structural and tectonic evolution of the umm gheig/ el-shush

Jo,,r,,al of African Earth Soences. Vol. 33. No. 2, pp. 199-209. 2001 o 2002 Elsewer Science Ltd

All rights reserved. Prmted I” Great Elrltaln 0899.5362101 S- see front matter

Structural and tectonic evolution of the Umm Gheig/ El-Shush region, central Eastern Desert of Egypt

S. IBRAHIM and J. COSGROVE” Department of Earth Sciences and Engineering, Royal School of Mines, Imperial College,

Prince Consort Road, London SW7 2BP, UK

ABSTRACT-The rocks of the Wadi Umm Gheig/El-Shush area in the central Eastern Desert of

Egypt form part of the Nubian Shield, a component of the Neoproterozoic Pan-African Orogeny.

The rocks have been divided into three units: (i) low-grade metamorphosed rocks, which consist

of metavolcanic rocks interleaved with slices of ophiolitic melange; (ii) high-grade metamorphic

rocks, which consist of syn-tectonic granitoids; and (iii) post-tectonic granites, which intrude into

both the low- and high-grade rocks.

Three distinct tectonic and two magmatic events have been deduced from the structural analysis

of the area. These are listed in chronological order: (i) the formation of the major D, sinistral strike-

slip El-Shush Shear Zone, which occurs within the granitoid rocks (six individual granitoid bodies,

all now intensely sheared, are thought to have been intruded into the active El-Shush Shear

Zone); (ii) the emplacement of the metavolcanic rocks over the granitoid rocks by major D,thrusting

along a low angle dip-slip shear zone; (iii) upright open folding of the rocks during D,; and (iv) the

intrusion of late stage granites, which are virtually undeformed.

A model for the tectonic evolution of the study area is proposed. It is argued that the major

strike-slip shear zone of the region is possibly related to island-arc accretion and that the

various granitoid rocks were intruded along this active strike-slip zone. During the collision

associated with island-arc accretion, ophiolite sheets were interleaved with the volcanic rocks

and together were thrust over the granitoid basement. Field observations show that an important

episode of folding occurred after thrusting and that the folds possess an axial planar fracture

cleavage. They are, therefore, the result of a sub-horizontal tectonic compression rather than

of diapirism as previously suggested. Thus, the domal structures of the Eastern Desert may

be the result of folding rather than diapirism. 0 2002 Elsevier Science Limited. All rights

reserved.

(Received 2418198: accepted 1 O/9/01 1

INTRODUCTION

The Pan-African basement of the Eastern Desert of Attempts have been made to determine the exact Egypt includes one of the oldest known relics of temporal and structural evolution of these rock com- oceanic crust, ca 800 Ma old (Kroner, 1985). This is plexes in these regions (e.g. Shackleton et a/., 1980; tectonically overlain by other magmatic rocks, which El-Gaby et al., 1984, 1988; Bennett and Mosely, include ophiolites in an assemblage characteristic of 1987; Greiling, 1987; Greiling and Ramley, 1990; present-day island-arcs located above active sub- Greiling et a/., 1994; O’Connor, 1994). The excellent duction zones. This petrological and geochemical exposures of metavolcanic rocks, igneous intrusions assemblage is now well recognised in northeast Africa and major shear zones in the Wadi El-Shush/Umm and in the Arabian Peninsula. Gheig area of the Eastern Desert of Egypt, combined

*Corresponding author

[email protected]

Journal of African Earth Sciences 199

S. IBRAHIM and J. COSGROVE

with its relatively easy access, make it an ideal area

to study elements of the tectonic evolution of the

Nubian Shield. The metavolcanic rocks and the igneous

intrusions of the study area are typical of an island-

arc. The aims of the work reported in this paper are

to use observations of small-scale structures in

combination with geological mapping to determine the

movement episodes on the shear zones and to

determine the most appropriate tectonic model for

the evolution of this part of the Eastern Desert.

