Formation of Neoproterozoic metamorphic core complexes during

Vol. 23, No. 3, pp. 311-329, 1996
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Formation of Neoproterozoic metamorphic core
complexes during oblique convergence
(Eastern Desert, Egypt)
H. F R I T Z , 1 E. W A L L B R E C H E R ,
1 A. A. KHUDEIR, 2
F. A B U EL E L A 2 a n d D. R. D A L L M E Y E R 3
1Department of Geology and Paleontology, University of Graz, Austria
2Department of Geology, Assiut University, Egypt
3Department of Geology, University of Georgia, Athens GA 30602, USA
portions of the Pan-African Orogen in the Eastern Desert of Egypt were
formed by island-arc accretion in the Neoproterozoic. These areas are characterized by their
lack of major crustal thickening. Metamorphic core complexes occur parallel to the strike of
the Eastern Desert Orogen. These domes exhibit polyphase metamorphism and deformation in
contrast to the structurally overlying nappes which include ophiolitic mdlanges and island-arc
volcanic rocks. These nappes show northwest directed, orogen-parallel thrusting in the internal
parts and west to southwest directed imbrication in the external parts of the orogen. Structures
related to exhumation of the metamorphic core complexes partition into different displacement
paths localized within a crustal-scale wrench corridor of the Najd fault system. Northwest
trending orogen-parallel, sinistral strike-slip faults define the western and eastern margins of
the domes. North and south dipping low-angle normal faults developed along the northern and
southern margins of the domes and form extensional bridges between them. 4°Ar/SSAr ages
obtained from syntectonic muscovites within the shear zones gave Neoproterozoic ages of
595.9_+0.5 and 588.2_+0.3 Ma. The synchronous activity of strike-slip and normal faults
suggests a regional east-west shortening which was accomodated by deep-level basal
decollement beneath the metamorphic core complexes and a coeval northwest-southeast,
orogen-parallel extension. This extension was accompanied by intramontane molasse
sedimentation and emplacement of calc-alkaline plutons.
Since the rapid exhumation of gneisses in the core complexes cannot be explained by thickening
of the crust, the authors favour a model which calls for enhanced heat flow along the Najd
fault system which would have enabled the formation of syn-extensional plutonism and triggered
the exhumation of the metamorphic core complexes. Lateral buoyancy forces were concentrated
within the Najd wrench corridor and enabled orogen-parallel extension. Copyright © 1997
Elsevier Science Ltd. All rights reserved
Abstract--Major
R~sum~--Dans le D~sert Oriental de I'Egypte des portions importantes de I'orog~ne pan-africain
ont ~t~ form6es par accretion d'arcs insulaires au N~oprotdrozoique. Ces r6gions se caract~risent
par I'absence d'~paississement crustal majeur. Des "metamorphic core complexes" y sont
orient,s parall~llement & I'allure g~n~rale de I'orog~ne. Ces dSmes montrent un m~tamorphisme
polyphas~ et une d~formation contrastant avec les nappes de charriage structuralement susjacentes, comprenant des m~langes ophiolitiques et des roches volcaniques d'arcs insulaires.
Dans les parties internes de I'orog~ne ces nappes ont des sens de chevauchement vers le
nordouest, parall~lement ~ I'orog~ne, alors que dans ses parties externes I'imbrication des
~cailles est dirig~e vers I'ouest ~ sudouest. Les structures li~es ~ I'exhumation des "metamorphic
core complexes" se regroupent sur base de leur cin~matique dans un couloir de cisaillement
d'~chelle crustale, appel~ syst~me de failles de Najd. Les bordures occidentales et orientales
des dSmes sont marqu6es par des failles de d~crochement senestre, de direction nordouest et
parall~les ~ I'orog~ne. Les bordures septentrionales et m~ridionales des d6mes sont soulign~es
par des failles normales peu inclin~es vers le nord ou le sud s~parant des domaines en extension.
Des mesures 4°Ar/3SAr obtenus sur des muscovites n6oform~es au sein des zones de cisaillement
ont donn6 des &ges n~oprot~rozoiques de 595,9 + 0 , 5 et 588, 2 + 0 , 3 Ma. Le d~veloppement
synchrone de failles d~crochantes et normales sugg~re un raccourcissement r~gional ouest-
Journal o f African Earth Sciences 311
H. FRITZ
e t aL
est accomod~ par un d6collement basal profond sous les "metamorphic core complexes" ainsi
qu'une extension concommitante nordouest-sudest parall~lement ~ I'orog~ne. Cette extension
a ~te accompagn6e d'une s6dimentation molassique de bassins intramontagneux, ainsi que
d'une mise en place de plutons calco-alcalins.
L' exhumation rapide de gneiss dans les "core complexes" ne pouvant s'expliquer par
~paississement crustal, nous preferons un module faisant appel ~ une augmentation d u f l u x de
chaleur le long du syst~me de failles de Najd permettant la formation de plutonisme synextension et provoquant I'exhumation des "metamorphic core complexes". Des pouss~es de
forces lat6rales concentr~es au sein du corridor cisaillant de Najd ont permis I'extension
parall~lement & I'orogene. Copyright © 1997 Elsevier Science Ltd. All rights reserved
(Received 14 November 1995: revised version received 30 May 1996)
INTRODUCTION
during continental collision. An alternative
It is widely accepted that crustal consolidation
mechanism has been proposed by Hill e t al.
in northeast Africa was achieved by accretion
( 1 9 9 5 ) w h o explains the d e v e l o p m e n t of
of island arcs during the Neoproterozoic (Engel
metamorphic core complexes by magmatic
et al., 1980; Gass, 1982; Kr6ner, 1984; Kr6ner
t h i c k e n i n g of the middle crust. H o w e v e r ,
et al., 1994). Accretion resulted in the formation
significant crustal t h i c k e n i n g accompanied
of a nappe assembly which includes ophiolites,
island-arc accretion and nappe stacking in the
arc volcanics and sedimentary rocks (Frisch and
Eastern Desert as indicated by shallow level
El Shanti, 1977; Ries et al., 1983; Kr6ner et
thrusting of oceanic rocks and the absence of
al., 1994) emplaced over polymetamorphosed
high-grade m e t a m o r p h i s m during late Panand polydeformed basement. The structural
African stacking (Stern, 1994). Consequently,
cover units (Pan-African nappes) developed in
a specific structural style and geodynamic
a thin-skinned t e c t o n i c style and suffered
setting with minor lateral buoyancy forces due
g r e e n s c h i s t m e t a m o r p h i c c o n d i t i o n s . This
to the absence of ove~thickened crust (e.g.
contrasts with the high-grade metamorphism
Molnar and Lyon-Caen, 1988; England and
and partial migmatisation within the structural
Houseman, 1989; Sandiford and Powell, 1991;
basement (Neumayr et aL, 1995). Basement
Zhou and Sandiford, 1992) has to be assumed.
domains are exposed within metamorphic core
This work presents a model to explain the
complexes (Fig. 1) bounded by n o r t h w e s t
evolution of metamorphic core complexes during
trending sinistral shear zones which is related
oblique convergence within an island-arc setting.
here to the Najd fault system (Stern, 1985).
