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Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal-scale strike-slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal-scale vertical strike-slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike-slip fault zone, and the segments were separated by another 30 km of lateral motion. Strike-slip core complexes like the Qazaz Dome may be common and may have an important local effect on crustal strength.This article is protected by copyright. All rights reserved.
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A strike-slip core complex from the Najd fault system,
Arabian shield
Sven Erik Meyer,
Cees Passchier,
Tamer Abu-Alam
and Kurt St
Institute of Earth Sciences, Johannes Gutenberg University Mainz, Mainz 55128, Germany;
Institut f
ur Geowissenschaften, Karl-
at Graz, Universit
atsplatz 2, Graz A-8010, Austria;
Norwegian Polar Institute, Hjalmar Johansens gt. 14, Tromsø NO-
9296, Norway;
Geology Department, Faculty of Science, Tanta University, Tanta, Egypt;
Egyptian Institute of Geodynamics, Cairo, Egypt
Metamorphic core complexes are usually thought to be asso-
ciated with regional crustal extension and crustal thinning,
where deep crustal material is exhumed along gently dipping
normal shear zones oblique to the regional extension direc-
tion. We present a new mechanism whereby metamorphic
core complexes can be exhumed along crustal-scale strike-slip
fault systems that accommodated crustal shortening. The
Qazaz metamorphic dome in Saudi Arabia was exhumed along
a gently dipping jog in a crustal-scale vertical strike-slip fault
zone that caused more than 25 km of exhumation of lower
crustal rocks by 30 km of lateral motion. Subsequently, the
complex was transected by a branch of the strike-slip fault
zone, and the segments were separated by another 30 km of
lateral motion. Strike-slip core complexes like the Qazaz
Dome may be common and may have an important local
effect on crustal strength.
Terra Nova, 26, 387394, 2014
Metamorphic domes surrounded by
low-grade metamorphic rocks are
commonly formed by the exhuma-
tion of medium- to high-grade meta-
morphic rocks from lower crustal
levels (Davis and Coney, 1979; Crit-
tenden et al., 1980). The most com-
mon mechanism of exhumation is
thought to be regional-scale exten-
sion (Wernicke, 1981; Davis et al.,
1986) and crustal thinning, where
higher grade rocks are brought up in
the footwalls of gently dipping shear-
zone systems oblique to the regional
extension direction (often termed
‘low-angle detachments’) forming so-
called core complexes (Lister et al.,
1984; Tirel et al., 2008; Huet et al.,
2010; Fig. 1a). Here, we present evi-
dence that core complexes can also
be locally exhumed along major ver-
tical strike-slip shear zones in areas
of crustal shortening without regio-
nal-scale crustal thinning, using an
example from the Najd shear-zone
system in Saudi Arabia (Abdelsalam
and Stern, 1996; Fig. 1b).
The Najd shear-zone system
The Arabian-Nubian shield (ANS) in
Egypt, Saudi Arabia and Sudan is
composed of ~870630 Ma Neopro-
terozoic juvenile arc terranes and
remains of ophiolite belts which
amalgamated during the closing of
the Mozambique Ocean and the
associated assembly of Gondwana-
land (Stern, 1994; Johnson et al.,
2004; Stern and Johnson, 2010).
Most of the ANS consists of
low-grade metavolcanics and metase-
diments with scattered intrusive arc-
type and a few A-type granitoids. All
units are affected by the Najd fault
system (NFS), a network of crustal-
scale sinistral strike-slip zones
2000 km long and 400 km wide,
which cut and partly reactivate older
tectonic elements in the shield (Stern,
1994; Fig. 2). Development of this
shear-zone network during and fol-
lowing the collision of West and East
Gondwana resulted in EW shorten-
ing with a northwards trend of
escape tectonics (Burke and Seng
1986; Stern, 1994; Abdelsalam and
Stern, 1996). This was accompanied
by the exhumation of metamorphic
domes (e.g. Blasband et al., 2000;
Fritz et al., 2002; Brooijmans et al.,
2003; Abd El-Naby et al., 2008;
Abu-Alam and St
uwe, 2009). We
studied one of these domes in NW
Saudi Arabia: the Qazaz Dome,
which is associated with the sinistral
Qazaz strike-slip shear zone, one of
the largest Najd structures with a
length of at least 140 km (Stern and
Johnson, 2010; Genna et al., 2002;
Figs 2 and 3).