The work involved a detailed study of the orientation

and intensity of the rock fabrics and of the contacts

between the various intrusions along the major shear

zone of the Wadi El-Shush area. In addition, a study

of the spatial and temporal relationship between the

rocks of Wadi Umm Gheig and Wadi El-Shush was

undertaken in an attempt to determine whether they

are separated by a major strike-slip or dip-slip shear

zone and how they relate to each other in a broader

tectonic setting.

THE GEOLOGY OF THE WADI UMM GHEIG/

EL-SHUSH AREA

The study area lies in the eastern part of the Eastern

Desert of Egypt (Fig. la) and is delimited by the

latitudes 25”25’-25”27’ and longitudes 34”08’-

34’22’. It forms a part of the exposed Pan-African

basement (Nubian Shield) of the central Eastern

Desert of Egypt and covers - 640 km*. The area is

intersected by many wadis, the two largest being

Wadi Umm Gheig and Wadi El-Shush, which drain into

the Red Sea to the east.

The rocks of the area comprise three mapable units

(see e.g. Khudier et a/., 1995). These are:

J underlying granitoid basement of the El-Shush area,

which has been metamorphosed to amphibolite-facies;

iii overlying metavolcanic rocks of the Umm Gheig

area, which are interleaved with sheets of ultramafic

rocks and which have been metamorphosed to

greenschist-facies; and

iii) alkali feldspar granites of the Umm Shadad area

(Fig. 1 a).

The granitoid rocks are exposed in Wadi El-Shush

where six individual elongate granitoid bodies, which

were emplaced syntectonically along the El-Shush

Shear Zone, have been recognised by the authors (Fig.

1 a). These are:

il amphibolite-migmatite association (oldest);

iii Umm Luseifa Gneiss;

iii) El-Shush gneissic tonalite;

iv) Abu Marakhat gneissic granodiorite;

v) Umm Luseifa porphyritic gneissic granite; and

vii El-Mirifiya garnetiferous gneissic granite

(youngest).

200 Journal of African Earth Sciences

The widths of these granitoid bodies vary from a

few metres to several kilometres. They are charac-

terised by an elongate outcrop shape (Fig. 1) and an

internal structure defined by a steeply inclined planar

fabric and a sub-horizontal linear fabric, the result of

ductile shearing. The intensity of the planar fabric

within these intrusions varies considerably and, in

places, mylonites have been formed. Small-scale

asymmetric folds occur within the mylonite bands

with axial planes approximately parallel to the my-

lonitic foliation. Their plunges range between steeply

to gently inclined.

The overlying metavolcanic rocks are exposed in

Wadi Umm Gheig and comprise a heterogeneous

assemblage of:

II acid micaceous-rich (biotite, sericite, +/-chlorite)

rocks termed the grey-metavolcanics;

iii green-metavolcanic rocks dominated by dark

green mafic minerals (amphibole + biotite and epidote,

plagioclase);

iiil serpentinite-talc rocks; and

iv) amphibolites.

Numerous ellipsoidal shear-bounded pods of low

deformation in the metavolcanic rocks reveal traces

of original volcanic features, such as stretched

amygdales and flattened feldspar porphyroclasts in

aphanitic lavas. For example, mafic and subordinate

acid lavas exhibiting flow lamination and auto-

brecciation structures, together with a coarse-grained

andesite body, occur in almost unstrained pods in

central Wadi Umm Gheig.

Grain size in the metavolcanic rocks varies irregularly

across strike from fine-grained to coarse-grained.

Although some of these variations are primary, it is

clear from field observations that grain size reduction

has also occurred as a result of shearing, particularly

near the boundary that separates the metavolcanic rocks

from the underlying granitoids where a shear zone up to

1 km wide has developed.

Coarse- to medium-grained epidote-amphibole

greenstones are abundant in central and upper Wadi

Umm Gheig. These are frequently interleaved with

amphibole-biotite-plagioclase schists. Included with-

in the metavolcanics is a minor group of recrystallised

amphibole-microgabbro and microdiorite, which

locally develop a striped banding of felsic and mafic

minerals.

