gravitational collapse origin of shear zones, foliations and linear

Journal of African Earth Sciences 38 (2004) 23–40

www.elsevier.com/locate/jafrearsci

Gravitational collapse origin of shear zones, foliations andlinear structures in the Neoproterozoic cover nappes,

Eastern Desert, Egypt

Abdel-Rahman Fowler a,*, Baher El Kalioubi b

a Department of Geology, Faculty of Science, United Arab Emirates University, P.O. Box 17551, Al-Ain, United Arab Emiratesb Department of Geology, Ain Shams University, Abbassiyya, Cairo, Egypt

Received 3 December 2002; received in revised form 6 April 2003; accepted 12 September 2003

Abstract

The Um Esh–Um Seleimat area lies to the west of the Meatiq Core Complex (MCC), in the Central Eastern Desert (CED),

Egypt, which forms part of the Neoproterozoic Arabian–Nubian Shield in NE Africa and Western Arabia. The study area is a NW-

trending zone of intensely foliated ophiolitic melange and molasse sedimentary rocks. There is a single regional foliation, S1, definedmainly by low- to very low-grade metamorphic phases, though grade increases to amphibolite facies in the areas bordering the

MCC. S1 is associated with shearing and passes directly into the mylonites of the MCC sheared carapace. The foliations and

mylonites together define an originally subhorizontal thick ductile shear zone of regional extent. The sense of shearing is top-

to-the-NW, parallel to NW–SE trending stretching lineations, L1. S1 is folded by open rounded symmetrical mesoscopic F2 folds withNW–SE trending subhorizontal hinges and variably dipping axial planes. F2 folds are folded by coaxial (i.e. NW–SE trending) but

non-coplanar close to tight macroscopic folds (F3). Subhorizontal S1 foliation formed continuously during F2 folding and perhaps

also into the early stages of F3 folding. This reflects top-to-the-NW shearing under laterally confined conditions produced by the

onset and gradual dominance of NE–SW shortening. SW-ward thrusts and NW–SE trending sinistral brittle faults are late stage

structures. The NW-ward shear translation of the ophiolite and molasse cover nappes results from gravitational collapse following

arc-collision and crustal thickening. A gliding–spreading nappe emplacement mechanism is most consistent with the field evidence.

The steep metamorphic gradient from low-grade cover rocks downwards into gneissic rocks is interpreted as a result of vertical

thinning of the ductile shear zone during collapse. Amphibolite facies conditions are found at the base of other top-to-the-NW low-

angle major shear zones associated with gneissic complexes in the CED (e.g. El-Sibai, El-Shalul complexes) suggesting that the

crustal level of the shear zone may be determined by thermally controlled rock rheological factors.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Shear foliations; Stretching lineations; Nappe transport; Neoproterozoic; Gravitational collapse; Egypt

1. Introduction

The basement rocks of the Eastern Desert of Egypt

form the western exposures of the Arabian–Nubian

Shield–a collage of intraoceanic island arc complexes

and microcontinental blocks. They were assembled as a

result of the Neoproterozoic extension, and subsequent

accretion and collision of East and West Gondwanaland(�900–700 Ma) to produce a southward narrowing zone

of complexly deformed juvenile crust, referred to as the

East African Orogen (Garson and Shalaby, 1976; Gass,

1977; Engel et al., 1980; McWilliams, 1981; Stoeser and

* Corresponding author. Tel.: +971-506935982; fax: +971-37671291.

E-mail address: [email protected] (A.-R. Fowler).

0899-5362/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2003.09.003

Camp, 1985; Kr€oner et al., 1987; Vail, 1988; Kr€oneret al., 1991; Stern, 1994; Abdelsalam and Stern, 1996;

Stern and Abdelsalam, 1998; Stern, 2002).

Recent tectonic models for the evolution of the

Eastern Desert have concentrated on the origin, signi-

ficance and mechanism of formation of several gneiss-

cored dome structures (Wadi Kid, Meatiq, Um Had,

Gebel El-Sibai, Gebel Um El-Shalul, Hafafit), with thecharacteristics of metamorphic core complexes (Sturchio

et al., 1983a,b; El Ramly et al., 1984; Habib et al., 1985;

Bennett and Mosley, 1987; El-Gaby et al., 1990; Greil-

ing et al., 1993; Wallbrecher et al., 1993; Kr€oner et al.,1994; Fritz et al., 1996; Greiling, 1997; Neumayr et al.,

1998; Blasband et al., 2000; Fowler and Osman, 2001;

Loizenbauer et al., 2001; Fritz et al., 2002).

mail to: [email protected]

24 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40

Shear zones play an important role in the evolution ofthese core complexes. The gneissic rocks of the core

complexes are separated from low-grade metamor-

phosed upper crustal rocks by low-angle mylonitic shear

zones (Sturchio et al., 1983a,b; Ries et al., 1983; Habib

et al., 1985; Blasband et al., 2000). Transcurrent ductile

shear zones and normal ductile shear zones framing the

core complexes have also been included in models for

Eastern Desert core complex exhumation involvingstrain partitioning between �internal’ and �external’ partsof the orogen (Fritz et al., 1996; Fritz and Puhl, 1996;

Neumayr et al., 1998; Fritz et al., 2000).

This contribution describes field relations of regional

foliations and associated linear structures in the Um

Esh–Um Seleimat area (Fig. 1), a zone of intensely

foliated cover rocks along the Qift-Quseir road, adjacent

to the Meatiq Core Complex (MCC) in the CentralEastern Desert (CED) of Egypt. The aim of this study is

to present new structural data from the cover rocks

adjacent to the Meatiq, and to develop a structural

model to explain their origin. Following this, the sig-

nificance of the structural model for the tectonics of the

CED is discussed, including consequences for existing

exhumation models of the MCC.

Fig. 1. Location map for the Um Esh–Um Seleimat area in the Central E

cambrian of the Eastern Desert’’ (O’Connor et al., 1996). M¼Meatiq Core C

Hammamat molasse deposits; WK¼Wadi Kareim molasse deposits; GS¼G

lines, broken lines are bedding trend lines.