Qazaz metamorphic complex
The Qazaz Dome developed in a
low-relief area with nearly continu-
ous exposure in the desert of Saudi
Arabia. It is a triangular dome of
medium- to high-grade gneisses sur-
rounded by low-grade mylonite zones
and very low-grade metapelite, con-
glomerate and volcanic rocks of the
Neoproterozoic Thalbah and Bayda
Groups (Fig. 3a). The Thalbah
group sediments have been deposited
unconformably on the Imdan plu-
tonic complex (660 4 Ma) and
were intruded by the Liban complex
(621 7 Ma), which appears to
bracket deposition between 660 and
620 Ma (Johnson et al., 2011). How-
ever, the age of the group is debated:
new U-Pb dates of detrital zircons
from two of the three formations
that make up the Thalbah group
(Bezenjani et al., 2014) suggest depo-
sitional ages of 596 10 Ma (Ha-
shim Formation) and 612 7Ma
(Zhufar Formation). The Qazaz
shear zone is locally 34 km wide
with a dominance of vertical folia-
tions and gently plunging stretching
lineations. It is an anastomosing
complex of high-strain branches with
Correspondence: Mr. Sven Erik Meyer,
Tectonophysics, Institute of Earth Sci-
ences, Becherweg 21, Johannes Gutenberg
University Mainz, Mainz 55099, Rhein-
land Pfalz, Germany. Tel.:
+49 6131 3920293; e-mail: meyersv@
©2014 John Wiley & Sons Ltd 387
doi: 10.1111/ter.12111
high-grade mylonitized rocks in the
core (Fig. 3a). Adjacent to the Qazaz
Dome, the shear zone splits into two
strike-slip zones with similar sinistral
shear sense, which flank the dome as
described below. The activity of the
Qazaz shear branches is bracketed
between ~630 and 580 Ma (Calvez
et al., 1984; Kennedy et al., 2009)
based on displacement of dated
The Qazaz Dome is characterized
by a dominant gently SWNE-dip-
ping mylonitic foliation with NWSE
or NS trending gently plunging
stretching lineations developed in
granitic gneisses (Fig. 3c). The age of
the gneisses (protolith) in the dome
itself is given by SHRIMP zircon
dating as 725696 Ma (Johnson and
Woldehaimanot, 2003). Towards the
southern detachment, the gentle
southern dip of the mylonitic folia-
tion increases to a maximum of 40°
(Figs 3c and 4). The detachment
includes parts of the Qazaz gneisses
and metasediments of the Thalbah
group with a strong south-dipping
mylonitic shape fabric (Fig. 3c). The
footwall contains high-strain migma-
tites and high-metamorphic-grade
mylonites with r-type feldspar clasts
and garnet clasts with strain shad-
ows, giving a dominant top to the
south shear sense (Fig. 5). These
high-grade shear sense markers and
shape fabrics are overprinted by
lower grade S-C shear bands and
chlorite veins indicating the same
movement direction and shear sense,
suggesting synkinematic exhumation
of the dome. The hanging wall of the
Qazaz Dome to the south and west
is composed of rocks of the Thalbah
Group. These are weakly to moder-
ately deformed with open upright
folds south of and alongside the
dome and the Qazaz shear zone, but
undeformed further away (Fig. 3c).
