Journal of African Earth Sciences 45 (2006) 1–15
www.elsevier.com/locate/jafrearsci
The El Mayah molasse basin in the Eastern Desert of Egypt
A. Shalaby
a
a,b,*
, K. Stüwe
a,*
, H. Fritz a, F. Makroum
b
Department for Earth Science, University of Graz, Heinrichstrasse 26, A-8010 Graz, Austria
b
Department of Geology, Mansoura University, Mansoura, Egypt
Received 8 September 2004; received in revised form 13 September 2005; accepted 6 January 2006
Available online 9 March 2006
Abstract
The El Mayah basin is one of several Pan-African, sedimentary basins that formed across several hundreds of kilometres of the Eastern Desert of Egypt. Its sedimentary record shows that deposition occurred in two stages: The earlier stage is characterised by the deposition of two units of fluviatile sediments deposited in a half graben structure. Initial deposition of volcaniclastics and later influx of
granitic clasts indicates that this part of the basin formed mostly after intrusion of what is known as the ‘‘older’’ granite generation
in the Eastern Desert around 650–610 Ma. The later stage in the basin evolution is characterised by basin inversion, tilting of the older
units and subsequent sedimentation of fluviatile sediments into a shallow synformal pull apart structure that formed after intrusion of the
‘‘younger’’ granite generation. This pull apart structure is interpreted to have formed in response to a brittle reactivation of one of the
largest shear zones in the Eastern Desert causing a linking of the Um-Nar Nogrus shear zone to the Sholul dome shear zone.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Molasse basin; Eastern Desert of Egypt; Pan-African evolution
1. Introduction
Molasse basins occur in different tectonic settings and
their record contains useful information on the tectonic
evolution of the adjacent orogen (e.g. White et al., 2002;
Spiegel et al., 2000). Molasse type sediments have been
described from pull-apart basins related to shear zone formation, and from large foreland basins related to elastic
flexure of plates in the orogenic foreland (Christie-Blick
and Biddle, 1985; Sachsenhofer et al., 2000). One area
where there is an obvious link between molasse basin formation and orogenic evolution is the Central Eastern Desert of Egypt (Fig. 1). There, the formation of molasse
basins is directly linked to the formation of metamorphic
core complexes during the late Pan-African orogenic evolution (e.g. Greiling et al., 1994; Fowler and Osman, 2001).
As such, the record of the basins can be used to constrain
*
Corresponding authors. Tel.: +43 316 380 5682; fax: +43 316 380 9870.
E-mail addresses: [email protected] (A. Shalaby), kurt.stuewe@
uni-graz.at (K. Stüwe).
1464-343X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jafrearsci.2006.01.004
the exhumation history of the belt. The sediments of these
basins are called ‘‘Hammamat molasse’’ – named after the
type locality in the Hammamat valley near the Meateq
dome. The basins record a complicated succession of
mostly fluvial sediments (Grothaus et al., 1979). Fritz and
Messner (1999) have shown that these molasse basins in
a broader sense include: (i) foreland, (ii) intramontane
and (iii) strike slip basins and they discussed the subsidence
mechanism of one of the intramontane basins. In this paper
we present a description of one of the strike slip basins –
the El Mayah basin – and relate its record to the tectonic
evolution of the adjacent region. It can be shown that the
basin formed during the activity of a shear zone system
that is also responsible for the large scale geometry of the
Wadi Mubarak belt – a belt of volcano-sedimentary rocks
that cross-cuts the general structural fabric of the Central
Eastern Desert and which forms therefore a key feature
in the understanding of the Central Eastern Desert (Shalaby et al., 2005). Our interpretation is discussed within
the general framework of molasse basin formation during
the Pan-African in the Central Eastern Desert.
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A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
Fig. 1. Tectonic map of the Central Eastern Desert of Egypt (modified after Fritz et al., 1996). Note that the metamorphic core complexes and molasse
basins are aligned along the Najd Fault system. The El Mayah basin is formed at the western end of the Wadi Mubarak belt which is made up of a
volcano-sedimentary sequence and is shown by the hatched area. Plutons labelled as ‘‘post-kinematic’’ and ‘‘syn-extensional’’ are also known as the
‘‘younger’’ and the ‘‘older’’ granite, respectively.