2. General geology

2.1. Location and setting

The Um Esh–Um Seleimat area is a NW-trending

strip of exceptionally well-foliated rocks extending from

Wadi Um Esh in the north to Wadi Um Seleimat in the

south, and bordered by the El-Sid Metagabbro on the

west, and the Meatiq Core Complex (MCC) on the east(Fig. 2a). Akaad (1996) has described this area as a

‘‘formidable shear zone’’. It consists of foliated ophio-

litic melange interrupted by NW–SE trending belts of

foliated clastic metasedimentary formations, generally

accepted as parts of the Hammamat molasse units. In

the east, the sheared ophiolitic melange lies structurally

above the MCC gneisses and is separated from them by

a thick westerly dipping schistose to mylonitic carapace(Fig. 2b). To the west of the study area, along Wadi

Atalla, the folded foliations of the melange are cut by

W- to SW-directed thrusts against the NE limb of an-

other gneiss-cored antiformal structure of the Um Had–

Um Effein area, described by Fowler and Osman (2001).

The Um Esh–Um Seleimat area lies in Fritz et al.’s

(1996) external part of the orogen, which they described

astern Desert, Egypt, adapted from the ‘‘Geological Map of the Pre-

omplex; F¼ Fawakhir Granite, UH¼Um Had Granite; WH¼Wadi

ebel El-Sibai. Bold lines are faults, short thin lines are foliation trend

Fig. 2. (a) Geological map of the Um Esh–Um Seleimat area showing the ophiolitic melange and conglomerate formations. Macroscopic fold axial

traces are shown. A–A0, B–B0 and C–C0 refer to cross-sections. (b) Cross-sections for (a). Fold axial planes are represented by dotted lines. Faults are

represented by bold lines. Lithological symbols as for (a). Thicker lines in section B–B0 near the boundary between mica schist and metabasalt

represent the mylonitic carapace separating the ophiolites from the Meatiq Core Complex.

A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 25

as a zone of W- to SW-directed thrust imbrication, in

contrast to the zone of NW–SE extension in the internal

part of the orogen (incorporating the MCC).

Detailed mapping of the Um Esh–Um Seleimat area

was carried out by Noweir (1968), Akaad and Noweir

(1969, 1980) and Akaad et al. (1996) from mainly


sedimentological, petrological and stratigraphic pointsof view. Various authors have included the Um Esh–Um

Seleimat area in their broader areas of study (El Ramly

and Akaad, 1960; Akaad and Shazly, 1972; Stern, 1979,

1981; Ries et al., 1983; El-Gaby et al., 1984; Habib,

1987; Bennett and Mosley, 1987; Wallbrecher et al.,

1993; Ragab et al., 1993; Greiling et al., 1996; Messner,

1996; Kamal El-Din et al., 1996; Ragab and El-Alfy,

1996; Fritz et al., 1996).

2.2. Stratigraphic units and lithology

The ophiolitic melange of the Um Esh–Um Seleimatarea consists of greyish metagabbro, metadolerite and

metabasalt blocks, up to several kms in dimension, and

appear as elongate or lenslike bodies incorporated

within foliated serpentine and dark grey pelite (Fig. 2a).

Associated minor lithologies are lenses of cherts, pelites,

tuffs, greywackes and rare limestone. Previously this

melange was referred to as the Abu Ziran Group, and it

was subdivided into stratigraphic formations (Akaadand Noweir, 1969, 1980), until Ries et al. (1983) dis-

covered that the ’’formations’’ are actually chaotic

blocks with no regular stratigraphic succession. The

melange has been referred to as Older Metavolcanics

(Stern, 1981), Eastern Desert Ophiolitic Melange (Ries

et al., 1983; Habib, 1987), or Umm Esh melange (Akaad

et al., 1996).

The conglomeratic formations attain a thickness of4000 m in the nearby Wadi Hammamat section (Akaad

and Noweir, 1980) and are probably much thicker over

the entire Qena-Quseir section (Messner, 1996). The

sediments consist of alternating purplish red and

greenish grey metaconglomerate, metagreywacke and

metapelite. There are clasts of mudstone, felsic to mafic

volcanics and pink granite (Akaad and Noweir, 1969;

Akaad et al., 1996; El Kalioubi, 1996). These late-oro-genic molasse sediments (Grothaus et al., 1979; Akaad

and Noweir, 1980; El-Gaby et al., 1984; El-Gaby, 1994)

accumulated in intermontane normal fault-bounded

basins as alluvial fan braided stream and lake deposits

(Grothaus et al., 1979; Messner, 1996; Fritz and Mess-

ner, 1999). The sediments were derived from a recycled

Pan-African orogen and dissected arc terrain (El Ka-

lioubi, 1996; Osman, 1996; Messner, 1996). Originallymost of the deformed conglomerates were identified as

Atud Formation of the Abu Ziran Group by Akaad and

Noweir (1980), though later authors regarded them as

higher strained stratigraphically lower sections of the

Hammamat Formation (Stern, 1979, 1981; Ries et al.,

1983; El-Gaby et al., 1984; Hassan and Hashad, 1990;

Messner, 1996; Ragab and El-Alfy, 1996; Fritz et al.,

1996). In a later revision (Akaad et al., 1996), the Atudequivalent formation (the Muweilih Conglomerate) was

found to occupy a more restricted area only to the south

of Wadi Um Seleimat. Fritz et al. (1996) identified the

Hammamat exposures of the areas west of Meatiq asforeland basin deposits affected by W- to SW-ward

directed thrusts in the external part of the orogen. They

were thus structurally distinguished from strike-slip

fault-controlled Hammamat basins (e.g. the Wadi

Kareim basin) which reflected NW–SE extension and

were associated with the rise of the CED core complexes

(Fritz and Messner, 1999).

High-grade metamorphic gneisses of the MCC werenot examined in this study. The mica schists forming the

thick sheared carapace of the MCC lie at the eastern

margin of the study area. These foliated rocks have been

referred to as the Abu Fannani Schists by Akaad and

Noweir (1969, 1980). Lithologies include garnet mica

schist, amphibolite and mica phyllonites.