To the SE, the Thalbah group is
invaded by small, mostly undeformed
monzogranite bodies that are not
affected by the main strike-slip or
detachment mylonitization, although
some minor shear zones occur at the
contact of the granite and the
metasediments. The gently dipping
mylonitic foliation in the Qazaz
Dome is affected by km-scale folds
with NNWSSE trending steep axial
planes, which are open in the SE but
become increasingly tight to the NW
(Figs 3c and 6ce). In the NW apex
of the Dome, a single tight to isocli-
nal antiform dominates the mylonite
zone and grades into the Qazaz shear
zone. Up to 20 km north of the
Qazaz Dome, this fold is still recog-
nizable because the strike-slip Qazaz
shear zone has gently plunging linea-
tions throughout, but a foliation that
changes from subvertical in the
shear-zone limbs to horizontal in the
centre (Figs 3d, 6e and 7 inset). Fur-
ther NW, only vertical foliations are
present in the Qazaz shear zone.
The west side of the Qazaz Dome
is flanked by a sinistral strike-slip
shear zone, which overprints the
high-grade mylonitic fabrics in the
upper/middle crust
lower crust
upper/middle crust
lower crust
Fig. 1 (a) Typical metamorphic core complex formed during regional extension; (b)
strike-slip core complex, formed during regional shortening by local extension asso-
ciated with a jog in a strike-slip shear zone.
Fig. 2 Overview map of the Najd shear zones and the gneiss complexes in the Ara-
bian-Nubian Shield; modified after Abu-Alam and St
uwe (2009).
388 ©2014 John Wiley & Sons Ltd
Strike-slip core complex S. E. Meyer et al. Terra Nova, Vol 26, No. 5, 387–394
Lithostratigraphy of the Qazaz Complex
Litharenite and Conglomerate, Thalbah Group
Litharenite and Siltstone, Bayda Group
Amphibolite Facies
Amphibolite F. overprinted by Greenschist F.
Greenschist Facies
Metamorphic grade of deformation zone
10 km
E36.60°E36.40° E36.80°
Very low grade Facies
Qazaz Dome
14 km
Red Sea
Lower hemi spher e
[L] Qazaz-West-ShearZone-Lineation.txt (poles to lines) n=40
[L] Qazaz-East-ShearZone-Lineation.txt (poles to lines) n=30
[L] Qazaz-Decollement-Lineation.txt (poles to lines) n=29
Lower hemisphere
[P(dd)] Qazaz-West-ShearZone-Foliation.txt (poles to planes) n=44
[P(dd)] Qazaz-East-ShearZone-Foliation.txt (poles to planes) n=29
[P(dd)] Qazaz-Decollement-Foliation.txt (poles to planes) n=30
35 70
Lower hemi spher e
[P(dd)] Qazaz_foliation_granite.txt (poles to planes) n=79
[L] qazaz_lineation granite.txt (poles to lines) n=70
Qazaz Dome
14 km
Red Sea
Grade of Deformation
Strongly deformed - Mylonized
Medium strain
Low strain
Undeformed rocks
Structural Symbols
10 km
Foliation S1 / Dip
Lineation L1, / Plunge
Qazaz - Eastern shear zone
Qazaz - Western shear zone
Qazaz - Detachment
Shear zones
pole of foliation
Shear Zones
Qazaz dome gneiss - Foliation
Qazaz dome gneiss - Lineation
Pole of foliation / Lineation
Brittle Fault
Sense of movement - Strike Slip
E36.60°E36.40° E36.80°
Sample P1
7.5 ± 0.5 kbar
560-640 °C
Sample P3
4.7 ± 0.3 kbar
400-600 °C
Sample P2
7.0 ± 0.5 kbar
570-630 °C
Sample P4
0.7 ± 0.2 kbar
430-450 °C
Gneiss Dome
Geothermobarometry, Sample location
Thalbah Group
Mylonite zones
n = 103
n = 99
n = 149
P4 P3
Fig. 3 (a) Lithological and metamorphic map of the Qazaz complex; (b, d) inset maps showing the shear zone northwest of the
Qazaz Dome; (c) structural map of the Qazaz complex.
©2014 John Wiley & Sons Ltd 389
Terra Nova, Vol 26, No. 5, 387–394 S. E. Meyer et al. Strike-slip core complex
western part of the gneiss dome with
a lower grade shear band cleavage.