2. Tectonic setting of the Hammamat molasse basins
Molasse basins similar to the Hammamat molasse are
common in Arabia (Genna et al., 2002) and in the Central
Eastern Desert of Egypt (Akaad and Noweir, 1969)
(Fig. 1). In the Central Eastern Desert of Egypt these
basins are distributed over a region that is several hundreds
of kilometres in north–south extent. They all lie on top of a
structural unit of Pan-African rocks known as ‘‘suprastructure’’ (El Gaby et al., 1990; Hassan and Hashad, 1990;
Akaad and Noweir, 1980). This suprastructure was intensely deformed during accretion of island- and magmatic
arcs onto the East-Sahara craton by an oblique collision
event in the late stage of the Pan-African orogeny (Gass,
1982). At this time, a sinistral wrench corridor developed
which is known as the Najd Fault system (Stern, 1985).
This wrench corridor has exposed a series of metamorphic
basement domes, known as the Metaeq, Sebai and Hafafit
domes (Fig. 1). The tectonic evolution of the domes can be
constrained in absolute time by a series of intrusive events.
In particular, there are two generations of granite, widely
known as the ‘‘older’’ and the ‘‘younger’’ granite generation. The older generation is deformed and intruded at variable times between 700 Ma and 600 Ma, in the Wadi
Mubarak belt around 600 Ma. The younger generation is
clearly identified by its pink colour and undeformed character and intruded around 580 Ma (see Shalaby et al.,
2005).
Several molasse basins formed adjacent to the basement
domes (Fig. 1) (Fowler and Osman, 2001; Fritz and Messner, 1999). The Hammamat basin sensu stricto is located
20 km west of the Meateq dome in a foreland position
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
(Akaad and Noweir, 1969, 1980; Grothaus et al., 1979).
The time of its initial subsidence is not well constrained,
but sedimentation ended around 580 Ma. The Karim
basin is located 4 km northwest of the Sebai dome in
an intramontane position within the Najd Fault system.
It is fault-bound with normal faults along the northwestern and southeastern margins and strike-slip faults in the
southwest and northeast (Fritz and Messner, 1999). The
onset of sedimentation is interpreted to be around
645 Ma and is related to the emplaced of the Sebai dome
between 650 and 585 Ma (Bregar et al., 2002; Fritz et al.,
2002).
The El Mayah basin (EMB) is one of several smaller
basins that also include the Igla and Atawi basins
(Fig. 1). The Igla and El Mayah basins differ from the larger molasse basins because the smaller basins are bound by
strike-slip zones and show typical patterns of strike-slip
basins with rapid subsidence coupled with high amount
of sediment delivery, narrowness and migration of depocenters (Messner, 1997). These small basins are also not
related to a metamorphic dome. As such, they may be better interpreted as pull-apart basins. Nevertheless, like the
larger molasse basins, they contain a similar succession of
fluvial sediments including gravels and various sand- and
siltstones. The El Mayah basin lies at the western end of
a crustal scale shear zone named the Um-Nar shear zone
3
that crosses the Wadi Mubarak belt and is intimately
related to its activity (Fig. 2). This shear zone is one of
two major shear zones in the Wadi Mubarak belt and the
deformation history of this shear zone has been constrained in some detail by cross-cutting relationships to
magmatic intrusions (Shalaby et al., 2005).
3. Geology of the El Mayah molasse basin
The El Mayah basin is located between 34°00 0 E–34°10 0 E
and 25°15 0 N–25°20 0 N at the western end of the Wadi
Mubarak belt. This belt is one of the few regions in the
Central Eastern Desert that strikes west–east and even
northeast–southwest and is therefore oblique to the general
fabric of the Eastern Desert (Fig. 1). Shalaby et al. (2005)
interpreted the strike of the Wadi Mubarak Belt as a conjugate direction to the Najd Fault system and showed that
its deformation history can be interpreted in terms of the
accepted scheme of deformation events for the Central
Eastern Desert of Loizenbauer et al. (2001).
The EMB is separated from this belt by a 4 km wide
zone of weakly deformed island arc rocks known as ‘‘undifferentiated metavolcanics’’ (Akaad et al., 1996) (Fig. 2).
The basin is elongated in an east–west direction for about
16 km and about 2 km in a north–south direction (Figs. 2
and 3). The basin was investigated along three profiles
Fig. 2. Simplified geological map of the western half of the Wadi Mubarak belt showing linking of the western end of the belt by the El Mayah basin
through the northwest–southeast trending sinistral shear of the Um-Nar shear zone. (simplified after Shalaby et al., 2005). Note the areal distribution of
the granitic rocks close to the El Mayah basin where they form at least two thirds of the outcrop.