The i m p o r t a n c e of e x t e n s i o n p r i o r to
They form a number of domal structures which
convergence, as shown by van Wees et al.
are arranged in a northwest direction, parallel
(1992), and the role of displacement partitioning
to strike of the mountain range (Fig. 1) and form
within
a crustal scale wrench corridor (Najd fault
the topographical highs in the Eastern Desert
system)
is outlined. Displacement partitioning
(e.g. Sturchio et al., 1983; El Gaby et al., 1990;
allowed
overall
orogen-parallel extension dunng
Wallbrecher et al., 1993a, b; Greiling et al., 1994).
a
bulk
compressive
regime. This study is based
However, the tectonic significance of theses
on
detailed
structural
analyses supplemented
domal structures is still controversial (El Gaby et
by
data
from
sedimentary
basins and synkinematic
al., 1990; Rashwan, 1991; Greiling et al., 1994;
granitic
plutons.
In
addition,
4°Ar/39Ar muscovite
Kr6ner et al., 1994; Neumayr et al., 1995).
ages
have
been
obtained
to
date the activity of
Several models for the exhumation and uplift
the
Najd
fault
system
and
to
relate deposition
of metamorphic core complexes in collisional
of
molasse-type
sediments
and
intrusion of
orogens have been proposed. These include
granitoids
to
e
x
h
u
m
a
t
i
o
n
and
uplift
of the
crustal thickening and gravitational collapse
metamorphic
core
complexes.
(Platt, 1 9 8 6 ; D e w e y , 1 9 8 8 ; HsQ, 1 9 9 1 ) ,
delamination of supracrustal rocks along a
rheologically weak zone (Wijbrans et al., 1993),
GEOLOGICAL SETTING
removal of the l i t h o s p h e r i c root beneath
Two
major
units
have been destinguished in the
collisional orogens (Vissers et al., 1995), and
E a s t e r n D e s e r t of E g y p t .
The l o w e r
displacement accomodation within strike-slip
tectonostratigraphical unit is referred to as
faults (Neubauer e t a / . , 1994). All these models
"infrastructure" (Habib et al., 1985; El Gaby er
outline the importance of crustal thickening
3 t 2 Journal of African Earth Sciences
Formation o f Neoproterozoic metamorphic core complexes during oblique convergence
25
30
35
EGYPT
200ka
\,\
\
Phanerozoic overstep sequence
[ I P&'l-African Nappes
Molasse sediments
[:77~ Synteetonio plutons
Basement domes
~arsa Alto
Thrust faults
~
Normal faults
Displacement directions during:
strike-slip faulting
normal faulting
Figure 7. Distribution o f metamorphic core complexes in the Pan-African Orogen in the Eastern Desert
o f Egypt compiled from maps by Klitzsch et aL (1987), Akaad and Noweir (1980), El Rarely et aL
(1993) and the authors' o w n field work. Stacking structures (black arrows) are resolved in an
anticlockwise displacement path. Structures are related to extension and exhumation o f the metamorphic
core complexes (white arrows) partitioned into strike-slip displacements and low-angle normal faults
along the Najd fault system. Insert: Neoproterozoic outcrops in the Eastern Desert o f Egypt and Saudi
Arabia. I: Israel; J: Jordania; SA: Saudi Arabia.
1990). It is exposed in metamorphic core
complexes which are bounded by sinistral strikeslip faults (Fig. 1) of the Najd fault system in
the Eastern Desert (Stern, 1985). The internal
structure of these core complexes comprises
several nappes (Habib e t a l . , 1985) summarized
as " b a s e m e n t
complexes".
The u p p e r
tectonostratigraphical units, here referred to as
a/.,
the "Pan-African nappe complex", include a
nappe assemblage of ophiolites, sedimentary
rocks, arc volcanics and molasse-type sediments
(Grothaus e t a l . , 1979; AI Shanti and Gass,
1983; Akaad and Noweir, 1980).
The m e t a m o r p h i c c o r e c o m p l e x e s are
distributed along a northwest trending zone that
extends for more than 400 kin. A simplified
Journal of African Earth Sciences 313
H. FRITZ eta/.
@
B
Phanerozoic sediments
Fr-q Post-tectonic granites
thrust faults and displacement
n o r m a l faults and displacement
strike-slip faults and displacement
"~'~.. anlaformaldome axes
~
synformal dome axes
Hammamat molasse sediments
Pan-African Nappes (undivided)
Synteconio grarfitoids
I I Meatiq Core complex (undivided)
1,2 sample locations for 40Ar/39Ar analyses
A
B so.ion me of profile
®
B
A
®®
o
o
tJr
i
c. S k m
Phanerozoic sediments
"~
thrt~ faults and displacement
Hammamat molasse sediments
~
(;)
normal faults and displacement
approaching block during strike-slip
Pau-Aflic~mNappes (undivided)
[
[ Meatiq core complex (tmdivided)
withdrawing block during strike-slip
Figure 2. Structural map (a) and cross section (b) o f the Meatiq metamorphic core complex
(after A k a a d and Noweir, 1980 and the authors' o w n obsevations). Stacking structure (black
arrows) follows an anticlockwise displacement path with external imbrication (SW) o f foreland
molasse units and n o r t h w e s t directed internal thrusting. Orogen-parallel extension is partitioned
into n o r t h w e s t trending strike-slip faults (white half-arrows) and northeast-southeast dipping
low-angle n o r m a l faults (white arrows).
314 Journalof African EarthSciences
F o r m a t i o n o f N e o p r o t e r o z o i c m e t a m o r p h i c core c o m p l e x e s during oblique c o n v e r g e n c e
stratigraphy of the basement complexes (for a
review see El Gaby et al., 1990; Hassan and
Hashad, 1990; Greiling e t al., 1994) includes,
from structural bottom to top:
i) A m p h i b o l i t e s , a m p h i b o l i t e m i g m a t i t e s ,
serpentinites and partially migmatitic gneisses
(Hassan and Hashad, 1990; Habib e t a L , 1985).
The mafic suite is interpreted to represent
obducted oceanic crust (Rashwan, 1991 ). Ages
ranging between 677 and 750 Ma were obtained
from trondhjemites and gneisses using the U/Pb
single zircon evaporaration technique (KrSner et
al., 1992, 1994).
ii) Gneisses, which range between tonalitic and
granodioritic composition. These intruded the
former suite (Rashwan, 1991; Neumayr et al.,
1995). Ages from these assemblages cluster
around 670 Ma (Stern and Hedge, 1985).
iii) Highly deformed
metasedimentary
sequences. These are exposed in upper structural
levels and include a l u m i n o u s m e t a p e l i t e s ,
metapsammites, and subordinate amphibolites.