The western shear zone shows a
steep mylonitic foliation with a west-
wards dip and stretching lineations
that gently plunge with a NWSE to
NS trend (Fig. 3c). In the SW cor-
ner of the Dome, the strike-slip shear
zone changes direction and grades
into the extensional detachment
described above; the stretching linea-
tions in both segments have the same
orientation with a plunge of 1035
degrees (Fig. 3c). There is no indica-
tion of any overprint, suggesting that
the strike-slip and detachment seg-
ments operated simultaneously as
one continuous shear zone. The
gently dipping mylonitic fabric in the
central parts of the dome gradually
steepens and grades into the western
shear zone and southern detachment
without overprint and without a sig-
nificant change in orientation of the
main lineations (Figs 3c and 7).
The eastern branch of the Qazaz
shear zone is a sinistral shear zone
with steep foliations and gently
plunging stretching lineations similar
to the western branch; it likewise has
a greenschist facies mylonitic fabric
with prominent shear band cleavage
that overprints the gently south-dip-
ping amphibolite-grade mylonites of
the central dome. Locally, the older
mylonitic foliation is folded, and two
parallel stretching lineations of dif-
ferent age can be found. The eastern
branch is therefore younger than the
detachment and formed later than
the other structures. The eastern
branch also transects the contact of
the dome with the Bayda group in
the south, interrupted by a sinistral
brittle fault in the wadi (Fig. 3c).
Depth of emplacement
Mineral exchange thermobarometers
(i.e. garnet-biotite of Hodges and
Crowley, 1985; muscovite-plagioclase
of Green and Usdansky, 1986 and
hornblende-plagioclase of Holland
and Bundy, 1994) and the Al-in-horn-
blende barometer of Johnson and
Rutherford (1989) were used to calcu-
late peak metamorphic conditions for
several samples across the shear zone
to determine the maximum depth of
burial. The hornblende-plagioclase
thermometer and the Al-in-horn-
blende barometer are based on cali-
brations of igneous systems, but can
be used for the metamorphic system
(e.g. Mancini et al., 1996; Sch
arer and
Labrousse, 2003). The calculated
pressure and temperature conditions
based on hornblende-bearing assem-
blages agree with the conditions that
were calculated using the garnet-bio-
tite and the muscovite-plagioclase
thermobarometers. A sample from
the core of the Dome (Fig. 3a, P1:
26.6956°N, 36.6996°E) attained peak
metamorphic conditions of 560
640 °C and 7.5 0.5 kbar (crustal
depth of 2428 km; for an overbur-
den density 2850 kg m
assuming lithostatic conditions). A
high-grade gneiss sample near the
Qazaz dome Detachment
Thalbah group
metasediments Foliation
Shear zone
Bayda group
metasediments Strike-slip shear zone
250 m
2 km
250 m
Shear zone Shear zone
Qazaz dome
–125 m
Fig. 4 Profiles through the Qazaz Dome, marked in Fig. 3c.
100 μm qtz
300 μm
300 μm
100 μm
(a) (b)
(c) (d)
Fig. 5 (a) Quartz (qtz) with irregular grain boundaries with lobate structures, devel-
oped by grain boundary migration (GBM) recrystallization. Amphibolite facies,
centre of the Qazaz Dome. (b) feldspar (fsp) porphyroclast replaced by epidote
(ep). Quartz shows a typical dynamic recrystallization fabric of subgrain rotation
(SGR). Upper greenschist facies, Western shear zone. (c) Quartz with subgrain
rotation (SGR) recrystallization and garnet crystals with irregular rims of plagio-
clase. Lower amphibolites facies, Western shear zone. (d) Relicts of GBM recrystal-
lization, overprinted by lower temperature bulging recrystallization (BLG). Gneiss,
detachment, S-side Qazaz Dome.