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Fig. 3. Stratigraphic and structural data collected in the El Mayah basin. (a) The lateral distribution of the lithostratigraphic units. (b) Lower hemisphere
stereonet projections of bedding. (c) Fault slip data analyses for different basin points. The fault slip data show normal faults and strike-slip faults
depending on closeness to the shear zone. The trends of the dyke swarms in the basin and analyses of extension gashes on the shear zone indicate
northwest–southeast extension throughout the basin.
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
across the basin. These profiles are constructed along the
Wadi Abu Besella, Wadi El Meyah and Wadi Um Lasifa
valleys (Fig. 3a). In all three profiles the basin sediments
lie atop undifferentiated metavolcanics.
The basin-fill can be sub-divided into two elongated subbasins separated by an unconformity (Fig. 3). The western
sub-basin is elongated in a NE–SW direction and is about
7 km long and 2 km wide. The sedimentary strata trend
NW–SE and dip is steep (Fig. 3b). The sedimentary succession of the western sub-basin can divided into two units
that are separated by a steep normal fault. The older Unit
I is exposed along the northern margin of the basin and is
northwards-younging (Fig. 3). The younger Unit II is
exposed in most of the western sub-basin and is also northwards-younging. Both units are fault-bound at both their
hanging wall and their footwall.
The eastern sub-basin is narrower and longer than the
western sub-basin and extends in a ENE–WSW direction.
It is about 10 km long and 112 km in width. It has a shallow
synformal geometry with a gently west-plunging axis. The
eastern sub-basin is filled by the third stratigraphic unit,
5
Unit III. Along the basin margins, strata dip more steeply,
but they dip gently towards the west in the centre. Unit III
can be further sub-divided into a lower and an upper subunit that are separated by an unconformity. All three units
are made up of fluvial sediments.
3.1. The lithostratigraphic sequence
Unit I contains no sedimentary detritus made up of granitic fragments. The unit has a thickness of about 375 m
and both the base (south) and the top (north) of the sedimentary succession are deformed by normal faults
(Fig. 4). The succession starts in the south with about
75 m of breccia composed of volcaniclastics within a matrix
of yellowish green and red coarse and fine sandstones
(Fig. 5a). The average diameter of the breccia components
is about 3 cm. Individual breccia beds are fining-upwards
with younging direction to the north. The topmost part
of the brecciated sand is overlain by an about 300 m thick
sequence of red sandstone (Figs. 3a and 4). The sandstones
are massive and free of sedimentary structures.
Fig. 4. Lithostratigraphic profiles drawn in the field, showing distribution of the sedimentary units and their boundary relationships. The locations of the
profiles are given in Fig. 3a.
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A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
Fig. 5. Field photographs showing the sedimentary and tectonic structures of the basin. (a) Successive fining-upward at the base of Unit I, showing
younging northward direction. Unit I is separated from Unit II by brittle normal faults. (b) Mud cracks at the base of Unit II. (c) Erosion contact at the
top of the lower coarsening-upward cycle at the base of Unit II. (d) High accumulation of younger granite fractions in the sediments of Unit III. (e) Crossstratification is abundant in the sediments of upper stratigraphic sub-Unit III. The red sandstone layers of the lower sub-Unit III are gradually changed to
yellowish white in the upper sub-Unit III. (f) The northern boundary of the basin in the western part is dominated by high angle normal faults crossing the
surrounding terrain. (g) The surrounding terrain in the southern contact of the basin shows semiductile shallow normal faults with north–south extension.
(h) The sediments of lower stratigraphic sub-Unit III on the Um-Nar shear zone are deformed by steep normal faults with sinistral shear sense. (i) Collapse
structures in the sediments of the lower sub-Unit III.
Unit II contains fragments of the older granite generation. Total thickness is about 1800 m. In the basal (southern) parts of profile A–A 0 , sedimentation starts with
laminated red sandstones that are intercalated locally with
thin lenses of claystones and siltstones. Desiccation cracks
are common within the siltstone interbeds (Fig. 5b). For
the next 600 m, the red sandstones become gradually coarser upwards. Gravel-supported conglomerates develop
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
which contain massive, unsorted and slightly rounded fragments (Fig. 4). The fragments are mainly composed of
older granite, with some talc-serpentinite and metavolcanic
pebbles. The conglomerates become coarser towards the
top (Fig. 4a). Close to the top of this coarsening-upward
sequence, 85 m of red sandstones contain some clasts of
reworked red sandstones and conglomerates. The red sandstones are overlain by coarse to pebbly sized green sandstones, which contain fragments of the red sandstones
and show evidence for cross-stratification and graded bedding. The contact between the red sandstones and green
pebbly sandstones is an erosive one (Fig. 5c).
