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).
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
26 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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
A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 27
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.
28 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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.
A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 29
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%).
30 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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.
A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 31
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).
32 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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.
A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 33
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.
34 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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.
A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 35
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
36 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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
A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 37
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
38 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40
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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