390 ©2014 John Wiley & Sons Ltd
Strike-slip core complex S. E. Meyer et al. Terra Nova, Vol 26, No. 5, 387–394
periphery of the Dome (P2: 26.7455°
N, 36.6193°E) gave a pressuretem-
perature range of 570630 °C and
7.0 0.5 kbar (crustal depth of 22
26 km). A schist sample from the wes-
tern Qazaz shear-zone branch (P3:
26.7347°N, 36.6034°E) reached peak
conditions of 400460 °C and 4.4
5.0 kbar (crustal depth of 15.5
17.5 km). A sample from the base of
the Thalbah group at a distance of
~120 m from P3, SW of the western
Qazaz shear-zone branch (P4:
26.7331°N, 36.6004°E), shows
greenschist facies conditions of 430
450 °C and 0.7 0.2 kbar (crustal
depth of 1.53.5 km). Clearly, the
local metamorphic gradient is tele-
scoped with significant uplift of the
dome with respect to the Thalbah
group due to movement on the shear
The metamorphic grade of myloni-
tization in the Qazaz Dome and the
shear zones was assessed from the
microstructure using recrystallization
characteristics of quartz and feldspar
as an indicator (Stipp et al., 2002;
Passchier and Trouw, 2005). The
microstructure varies consistently
with the barometry (Fig. 3a). The
highest grade fabrics are found in
the central and southern parts of the
dome, with typical high-temperature
grain boundary migration (GBM)
recrystallization of quartz with lobate
grain boundaries, coarse-grained
(>350 lm) recrystallization of feld-
spars, and bulbous feldspar-quartz
boundaries (Stipp et al., 2002;
Fig. 5a). These fabrics grade to the
south and west into greenschist facies
mylonitic fabrics, with subgrain
boundary rotation or even bulging
recrystallization of quartz, recrystalli-
zation to fine grain-size and brittle
deformation of feldspar, and abun-
dant development of shear band cleav-
age (Stipp et al., 2002; Fig. 5bd).
The greenschist facies mylonites along
the western, eastern and southern
sides of the core complex show
remains of an earlier high-temperature
fabric (Fig. 5d). The overprinting of
coarse-grained quartz bands with
lobate grain boundaries by shear band
cleavage and bulging recrystallization
indicate exhumation and cooling dur-
ing ongoing mylonitization (Fig. 5d).
Garnet crystals in samples from close
to P3 in the western shear zone show
irregular rims of plagioclase (Fig. 5c)
in a primary volcanic rock, an indica-
tor of decompression during the
growth of the crystals. The calculated
peak metamorphic conditions of P3
classify this part as a middle crustal
segment, which could be the deeper
part of the Thalbah group. The kine-
matics of the shear zone and a steep
metamorphic gradient of over 200 °C
and 4.0 kbar in an EW profile
between the gneiss and the metase-
diments of the Thalbah group show
that the southern Qazaz complex is
a typical extensional detachment
Field and microstructural evidence
shows that the Qazaz Dome is a
high-grade metamorphic dome that
has been brought into contact with
Jog forms
Exhumation of
lower crust during
New shear zone
transects the
Present state
20 km
Fig. 6 (ae) Proposed sequence in the development of the Qazaz complex along the
Qazaz strike-slip shear zone.
©2014 John Wiley & Sons Ltd 391
Terra Nova, Vol 26, No. 5, 387–394 S. E. Meyer et al. Strike-slip core complex
adjacent low-grade metasediments by
movement on ductile shear zones.
The emplacement of the footwall
high-grade dome by exhumation
along a gently S-dipping detachment,
and emplacement against low-grade
hanging wall metasediments are typi-
cal of a metamorphic core complex
(Tirel et al., 2008; Huet et al., 2010).
However, the relationship with
strike-slip shear zones is different
from other core complexes (Fig. 1).