The upper (northern) part of Unit II is marked by an
approximately 1100 m thick section of a continuously
upward-fining sequence (Figs. 3a and 4). Initially, there is
an 85 m thick conglomerate with boulders up to 50 cm in
diameter. These boulders are derived mostly from older
granite and metavolcanics. Reworked pebbles derived from
red sandstones are rare, especially close to the top of the
conglomerate layer. The gravels are well rounded but badly
sorted. A few lenses of red sandstones are intercalated with
the conglomerates. The roundness of the clasts increases
upwards. Towards the top, the boulders decrease gradually
in size to diameters around 10–15 cm and intercalations of
red sandstone beds become abundant. About 720 m of red
sandstone beds overlie conformably the gravel-supported
7
sediments. The contact is gradational and is marked by
wide intercalations of green pebbly to coarse sandstones.
Siltstone intercalations are common close to the top of
the sequence.
Unit III is about 1100 m thick along profile B–B 0 and
about 850 m along profile C–C 0 (Fig. 4). The unit is characterized by reworked sediments form the other two units.
The lower stratigraphic sub-unit is exposed in the easternmost part of the basin (Fig. 3b). It overlies Unit II unconformably. The sequence starts with 260 m of boulders,
gravels and conglomerates. The diameter of fragments
reaches 50 cm but a gradual decrease in the diameter is
recorded towards the top of the sequence (Fig. 4). The fragments include components of metavolcanics, younger and
older granites, serpentinite, talc and diorite (Fig. 5d). Boulders of reworked red sandstone also occur. Fragments of
serpentinite, talc and diorites are common close to the
southern margin of the basin, but they are not recorded
at higher stratigraphic levels of the lower sub-unit. The
degree of roundness and sphericity increases gradually to
the top of the lower sub-unit. Up to a stratigraphic height
of 300 in Wadi El Meyah (Fig. 3) the boulders get finer and
reach gravel size. Beds of red sandstones are widely intercalated with the gravel-supported conglomerates. The
sequence of the lower sub-unit ends with about 320 m of
red sandstones. The contact between the sandstones and
Fig. 6. Interpretive schematic summary cartoon of the tectono-stratigraphic sequence of the El Mayah basin. The figure shows the sedimentary units with
the local and regional variations in their lithology and structures depending on the litho-type components, type of intrusions and the characteristic
structures affecting the basin units.
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A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
the underlying gravel-supported layer is gradational.
Lenses of pebbly and gravely sandstones intercalate the
red sandstones locally, and contain some pebbles of
reworked red sandstones and older and younger granites.
Close to the top of this sequence, the red sandstones gradually change to yellowish white sandstones, with intercala-
Fig. 7. Strain analysis for deformed pebbles of the sediments of lower sub-Unit III along the Um-Nar shear zone. These pebbles are used as strain markers
because they are highly deformed. (a) Fry analyses along the three orthogonal principle finite strain planes. (b) Flinn’s diagram showing that the strain is
different from the west to the east, along the shear zone. (c) Orientation of the principle finite strain axes and planes indicating that the extension was in a
northwest–southeast direction.
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
tions of a few layers of red sandstones (Figs. 3a – profiles
B–B 0 and C–C 0 and 4). The yellowish white sandstones
are pebbly at the base and change to white silty sandstones
at the top. Cross-stratification and graded bedding structures are common (Fig. 5e).
The upper stratigraphic sub-unit of Unit III is different
to the lower sub-unit in that it contains a large amount
of reworked sediments that are often very coarse-grained.
This sub-unit is exposed in the centre of profile B–B 0 – in
the core of the synformal structure of the eastern sub-basin
(Figs. 3a and 4). Sedimentation started with deposition of
green pebbly to gravely conglomerates, which comprise
fragments of reworked red sandstones, younger and older
granites and metavolcanics. Reworked sediments of different compositions are common. Intercalations of red sandstone layers are rare. Cross-bedding and graded bedding
structures are characteristic of this sub-unit. The gravel size
decreases gradually towards the top of the sequence, where
a change to coarse and pebbly sands intercalated with
gravel rich conglomerates is apparent. Sandstones are less
abundant than in the lower sub-Unit III. The sedimentary
succession described above is interpreted in terms of an
integrated basin evolution in Section 4.
3.2. Observations on structure and intrusions
Several intrusive rocks cut various parts of the sedimentary succession and provide relative time markers for its
evolution. Some of these intrusions correspond in character to the ‘‘older’’ and ‘‘younger’’ granite generation as
described in Section 2. Unit II is intruded by an undeformed andesite stock at the southern basin margin (Figs.