2.3. Petrography and metamorphism

Petrographic data from the ophiolitic melange

lithologies of the Um Esh–Um Seleimat area are sum-

marized in Table 1, and illustrated in Fig. 3. Theophiolitic melange of the CED, in general, is charac-

terized by mainly greenschist facies assemblages with

metamorphic temperatures (adjacent to Meatiq) >350

�C but <540 �C (Fritz and Puhl, 1996; Neumayr et al.,

1998) and pressure conditions <4 kbar. From their

transect along Wadi Um Esh, Ries et al. (1983) reported

that the metamorphic grade of the melange nappes in-

creases downward towards the shear zone separating thenappes from the Meatiq gneisses. They also noted the

presence of hornblende and garnet in the lower sections

of the sheared melange. Both Ries et al. (1983) and El-

Gaby (1994) drew attention to the steep thermal gra-

dient from the melange nappes downwards into the

gneisses. Ries et al. (1983) also recognized that the

metamorphic isograds dip more steeply than the tectonic

units suggesting that metamorphic heating continuedafter folding. The zone of hornblende stability extends

up to 2.5 km away from the MCC margin.

A brief description of the petrography of Hammamat

lithologies from the Um Esh–Um Seleimat area is pre-

sented in Table 1 and illustrated in Fig. 3. The CED

Hammamat metasedimentary rocks in general show

anchizonal metamorphic grade indicated by the pres-

ence of montmorillonite and pumpellyite (Soliman,1983; Osman et al., 1993; Osman, 1996). However,

chlorite zone greenschist metamorphic facies is also

found (El Kalioubi, 1996; Greiling et al., 1994; Neumayr

et al., 1996; Fritz and Messner, 1999), with biotite zone

or higher grades at distances up to 5 km from the MCC

margin. Messner (1996) described somewhat higher

grades in the lower stratigraphic sections of the Ham-

mamat in the Wadi Hammamat–Wadi El-Qash area,and correlated this with increased shearing effects.

Fowler and Osman (2001) also noted the increased

shearing effects in the lower stratigraphic section of the

Table 1

Brief petrographic description of the main lithologies of the Um Esh–Um Seleimat area

Meatiq gneisses Quartz-, feldspar-rich garnet muscovite, red–brown biotite schists. There are signs of

retrogression of garnet to chlorite. There are also mica schists with quartz and

plagioclase porphyroclasts, which are probably sheared syn-kinematic intrusions into the

carapace of the Meatiq dome

Ophiolitic melange Ultramafics Include original pyroxenite and peridotite, now represented by serpentinite, talc-

tremolite schist and rocks composed entirely of actinolitic amphibole

Mafic rocks Originally gabbros, dolerites, basalts. These lithologies are represented by greenschist to

amphibolite

Amphibolites consist of blue-green hornblende, brown biotite, plagioclase, quartz and

minor epidote, sphene and chlorite. Blue–green hornblende mantles and replaces an

earlier brownish hornblende phase

Greenschist facies rocks are dominated by actinolite, chlorite, epidote, plagioclase,

quartz and sphene, and may contain minor calcite and biotite or stilpnomelane. In some

examples blue–green hornblende (itself containing traces of brownish hornblende)

remains as relicts partly replaced by actinolite

Metagabbros have preserved igneous fabrics under low strain conditions and may even

appear hornfelsic (Fig. 3(c)). Progressive development of foliation in metagabbros begins

with crude foliation defined by parallel alignment of hornblende+biotite or actino-

lite + chlorite outlining lenses and boudins of recrystallized and strained mafic and

plagioclase grains (Fig. 3d). As foliation enhances, these lenses, augen and boudins are

progressively reduced and recrystallization decreases their grainsize. Ultimately mafic

mylonites are produced. There are isoclinally folded, boudinaged and sheared quartz

veins in the highly strained metamafic rocks (Fig. 3f)

Sedimentary rocks Include black cherts, grey pelites, rare limestone blocks. The grey pelites are now

composed of metamorphically grown sodic plagioclase and chlorite, and minor

polygonal quartz. They contain significant quantities of hematite and calcite

Conglomerate and

associated rocks

Greywacke and

conglomerate matrix

Composed mainly of sand-sized angular clasts of plagioclase, quartz and silicic and

intermediate volcanic groundmass. The sand grains are set in a finer matrix of

plagioclase, quartz, white mica, epidote, chlorite, magnetite and sphene (Figs. 3 a and b)

Traces of tourmaline and zircon are usually present. Assemblages of white

mica+greenish-brown biotite, or biotite + chlorite are found at higher metamorphic

grade. The mica and chlorite phases define weak to extremely strong foliations enclosing

strained and recrystallized clastic grains (Figs. 3a and b). Pebble lithologies are mainly

felsic volcanics as described below. There are also pebbles of alkali granite

Felsic volcanics Contain quartz and plagioclase phenocrysts in a groundmass of felsitic, spherulitic and

microgranular textures. Groundmass phases include quartz, plagioclase, white mica,

chlorite, epidote and sphene. Others have olive green biotite. Foliations may be weak or

intense. Some examples look mylonitized and have mainly white mica foliations


Hammamat along Wadi Muweih. Based on different

strain intensities, El Ghawaby (1973) divided the Ham-

mamat into a lower and upper unit in the Wadi Zeidun

area. The lower unit contains flattened and stretched

pebbles that are absent in the upper unit. Andrew (1939)

and Ries et al. (1983) described biotite zone grades,

correlating with higher strains in Hammamat metase-

dimentary units along Wadi Um Esh.

3. Structure

3.1. Bedding (S0)

Bedding ðS0Þ is excellently preserved in the con-glomeratic formations where it is represented by pebbly

bands within the greywacke (Fig. 4a and b). It is rarely

observed in the melange itself, though it occurs as thin

layering of black cherts, in dark coloured pelites, and as

felsic tuffaceous or pebbly bands in the pelites. Bedding

planes trend generally NW–SE and have variable dips

(Fig. 2a). On equal area plots they form a girdle of poles

defining a gently SE-plunging p-axis (Fig. 5).

3.2. Tectonic planar fabrics (S1 and Sm)

The study area is dominated by a regionally devel-

oped, commonly penetrative phyllitic cleavage to schis-

tosity, termed S1 (Figs. 3a and 4a–e), which is best

developed in the pelites, tuffs and serpentinites, and

usually poorly developed in the metabasalts and meta-

gabbros. S1 is locally parallel to the bedding (Fig. 4a)but more commonly subtends an angle of 45� or less tobeds (Fig. 4b). S1 orientations vary from gently to

steeply dipping (Fig. 2a). Where stretching lineations

Fig. 3. Thin section photomicrographs of the lithologies of the Um Esh–Um Seleimat area. (The short edge of each photograph represents a length

of 3 mm, except for photograph e, with a short edge of 0.75 mm, and photograph f , with a short edge of 2.45 mm). (a) Foliated metagreywacke with

quartz and plagioclase clastics and white mica and biotite foliation. (b) Mylonitized metagreywacke with foliation defined by biotite and epidote

grains. (c) Isotropic polygonal granoblastic fabric of recrystallized metagabbro containing plagioclase, quartz and hornblende. (d) Foliation in

metagabbro is defined by stretched plagioclase grains (white) and actinolite (pale green). (e) Metabasalt showing foliation defined by hornblende.