The Umm Gheig metavolcanic belt is frequently

interleaved with variably altered ultramafics in the

form of wedges or slices usually in association with

bluish-grey talc-graphite schists. These serpentinites

have sharp tectonic contacts with the adjacent

metavolcanics and occur in numerous colours and

textural formats, the most common being one which

exhibits a cream-khaki colour and which displays

Structuraland tectonic evolution of the Umm Gheig/El-Shush region, central Eastern Desert of Egypt

..*.**. l . . ..**.*..* .*.**. . ..*..*..t**

l .*... *....*.***t*

..*.*...**t.*

l **tt**t+**t

l ...**..*. .*.t*.

.*....t

. . ..t..

b)

y Main thn&t of Khor

cl Umm Rakham

El-MMva gameriferous gneissic granite

3 Umm Lusetfa porphyritic 9

gneissic granodtorite b e 5

Abu Marakhat gneisstc (I)

granodiorite

Ophiolitic Melange rocks

Metavolcanic rodts

Whush grwissic tonslite 3 El-Sibs poclt tectonic granite

Umm Luseifa gneiss 8 Umm Shaded late tectonic granite

Amphibolit~migrnatite assouatlon

Figure 1. la) Geological map of rbe El-Shush/Umm Gbeig area, Eastern Desert, Egypt. lbl Geological cross-section IA ‘8 7 along the Umm Gheig/El-Shush area. /c/ Geological cross-section (C’-D7 along the Khor Umm Rakham area

Journal of African Earth Sciences 20 1

S. IBRA HIM and J. COSGRO VE

carious weathering. Serpentinites locally contain

patches of cauliflower magnesite. A less common

variety of ultramafic rock, and one representing a

more mature state of alteration, is the ankerite-silica-

graphite schist, Talc-graphite schist slices are nearly

always present in the vicinity of khaki schistose

Within the metavolcanics of Wadi Umm Gheig, a

few exposures of ophiolitic melange are found. These

rocks are characterised by blocks of various com-

positions and sizes set in a strongly sheared matrix.

It is clear from both field observations (Fig. 1 a) and

satellite images that the area has been intruded by

alkali feldspar granites, which form approximately

circular outcrops up to 22 km in diameter, Fig. 1 a.

These rocks, which are virtually undeformed, have

intrusive contacts with the basement granitoids and

overlying metavolcanics.

STRUCTURAL ELEMENTS IN

THE EL-SHUSH AREA

The dominant structural pattern of the basement

rocks, which crop out in the El-Shush area, is

controlled by a steeply dipping D, west-northwest-

east-southeast-trending shear zone (Fig. 1). This is

made up of a network of anastomosing bands of

generally highly-strained rocks in which folding and

linear fabrics are common. Field observations within

this El-Shush Shear Zone show that at least six

individual granitoid intrusions have been deformed by

the shear zone and, therefore, either predate or were

emplaced during its formation. This shear zone is

defined by a planar mylonitic fabric with a consistent

west-northwest-east-southeast orientated sub-

horizontal lineation, (Fig 2a, b), which is either a D,

mineral stretching lineation (L,) on the S, schistosity

or a local constrictional fabric. In addition, slickenside

lineations, which consist of fibrous mineral aggregates

composed of feldspar, quartz and biotite, are also

found on some of the schistosity planes within the

shear zone. They form a marked lineation sub-parallel

to the stretching lineations and plunge 2O’toward

the west-northwest.

Porphyroclasts are found within the shear zone, and

they act as excellent kinematic indicators. Both o-

type and &type porphyroclast systems occur. The o-

type porphyroclasts are invariably feldpars and

characterised by wedge-shaped tails of recrystallised

material with an internal monoclinic stair-stepping

symmetry (Fig. 3a). &type porphyroclast systems (Fig.

3b) are also found within the ultramylonites. Both

indicate a sinistral sense of displacement.