3a and 4). However, note that there are no andesitic clasts
within this unit. A stock of diorite intrudes the southern
contact of Unit III with the basin-margin (Figs. 3a and
4). The diorite is deformed by brittle normal faults. The
northern boundary of the western half of the basin is
intruded by younger granite. This intrusion of the younger
granite is not exposed in a map view because the red sandstone beds of Unit I cover the granite. The granite sends
offshoots into the red sandstones (Fig. 6). The early sediments are intruded by rhyolitic, white aplitic and basaltic
dykes. The aplite dykes locally cross-dykes of rhyolitic
composition. An aplite dyke of about 5 m thickness
crosses the younger granite and the overlying red sandstone of Unit I close to the northern boundary of the
basin (Figs. 4 and 6). Several aplitic and basaltic dykes
also cross the lower sub-Unit III. A large aplitic dyke
observed in Units I and II has also been found in the
lower sub-Unit III but has not been found in the upper
sub-Unit III (Fig. 6).
The EMB is entirely bound by normal faults that dip
steeply to the south along the northern boundary, and it
shallow angles to the north along its southern boundary
(Figs. 4 and 5f, g). However, the northern and southern
contacts of Unit III are very different, giving the basin
an overall asymmetry (Fig. 6). The northern boundary is
9
characterised by a series of steep early normal faults that
appear to be associated with the early basin subsidence at
the time of the deposition of Unit I. However, these faults
are overprinted by a large east–west trending sinistral
shear zone that cut the basin after deposition of Unit
III. Strongly deformed pebbles in this shear zone show
that there is a strain gradient both along and normal to
the shear zone. The shear zone and associated normal
faults are coated with chlorite, suggesting deformation
under greenschist metamorphic conditions. Fault plane
solutions derived from slickenside striations on brittle
faults indicate NW–SE extension on the northern boundary of the basin (Fig. 3c – diagram no. 3). On the shear
zone the maximum and minimum principle stress axes
are oriented horizontally NE and NW, respectively
(Fig. 3c – diagram no. 1). In contrast, the southern contact is depositional. However, there are several northdipping normal faults within the basin-fill that constitute
a horst and graben architecture. These faults are also
coated by chlorite and are likely to be related to the shear
zone along the northern margin. Nevertheless, r1 near the
southern boundary plunges in a vertical direction while
both r2 and r3 are oriented horizontally, indicating
NNW–SSE extension at the southern boundary of the
basin (Fig. 3c – diagram nos. 3 and 4).
3.3. Strain analysis
The shear zone along the northern margin of the basin is
part of a major shear zone system called the Um-Nar shear
zone. In the El Mayah basin, this shear zone does not
deform sediments of Units I and II, but strongly affects
pebbles of Unit III. This shear zone was discussed by Shalaby et al. (2005) and forms an important time marker for
the interpretation of the Wadi Mubarak belt and thus for
the tectonics of the Eastern Desert. In view of the fact that
it also plays an important role in the evolution of the EMB,
its genetic evolution forms a crucial part of our overall
understanding of the significance of the basin. Therefore
we performed a careful strain analysis on selected outcrops
in the shear zone.
Finite strain was obtained by measuring the axis-lengths
of deformed pebbles extracted in the field from their
matrix, in three orthogonal directions and plotting them
onto a Flinn diagram. On Fig. 7b it may be seen that the
data have a wide scatter around the plane strain line. However, it can be noticed that the strain changed from constriction strain at the western part of the shear zone to
flattening strain at the eastern part of the shear. This
indicates that the strain is heterogeneous along the shear
zone. The orientation of the deformed pebbles of the conglomerates was measured in the field to determine the orientation of the principal finite strain axes developed in the
shear zone. The three-dimensional strain data are stereographically projected on a lower hemisphere equal area
net (Fig. 7c). The X, Y and Z axes of the finite strain ellipse
plunge in the directions of 15°/300°, 50°/190° and 28°/45°,
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A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
respectively. The orientations of the XY, YZ and XZ
principle planes of bulk strain dip in the directions
50°/220°, 55°/125° and 35°/5° (Fig. 7c). The X and Z axes
tend to be sub-horizontal while the Y-axis tend to be subvertical, indicating that the northwest–southeast extension
and northeast–southwest compression correspond to sinistral shearing (Fig. 7c). This is consistent with the fault
plane solutions shown in Fig. 3c (diagram no. 3).