Rocks within the basement complexes suffered
polyphase d e f o r m a t i o n and m e t a m o r p h i s m
(Neumayr et al., 1995) with minumum
temperatures of 7 5 0 ° C within the lowermost
migmatic amphibolites and P - T conditions of
>600°C
and 8 kbar w i t h i n t h e u p p e r
metapelites. Greenschist facies metamorphism
in the uppermost structural levels is related to
overthrusting of the Pan-African nappes (Greiling
et al., 1994; Neumayr e t al., 1995).
The Pan-African nappe complexes in the upper
structural levels consist of ophiolites, volcanic
rocks and molasse-type sediments (for a review
see El Gaby et al., 1990; Greiling et al., 1994).
The nappe complex east of Quseir (Figs 1 and
2) includes, from structural bottom to top:
i) An ophiolite sequence of MORB or IAT type
affinity and volcano-sedimentary deposited from
a melange setting (AI Shanti and Gass, 1983;
Akaad and N o w e i r , 1 9 8 0 ; Khudier, 1983;
Hassan and Hashad, 1990). The late Pan-African
age of the o b d u c t i o n is indicated by the
incorporation of molasse-type sediments.
ii) A sequence of "younger metavolcanics"
consists mainly of felsic porphyritic rocks with
calc-alkaline geochemical affinity similar to rocks
of modern convergent plate margins (El Aref et
al., 1985).
iii) The uppermost tectonic unit is dominated
by subduction-related plutonic and volcanic rocks
(Akaad and El Rarely, 1960) which have been
dated between 655 Ma (El Shazly et al., 1973)
and 622 Ma (Stern and Hedge, 1985). In
addition, syn-kinematic calc-alkaline intrusions
are concentrated along the Najd fault system
and range in age between 614 and 620 Ma
(Stern and Hedge, 1985).
iv) Molasse-type sediments developed either
in the western portion of the Eastern Desert or
in intramontane basins (Grothaus et al., 1979;
Akaad and Noweir, 1980; Rice et al., 1993;
Messner et al., 1996) and are intruded by post
kinematic plutons which cluster around 580
Ma (for a review see Hassan and Hashad, 1990).
THE PAN-AFRICAN NAPPES
The late Pan-African stacking resulted in a
partitioned displacement with northwest directed
thrusting in the internal portions and west to
southwest directed thrusting in the external
portions of the orogen. The tectonic style in the
external Pan-African nappes around the Meatiq
area (Fig. 2) is that of a fold and thrust belt. The
w e s t e r n (external) parts are occupied by
molasse-type sediments of the Hammamat
Group w h i c h are imbricated with volcanosedimentary sequences. The orogen front is not
e x p o s e d , because it is buried under the
Phanerozoic Nubian sandstone (Fig. 2). However,
the presence of weakly deformed molasse-type
sediments in the westernmost position suggests
their presence close to the foreland. Intensity of
deformation within the Pan-African nappes
increases towards individual single thrust planes.
Moreover, there is a slight increase in the degree
of metamorphism and deformation from west
to east. Kinematic indicators such as cleavage/
bedding relations, asymmetrical shear fabrics
(Simpson and Schmid, 1983), and flat-ramp and
duplex geometries (Boyer and Elliot, 1982)
indicate a west to southwest tectonic transport
direction (Ries et al., 1983; Wallbrecher et al.,
1 9 9 3 ) . S t r e t c h e d pebbles in the external
Hammamat Group trend east-northeast - westsouthwest; the shape of these pebbles indicates
plane strain geometry.
Structures related to stacking of Pan-African
nappes are overprinted by orogen-parallel,
northwest trending, strike-slip faults. These
faults developed preferentially in anisotropic
rocks and in zones of high competence contrast.
Shear sense along these faults is mainly sinistral
as suggested by oblique fabrics (Fig. 1 and 2).
S t r e t c h e d p e b b l e s in s e d i m e n t s of the
Hammamat Group which were deformed by
faults indicate constrictional finite strain with
maximum stretch towards the northwest.
Journal of African Earth Sciences 315
H. FRITZ eta/.
In the internal portions of the orogen, remnants
of Pan-African nappes with mylonite zones at
their bases occur as klippen on top of the
metamorphic core complexes (Fig. 2). These
mylonite zones represent the basal thrust during
s t a c k i n g of P a n - A f r i c a n n a p p e s o v e r t h e
"basement complex" and contain a pronounced
n o r t h w e s t trending stretching lineation. The
sense of shear w a s d e t e r m i n e d from fabric
asymmetries, the geometry of strain shadows
around rigid objects (Passchier and Simpson,
1 9 8 6 ; S i m p s o n and de Paor, 1 9 9 3 ) and
crystallographic texture in quartz (Fig. 3). Quartz
c-axes define girdles oblique to the trace of the
f o l i a t i o n and i n d i c a t e n o r t h w e s t t e c t o n i c
transport of the Pan-African nappes across the
metamorphic core complexes (Fig. 3, pattern
1, 2). Syntectonic recrystallization and coremantle s t r u c t u r e s in quartz grains indicate
dislocation glide and climb as the dominant
d e f o r m a t i o n mechanism. This mechanism is
sensitive to temperatures of 3 4 0 - 4 0 0 ° C (Koch
e t al., 1989) at strain rates and differential
stresses appropriate for geological processes.
THE METAMORPHIC CORE COMPLEXES
The margins
The " b a s e m e n t c o m p l e x e s " in the Eastern
Desert of Egypt occur in a n o r t h w e s t trending
zone, parallel to the general trend of the orogen.
They are bounded by sinistral shear zones along
their s o u t h w e s t e r n and northeastern margins
and by gently dipping normal faults along their
northern and southern margins (Figs 1 and 2).
These northwest trending and steep shear zones
are dominant features in the Eastern Desert.
They are part of the Najd fault system in the
Arabian-Nubian Shield (Stern, 1985). Riedl shear
zones occur as en e c h e l o n orientated minor
west-northwest trending faults (Figs 1, 2). There
are also local f l o w e r structures associated with
the Najd fault system. The one w e s t of Marsa
Alam (Fig. 1) shows bending of the n o r t h w e s t
striking strike-slip faults t o w a r d s the northeast.
The sinistral slip movement in the Najd fault
system is proven by asymmetries of small-scale
structures and by the lattice preferred orientation
of quartz. Girdle distributions oblique to the
foliation (Fig. 3, pattern 3, 7, 8 and 9) are
common
and s u g g e s t n o n - c o a x i a l
low
temperature flow (Lister and Hobbs 1980; Lister
e t al., 1980). Low temperature f l o w and high
fluid activity during deformation are evident from
syntectonic mineral growth. Serpentinites were
transformed to chlorite and/or talc schists, and
3 1 6 J o u r n a l o f A frican Earth Sciences
feldspars in gneisses were entirely replaced by
white mica.
Low angle normal faults along the northern
margins of the domes indicate a n o r t h w a r d
tectonic transport. Kinematic indicators include
widely spaced extensionary crenulation
cleavages, asymmetrical boudin structures and
asymmetrical quartz textures. Low temperature
deformation is indicated by the dissolution and
precipitation of quartz in asymmetrical boudin
necks and chlorite-coated discrete shear bands.