In addition, small-scale F, folds, generated during

the formation of the El-Shush Shear Zone, have an S-

shaped profile, which also indicates a sinistral sense


of shear. However, it should be noted that many of

these minor folds have sub-horizontal axes. The

reason for this is probably related to the formation of

sheath folds and is discussed in greater detail later in

the paper.

Locally, the mylonite foliation is cut obliquely by a

second planar fabric with the same dip, which strikes

approximately east-west. The relationship between

the two foliations is that of the well-known S-C fabrics

that characterise many shear zones (see e.g. White

et al., 1986) and which can be used as kinematic

indicators. They confirm the sinistral sense of

movement on the El-Shush Shear Zone declared by

the porphyroclasts shown in Fig. 3.

STRUCTURAL ELEMENTS IN

THE UMM GHEIG AREA

The relationship between the granitoid basement of

Wadi El-Shush and the metavolcanic rocks of the Wadi

Umm Gheig area can be clearly seen in the geological

cross-sections shown in Fig. 1 b, c. The cross-sections

show a major low angle fault zone separating the

granitoid basement from the overlying deformed

metavolcanic rocks. This fault has been interpreted

by the authors as a thrust, which carried the meta-

volcanic rocks over the granitoids (Fig. 1 c). Within

the study area, the outcrop of the thrust can be traced

for 60 km (Fig. 1 a). During the overthrusting of the

metavolcanic nappes, most of the deformation in the

thrust zone seems to have been concentrated in the

granitoid rocks where, close to the thrust, very high

strains have produced a mylonitic foliation. Within this

zone of deformation, retrogression to greenschist-

facies has occurred.

In the metavolcanic nappes of Wadi Umm Gheig,

two planar fabrics are found. S, the earliest and most

dominant, is sub-parallel to bedding and is associated

with the thrusting of these rocks over the granitoid

rocks of the basement. The later fabric, S,, is an

axial planar fabric associated with the formation of

west-northwest-east-southeast-trending upright

folds. These are shown in Fig. 1 a, b: They fold the

metavolcanic nappes and, therefore, post-date their

emplacement. The folds are, therefore, related to a

third deformation phase D,.

Although the thrust that occurs at the junction

between the granitoid basement and the overlying

metavolcanic rocks is the main thrust, thrusts develop

on a variety of scales. Smaller thrusts occur within

the metavolcanics, as for example the Kab Ahmed

thrust zone (Fig. 1 a). An outcrop size example from

the same area is shown in Fig. 4a. The two faults at

Wadi Kab Ahmad, shown on the map in Fig. 1 a, are

parallel to bedding, which in this area dips steeply to

Structuraland tectonic evolution of the Umm Gheig/EJ-Mush region, central Eastern Desert of Egypt

Figure 2. (al Contours of mineral stretching line&ions measured on mv/onitic

foliation in the granrtoid basement of the Wadt El-Shush. Ibl Rose histogram

of the mfnerai stretchmg heations shown in (al. (cl Contours of mineral

stretchrng linearion measured on the LIZ thrust related foliation in the

metavolcanic rocks of Wad! Umm Gheig. Idl Rose histogram of the mineral

stretching lineations shown in /cl. lei Poles to bedding I*J, poles to cleavage

(01 and minor fold axes Ix) in the metavolcanic rocks of Wadi Umm Gheig.

The axial plane of the F3 folds can be seen lo dip steeply towards the

northeast.

the northeast as a result of the late D, folding. In the

following section, the authors present evidence that

suggests that these faults are thrust zones.

The thrust-generated schistosity within the meta-

volcanic rocks is not uniformly developed and this is

probably the result of the variation in the original rock

type and the localisation of slip along the boundaries

between adjacent lava flows. This S, fabric has a

well-developed mineral stretching lineation, which, as

can be seen from Fig Zc, d, has an average trend of

west-northwest-east-southeast. Figure 26 shows

that the mineral lineation plunges more toward the

southeast than the northwest. Kinematic indicators,

such as small-scale thrusts and asymmetrical F, folds

in the mylonitic fabric (Fig. 4a) and in quartz veins

containing well-developed pinch-and-swell structures

[Fig. 4b), indicate that thrusting was top to the west-

northwest.