The Fry method (Fry, 1979) is used to estimate the
superimposed tectonic strain (Rs). Selected clasts were
traced on the outcrops – using the finite strain planes as reference planes – and copied on a transparent sheet. The copied grains on the sheets were digitized and analysed using
the Structural Geology – Mapping/GIS Software (http://
www.earthsciences.uq.edu.au/~rodh/software/index.html#
geosymbol). The centres of the identifiable clasts were
clicked by using the mouse. The program processed the
input data, which are represented by a map of clast centres.
The points that are processed by the Fry program outline
a vacancy in the middle part of the output diagram
(Fig. 7a). We sought the best ellipse to fit the formed
vacancy. The axial ratio of the ellipsoidal vacancy estimates tectonic strain. It is clear that tectonic strain on
the XZ plane of finite strain is higher than those measured
on the YZ and XY planes (Fig. 7a). The values of Rs on the
XY and YZ planes are relatively close to each other. This
Fig. 8. Tectonic evolution model of the El Mayah basin. The insets show the interpreted stress regime during the different stages of the basin evolution.
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
means that the deformation of the conglomerates tends to
plane strain.
4. Inferred basin evolution and subsidence mechanism
The El Mayah basin evolved in two important stages.
The first stage started with formation of a northwards
tilted half-graben in the western part of the basin
(Fig. 8a). The fault population and slickenside striation
analyses of early syn-sedimentary faults indicate bulk
north–south extension (Fig. 3c – diagram no. 2) which is
consistent with the maximum extension direction during
the formation of the Najd Fault system. This event is characterised by an overall northwest–southeast distributed
sinistral shear regime in the Central Eastern Desert and is
often referred to as D3 (e.g. see summary by Loizenbauer
et al., 2001). The orientation of this sinistral shear produces
maximum horizontal compression in an east–west direction
and maximum extension in a north–south direction
(Fig. 8a inset). This event was also associated with the
intrusion of the older granite generation in the Eastern
Desert around 615–650 Ma (e.g. Loizenbauer et al., 2001;
Bregar et al., 2002; Fritz et al., 2002). As there are no
granitic clasts in this unit, we suggest that rifting of the
western sub-basin occurred just before emplacement of
these older granites. Intrusive bodies of the older granite
generation do occur in the Um-Nar shear zone only few
kilometres east of the EMB (Fig. 2).
Subsidence rates and relief energy further increased during deposition of Unit II. The depocenter shifted southwards. The coarsening-upward sequence at the base of
Unit II indicates that the basin subsidence was active during the early deposition with subsequent uplift of the surrounding relief. The deposition of red sandstone and the
coarse green sandstones with the existence of reworked
conglomerates and red sandstone fragments indicate unstable and active fluvial conditions. However, Unit II contains
granitic clasts so that we suggest that sedimentation of this
unit continued after the intrusion of the older granite generation. It can also be said that Unit II was deposited
before the emplacement of the andesite intruding the
southern boundary. Such andesite stocks are typical for
the so-called Dokhan volcanics that mark the late stages
of the Pan-African evolution of the Central Eastern Desert
(El Gaby et al., 1990; El Gaby, 1994). Siltstone intercalcation towards the top of Unit II indicate that the relief
decreased gradually and the basin stabilised in the last
stages of the deposition of Unit II.
After deposition of Units I and II the basin was inverted
and exhumed. The steepness of bedding in Units I and II
indicates that the basin had undergone passive rotation
before the deposition of the Unit III. During this process
the sedimentary sequence was tilted vertically, probably
in a north–south compressional environment and later
intruded by a small granite, a diorite and rhyolitic dykes,
all of which belong to what is known as the younger granite
generation in the Eastern Desert (El Gaby et al., 1990).
11
The onset of sedimentation of Unit III marks the start of
the second stage of the basin evolution. The eastern half of
the basin subsided rapidly. Sediments contain clasts of diorite from the younger granite generation and reworked sediments from Units I and II. Thus, we suggest that Units I
and II remained exhumed and we infer a normal fault
between the western sub-basin and the eastern sub-basin
(Fig. 8b). We suggest that the subsidence during this later
stage is related to a late-stage kink in the Um-Nar shear
zone (Fig. 8b). This kink in the Um-Nar shear zone is of
overall importance to the structural geometry of the Eastern Desert. It is part of the northwest–southeast trending
Najd Fault system, that caused exhumation of the basement domes throughout the Eastern Desert (Stern, 1985;
Fritz et al., 1996; Shalaby et al., 2005) (Fig. 1). During
D3, the shear zone was a sinistral strike slip zone and
was related to the exhumation of the Hafafit dome in the
south (Shalaby et al., 2005). At the same time, an en echelon branch of the shear zone further north was related to
the exhumation of the Sholul dome (Fig. 1). We suggest
that a later re-activation of the shear zone connected the
Um-Nar shear zone and the Sholul shear zones with an
east–west oriented branch in the region of the El Mayah
basin. This east–west directed sinistral shear caused maximum compression in a north–south direction and exhumation of the western sub-basin (Fig. 8b inset). A
subsequently formed step-over in the shear zone formed a
pull-apart structure and caused subsidence of the eastern
sub-basin.