A shear structure offset the larger amphibole grain. (f) Foliated metabasalt showing abundant slender hornblende grains defining a foliation, and

a sheared quartz vein with quartz subgrains oblique to the vein margins. Both (e) and (f) give a shear sense of top-to-the-right.


ðL1Þ are evident, L1 invariably lies within S1. Poles to S1define a complete girdle with gently SE-plunging p-axis(Fig. 5). Flattened pebbles have long and intermediate

axes parallel to S1 foliation. Rare isoclinal rootless F1fold axial planes and boudinaged layers are also parallel

to the foliation. S1 is parallel to the principal plane of

flattening in the rocks, and is not itself a shear plane

structure, however, it has a spatial, temporal and kine-

matic association with shearing in that: (a) it is betterdeveloped in areas bordering sheared contacts, e.g. be-

tween metabasalts and conglomerate units, and locally

merges with mylonitic foliations ðSmÞ at higher strains;

(b) the stretching lineations on the foliation are parallel

to nearby mylonitic lineations ðLmÞ (Fig. 6); (c) some

foliation planes have developed into striated slip planeswith the striations approximately parallel to the

stretching lineation; (d) the foliation forms the S element

of S–C structures; and (e) there is evidence for local slip

along the foliations (Fig. 3e). The mylonitic foliation

(Sm) forms numerous thin shear zones in the ophiolitic

melange near the MCC carapace. Fine-grained horn-

blende defines the mylonite foliation Sm (Fig. 3f) and

lineations Lm indicating that hornblende crystallizationtemperatures existed during ductile shearing. Our results

agree with those of Ries et al. (1983), Habib et al. (1985)

and Neumayr et al. (1996) in finding that amphibolite

facies conditions existed in the melange adjacent to the

MCC.

Fig. 4. (a)–(b) Field photographs of bedding ðS0Þ and foliation ðS1Þ relations in the Um Esh–Um Seleimat area. (a) Bedding defined by conglomerate

layers in greywacke. S1 foliation is parallel to the bedding. (b) Conglomerate bed within greywacke. The S1 foliation lies 15� clockwise from the

bedding. Direction of observation is NW, along the line of intersection of the bedding and foliation. (c)–(e) Mesoscopic shear sense indicators from

the Um Esh–Um Seleimat area. (c) Foliation ’fish’, top-to-right shear sense (looking W ). (d) Shear boudins, top-to-right shear sense (looking W ).

(e) Imbricate extensional small scale shears inclined to the foliation, top-to-left shear sense (looking NE). All indicators are consistent with top-to-

the-NW shear sense.

Fig. 5. Schmidt net stereograms of poles to planar structures from the Um Esh–Um Seleimat area. Bedding ðS0Þ: 59 measurements (density contours

2%, 4%, and 8%; p-axis is 7� towards 153�). Foliations ðS1): 271 measurements (density contours 1%, 2%, 4%, 8%; p-axis is 5� towards 144�).Mylonitic foliations ðSmÞ: 25 measurements (p-axis is 6� towards 145�). Mesoscopic F2 fold axial planes: 32 measurements (p-axis is 3� towards 148�).Approximate great-circle girdles of best fit (and their corresponding p-axes) to the data are shown.


Fig. 6. Schmidt net stereograms of linear structural data from the Um Esh–Um Seleimat area. F2 fold hinge lines: 47 measurements (density contours

3%, 6%, 12%, 24%). Bedding–cleavage intersection lineations ðL01Þ: 23 measurements, filled and unfilled square symbols represent clockwise and

anticlockwise orientation of beds relative to foliations, with direction of observation along the lineation looking approximately NW. Mylonitic

lineations ðLmÞ: 20 measurements. Pebble long-axis lineations ðL1Þ: 37 measurements (density contours 3%, 6%, 12%, 24%, 48%). Pencil axes: 81

measurements (density contours 2%, 4%, 8%, 16%, 32%). Stretching lineations (other L1 than pebble long axes): 104 measurements (density contours

1%, 2%, 4%, 8%, 16%, 32%).


3.3. Shear sense indicators

There are a number of kinematic indicators within

the S1 foliation in the study area. Rare S–C structures

and foliation ’fish’ indicate top-to-the-NW shear sense

(Fig. 4c). Another shear sense criterion is shear-seg-

mented veins (Fig. 4d) which may superficially appear asboudins. Asymmetric intrafolial F1 folds are also con-

sistent with top-to-the-NW shear sense. All of these

shear sense indicators have been rotated along with the

S1 foliation about later NW–SE trending fold axes, as

explained by Fowler and Osman (2001) for the Um Had

area to the west, and detailed below. As a result of this

later folding the F1 asymmetric folds now appear to

indicate dextral or sinistral shear sense which reversesacross the later fold hinges. Microscopic top-to-the-NW

shear sense indicators include mica ’fish’; asymmetric

pressure fringes (beards) on porphyroclasts; S–C fabrics;

grain elongation fabrics inclined to mylonitic foliations

(Fig. 3f); and microshear offsets (Fig. 3e). These kine-

matic indicators are consistent with the S1 foliations

being associated with NW-ward shear translation, and

not with SW-ward thrusting as in current structural

models for the area to the west of the MCC (Wallbre-

cher et al., 1993; Fritz et al., 1996).

3.4. Stretched particle lineations (L1)

Lineations defined by the long axes of particles, are

common throughout the study area. The most impres-

sive are those defined by pebbles in the conglomeratic

formations. The pebbles have ’’beards’’ produced by S1foliation anastomosing around them. Most examples of

elongate pebble fabrics show features consistent with

extension parallel to the pebble long axis (e.g small scale

normal faulting; extension fractures with fibres parallel

to the pebble long axis; and rare examples of boudi-

naged pebbles). On this evidence L1 is regarded as a

stretching lineation. Since L1 always lies within S1 it

appears that the elongation producing these lineations isrelated to the same events that produced the foliations,

and therefore that extension and shearing are contem-

porary deformation effects.