S. IBRAHIM and J. COSGRO VE

Figure 3. Kinematic indicators in mylonites within the El-Shush Shear Zone. (al O- type feldspar porphyroclast. Ib) &type feldspar porphyroclast. These structures show the sinistral sense of movement along the El-Shush Shear Zone.

STRUCTURAL SYNTHESIS Based on the structural study outlined in this paper,

it is concluded that the tectonic evolution of the study

area involved three major tectonic events. These are:

iI formation of the El-Shush sinistral shear zone in

the basement during D,;

ii) later D, thrusting of the metavolcanic rocks over

the granitoid basement; and

MJ formation of the west-notthwest-east-southeast-

trending upright folds, which fold the major thrust D,.

The earliest deformation, D,, only affects the

basement granitoid rocks and the most important D,

structure is the El-Shush Shear Zone and the asso-

ciated planar mylonitic fabric, which dips steeply (65-

85”) to the southwest. It contains two sub-horizontal

lineations, i.e. a mineral stretching lineation and crystal

fibre ‘slikensides’. These two lineations indicate that

both ductile and brittle deformation occurred during

shearing along the shear zone. This observation is

compatible with the conclusion reached from the study


Structuraland tectonic evolution of the Umm Gheig/El-Shush region, central Eastern Desert of Egypt

WNW ESE

-- --z’,, ’ lo cm ’ a

WNW ESE 1

z

63 , IOcm ,

b

Figure 4. Kinematic indicators in the metavolcanic rocks.

(al Folding of the mylonitic fabric in Wadi Kab-Ahmed. (b) Folds in quartz veins containing well-developed pinch-and- swell structures, Wadi Umm Ghamis. Both structures show thrusting top to the west-northwest.

of these rocks in thin section, which shows that a

crystal plasticity texture is destroyed in places by

bands of cataclasis (Ibrahim, 1996). It is suggested

that these observations indicate either an increase in

strain rate after the formation of the mylonitic fabrics,

which caused the onset of brittle deformation, or that

the two textures were developed during non-syn-

chronous deformation events such as, for example,

an early event associated with strike-slifjmovement

along a relatively deep-seated region of a fault and

latter movement on the same part of the fault system

when it has been cooled as a result of partial exhu-

mation. In this regard, it is relevant to note that a

retrograde metamorphism is associated with the

shearing events that produced the cataclasis within

the shear zone.

During mylonitisation, minerals in the rock under-

went dynamic recrystallisation with the result that a

reduction in grain size took place, and strong patterns

of preferred orientation of crystals (e.g. quartz, feld-

spar) developed. Kinematic indicators found along the

El-Shush Shear Zone include S-C mylonites,

asymmetrical porphyroclast systems (Fig. 3) and

asymmetrical minor folds. These indicators point to

a dominant sinistral sense of shearing.

The second deformation to affect the study area,

G2, is associated with the emplacement by thrusting

of the metavolcanic rocks over the granitoid basement. The major D, structure is the shear zone, up to 1 km. wide, that separates the metavolcanics from

the basement rocks. This important thrust can be

traced more than 60 km in the study area along the

contact between these two rocks (Fig. la). The

distribution of the D, deformation in the metavolcanic

nappes is uneven. Numerous shear-bounded pods of

low deformation occur, and these contain almost

undeformed primary features, such as amygdales and

feldspar porphyroclasts. Interleaved with the meta-

volcanic rocks are slices of ultramafic material. Sharp

tectonic contacts separate the two rock types, and

it is suggested that the ultramafics are imbricate

slices, possibly of ophiolitic origin.

The planar fabric associated with thrusting has a

mineral stretching lineation trending west-northwest-

east-southeast and plunging gently to the east-

southeast (Fig. 2c, d) and small-scale thrusts (Fig.