After deposition of the lower sub-unit of Unit III, the
Um-Nar shear zone was reactivated resulting in formation
of the regional synformal structure of sub-Unit III (Fig. 6).
This period is also marked by intrusion of aplitic and basic
dykes that crossed sub-Unit III and all previous structures
(Fig. 6). The sediments of the upper sub-Unit III were
deposited in the core of this synform and contain abundant
clasts of reworked sediments of all older units. At the base
of sub-Unit III clasts of talc and serpentinite occur which
we interpret as an indication for proximity of the source
rock to the southern contact of the basin. This is also confirmed by the fact that the roundness and sphericity of the
boulders at the base of the sequence is low and clasts are
badly sorted. The occurrence of dykes that cross-cut all
sedimentary units except the upper sub-Unit II suggests
that the basin was under late regional bulk north–south
extension that ceased before sedimentation of the last
sub-unit (Fig. 3c – diagram no. 5).
5. Discussion
5.1. Temporal correlation with other molasse basins in the
Eastern Desert
The tectonic evolution of the El Mayah basin can be
correlated well with the other molasse basins to make some
orogen-wide interpretation on the evolution of molasse
basins in the Eastern Desert in Pan-African times. For this,
12
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
it is important to underline that much of the evolution of
the Eastern Desert may be related to a time framework
given by an ‘‘older’’ and a ‘‘younger’’ granite generation
that are 650–610 Ma and 580–590 Ma, respectively and
that both occur throughout the Eastern Desert (El Gaby
et al., 1990; Hassan and Hashad, 1990; Bregar et al.,
2002). We begin with a summary of the evolution of the
most important of the other molasse basins.
The Hammamad basin is the largest of the molasse
basins of the Eastern Desert and formed in a foreland position near the Meateq dome. Sedimentation started well
before the exposure of the older granite suite at the surface
(Abu Ziran older granite 614 Ma, which intruded the
Pan-African nappe at a depth of about 12 km, Loizenbauer
et al., 2001; Fritz and Puhl, 1996) (Fig. 9). It was therefore
not available for erosion at this stage (Messner, 1997).
Thus, onset of sedimentation is not very well constrained.
During a second phase, rapid hinterland uplift increased
the relief of the surrounding terrain. This is followed by
basinward migration of normal faults which rejuvenated
the source areas. This stage is dated to have occurred
around 588–595 Ma, because the normal faults are also
related to the emplacement of the Meateq dome, which is
dated at this time (Fritz et al., 1996; Loizenbauer et al.,
2001). During the third phase the Pan-African nappes were
detached from the dome and the Hammamat basin was
incorporated into the late-orogen tectonic regime (Fig. 9).
The Hammamat basin is intruded by one of the younger
granites at around 590 Ma (Rice et al., 1993) indicating
that the basin evolution was completed by then.
The onset of sedimentation in the Karim basin is related
to formation of a set of ‘‘outer’’ strike slip zones around the
Sebai dome that record the onset of exhumation of the
Sebai dome (Fritz et al., 2002). These shear zones are
related to the emplacement of the central gneiss, dated at
650 Ma (Bregar, 1996; Bregar et al., 2002). Thus, initial
subsidence in the Karim basin is well constrained to be at
650 Ma (Fig. 9). Sedimentation started with shallow water
lacustrine facies, with low sedimentation rates. A subsequent uplift of the Sebai dome caused rapid sediment
accumulation in the basin, with a coarsening-upward
megacycle (Messner, 1997; Fritz et al., 2002). This stage
is followed by hinterland denudation and deposition of a
fining-upward megacycle (Fritz and Messner, 1999). Intrusions of the late stage of the younger granites occurred
around 580 Ma (Hassan and Hashad, 1990) and marks
the end-phase of rapid uplift of the Sebai dome. This phase
is associated with accumulation of late fanglomerate in the
top sequence of the Karim basin (Fritz and Messner, 1999)
(Fig. 9).