Quartz t e x t u r e s (Fig. 3, p a t t e r n s 4 and 5)
s u g g e s t both coaxial and n o n - c o a x i a l l o w
t e m p e r a t u r e plastic f l o w as the d o m i n a n t
deformation mechanism (Langdon, 1985). The
southern margins of the domes are defined by
s o u t h d i p p i n g s h e a r z o n e s (Fig. 4) w i t h
southward tectonic transport. Subvertical
flattening strain is derived from refolding of the
penetrative mylonitic foliation around horizontal
west trending fold axes and from quartz c-axes
patterns. Local low-angle normal faults
developed subparallel to the limbs of these
upright folds. Crystallographic textures in quartz
from the shear zones at the southern margins
of the domes s h o w minor texture asymmetries
(Fig. 3, pattern 10, 11, 12, 13 and 14). Cross
girdle distributions and small-circle distributions
around the pole of the foliation are common.
They are interpreted to reflect a high component
of flattening strain (Lister and Hobbs, 1980).
THE CORES A N D DECOLLEMENTS
The structures in the interior of the metamorphic
core complexes are dominated by northwest
t r e n d i n g a n t i f o r m s w h i c h refold the older,
subhorizontal mylonitic fabrics (Figs 2, 4 and
5). These antiforms suggest updoming which
is responsible for the formation of the structural
and topographical relief in the Eastern Desert
of Egypt. The northwest trending anticlinal dome
axes (Fig. 2) s u g g e s t n o r t h e a s t - s o u t h w e s t
d i r e c t e d bulk c o m p r e s s i o n . The a m o u n t of
shortening and the depth of the decollement
b e n e a t h t h e s e a n t i f o r m a l d o m e s can be
estimated
by i d e n t i f y i n g
and t r a c i n g
stratigraphical markers across the domes and
by the balancing of profiles. T w o sections have
been chosen across the Meatiq and Hafafit
domes (Fig. 5a, b) where distinct stratigraphical
markers can be defined. To calculate the depth
of the decollement, the excess area technique
was used (Chamberlin, 1910; Laubscher, 1965;
Epard and Groshon, 1993). There are, however,
sources of error in this technique which arise
Formation o f Neoproterozoic me t amor phi c core complexes during obfique convergence
d
Y~ b<
Hammamat molasse sediments
Pan-African nappes
syn-teconic granitoids
[
~,~,
[ Meatiq core complex (undivided)
3
3.
thrust faults
normal faults
strike-slip faults
antiformal axial trace
synformal axial trace
Figure 3. Quartz-c axes plots (lower hemisphere equal area plots) from the the basal Pan-African thrust
and the margins o f the Meatiq metamor phi c core complex. X is southeast (trace o f the foliation) and Z
is d o w n for a//pattern and exemplarily s h o w n in pattern 1. Italic numbers in the l o w e r left o f insets mark
the multiple o f r a n d o m c o n t o u r (MRD) o f the black domain (e.g. pattern 1: black domain 2.8 MRD)
Sinsitral shear and major noncoaxial f l o w characterize strike-slip faults (pattern: 3, 7, 8, and 9). Quartz
c-axes pattern along n o r m a l faults (5, 10-14,), exhibit mi nor texture asymmetries and are interpreted to
reflect enhanced flattening strain. For further informatzon, see text.
from the assumptions which have to be made:
neglecting both the internal strain, as well as
the possibility of folding of the decollement
itself, can lead to an overestimation of the depth
of the decollement. On the other hand, the
assumption that the inflection points along the
margins of the domes (in the vicinity of the
s t r i k e - s l i p f a u l t s ) are f l a t can lead to an
underestimation
of d e p t h of t h e b a s a l
decollement. Nevertheless, even a nonquantitative approach reveals important
information: basement domes within the Eastern
Desert of Egypt are characterized by a minor
amount of shortening, together with very large
excess areas. This would indicate deep levels
of basal decollement.
In the Meatiq metamorphic core complex the
boundary b e t w e e n the u p p e r m o s t structural
levels of the "basement c o m p l e x " and the PanAfrican nappes is marked by a strong mylonite
zone defining the thrust plane. The Pan-African
nappes occupy the eastern and western flanks
of the dome, and form klippen preserved in
synformal structures of the dome. This thrust
plane is slightly folded into t w o open, northwest
trending antiforms (Figs 2 and 5a). The bulk
shortening calculated from the balancing of the
Journalof African EarthSciences317
H. FRITZ et al.
®
(
Contour intervals: 1; 3; 5.19; 9 MRD
at~ points
Contoux intc~als: 1; 5.2; 11.9; 27.1 MRD
Figure 4. Foliation
complex, Contour
macrosca/e folding
mylonitic foliation
Foliation poles o f
exemplarily s h o w n
1; 3.1; 5.4; 9.4 MRD
ata points
Contoz~rintervals: 1; 5.1; 11.6; 26.3 MRD
poles (a) and stretching /ineations (b) from the Meatiq metamorphic core
intervals are given in multiples o f random distribution (MRD), Note the
along n o r t h w e s t trending fold axis (a) with a steepening o f the subhorizontal
(central cluster) towards the margins o f the domes (marginal domains).
l o w angle shear zones (c) and associated stretching /ineations (d) are
from the southern margin o f the Sibai dome,
profile s h o w n in Fig. 5a is a b o u t 4 . 5 % . The
p e r c e n t a g e of s h o r t e n i n g (S%) is given by:
S% = (IJl o) x 100,
w h e r e I° is the original length of the profile and
I~ the d e f o r m e d length b e t w e e n t w o pinpoints.
These pinpoints are c h o s e n at the margin of
the domes. The excess area (AE) is 22 km 2 in
the profile s h o w n in Fig. 5a, w h i c h is d r a w n in
a w e s t - e a s t section. Using these values of 4 . 5 %
shortening and 22 km 2 e x c e s s area (AE) in the
equation:
D = AE/(I ° -[d),
the d e p t h (D) of the d e c o l l e m e n t b e n e a t h the
Meatiq d o m e is f o u n d to be a b o u t 20 km.
318 Journalof African EarthSciences
Contouxin~:
624 Data points
This calculation w a s also used to calculate
the depth of the decollement beneath the Hafafit
d o m e . T h e p i n p o i n t s w e r e c h o s e n at t h e
n o r t h e a s t e r n margin and close to the w e s t e r n
m a r g i n of t h e d o m e (Fig. 5b). T h e w e s t s o u t h w e s t - e a s t - n o r t h e a s t section t h r o u g h the
dome includes three marker horizons w h i c h were
used for the calculation:
i) The w e l l - d e f i n e d b o u n d a r y b e t w e e n the
mafic/ultramafic
s e q u e n c e and t h e b i o t i t e
schists in the deeper structural levels;
ii) The b o u n d a r y b e t w e e n the biotite schists
and the psammitic gneisses, also in the deeper
structural level; and
iii) The b o u n d a r y b e t w e e n the " b a s e m e n t
c o m p l e x " and the Pan-African nappes in the
higher structural level.