4a). Folds indicate that movement on the thrust was

top to the west-northwest. In the previous section, it

was suggested that the two faults in Wadi Kab Ahmad

(Fig. 1 a) are thrusts. A brief summary of the argu-

ments used to support this view are given below.

The more southerly of the faults defines the boun-

dary between the underlying granitoids and the

metavolcanics; the other lies within the metavol-

canic rocks. Inspection of the map shows that the

fault separating the granitoids and metavolcanics

can be traced for over 50 km and is broken in only

one place by the intrusion of a post-tectonic granite.

As this contact is known to be a thrust from field

data at Wadi Umm Ghamis and Umm Gheig, it is

argued that the faults at Wadi Kab Ahmad are also

thrusts. At present, these faults dip steeply to the

northwest and it is argued that this is the result of

folding during the D, deformation (see Figs 1 b and

2e). An axial plane cleavage, associated with these

F, folds, confirms that they are the result of a

tectonic compression orientated approximately nor-

mal to the fold axes, which trend west-northwest-

east-southeast.

The thrusting of the relatively dense metavolcanics

and ultamafics over the granitoid basement during D,

gave rise to a density inversion and a thermal

blanketing effect. It has been suggested by earlier

workers (Kroner, 1985) that this lead to the for-

mation of large-scale domes (several kilometres in

diameter), the erosion of which has resulted in the

formation of isolated exposures of the underlying

basement, which characterise the central Eastern

Desert. However, field observations show that an

important episode of folding occurred after thrusting

and that the folds, which have a trend of west-

northwest-east-southeast, possess an axial planar

Journal of Afrjcacan Earth Sciences 205


fracture cleavage. These D, structures are, therefore,

clearly the result of a sub-horizontal tectonic com-

pression rather than of diapirism, and it is suggested

that the major domal structures of the Eastern Desert

may be the result of folding rather than diapirism. It

is difficult to know if the density inversion contributed

in any way to the amplification of the folds, or whether

the folding event amplified pre-existing domal

structures.

It can be seen from cross-section A’-B’ (Fig. 1 b)

that both the granitoid basement and the overlying

metavolcanic nappes have been affected by a later,

i.e. D, deformation. The S, thrust-generated fabric,

which has a regional strike approximately north-

northeast-south-southwest and which originally

dipped gently to the east-southeast, has been folded

about a west-northwest-east-southeast fold axis

into upright open large-scale F, folds. The folds plunge

variably (O-32? toward east-southeast and west-

northwest and have a weak S, axial plane fracture

cleavage which is sub-vertical. In addition, minor folds

associated with this deformation occurred within the

S, shear zone fabrics of the El-Shush Shear Zone and

the S, thrust separating the granitoids and meta-

volcanic nappes.

The formation of minor folds associated with the

three major episodes of deformation, i.e. movement

on the El-Shush Shear Zone, the thrusting of the

metavolcanics over the basement and the late folding

events, has lead to some ambiguity in the structural

interpretation of the region. This problem is discussed

in the following section.

The likely stress regimes associated with the three

deformation events are shown in Fig. 5. The maximum

principal compression that generated the El-Shush

Shear Zone (the earliest structure), based on the

Mohr-Colomb criteria of shear failure (see e.g.

Anderson, 19511, is assumed to be - 30” to the shear

zone, i.e. E5’N-W5”S. The maximum principal

compression, associated with the thrusting of the

metavolcanics over the sheared granitoids, is

assumed to be parallel to the stretching lineation, i.e.

west-northwest; and the principal compression

associated with the folding of this thrust is assumed

to be normal to the fold trend and axial plane cleavage,

i.e. north-northeast-south-southwest.