3.5. Mesoscopic folds (F2)

The beds and S1 foliations are folded about uniformly

NW–SE trending, gently plunging mesoscopic folds.

These folds are usually rounded, with one to tens of

metres wavelengths. Almost all of these folds are gentle

to open, and they may be found in upright, inclined and

recumbent orientations (Figs. 7a–c). Some rare tight F2folds have an axial planar crenulation cleavage ðS2Þ.More commonly the F2 folds have pencil structure in

their cores (Fig. 7d). The relationship between the

mesoscopic F2 folds and the macroscopic folds shown on

the cross-sections is discussed in Section 3.6.

3.6. Macroscopic folds (F3)

The map and cross-sections of the study area dem-

onstrate the existence of macroscopic folds ðF3Þ (Fig. 2a

Fig. 7. (a)–d) Photographs of folds in the Um Esh–Um Seleimat area. (a) U

the area. (b) Recumbent open low amplitude F2 folded foliations from the eas

folded metaconglomerate and metagreywacke beds from the northern part

(e) Pencil structure in metagreywacke.

and b), which are coaxial with the F2 folds (ie also NW–SE trending and gently plunging). The macrofolds,

however, are moderate to tight, and have consistently

upright to steeply inclined axial planes (Fig. 2b). F3 foldshave no axial plane foliations. There are no asymmetry

variations in the F2 mesoscopic folds systematic with

their position on the limbs of the F3 folds. This is be-

cause the axial planes of the F2 folds are almost always

nearly normal to the average orientation of the layering.The broadly variable orientation of the F2 fold axial

planes (Fig. 5) is a result of coaxial non-coplanar

refolding of the F2 folds by F3 folds.Combining the cross-section of Fig. 2b with the results

of Fowler and Osman (2001) for the Um Had area to the

west, and the profile shown by Habib et al. (1985) for the

MCC to the east, allows the construction of the profile

shown in Fig. 8. This interpreted cross-section shows asingle thick shear zone defining the MCC carapace and

pright open low amplitude F2 folded foliations from the central part of

tern part of the area. (c) Open inclined low amplitude F2 symmetrically

of the area. (d) Pencil structure parallel to the hinge of an F2 fold.

SW NE

Wadi Muweih

Um Had gneisses Meatiq gneisses

Wadi Atalla SW-ward thrusting

Wadi Abu Diwan

Um Esh - Um Seleimat area

?

?

Fig. 8. A schematic NE–SW projected cross-section through the Um Esh–Um Seleimat area extending from Wadi Muweih (where basement rocks

first appear from beneath the Nubia Sandstone) to the NE flank of the Meatiq Core Complex (MCC) at Wadi Abu Diwan. The figure interprets the

continuity of the originally subhorizontal S1 foliations and mylonites associated with top-to-the-NW regional shear translation. Data for the section

fromWadi Muweih to Wadi Atalla are derived from Fowler and Osman (2001). The Um Esh–Um Seleimat section is based on Fig. 2a (Section A–A0)

and shows the upright F3 macroscopic folds. Meatiq cross-section is modified from Habib et al. (1985). Close-spaced lines represent sheared ophiolitic

and conglomeratic cover rocks. Thick grey line represents mainly mica schists of the sheared carapace of the MCC. No examples of these latter

schists have been found in the more tightly folded synformal structures between the Um Had and Meatiq antiforms. Line of the cross-section

represents very approximate level of present surface exposure. SW-ward thrusts in Wadi Atalla and NE-ward thrusts near wadi Muweih are also

shown, and interpreted to be of similar age. Transcurrent faults are also shown with circle symbols indicating sense of slip (block with circled Xsymbol moves forwards, relative to the observer).


sheared overlying nappes continuing into the Um Esh–

Um Seleimat area, and from there wrapping over the Um

Had gneisses and being folded in the Wadi Muweih area,

west of Um Had. The implication is that this thick shear

zone was subhorizontal before folding and core complexrise, and of regional extent. A similar, but orogen-par-

allel, continuity of the same shear zone from Hafafit

through Sibai to Meatiq was suggested in interpretations

by Greiling and El Ramly (1985).

3.7. Bedding–cleavage intersection lineations (L01) and

pencil structures

Bedding–cleavage intersection lineations ðL01Þ are

subparallel to the F2 and F3 fold hinges in the study area(Figs. 6 and 9). Three possible interpretations for bed-

ding–cleavage intersections parallel to fold hinges are

shown in Fig. 10. The simplest interpretation (Fig. 10a),

with the S1 foliation as axial plane to the F2 folds, must

obviously be rejected because S1 is folded by F2 and F3folds, and is not axial plane to them (Fig. 7a and b).

Another interpretation is shown in Fig. 10b, where

bedding and an inclined foliation are coaxially folded. Amodel of this kind was inferred by El-Gaby et al. (1984)

when they considered the NW–SE folds deforming

imbricate SW-vergent thrusts and associated foliations.

However, the data on angular relations between bedding

and S1 foliations do not support this model. According

to Fig. 10b, the bedding should lie consistently anti-

clockwise of the foliation, looking NW. The data from

the study area are roughly evenly divided betweenclockwise and anticlockwise angular relations between

S1 foliation and bedding, looking NW (Fig. 6). The best

model to explain the bedding–cleavage relations is

shown in Fig. 10c, where gentle folds are transected by

subhorizontal foliations. In the study area this repre-

sents continued formation of S1 foliations into the stage

of F2 or even F3 folding. For simplicity, we have called

the foliation S1 although it is not confined to the firstdeformation event. This model also explains the gene-

rally low angle between beds and foliations (75% of the

measurements lie between 0� and 40�) by supposing that

the subhorizontal S1 foliation continued to form only up

to the stage of F2 folding or as far as the early stages of

F3 folding, when limb dips did not exceed 40�. This

model supposes a stage of overlap of the top-to-the-NW

shear displacement event and the NW–SE trendingfolding event. Tectonic aspects of this overlapping

shearing and folding model are briefly discussed below.