The s e c t i o n of Fig. 5b w a s c o m p i l e d using
orientation data published by El Ramly er al.
F o r m a t i o n o f N e o p r o t e r o z o i c m e t a m o r p h i c core c o m p l e x e s during oblique c o n v e r g e n c e
(•)
Meafiq cross section
excess area calculation
A (svo
A" (~lE)
excess
8~ea
ca. lOkm
@
B (SW)
Haffafit cross section
excess area calculation
B" (NE)
@
line shortening c. 6%
Excessarea calculation
Ha~t
dome
:[
decollemem depth c. 16kin
~3o[
ca. 10kin
i '°IOF.
.
.
.
-20 -15 -10 -5
0
5
Depth of structural relief(in 1000m)
Figure 5. Line b a l a n c i n g o f (a) the M e a t i q d o m e a n d (b) the H a f a f i t d o m e (section lines
m a r k e d in the insets). For the M e a t i q d o m a l m a s s I a = 2 0 . 8 k m ; Io = 2 1 . 8 k m ;
excess area = 2 2 . 3 k m 2 a n d S % = 4 . 6 % w h i c h results in a d e p t h o f d e c o l l e m e n t o f
2 2 , 3 kin. For the H a f a f i t : I~ = 2 0 . 3 k i n ; Io = 2 1 . 6 k m ; S % = 6 . 2 % ; a x c e s s area =
2 0 . 7 k m 2, a n d t h e r e f o r e d e p t h o f d e c o l l e m e n t = 1 5 . 9 k m . (c) Excess area o f
s t r a t i g r a p h i c a l m a r k e r levels p l o t t e d a g a i n s t the d i s t a n c e to a c h o s e n r e f e r e n c e l e v e l
(sea level). The regression line t h r o u g h d a t a p o i n t s c u t s the zero level a t 15 kin. This
is m terpre t e d to r e p r e s e n t the basal decollemen t level (technique o f Epard a n d Groshong,
1993).
(1993) and the authors' o w n o b s e r v a t i o n s . The
depth of the d e c o l l e m e n t beneath the H a f a f i t
dome is calculated to be about 16 km.
The presence of well-defined marker horizons
allows the control of these calculations by applying
a method suggested by Epard and Groshong
(1993). In this method, the amount of excess
area at a certain stratigraphical level is plotted
against the distance to a chosen reference level
(in this case the recent sea level). The amount of
excess area calculated for a stratigraphical marker
is a linear function of the distance to the basal
decollement. A t the level of the basal decollement
the amount of excess area is zero. The excess
area values determined for the three stratigraphical
markers in the Hafafit dome were plotted against
the distance to the reference level. The intercept
of the regression line w i t h the level of zero excess
area gives a depth to the basal decollement of
about 17 km (Fig. 5c).
Journal of African Earth Sciences 319
H. FRITZ e t al.
extension
b
Meatiq core complex
Panafrican nappes
I
Abu Ziran pluton
%
strike-slip faults
/
normal faults
~
d
clil
West
1.6
/
/
,//k=l
o- " # W ' ,
o
0-~
A
• • //
1.2
East
sy'nteetonic
pluton
l
50
--
go
.
.
.
.
.
.
go
Si02 (vat %)
$.4
o2s
1.-i
f.~
m (y/z)
A West
• Centre
• East
Figure 6. (a) Geological map o f the A b u Z/ran P/uton south o f the Meatiq met amor phi c
complex (see Figs 1 and 2 for location). The ar r ow indicates decreasing magmatic
strain and increasing S/O 2 content o f the magma from west to east. (b) The magmat/c
b o d y intruded in a shear extensional reglTne controlled by simultaneous activity o f
strike-slip and n o r m a l faults. {c) Different/at/on trend w/thin the A b u Ziran gran/to/d
b o d y with an increase o f the SiO 2 content f r o m west to east. (d) Logarithmic ratios o f
principal axes derived from strained xenofirhs in a F/inn graph. X, Y, and Z are the long,
intermediate and short principal strain axes, respectively. The orientation o f the X axes
are west-east, parallel to the magmatic differentiation trend.
THE SYN-TECTONIC
GRANITOID$
The Abu Ziran granitic body to the south of
the Meatiq dome (Figs 2 and 6a) occurs as a
w e s t - e l o n g a t e d , c a l c - a l k a l i n e i n t r u s i o n of
about 15 kin. The w e s t e r n part of the pluton
is of tonalitic to dioritic composition, occupies
a w i d t h of only a f e w metres, and is entirely
bounded by faults of Riedl shear orientation
c o n n e c t e d w i t h the s o u t h w e s t e r n marginal
shear zone of the Meatiq (Fig. 6a). The eastern
320 Journal of A frican Earth Sciences
parts of the pluton are about 4 km wide and
of granodioritic composition. They cross-cut
f a u l t s and o l d e r m a g m a t i c
phases. A
differentiation
t r e n d (Fig. 6) i n d i c a t e s
progressive SiO 2 e n r i c h m e n t t o w a r d s the east
(Fritz, 1 9 9 5 ) . Based on the AI c o n t e n t of
magmatic hornblende in equilibrium w i t h the
melt ( S c h m i d , 1 9 9 2 ) , the p r e s s u r e during
magma crystallisation was estimated to be 4
to 5 kb (Fritz, 1995).
Formation o f Neoproterozoic met amor phi c core complexes during oblique convergence
650 .
i
.
.
.
.
.
.
.
.
.
Meatiq; westem shear zone
~
600
=='===~
r'-
Plateau Age = 588.2 + 0.3 Ma
550
Total-Gas Age = 588.0 + 0.4 Ma
500
i
0
i
i
~
20
I
i
40
I
60
I
I
80
100
Cumulative Percentage 39Ar Released
650
.
(~
.
.
.
.
.
.
.
Meatiq; southem low-angle normal-fault
600
Plateau Age = 595.9 + 0.5 Ma
550
<
Total-Gas Age = 594.9 4- 0.5 M a
500
i
0
I
20
P
40
I
J
60
I
I
I
80
100
Cumulative Percentage 39Ar Released
Figure 7. 4°Ar/39Ar incrementa/-release age spectra o f muscovite concentrates
f r o m the strike-slip zone west o f the Meatiq dome (1) and the n o r m a l fault
south o f the Meatiq (2). Analytical uncertainties (intralaboratory) s h o w n bv
the vertical width o f the bars. Experimental temperatures increase f r o m / e f t
to right. Total gas and plateau ages are listed on each spectrum. For sample
locations see Fig. 2.
The Abu Ziran Pluton contains mafic xenoliths
and late aplitic veins. Strain analysis of the mafic
xenoliths indicates a decrease of strain induced
by magmatic e m p l a c e m e n t from w e s t to east
(Fig. 5d). The overall strain geometry is plane
strain with a west-east orientated semi-principal
extensional axis of the strain ellipsoid. This is
parallel to the magmatic differentiation trend.
This, in t u r n , s u g g e s t s an e a s t d i r e c t e d
magmatic f l o w with a concentration of the f l o w
stresses in the western part of the pluton. Solid-
state deformation deduced from the orientation
of extension veins and from late magmatic aplitic
dykes indicates a north-south extension during
the late magmatic stage.