AMBIGUITIES IN THE STRUCTURAL

INTERPRETATION OF SOME MINOR STRUCTURES

Because of the partilelism of the fold axes associated

with the late (D,) deformation, the direction of thrusting

associated with the D, deformation and the mineral

stretching lineation (L,) in the El-Shush Shear Zone, a

D, structure, some ambiguity exists when attempting


Figure 5. Simplified map of the study area, showing the stress orientation associated with the three major tectonic events. a: Formation of the El-Shush Shear Zone; b: the overthrusting of the metavolcanic rocks; c: the late-stage west-northwest-east-southeast-trending folding.

to interpret the structural significance of the minor

folds within the study area. For example, the folds

associated with the strike-slip shearing along the El-

Shush Shear Zone would ideally have vertical axial

planes and vertically plunging hinges [Fig. 6a(i)l.

However, it is probable that sheath folding occurred

during the evolution of the shear zone and that, as a

result, some of the fold hinges would be rotated into

a sub-horizontal, west-northwest-east-southeast

orientation [Fig. Ga(ii)l. It can be seen that, unless

one is fortunate enough to find the ‘nose’ of the fold,

the folds would appear in the field as upright folds

with approximately horizontal axes trending west-

northwest-east-southeast. Similarly, the minor folds

associated with the later thrusting would ideally have

gently-dipping axial planes and hinges trending normal

to the movement direction [Fig. 6bfi)l. The subsequent

evolution of sheath folds would re-orientate the fold

hinges into a west-northwest-east-southeast

direction [Fig. Gbfii)]. Thus, unless the nose of the

fold was observed in the field, these folds would be

recorded as recumbent folds with west-northwest-

east-southeast-trending horizontal axes.

The effect of the late folding on these two sets of

structures would be different. The minor folds within

the El-Shush shear-zone, which have sub-vertical axial

planes, would simply be flattened by this later north-

northeast-south-southwest compression. In contrast,

the sub-horizontal axial planes of the minor folds within

the major thrust zone would be folded into upright

west-northwest-east-southeast-trending folds [Fig.

6cfii)l. Thus, the three sets of folds would tend to

have the same geometry and orientation and may be

difficult to distinguish in the field. Clearly, the large-

scale late folds that can be seen on the cross-section

to fold the thrust can be confidently recognised as F,

Structuraland tectonic evolution of the Umm Ghei$/Ei-Shush region, central Eastern Desert of Egypt

Figure 6. alil ldealised isoclinal Fl folds associated with the early shear zone. Their axes

are normal to the sense of movement. a(iiJ The more likely orientation of the fold axes as a

result of the formation of sheath folds. The folds have sub-horizontal west-northwest-east-

southeast-trending hinges, except at the nose. blil Recumbent, ideally orientated, F2 folds

with axes trending normal to the thrust movement direction. b/ii) The more likely orientation

of the fold axes as a result of the formation of sheath folds. These folds have sub-horizontal

west-northwest-east-southeast-trending hinges, except at the nose. clii F3 folds trending

west-northwest-east-southeast associated with the later deformation. c(C) The effect of

the F3 folds on the F2 folds shown in b(iiJ.

folds. However, minor F,, F, and F, folds would be

difficult to differentiate.

Another problem of ambiguity relating to the mineral

stretching lineations should also be considered. As

discussed earlier, the mineral stretching lineation in

both the El-Shush Shear Zone and the thrust, which

separates the metavolcanic nappes from the

granitoids, are parallel. As mentioned above, the effect

of the D, north-northeast-south-southwest compres-

sion on the low angle thrust and the steeply-dipping

west-northwest-east-southeast-trending El-Shush

Shear Zone would be very different. The thrust would

have been folded (see cross-section A-B’: Fig. 1 b) so

that now parts of it would be steeply dipping. The

effect on the El-Shush Shear Zone, however, would

be simply to flatten it and accentuate any pre-existing

planar fabric. Thus, during this folding event, it is

possible to rotate part of the thrust into parallelism

with the El-Shush Shear Zone. The authors are thus

obliged to consider the possibility that the El-Shush

Shear Zone may itself represent a folded thrust (Fig.