The Atawi basin is formed at the southwest corner of the
Sebai dome and it is rimmed by the western Outer Sebai
Shear Zone (Fig. 1). The Atawi basin formed as a pullapart basin synchronously with the emplacement of the
younger granite suite of Gabel Atawi of about 580 Ma
(Hassan and Hashad, 1990) at the southwestern corner of
the western Outer Sebai Shear Zone (Pelz, 1996) (Fig. 9).
Therefore, we interpret that the Atawi basin formed during
the second phase of the emplacement of the Sebai dome
and that it represents the continuation of the Karim basin
(Fig. 9). Later, both Karim and Atawi basins were
deformed close to the outer shear zones, indicating tectonic
reactivation of the dome and basins, coeval with or after
intrusion of the Younger Granite (Fritz et al., 1996; Pelz,
1996).
The Igla basin is also a pull-apart basin formed at the
late stage of tectonic evolution of molasse basins in the
Central Eastern Desert (Rice et al., 1993; Messner, 1997).
The pink Younger Granite predates the deposition of the
early Igla Formation, although later minor pale granites
cut the sediments (Rice et al., 1993). On the other hand
the basin-fill contains fractions of red to purple Dokhan
volcanics. Therefore, we interpret that the basin formed
coeval with or after intrusion of the younger granite in
the Central Eastern Desert, and after the Dokhan volcanics
(Fig. 9).
The El Mayah basin commenced sedimentation prior to
the exposure of the older granite generation at the surface,
and outlasted the intrusion of the younger granite generation. As such, the major part of its sedimentary record was
deposited in the time between 650 Ma and 580 Ma. This is
the time period in which most other molasse basins of the
Eastern Desert were also filled and we can conclude that
the El Mayah basin is similar in age to all the other basins.
Indeed, the coarsening-upward sequence of Unit II and
later fining during basin stabilisation is very similar to
the sedimentology of the Karim basin-fill. However, the
sedimentation in the El Mayah basin outlasted the other
basins with deposition of Unit III. This unit was deposited
in a pull-apart structure after the intrusion of the younger
granite generation and occurred therefore later than sedimentation in any of the other basins. We suggest that this
late stage in the sedimentary evolution of the EMB is
related to a late-stage reactivation of the Um-Nar shear
zone, during which it bridged the earlier shear zones
bounding both the Hafafit and the Sholul domes along
their eastern margins (Shalaby et al., 2005).
5.2. Orogen-wide interpretation of the depositional
environment
In summary, we suggest the following temporal evolution for the molasse basins of the Eastern Desert: (i) Onset
of sedimentation in all molasse basins is related to the
NW–SE striking Najd Fault system, that was active during
a long-lived period known as D3, 650–620 Ma. The age of
its activity is known as it is related to the emplacement of
the older granite generation and exhumation of the core
complexes in the Eastern Desert. (ii) Sedimentation terminated before intrusion of the younger granite generation in
all but the El Mayah basin, where Unit II was deposited on
top of this younger granite. (iii) The deposition of Unit III
in the EMB is related to a late-stage kink in the Um-Nar
Shear zone that links the eastern margins of the Hafafit
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
Fig. 9. Correlation chart for the evolution of molasse basins in the Central Eastern Desert of Egypt, and the relationships between them and the evolution of the surrounding terrain.
13
14
A. Shalaby et al. / Journal of African Earth Sciences 45 (2006) 1–15
and Sholul domes and that is not observed in any of the
other molasse basins.
All molasse basins in the Eastern Desert were characterised by fluviatile environments and are likely to have
formed as individual basins in different individual tectonic
settings, rather than as a single large basin covering the
entire area. This is indicated by different subsidence mechanisms for different basins and also by the palaeocurrent
directions in various basins (Grothaus, 1979). The fact that
no marine influence has been found in any of the basins
indicates that none of the basins ever subsided to large
depths. This may be interpreted as evidence for a largely
thin skinned tectonic environment during the Pan-African
orogen.
small basins rather than remnants of a larger super-basin.
We infer that the absence of marine basin-fill indicates a
thin skinned nature for the Pan-African orogen in the
region.
6. Conclusion
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evolution of this orogen. Our investigations show that
the basin evolution involved two phases. During the earlier
phase the basin developed as a half-graben. Two sedimentary units with a total thickness of 1500 m were deposited
during this phase. The earlier unit contains no granitic
clasts and are therefore interpreted to have been deposited
prior to exhumation of an older granite generation (650–
610 Ma). The later unit contains clasts from this granite. At
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Structural investigations show that the first stage of the
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The Austrian Academic Exchange service ÖAD and the
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