The rocks of the study area show remarkably good

development of pencil structures in most lithologies

(Figs. 7e and 9), particularly the metabasalts and

metagabbros. There is a strong spatial association be-

tween pencil structure and F2 fold hinges (Fig. 7d), and

fold hinges are rarely more than 10� different in orien-tation from pencils (Fig. 9). Pencil structure is normally

the result of a weak flattening strain (here related to F2or F3 folding) superimposed on a weak pre-existing

planar fabric.

3.8. SW-ward thrusting and strike-slip faulting

West of the study area, along Wadi Atalla, there are

E- and NE-dipping thrusts which slice through (i.e.

post-date) the folded S1 foliations. These structures havebeen described by Stern (1979, 1985), El-Gaby et al.

(1984), Kamal El-Din et al. (1996) and Fowler and

Osman (2001). West of Wadi Um Seleimat there are

Fig. 9. Map showing the relatively uniform NW–SE trending orientations of mesoscopic F2 fold hinges, pencil axes and L1 extension lineations

(including pebble long axes) in the Um Esh–Um Seleimat area.

Fig. 10. Sketches showing three simple models for the significance of bedding–cleavage intersection lineations ðL01Þ being approximately parallel to F2

and F3 fold hinges in the Um Esh–Um Seleimat area. See text for discussion. (a) The foliations have axial plane relations to the folds. (b) The beds

and inclined foliations are folded coaxially. (c) The subhorizontal foliations transect the folds.


N–S trending sinistral strike-slip faults at the western

margin of the study area (Fig. 2a). Also, NW to WNW

trending brittle sinistral strike-slip faults interrupt the

folded S1 foliations throughout the study area and are

associated with much kinking of this foliation. Abrupt

anti-clockwise deflections in S1 and L1 trends in the SW

part of the study area (Fig. 9) are consistent with fault

block rotation associated with the sinistral movements.


4. Discussion

4.1. Structural model for the Um Esh–Um Seleimat area

A summary of structural events is given below, and is

followed by a discussion of key aspects relevant for re-

gional tectonics in the CED.

Stage 1: Top-to-the-NW shear translation occurred

on a thick regionally extensive subhorizontal ductileshear zone generating initially subhorizontal S1 folia-

tions and Sm mylonitic foliations. These foliations con-

tain F1 isoclinal rootless intrafolial folds, and NW–SE

trending mylonite lineations ðLmÞ and stretching linea-

tions ðL1Þ in the direction of transport (Fig. 11a). We

prefer the terms ‘‘top-to-the-NW translation’’ or ‘‘NW-

ward nappe translation’’ over Ries et al. (1983) ‘‘NW-

ward thrusting’’ since the latter requires that theshear surfaces dip consistently SE. Our conclusions are

that the thick ductile shear zone is essentially subhori-

zontal.

Stage 2: Before the end of the top-to-the-NW

shearing, low amplitude rounded open F2 mesoscopic

folds began to form with axes parallel to L1 (Fig. 11b).

The onset of F2 folding reflects a progressive NE–SW

shortening, which eventually came to dominate thedeformation. The transection of the F2 (and probably

also early stages of F3) folds by continuing formation

of S1 foliations produced bedding–cleavage lineations

L01 (Fig. 11c). Pencil structures formed during stages 2

and 3.

Stage 3: Top-to-the-NW shearing ceased as NW–SE

trending tight upright macroscopic F3 folds deformed

the earlier structures (Fig. 11d). These fold wereaccompanied or followed by W- to SW-ward thrusts at

the western margin of the area.

Stage 4: Sinistral shearing along N to NW trending

transcurrent brittle faults generated local kink zones

and produced some fault block rotations (Figs. 2a and

9).

The simultaneous top-to-the-NW shear translation

and NW–SE trending folds (stage 2) has been treated inFowler and Osman (2001). In the following discussion

we will concentrate on the implications of the simulta-

neous NW-ward nappe translation and NW–SE exten-

sion (stage 1).

4.2. Simultaneous top-to-the-NW nappe translation and

NW–SE extension event

Many of the tectonic models for the Pan-African

basement which attempt to explain the orogen-parallel

NW-ward nappe translation are essentially rear com-pression thrusting mechanisms (Ries et al., 1983; Greil-

ing et al., 1994; Shackleton, 1994; Fritz et al., 1996;

Neumayr et al., 1998; Loizenbauer et al., 2001). In these

models NW-ward thrusting is pictured as resulting fromthe same compressional events as those that formed the

arc collision-related sutures. From such models one

would expect the thrusting and the arc collision events

to be approximately synchronous.

Blasband et al. (2000) has dated arc collision in the

Sinai to 750–650 Ma, and this is comparable with the

800–700 Ma range for arc collisions in the Arabian–

Nubian Shield reported by other authors (Bentor, 1985;Stern and Hedge, 1985; Stoeser and Camp, 1985;

Kr€oner et al., 1992; Stern, 1993; Greiling et al., 1994;

Abdelsalam and Stern, 1996). Blasband et al. (2000)

also dated the activity on subhorizontal mylonitic foli-

ations with top-to-the-NW shear sense in the Sinai to

620–580 Ma, i.e. coeval with other NW–SE directed

extensional phenomena in that area (dyke swarms, NE–

SW striking graben formation, post-orogenic A-typegranites). They also presented field evidence that these

subhorizontal shear foliations cut across the fold struc-

tures associated with arc accretion and crustal thicken-

ing, and are therefore distinctly younger than the

arc accretion event. Elsewhere in the CED, structures

belonging to the top-to-the-NW nappe translation

event commonly affect Hammamat and Dokhan litho-

logies, which are generally accepted as having beendeposited in a post-collision extensional tectonic setting

(Abdeen et al., 1992; Rice et al., 1993; Greiling et al.,

1994; Stern, 1994; Naim et al., 1996; Fowler and Osman,

2001). Greiling et al. (1994) opted to have another

NNW–SSE compressional event following post-collision

extension in order to explain the NNW-ward thrust-

ing in Hammamat basins, but simultaneous NNW–SSE

compression and NNW–SSE stretching are incom-patible.

