

mation of chlorite along fractures in hornblend

as well as the sericitization of feldspar.

6. Interpretation of AMS data and

microstructures

The axes of the AMS-ellipsoids are general

subparallel with the fabric axes with max parall

to the mineral or stretching lineation and mnormal to the metamorphic foliation (Figs. 3 an5). In samples with high susceptibility this a

isotropy is due to anisometric magnetite grain

elongated in the direction of lineation (Grégoi

et al., 1995), and to an anisotropy in distributio

of magnetite (Hargraves et al., 1991). In sampl

dominated by a paramagnetic fabric the coinc

dence is caused by the preferred orientation

biotite and hornblende, which are minerals wit

high single crystal susceptibility anisotropi

(Friedrich, 1995; Siegesmund et al., 199

Hrouda et al., 1997). The magnetic fabric, therfore, can provide information about the orienta

tion of the finite strain in the HZ. However,

has to be taken into account, that the AM

method integrates across the whole fabric an

therefore, may reflect resultants of interferin

subfabrics (Rochette and Vialon, 1984; Hroud

and Potfaj, 1993). For example in a S–C geom

try (sensu Berthé et al., 1979), deflections b

tween the magnetic foliation (S) and the she

foliation (C-plane) may occur (Aranguren et al

1995). However, in the samples investigated hersuch a shear fabric is only weakly developed an

the resulting deflections are considered to be ne

ligible. Another problem interpreting AMS da

can be caused by mineralogical components th

produce an inverse magnetic fabric as it is occ

sionally observed for hornblende with max pe

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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 t

show the steep dip of the foliation within the HZ (cliffs in A) and the shallowing dips away from the HZ (towards W, B). The centr

part 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 Examp

of banded gneiss with amphibolite layers (black), showing (broadly symmetric) boudinage, which indicates N–S extension and E–W

shortening. East side of Um Rasein, Fig. 2 for location; hammer for scale (above central boudin). D Handspecimen of amphiboli

from the HZ (P 26) cut parallel with a mineral lineation marked by elongated feldspar porphyroclasts (structural X direction; X

section) 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-graine

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 sen

of shear (sample P 21, X polarizers). (F) Dynamically recrystallized quartz. Quartz grains and subgrains are oriented oblique to t

earlier foliation and indicate a dextral shear deformation (sample P 23, X polarizers).

pendicular to the long mineral axis (Rochette et

al., 1992; Friedrich, 1995). For the investigatedamphibolites and hornblende-bearing gneisses, a

parallel orientation of the magnetic lineation

with the macroscopically visible mineral lin-

eation was deduced so that a contribution of

inverse fabric elements can be excluded.

Microstructural analyses and AMS-directional

data and shape parameters indicate a variatio

in deformation fabric along the HZ ranginfrom strongly oblate to strongly prolate g

ometries. Simple shear deformation is conce

trated in discrete zones. Between these zones th

fabric shows mostly a pronounced stretching lin

eation but locally also flattening fabrics with n

macroscopically visible mineral lineations.

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As a consequence of strong simple shear defor-

mation a divergence between the high strain and

low strain geometries should be expected, since

the angle between the shear zone boundary and

the X-axis of the strain ellipsoid decreases from

45° towards a subparallel orientation with pro-

gressive simple shear deformation. Such oblique

magnetic fabrics have been observed in regions

were granite emplacement is related to strike slip

fault zones (Djouadi et al., 1997) or to transform

geometry (Bouillin et al., 1993).

However, in the present study, the lineations in

high strain and low strain areas are parallel, for

example 352/06 for the nearly undeformed meta-

granitoid (P 10) and 359/20 for a strongly de-

formed orthogneiss (P 21). This supports the

results of microstructural observations that non-

coaxial deformation is a minor component of the

bulk 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 of

the traces of the magnetic lineation trajectories is

presented. This deflection and progressive rota-

tion of the magnetic lineation in the SW-part of

the study area are accompanied by low tempera-

ture deformation as described in the chapter

above (fractures in feldspar, strong undulose ex-

tinction and localized recrystallization in quartz).

For some samples a sense of shear could be

determined and is indicated on the map (Fig. 9).

7. Discussion

From the regional tectonic context and from

geologic maps, a strike-slip movement along the

HZ has been inferred (Vail, 1985, 1988; Kröner et

al., 1987; Greiling et al., 1994; Nasr et al., 1998).

However, at least some field-based structural

studies (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 the

HZ. These latter results are partly in accordance

with 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 sho

strong variations across the HZ.