From field and microstructural data,
we envisage the following scenario
for the development of the Qazaz
Dome (Fig. 6). During development
of the crustal-scale vertical strike-slip
Qazaz shear zone between ~630 and
580 Ma, a 10 km size jog formed as
a gently south-dipping detachment,
probably in response to a local pre-
existing fabric (Fig. 6a). Progressive
strike-slip after the jog formed led to
exhumation of the middle and lower
crust underlying the jog zone
(Fig. 6b), which in turn considerably
changed the local strength profile of
the crust. The developing core com-
plex was affected by regional short-
ening oblique to the strike-slip
shear-zone segments. The uplifted
high-temperature dome underwent
ductile EW internal shortening lead-
ing to folding of the previously
gently dipping planar fabric of folia-
tion in the dome, at some distance
from the detachment (Fig. 6c). Since
the earlier exhumed part underwent
lateral constriction for a longer time,
folding is tightest furthest away
from the detachment zone, leading to
the triangular shape of the dome
(Figs 6e and 7). This process created
an unusual strike-slip shear-zone seg-
ment with an internal antiformal
folded foliation in its core (Figs 3c
and 7 inset).
If the detachment shear zone has a
dip of 40°throughout, as observed
at the surface, 25 km of vertical
exhumation may have been accom-
modated by 30 km of horizontal slip
on the Qazaz shear zone. The trian-
gular core complex that formed in
this way alongside the strike-slip
zone was subsequently transected by
a newly developing eastern branch of
the Qazaz shear zone, which sepa-
rated and displaced its NE side
(Fig. 6d,e). This may have happened
in response to cooling and strength-
ening of the distal side after slip on
the western shear zone ceased. Sinis-
tral displacement on the eastern
branch is estimated to be at least
30 km, based on strain intensity in
the shear zone and displacement of
marker horizons in satellite images
(Fig. 6d). The accumulated displace-
ment along the Qazaz shear zone is
therefore at least 60 km including
the estimated 30 km of strike-slip
displacement associated with exhu-
mation of the central Qazaz Dome.
Finally, a brittle sinistral strike-slip
fault offset the eastern branch itself
in a late stage of Najd shear-zone
activity (Figs 3c and 6e). The defor-
mation pattern in the low-grade
Thalbah and Bayda Group metasedi-
ments surrounding the Qazaz Dome
(Fig. 3c) also supports a model
where the Qazaz Dome formed in a
regime of strike-slip linked detach-
ment, accommodating regional E-W
crustal shortening (Figs 2 and 3c). It
is even conceivable that deposition of
part of these metasediments was con-
nected with development of the
extensional jog.
The most likely candidates that
may show a similar development to
the Qazaz Dome are other gneiss
domes in the Najd Fault system such
as the Hafafit complex (Fowler and
Osman, 2009) and the Kirsh gneiss
Dome (Al-Saleh, 2012; Fig. 2). The
Sha’it-Nugrus shear zone of the Ha-
fafit dome shows an identical transi-
tion from a strike-slip to a
detachment fault as does the Qazaz
Dome (Fowler and Osman, 2009).
Gneiss domes superficially similar
to the Qazaz Dome also occur in a
number of other tectonic settings. A-
type domes in the Aegean (Jolivet
et al., 2004b, 2010; Le Pourhiet
et al., 2012) and the gneiss domes in
the central Pyrenees (van den Eeckh-
out and Zwart, 1988; Den
ele et al.,
2007) superficially resemble the
Qazaz Dome, but have a more com-
plex history and no permanent link
to crustal-scale strike-slip shear
zones. Core complexes along the
Lewis and Clark fault zone in the
Rocky Mountains (Foster et al.,
2007) differ from the Qazaz Dome,
in that the core complexes are the
dominant structure, with strike-slip
faults playing an accommodating
role. The Ni
gde massif along the
Central Anatolian fault zone in Tur-
key (Whitney et al., 2007) and
domes along the Red River Shear
Zones in Southern China and Indo-
china may have formed in a similar
way to the Qazaz Dome, but are
reported to have formed with a
more complex history including a
transtension phase (Jolivet et al.,
2001). Further research on these
structures, and along major strike-
slip fault systems, will show whether
10 km
°09.62N °
Fig. 7 Annotated satellite image and 3-D cartoon of the present structure of the
Qazaz complex. Black dashes indicate the orientation of the stretching lineation in
the mylonites. Folding in the internal part of the dome is highlighted by an EW
392 ©2014 John Wiley & Sons Ltd
Strike-slip core complex S. E. Meyer et al. Terra Nova, Vol 26, No. 5, 387–394
other isolated metamorphic domes
formed in a similar way to the
Qazaz Dome.