The above facts argue strongly that the top-to-the-

NW shearing displacement is post-arc collision and

associated with an extensional tectonic event. Blas-

band et al. (2000) have argued that the evidence in

the Wadi Kid area, Sinai, favours an extensional col-

lapse NW-ward nappe transport mechanism, and

that this led to crustal thinning and isostatic rise ofthe CED core complexes, in a similar fashion to the

north American core complexes. Fritz et al. (2002)

and Bregar et al. (2002) argue against Blasband et al.’s

model on the grounds that there is insufficient evi-

dence for collision-related crustal thickening in the

CED. They preferred a magmatic assisted core com-

plex rise leading to a magmatic core complex, in the

case of the Gebel El Sibai complex. Fritz et al.(2002) proposed that crustal thickening during arc

accretion is compensated by the tendency for lateral

flow away from the collision zone of magmatically

softened crust in an oblique collision zone. We will re-

turn to this problem in Section 4.4. after discussing the

possible mechanisms for NW-ward nappe translation in

the CED.

Fig. 11. Sketch showing a structural model for the progressive development of the main structures in the Um Esh–Um Seleimat area, clarifying the

time relations between shearing and folding. Elliptical ornaments represent pebble stretching lineations. Stages 1–4 are discussed in the text. Arrows

indicating top-to-the-NW shearing decrease in size from (a) to (c) representing a gradual phasing-out of this deformation. Arrows indicating NE–SW

shortening increase in size from (b) to (d) representing a gradual phasing-in of this deformation. (a) Subhorizontal S1 and Sm mylonitic foliations with

top-to-the-NW shear sense and L1 and Lm lineations. (b) The subhorizontal foliations are gently folded about NW–SE trending symmetrical upright

mesoscopic F2 folds. (c) Continued top-to-the-NW shearing during F2 and probably early F3 folding produces transection of these folds by

progressive formation of subhorizontal S1 foliations. (d) NW–SE F3 macroscopic folding continues after cessation of top-to-the-NW shearing.

Mesoscopic folds are passively rotated to variable axial plane orientations but remain symmetrical. Some pencil structures are formed by continuous

cross-cutting of S1 folded foliations by other S1 foliations, some are formed by F2 fold-related shortening. SW-ward thrusts form during stage 4.


4.3. Nappe emplacement mechanisms

Nappe emplacement mechanisms are detailed in

Merle (1989, 1998). Merle classified three main (non-

brittle) nappe transport mechanisms: ductile gliding,

spreading mechanisms, and rear compression. The

spreading mechanisms were subdivided into gravita-

tional spreading (relieving the steepened topography at

the top of the nappe mass); spreading–gliding (combin-

ing spreading with gliding induced by a slope of the base

of the nappes); and extruding–spreading (combining rear

compression and spreading). The nappe emplacement-

related deformation is a combination of pure shear

(vertical thickening or thinning, and a ‘‘pull-from-the-

front’’ effect) and simple shear (related to nappe trans-lation). Identifying the operating nappe emplacement


mechanism requires recognition of patterns of foliationtrajectories (especially in vertical sections parallel to the

displacement direction); finite strain gradients; strain

regime (coaxial/non-coaxial); displacement gradients;

and relations between stretching lineations and dis-

placement directions.

Some structural characteristics of the Um Esh–Um

Seleimat and surrounding areas in the CED which assist

in constraining the identity of the NW-ward nappetransport mechanism in the CED are represented in

Table 2, along with essential features of the ideal nappe

transport mechanisms identified by Merle (1998). The

structural evidence is presently insufficient to identify

the CED nappe mechanism but field evidence supports a

gravitational collapse mechanism (probably gliding–

spreading) (Table 3).

Table 2

Comparison of characteristics of the various nappe emplacement mechanism

Nappe emplacement

mechanism

Summary of main structural characteristics of

emplacement mechanism

Ductile gliding • Downwards increasing simple shear strain

• No vertical shortening

• Concave foliation trajectories with foliation

exceeding 40�• Parallel extension lineation and nappe transl

directions

Gravitational spreading

(3D model)

• Downwards increasing simple shear strain

• Vertical shortening

• Concave foliation trajectories with foliations

ing vertical orientations in the upper parts o

• Nappe translation direction at 90� to extensio

near the top of the nappe but parallel to the l

the lower parts

Gliding–spreading • Shear strain increases downwards and towar

• Vertical shortening

• Convex (hindmost), sigmoid and concave (fo

foliations trajectories that are progressively m

concave as deformation proceeds. Foliations

less than 45� dip• Parallel extension lineation and nappe transl

tions

Extruding–spreading • Shear strain increases downwards and towar

• Vertical thickening at rear passes forwards in

shortening

• Concave foliation trajectories with upright o

near the top of the nappe

• nappe translation direction at 90� to extensio

near the top of the nappe but parallel to the l

the lower parts

Rear compression • Shear strain increases downwards and towar

• Vertical thickening at the rear, decreasing to

front

• Concave foliation trajectories with increase i

foliations to vertical near the top of the nappe

are never horizontal and increase in dip towar

of the nappe

• Extension lineations are best developed at th

are approximately vertical

4.4. Effects of collapse on arc collision-related structures

Blasband et al. (2000) identified early upright isocli-

nal NE–SW trending folds as evidence for crustal

thickening associated with arc collision in the Wadi Kid

area of the Sinai. If similarly oriented upright isoclinal

folds have existed elsewhere in the CED they may have

been deformed by later gravitational collapse as

explained by Aerden and Malavieille (1999) for theVariscan Orogeny. The latter authors discovered that

collision-related upright isoclinal folds associated with

crustal thickening had been rotated to recumbent

orientations in the deeper parts of a zone of subhori-

zontal foliation, which formed a decollement for cover

nappes transported during gravitational collapse. These

recumbent folds were then extended in the direction

s with field observations from the central Eastern Desert

the Field structural observations from the central Eastern

Desert

dips not

ation

(1) Downwards increasing strain. This is shown by the

increase in degree of development in S1 foliation down-

wards in both the ophiolitic melange and conglomeratic

formations. Increasing simple shear strain with depth is

supported by the predominance of mylonite zones in the

lower parts of the shear zone

approach-

f the nappe

n lineations

ineations in

(2) Vertical shortening. As discussed in text Section 4.5.

the steep metamorphic gradient from greenschist or

lower temperature facies nappe cover to gneissic rocks of

amphibolite or higher temperature facies (Ries et al.,

1983), is interpreted as being due to vertical shortening

of metamorphic zones within the foliated zone separat-

ing these rocks. This vertical shortening is responsible for

the subhorizontal S1 foliations and is associated with

NW–SE extension

ds the front

remost)

odified to

generally

ation direc-

(3) Concave foliation trajectories. Equivalent foliations in

the stratigraphically higher parts of the Hammamat

sequence in the Um Had area west of the study area were

originally gently to moderately dipping to the S and SE

(Fowler and Osman, 2001). The deeper parts of the same

sequence have foliations approaching parallelism with

the bedding. Together these are consistent with upward

concave foliation trajectory though further work is

necessary to determine if there is a gradation in foliation

dip values between these two levels

ds the rear

to vertical

rientations

n lineations

ineations in (4) Foliation dips less than 45�. The steepest foliations

which can be related to the top-to-the-NW nappe

translation do not have dips much greater than 40�(Fowler and Osman, 2001)

ds the rear

wards the

n dip of

. Foliations

ds the front

e rear and

(5) Extension occurs in the direction of nappe transport.