The east directed magmatic flow, the west-east
differentiation trend with mafic phases in the west,
more evolved magmas in the east, and the
structural frame of the Abu Ziran Pluton all suggest
emplacement with shear extension gashes as the
feeding system (Fig. 5b). Emplacement was
related to bulk northwest-southeast extension.
Journalof African EarthSciences321
H. F R I T Z e t al.
MOLASSE-TYPE SEDIMENT BASINS
T h r e e t y p e s of m o l a s s e - t y p e
b a s i n are
distinguished on the basis of their positions in
the Pan-African Orogen of the Eastern Desert.
Foreland basins are incorporated in the w e s t to
southwest directed thrusting (Fig. 2). The other
t w o types are intramontane basins related to
exhumation of the metamorphic core complexes:
i) Basins bounded by the sinistral northwest
striking Najd fault system (e.g., west of the Sibai
dome; Fig. 1) which show pull-apart geometry.
ii) Basins linked with low-angle normal faults
to the n o r t h w e s t a n d / o r s o u t h e a s t of the
metamorphic core complexes (e.g. north of Sibai;
Fig. 1) w h i c h reflect orthogonal n o r t h w e s t southeast extension.
These intramontane basins contain sediments
d e p o s i t e d by high d e n s i t y mass f l o w s and
braided streams (Messner et al., 1996), poorly
sorted, coarse pebbles derived from the PanA f r i c a n n a p p e s and f r o m t h e e x h u m e d
m e t a m o r p h i c c o r e c o m p l e x e s . The large
proportion of coarse grain sizes throughout the
sequences and the reworked sedimentary strata
(re-sedimentation) suggest high e x h u m a t i o n
rates in the source region.
The intramontane pull-apart basin to the west
of the Sibai m e t a m o r p h i c core c o m p l e x is
bounded by a northwest trending strike-slip fault
at the margin of the dome (Fig. 1). The basin
axis is roughly parallel to the shear zone. The
long axes of deformed pebbles are northwest
trending and thus parallel to the stretching
lineation in the marginal shear zone of the dome
(Bauernhofer et al., 1995).
The basins north and south of the metamorphic
core complexes suggest n o r t h w e s t - s o u t h e a s t
extension parallel to the trend of the orogen.
The largest of these basins occurs north of the
Sibai metamorphic core complex. Small remnants
of molasse-type sediments are incorporated in
low-angle normal faults north of the Meatiq
metamorphic core complex. The presence of
coarse fragments and synsedimentary normal
faults in these basins suggest rapid exhumation
of the hinterland. Structures within the molassetype sediments include the stretching of pebbles
w i t h their long axes o r i e n t a t e d n o r t h - s o u t h
(Akaad and Noweir, 1969) and e x t e n s i o n a l
crenulation cleavages within the matrix.
AGE RELATIONS
The ages of activity of strike-slip
normal faults, of deposition of
basins, and of emplacement of
granitic body are constrained
322 Journal of African Earth Sciences
and low-angle
intramontane
the Abu Ziran
by previously
published geochronological data and new 4°Ar/
39Ar mineral ages. Previous geochronological data
from the Meatiq metamorphic core complex
included a 614_+8 Ma U/Pb zircon age from the
Abu Ziran granitic body. This age is interpreted
to define the age of granitic emplacement (Stern
and Hedge, 1 9 8 5 ) . T h e extrusion of the Dokhan
volcanics is proven to be synchronous with the
d e p o s i t i o n of the H a m m a m a t molasse tvpe
sediments (El Gaby et al., 1990). Ries and
D a r b y s h i r e ( r e p o r t e d in Ries e t al., 1 9 8 3 )
obtained Rb/Sr whole rock isochron ages of
602_+ 13 Ma and 6 1 6 + 9 Ma, respectively, from
these rocks. The upper bracket of Pan-African
tectonic activities is determined by the age of
post-tectonic granitoids. The M e a t i q granite
(Fig. 2) gave an U/Pb zircon age of 585_+ 14 Ma
(U/Pb: Sturchio e t a / . , 1993). Ages of other ring
c o m p l e x e s w h i c h are y o u n g e r t h a n t h e
H a m m a m a t m o l a s s e - t y p e s e d i m e n t s cluster
around 570 Ma.
T w o muscovite concentrates were prepared
in order to date the activity of the northwest
trending sinistral strike-slip zone (Sample 1) and
of the extensionary shear zone south of the
Meatiq dome (Sample 2). 4°Ar/39Ar incremental
heating technique applied to synkinematically
grown muscovite was used for this purpose.
Sample locations are s h o w n in Fig. 2; the
analytical data are provided in Table 1. The
laboratory techniques were as follows.
The mineral concentrates were wrapped in
aluminium-foil packets, encapsulated in sealed
quartz vials, and irradiated for 80 hr in the central
thimble position of the TRIGA Reactor in the US
Geological Survey, Denver. Variation in the flux
of neutrons along the length of the irradiation
assembly was monitored with several mineral
standards. The samples w e r e i n c r e m e n t a l l y
h e a t e d u n t i l f u s i o n in a d o u b l e - v a c u u m ,
resistance heat furnace. Temperatures were
monitored with a direct-contact thermocouple,
controlled to _+ 1 °C between increments and are
accurate to _+5°C. Measured isotopic ratios
were corrected for total blanks and the effects
of mass discrimination. Interfering isotopes
produced during irradiation were corrected using
factors reported by Dalrymple et a/. (1981) for
the TRIGA Reactor. Apparent 4°Ar/39Ar ages were
calculated from corrected isotopic ratios using
the decay constants and isotopic ratios listed by
Steiger and J~ger (1977) following the methods
described in Dallmeyer and Takasu (1992).
Intralaboratory uncertainties reported here have
been calculated by statistical propagation of
uncertainties associated with measurement of
each isotopic ratio through the age equation.