Macroscopic fabric elements and the magnet

fabric in the study area show a good correlation

For example, magnetic lineations are subparall

with the stretching or mineral lineations and mag

netic foliations are subparallel with the metamo

phic banding. AMS and field data can thus b

combined for an interpretation of the deformatio

regime in the zone of concentrated deformation

In structural domain I (Figs. 6 and 9), the majo

part of the HZ, the foliation poles form a gird

distribution around the orientation of a we

defined subhorizontal N–S trending lineation th

is developed locally as a stretching lineation. Th

geometry is attributable to a constriction

regime, in contrast to a flattening regime, where

well-clustered foliation and a more scattered lin

eation should be expected. These results are

conflict with the interpretation of Miller anDixon (1992) who described a down dip directio

for the lineation and inferred a predominance o

crustal shortening by folding and thrusting. How

ever, we agree with these authors that the ducti

deformation in the HZ is mainly coaxial an

strike slip deformation is limited.

The curvature and bending of pre-existin

structural units towards the HZ and into para

lelism with the HZ-trend (Fig. 1) may be sugge

tive of a ‘drag’, as it is produced by (strike-slip

fault movement. However, this ‘drag’ is broadsymmetrical on both sides of the HZ and thus no

indicative of any significant relative movemen

across the HZ. Instead, our data imply that th

outcrop pattern is that of an antiform, whic

becomes progressively tighter at the HZ. A

though this antiform is almost horizontal at

local scale (Fig. 3), it plunges gently towards th

south at a regional scale, thus producing th

observed outcrop pattern of fold limbs bendin

away from the tight fold closure at the HS

towards the NW and NE (Fig. 1).No deviation of magnetic lineation orientation

between the different lithologies is observed. Th

holds also for the deformed granitoids. Therefor

the intrusion of the granitoids is considered

have taken place before or during the deformatio

along the HZ. Consequently, such high stra

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zones as the HZ are not only important late- to

post-tectonic elements but may have played a

major role also during and after the accretionary

orogenic 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 in

the NE– SW-trending structural domain II by

dextral shear deformation is geometrically related

to the NE – SW-trending dextral shear zone, de-

scribed by Stern et al. (1990) (Figs. 1 and 9) as

late structures truncating the N–S-trending fabric

of 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 late

stage of HZ deformation. However, our study

shows that the structures in the HZ are related to

two 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 overprint

under considerably lower temperature, greenschist

facies metamorphic conditions (structural domain

II).

The late overprint could be related to stress

fields active during the break-up of the Red Sea,

where continental high strain zones may have

been reactivated (Dixon et al., 1987; Talbot and

Ghebreab, 1997). However, earlier work on post-Pan-African basement deformation showed a

more brittle faulting during Red Sea tectonics

(Greiling et al., 1988). Therefore, it appears to be

more likely that also the domain II deformation is

related to a latest Pan-African deformational

episode. Consequently, the NE– SW-trending,

dextral strike-slip zone at domain II is interpreted

as 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 be

more widespread than it was known previously,

although their regional importance is limited.

In conclusion, the present results show the HZ

as a zone of high strain but with only minor

components of wrench faulting. Deformation is

dominated by pure shear producing E–W-shor

ening, with a strong N–S-extensional componen

resulting in constrictional strain. It is only a sub

sequent overprint, which produced minor dextr

wrench movement along a discrete, NE–SW

trending zone (domain II on Fig. 9).

Since these structures overprint the ophioli

nappes of the Allaqi-Sol Hamed suture, it can b

inferred that the E–W-shortening which produce

the HZ took place after suturing of the Gabgab

terrane with the terrane adjacent towards north

In a wider regional context, the present resul

provide an important complement to the tecton

history of the ANS: the Allaqi-Sol Hamed sutur

trending broadly E– W, is overprinted by E– W

shortening during the formation of the HZ. Suc

an E – W shortening has been proposed during th

suturing of the Gabgaba terrane with older cont

nental crust towards W (Keraf suture, Abde

salam et al., 1998, Fig. 1). As a consequence, thaccretion of the ANS in northern Sudan–south

ern Egypt, can be confirmed as polyphase or

least a two-phase event with early suturing b

N– S convergence (formation of Allaqi-S

Hamed suture) and a later stage with c. E– W

convergence leading to collision of the earli

assembled terranes with a craton towards we

and the formation of the Keraf suture (Abde

salam et al., 1998). These results preclude earli

views of the Pan-African evolution as a broad

continuous episode of transpressional orogen(O’Connor et al., 1994). Neither are there trac

of large-scale escape movements in the prese

area to the south of the Najd Faults (Stern, 198

1994) in a wider sense.

Acknowledgements

This work is a part of a cooperation betwee

the Geologisch-Paläontologisches Institut, Heide

berg, and the Egyptian Geological Survey anMining Authority (EGSMA), Cairo, supported b

the BMBW through the Forschungszentru

Jülich, International Bureau. The financial assi

tance is gratefully acknowledged. We thank th

Chairmen of EGSMA, G.M. Naim and M. E

Hinnawy for their generous support. Man

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thanks to L. Nano for his assistance in laboratory

work and L.N. Warr, A. Kontny and A. Maklouf

for helpful discussions. M. El Eraki provided

some of the samples. We thank reviewers J. Meert

and R.J. Stern for detailed comments and sugges-

tions that considerably improved the quality of

the paper.

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