7a, b) which cuts the basement. It has already been

noted that thrusts sub-parallel to the main thrust occur

within the metavolcanic rocks. It is, therefore, possible

that similar thrusts may also have developed in the

underlying granitoid rocks. The latter folding would

rotate the thrust zones, which in places would become

steeply inclined. As can be seen from Fig. 7, if the El-

Shush Shear Zone is a folded thrust, then the trace

of its outcrop pattern would be parallel to the meta-

volcanic/granitoid contact, i.e. the main D, thrust.

However, inspection of satellite images and fieldwork

observations (Fig. 1 a) indicate that they are related

as shown in Fig. 6c and that the folded thrust

hypothesis is very unlikely. In addition, kinematic

indicators show that movement on the El-Shush Shear

Zone, which dips steeply towards the southwest, is

dominantly sinistral. Thus, if it was a folded thrust,

the movement direction, when it was rotated back to

the horizontal, would be top to the east-southeast.

This is at variance to the majority of kinematic

indicators examined in the thrust zone between the

metavolcanic nappes and the granitoid basement,

which show that movement along the thrust was top

to the west-northwest.

DISCUSSION

Having considered the structural evolution of the

study area, it is interesting to consider its relationship

Journal of African Earth Sciences 207


C) not a folded thrust map view

Figure 7. (al and fbl The El-Shush Shear Zone as a folded thrust. The outcrop pattern (bl shows a parallelism between the thrust at the metavolcanic/granitoid basement contact and the thrust within the basement. Ic) The relationship between the metavolcanic/granitoids contact and the El- Shush Shear Zone, if it is assumed that the El-Shush Shear Zone is a pre-thrust strike-slip fault.

to the tectonic evolution of the Eastern Desert as a whole and, in particular, to the origin of the present- day distribution of the granitoid basement and overlying metavolcanics.

Inspection of the geological map of the Eastern Desert (Fig. 8) shows that the granitoid rocks crop out as a result of the erosion of a series of elongated west- northwest-east-southeast-trending domes. It has been argued that these large periclinal undulations are the result of diapirism caused by the thrusting of the more dense metavolcanic nappes over the granitoids (Kroner, 1985). However, as pointed out in this paper, these periclines and the smaller scale undulations have a well-developed axial plane cleavage, which is incompatible with this suggestion. Although the density inversion between the two rocks may have been a contributing factor to the formation of the larger elongated domes, there can be no doubt that their formation was also associated with a major sub:horizontal tectonic compression, which acted in a north-northeast-south-southwest direction (see Figs 1 band 8).

Mediterranean Sea /

Uweinat-Bir Safsaf-Aswan

29”

23”

I 3io 3j”

Figure 8. Map showing the rocks of the Pan-African belt and the exposed granitoid basement (compiled from El-Gaby et al., 1988; Abdel-Khalek et al., 7992; Greiling et al., 19941.

As discussed earlier, the evidence presented in this paper indicates that at least three important tectonic events have affected the rocks of the study area. A variety of workers (e.g. Shackleton et al., 1980; Gass, 1981; KrGner, 1985; O’Connor and Mosely, 1986; Greiling era/., 1994) have attempted to understand the tectonic evolution of the Eastern Desert in terms of a succession of island-arc accretions onto an ancient continental margin. However, because of the relatively small area studied, and bearing in mind the stress orientations associated with the three major phases of deformation, it is extremely difficult to place thesdeformation history into a specific plate tectonic setting.

ACKNOWLEDGEMENTS

The authors would like to express their thanks to Dr E. O’Connor of the British Geological Survey for supporting this topic of study. They would like to acknowledge the support and encouragement received from the Egyptian Geological Survey and Mining Authority (EGSMA), in particular Dr. A.A. Dardir and


Strucrural and tectonic evolution of the Umm Gheig/Ei-Shush region, central Eastern Desert of Egypt

colleagues in EGSMA. The authors also gratefully

acknowledge the British Council on behalf of Overseas

Develapment Administration (O.D.A.), who funded part of this research work. The use of Landsat TM

imagery supplied by the Remote Sensing Group of the British Geological Survey is gratefully acknow-

Iedged.

Editorial handling - P. Bo wcien

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