Formation o f Neoproterozoic m e t a m o r p h i c core complexes during oblique convergence
Table 1. +°Ar/39Ar analytical data for incremental heating experiments on muscovite
concentrates from mylonite granite, Meatiq Dome, the Eastern Desert, Egypt
Release
temp
(°C)
( 4°Ar/
(~--~
39Ar)*
39Ar)*
Sample 1' J = 0.010275
470
30.00 0.01864
530
38.81 0.00312
565
38.14 0.00027
600
37.94 0.00067
635
37.63 0.00029
670
37.57 0.00041
700
37.66 0.00036
730
37.60 0.00023
760
37.69 0.00076
790
37.67 0.00063
825
37.75 0.00043
860
37.78 0.00117
900
37.87 0.00033
Fusion 37.50 0.00137
Total
37.72 0.00071
Total without 470-600°C
900°C - fusion
Sample 2: J = 0.010218
470
33.74 0.00705
530
39.97 0.00265
560
39.13 0.00116
590
38.70 0.00057
620
38.53 0.00015
650
38.37 0.00046
680
38.36 0.00052
710
38.40 0.00049
740
38.48 0.00048
770
38.44 0.00030
880
35.42 0.00082
830
38.51 0.00039
865
38.58 0.00052
900
38.72 0.00051
Fusion 38.98 0.00351
Total
38.47 0.00080
Totalwithout 470-590°C
900°C - fusion
y~-gAr%
of total
39Ar)e
%4°Ar 3 6 A r e a
Apparent
non%
Age (Ma)**
atmos. +
0.464
0.124
0.055
0.058
0.050
0.088
0.071
0.034
0.037
0.103
0.064
0.051
0.034
0.681
0.067
0.51
4.24
2.70
4.82
7.32
10.92
15.53
11.10
10.83
7.47
9.26
9.21
4.74
0.31
100.00
81.68
81.75
97.64
99.79
99.47
99.77
99.69
99.72
99.81
99.40
99.51
99.66
99.02
99.74
99.05
99.42
0.65
1.08
5.68
2.33
4.70
5.82
5.37
4.01
1.34
4.43
4.07
1.18
2.85
13.53
3.77
405.5
593.3
595.4
591.2
588.6
587.4
588.7
588.4
587.5
587.8
589.6
587.1
591.6
583.5
588.0
588.2
+
+
+
+
+
+
+
+
+
+
+
+
+
+
_+
+
2.7
0.8
0.6
1.1
0.3
0.5
0.3
0.5
0.1
0.3
0.2
0.4
0.5
5.7
0.4
0.3
0.327
0.108
0.069
0.041
0.068
0.035
0.051
0.087
0.073
0.062
0.050
0.132
0.194
0.049
0.388
0.087
2.44
5.08
5.42
4.68
8.36
9.54
11.50
10.84
10.49
6.90
4.79
8.17
8.01
3.39
0.37
100.00
78.62
93.88
95.05
99.12
99.56
99.66
99.64
99.59
99.63
99.63
99.77
99.36
99.71
99.63
99.60
97.40
99.37
1.26
1.11
1.62
1.95
4.17
2.07
2.68
4.83
4.15
5.65
1.66
9.18
10.19
2.62
3.01
4.22
505.9
607.7
602.3
598.8
597.1
594.9
594.5
593.3
596.3
596.5
594.1
597.1
597.7
599.4
591.6
594.9
595.9
+
+
+
+_
+
+
_+
+
+
+
+
+
+
+
+
+
_+
2.4
0.3
0.1
0.6
0.3
0.4
0.5
0.4
0.7
0.4
0.3
0.5
0.5
0.4
1.7
0.5
0.5
* measured
¢ corrected for post-irradiation decay of 37 Ar (35.1 day 1/2-1ire)
+[4°Artot - (36Aratmos.) (295.5)] / 4°Artot
**calculated using correction factors of Dalrymple et al. (1981); two sigma, intralaboratory errors.
Journal of African Earth Sciences 323
H. FRITZ et a/.
older than 620 Ma
( ~ ) c. 600 Ma
Stage l: Oblique Thrusting
Stage 2: Further foreland imbrication and
hinterland extension
SE
Metamorphic core complexes
PanafricanNappes
Molasse Sedimcmts
Figure 8. Two-step three dimensional sketch illustrating the evolution o f PanAfrican nappes and formation o f metamorphic core complexes m the Eastern
Desert o f Egypt by oblique convergence. (a) obfique northwest directed
thrusting. (b) Ongoing thrust propagation within the western foreland and
coeval orogen parallel extension in the hinterland. Note that west-southwest
directed thrusts and sinistra/ strike-slip along northwest trending steep shear
zones operated simultanously. Orogen parallel extension triggered emplacement
of ca/c-alkaline magmas and the deposition of intramontane molasse basins.
Interlaboratory uncertainties are c a . . + 1 . 2 5 1.5% of the quoted age. Total gas ages have
been computed for each sample by appropriate
weighting of the age and percent 39Ar released
within each temperature increment. A "plateau"
is considered to be defined if the ages recorded
by three or more continuous gas fractions each
representing > 4% of the total 39Ar evolved (and
324 Journal o f African Earth Sciences
together constitute > 5 0 % of the total quantity
of 39Ar evolved) are similar w i t h i n a + 1 %
interlaboratory
uncertainty.
A detailed
d e s c r i p t i o n of t h e p r o c e d u r e is g i v e n in
Dallmeyer e t a / . (1992).
All gas fractions evolved from the muscovite
concentrates are characterized by very large
a p p a r e n t K / C a r a t i o s . T h e s e d i s p l a y no
F o r m a t i o n o f N e o p r o t e r o z o i c m e t a m o r p h i c core c o m p l e x e s during oblique c o n v e r g e n c e
Table 2. Structural styles during stacking and extension in the Eastern Desert
Stackin~l structures
Location
Style
Kinematic
Western thrust and fold
Localized shear, folding
TopW-SW
belt
duplex structures
Meatiq dome
Penetrative mylonitic
(upper structural level)
foliation
Top NW
Exhumation structures
Location
Style
Kinematic
Western dome margins
Localized NW-striking
Sinistral
steep shear zones
Eastern dome margins
Localized NW-striking
Sinistral
steep shear zones
Northern dome margins
Localized low-angle
Top North
shear zones
Southern dome margins
Localized low-angle
Top South
shear zones
significant intra sample variations. For this
reason, apparent K/Ca spectra are not presented
with the apparent age spectra in Fig. 7. The
t w o muscovite concentrates gave internally
concordant 4°Ar/39Ar age spectra which define
plateau ages of 588.2_+0.3 Ma (Sample 1) and
595.9_+0.5 Ma (Sample 2). These muscovites
were newly formed at the expense of felspar
during shear deformation and high fluid activity.
Quartz textures within the shear zones suggest
d i s l o c a t i o n as the d o m i n a n t d e f o r m a t i o n
mechanism. Dislocation glide is sensitive to
temperatures in the range between 330 and
4 0 0 ° C (Koch e t al., 1989) and suggests that
d e f o r m a t i o n t o o k place under g r e e n s c h i s t
metamorphic conditions. This metamorphic
grade is also indicated by the syndeformational
growth of pale hornblende, epidote, and zoesite
in adjacent rocks. A l t h o u g h not rigorously
calibrated, the experimental data of Robbins
(1972) in the diffusion equations of Dodson
(1973) suggest that temperatures of about 3754 0 0 ° C are appropriate for Ar retention in
muscovites of normal composition. Cliff (1985)
and Blanckenburg e t al. (1989) estimated a
temperature range of 3 5 0 - 4 1 0 ° C as appropriate
temperatures for Ar retention within muscovites
based on field studies. Therefore, the 4°Ar/39Ar
plateau ages from this study are interpreted to
date the age of muscovite crystallisation during
shearing rather than the cooling age after mica
crystallisation. Hence, the 4°Ar/39Ar data are
interpreted to define the age of the Najd fault
system and the age of beginning exhumation
of the core complexes.