Nappe transport direction indicators include striations,

duplexes etc. showing thrust nappe transport to the NW,

i.e. in the same direction as the stretching lineations

Table 3

Consistency of field observations (1)–(5) in Table 2 with characteristics of nappe emplacement mechanisms

Ductile gliding Gravitational

spreading

Gliding–

spreading

Extruding–

spreading

Rear compression

(1) Downwards increasing strain Yes Yes Yes Yes Yes

(2) Vertical shortening No Yes Yes Yes? No

(3) Concave foliation trajectories Yes Yes Yes? Yes Yes

(4) Foliations dipping less than 45� Yes No? Yes No? No

(5) Extension in direction of translation Yes No? Yes No? No


of nappe transport. Fig. 12 shows a similar model

for the formation of the nappes and Hammamat basins

in the CED, and offers another possible mechanism

for the formation of recumbent sheath folds in the

gneissic rocks at Hafafit described by Fowler and El

Kalioubi (2002). If this is so, then some key structural

evidence for collision-related crustal thickening in theEgyptian Eastern Desert may have been incorrectly

included in the category of structures associated with

the NW-ward nappe translation event. Fig. 12 also

shows how subhorizontal shearing could produce inter-

leaving of slices of ophiolitic melange and Hammamat

metasediments seen in the cross-sections of the study

area (Fig. 2b).

Fig. 12. (a) Idealized longitudinal section (oriented NW–SE along the direc

stippled areas represent Hammamat molasse in extensional basins. Areas with

volcanic formations. Unornamented areas of folding beneath the melange an

and Sm foliations produced during extensional collapse of the orogen. Crus

deformation in the stratigraphically lower parts depending on the degree of te

with the basin on the right). In the gneissic rocks, originally upright folds

recumbent orientations as a result of the gravitational collapse. Figures (b)

Hammamat units and ophiolitic melange, seen on the Um Esh–Um Seleima

Hamammat basin is shown composed of ophiolitic melange fault blocks that

planes labelled ‘‘s’’ in (b)) at the lower levels of the Hammamat basins juxtap

ophiolitic melange.

4.5. Thermal control on the level of the decollement

underlying the cover nappes

The foliated and mylonitic shear zone accommodat-

ing top-to-the-NW nappe transport in the study area

straddles the greenschist to amphibolite transition. The

gneissic complexes at Wadi Kid (Sinai) and Wadi UmHad, Gebel Meatiq, Gebel El Sibai and Gebel Um El-

Shalul (Central Eastern Desert) also have mylonitic

carapaces, with NW–SE trending stretching lineations,

some with top-to-the-NW kinematic indicators, with

amphibolite facies sheared rocks at the base of the shear

zone and greenschist facies rocks in the upper parts and

above. Metamorphism is roughly coeval with shearing

tion of nappe transport) for the Eastern Desert basement rocks. Grey

v-symbol ornament represent ophiolitic melange and Pan-African arc

d volcanics are gneissic rocks. Dashed lines represent subhorizontal S1t is shown thinning to the NW. Hammamat molasse sequences show

ctonic thinning of the crust beneath them (compare the basin on the left

related to arc collision and crustal thickening have been rotated to

and (c) provide a possible explanation for the interleaving of slices of

t cross-sections (Fig. 2b). In (b) and (c) an irregular basin floor of the

were juxtaposed during basin opening. Progressive shear slicing (shear

oses and sandwiches slices of sheared Hammamat metasediments and


at the base of the cover nappes in the study area on theevidence of greenschist and amphibolite facies phases

defining the mylonitic foliations (Sm) and lineations (Lm).

One interpretation of these facts is that after collision-

related crustal thickening, thermal re-equilibration of

the thickened Pan-African crust, involving upward

transfer of heat (perhaps magmatically as described by

Fritz et al., 2002), led to decreased shear strength of the

rocks via the effect of temperature on rock rheology. Ata temperature-controlled depth (evidently roughly cor-

responding to amphibolite facies conditions) a subhor-

izontal regional ductile shear zone formed in the

thermally softened rocks to accommodate the sliding of

gravity-driven cover nappes.

The apparently steep metamorphic gradient from

cover nappes to the MCC gneisses through the mylon-

itic shear zone, noted by Ries et al. (1983) (see abovein Section 2.3), and also evident at the Wadi Kid com-

plex (Blasband et al., 2000) and other complexes in

the CED, are an expected result of the intense thinning

of the shear zone during vertical collapse and horizon-

tal stretching. A similar feature was noted by Aerden

and Malavieille (1999) in their example of gravita-

tional collapse generated decollement. The evidence

that metamorphic heating continued after folding ofthe shear zone (see Section 2.3) is best explained by

the arrival of granitoids into the cores of the rising

gneissic complexes as explained by Fritz et al. (1996),

Neumayr et al. (1998), Fritz et al. (2002) and Bregar

et al. (2002).

Acknowledgements

The authors would like to thank Dr Khaled Gamal

Ali for rewarding discussions on the evolution of this

area and help in the field work. We acknowledge the

logistical assistance provided during this work by Mes-

seurs Sayyid Mohammed Mansour, Ahmed Mahmoud

Badawi, and Rida’. Thanks are due to Dr. Imbarak

Hassen of Suez Canal University for arrangementof thin section cutting. The paper has benefitted signif-

icantly from the comprehensive reviews of M.G. Ab-

delsalam, H. Fritz and an anonymous referee.

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