INTERPRETATION
The data from this study indicate that the final
emplacement of Pan-African cover nappes, the
exhumation of metamorphic core complexes,
the formation of intramontane molasse-type
basins, and the syn-tectonic intrusion of plutons
were related to a continuous process between
c a 620 and 580 Ma. In this process, internal
portions of the orogen suffered extension
coevally with foreward propagation of thrusts
in f o r e l a n d d o m a i n s ( T a b l e 2). T h r u s t
emplacement resulted from oblique convergence
and is characterized by an a n t i - c l o c k w i s e
displacement path (Fig. 8). Tectonic transport
directions change from northwest in the internal
parts of the orogen to west to southwest in
foreland regions (Fig. 2). This partitioned
displacement as a result of oblique convergence
is in agreement with that observed in other
Journal of African Earth Sciences 325
H. F R I T Z e t a / .
transpressional orogens (Fritz and Neubauer,
1993a).
The e x h u m a t i o n of m e t a m o r p h i c core
complexes is related to northwest-southeast
oblique convergence which was accomodated
by d i s p l a c e m e n t p a r t i t i o n i n g and s t r a i n
partitioning within the Najd fault system. Bulk
strain resolved local strain and displacement
fields (Tikov and Tessier, 1994) with strike-slip
domains dominated by simple shear and pure
shear dominated components. The strike-slip
component operated on n o r t h w e s t trending
sinistral faults of crustal scale. The pure shear
c o m p o n e n t , o r t h o g o n a l to orogen strike,
resulted in northeast-southwest compression or
coeval northwest-southeast extension (Fig. 8).
These strain components are preseved in the
form of northwest trending antiforms in the
cores of the metamorphic complexes and in
n o r t h w e s t - or s o u t h e a s t dipping low-angle
normal faults w h i c h d e v e l o p e d along the
northern and southern margins of the domes.
This model c o n t r a s t s to that proposed by
Greiling (1985), Greiling e t al. (1988) and El
Rarely e t al. (1993) who interpreted the faults
to the north and east of the Hafafit as thrust
f a u l t s . T h e s e a u t h o r s related a n t i f o r m a l
structures with ramp anticlines during northwest
directed nappe transport.
Displacement partitioning within the Najd fault
system
resulted
in t r a n s t e n s i o n
and
transpression. The transtensional regime was
accomodated by low-angle normal faults and
tension gashes. These transtensional domains
were sites for the deposition of intramontane
Hammamat molasse-type sediments (Messner
e t al., 1996) and for syn-extensional plutons
(Fritz 1995). The geometry, differentiation trend,
and magmatic and solid state deformation in
the Abu Ziran pluton suggest that intrusion was
controlled by the northwest trending sinistral
strike slip fault west of the Meatiq metamorphic
core complex and by the low-angle normal faults
to the south. The minimum penetration depth
of the s h e a r - e x t e n s i o n s y s t e m (Najd fault
system) feeding the Abu Ziran Pluton is derived
by the hornblende barometry. Pressures of 4-6
kbar are interpreted to represent crystallisation
during extension and correspond to ca 12-15
km depth. The calc-alkaline magmatism is not
necessarily related to subduction. Hopper e t a/.
(1995) explained that calc-alkaline magmatism
can result from lithospheric extension in areas
with a long history of previous subduction
events. This scenario is similar to that proposed
for the evolution of the Pan-African orogenic
belt in the Eastern Desert of Egypt.
326 Journal of African Earth Sciences
The transpressional regime resulted in the
formation of strike-slip faults and northwest
trending antiforms. These, in turn, were
related to buckling and updoming in the
metamorphic
core c o m p l e x e s .
Basal
decollements at a depth between 16 and 20
km beneath the metamorphic core complexes
accomodated buckling.
Physical conditions of the lithosphere during
orogen-parallel extension and exhumation of
metamorphic core complexes in the Eastern
Desert of Egypt can be crudely estimated on
the basis of the following observations:
i) There was no extensive thickening of the
crust as indicated by the weak metamorphism
in the Pan-African nappes and the thin-skinned
thrust regime;
ii) Denudation of the metamorphic core
complexes was achieved by extension along
normal faults;
iii) Exhumation was rapid, as indicated by
sedimentary parameters within molasse-type
basins;
iv) Intrusion of syn-extensional, calc-alkaline
granitic bodies suggests an enhanced heat
flow in the lithosphere (> 8 0 0 ° C at the base
of the crust);
v) The lower crust in the Arabian-Nubian
Shield consists of mafic igneous rocks which
contrasts to the granulitic lower crust in the
Mozambique Belt further south (McGuire and
Stern, 1 9 9 3 ) . This led Stern ( 1 9 9 4 ) to
postulate a relatively mild terrane accretion
in the Arabian-Nubian Shield which, together
with a continental collision in the Mozambique
Belt formed the East African Orogen; and
This model explains the significant amount
of calc-alkaline magma along the Najd fault
system by changing the starting conditions,
including enhanced heat flow prior to Late
Pan-African thrusting. Van Wees e t a / . (1992)
demonstrated the importance of pre-thrusting
extension in the Betic Cordillera to explain
enhanced heat flow in the lithosphere. This
scenario is likely because the Pan-African
Orogen can be interpreted as a mobile belt
formed by the sucessive opening and closing
of oceanic domains (Gass, 1982; Kr6ner e t
al., 1994) and because the Najd fault system
can be interpreted as an inherited rift-related
transform system (Stern, 1985).
The exhumation of the metamorphic core
c o m p l e x e s is e x p l a i n e d by l i t h o s p h e r i c
thinning and is tectonically supported by
m a c r o s c a l e b e n d i n g of the c r u s t w i t h
wavelengths exceeding 20 km. Extension was
not necessarily driven by lateral buoyancy
Formation of Neoproterozoic metamorphic core complexes during oblique convergence
forces due to thickening.
Sandiford
and Powell
(1 9 9 1 ) a n d Z h o u a n d S a n d i f o r d (1 9 9 2 ) p r o p o s e d
that orogen-perpendicular
exceed
extensional forces can
orogen-perpendicular
compressive
forces during thickening
of the lithosphere.
P a r t i t i o n e d d i s p l a c e m e n t in t h e N a j d f a u l t s y s t e m
allows
orogen-parallel
parallel
perpendicular
without
to
rather
tectonic
compensated.
required
extension.
component,
driving force,
Therefore
explain
significant
The orogen-
than
orogenhas to be
no major thickening
denudation
lithospheric
is
by extension
thickening.
ACKNOWLEDGEMENTS
The authors
acknowledge
fruitful
discussions
w i t h A . B a u e r n h o f e r , M . B r e g a r , S. El G a b y , R.
O. G r e i l i n g , M . M e s s n e r , K, Pelz, A . A . R a s h w a n
a n d R. S t e r n .
improved
by
Abdelsalam,
was
The paper itself
critical
reviews
was
by
greatly
M. G.
J. V a i l , a n d S. El G a b y . T h i s w o r k
financially
supported
Science Foundation
by
the
Austrian
(FWF Grant: P09703-Geo)
t o E. W a n d H. F.
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