The development of metamorphic
core complexes is generally thought
to involve crustal-scale extensional
processes of crustal thinning with
exhumation of the lower crust. The
Qazaz Dome shows that core
complexes can form along crustal-
scale strike-slip shear zones which
accommodated crustal shortening.
The synchronous activity of strike-
slip shear zones and a detachment
jog is an extremely effective way to
exhume deeper crustal rocks under
constrictional conditions. Strike-slip
core complexes similar to the Qazaz
Dome may therefore be present,
unrecognized, along many other
crustal-scale strike-slip shear zones
where they have been transposed by
ongoing strike-slip deformation.
Development of a strike-slip core
complex will locally change the ther-
mal profile of the crust and can have
far reaching effects on local crustal
strength and the functioning of crus-
tal-scale strike-slip shear zones.
This project was financed by the Geocy-
cles and SRFN programs at the Univer-
sity of Mainz. We thank the Saudi
Geological Survey (SGS) for logistic sup-
port in the field and further cooperation.
Special thanks to the President of the
SGS, Dr Zohair Nawab and to the direc-
tor, Dr Khalid Kadi. We thank Saad M.
S. Al-Garni, Mubarak M. M. Al-Nahdi
and Wiesiek Kozdr
oj for their support.
This project is part of and was supported
by the Swedish JEBEL research initiative.
We gratefully acknowledge support by
FWF Project P22351-N22 and by the
urmann Foundation.
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... In the region where the Yanbu Suture is dislocated by the Qazaz Fault Zone, Johnson and Kattan (2012, their fi gure 6-3) name three distinct NW-trending shear zones (here referred to as faults); from southwest to northeast: (1) Dhawrah Fault; (2) The Thalbah Basin is the largest Neoproterozoic sedimentary basin in the Qazaz Fault Zone. The structural setting of the basin and surrounding region was discussed by Davies (1985), Genna et al. (2002), Johnson et al. (2013) and Meyer et al. (2014). The sediments of the Thalbah Group are moderately folded with a NS-axis with beds dipping between 10° and 70° and, in places, are overturned near the northwestern side of the basin (Figure 4; Davies, 1985). ...
... The two features are here referred to as the "Greater Qazaz Complex" (Figure 3). Meyer et al. (2014) presented a new interpretation whereby the Greater Qazaz Complex is attributed to EW-directed compression and NW-directed transpressional sinistral shearing. They consider the structural development of this feature to be coeval to the deposition of the Thalbah Group, which based on the literature, they suggest may be ca. ...
... 630-580 Ma (see "Age of Greater Qazaz Complex" below). Meyer et al. (2014) collected four samples and analyzed them using mineral exchange thermobarometers to calculate peak metamorphic conditions and maximum depths of burial ( Figure 3, Table 2). They indicate that amphibolite-facies rocks dominate the core of the Qazaz Dome (Sample P1); its periphery is amphibolite overprinted to greenschist (P2), and sheared rocks that bound the southwestern edge of the dome are schists (P3). ...
This paper starts with a bibliographic review of the lithostratigraphy and radiometric dating of the Ediacaran Thalbah Group in the northwestern Arabian Shield, Saudi Arabia. It seeks to establish the spatio-temporal position of the group in the ongoing compilation and correlation of Ediacaran–Cambrian sedimentary time-rock units in the Middle East Geologic Time Scale (Al-Husseini, 2010, 2011, 2014). The group is defined and described in the Thalbah Basin, which crops out in the Al Wajh Quadrangle, and is approximately 100 km (NW-SE) by 40 km (SW-NE) in extent (Davies, 1985). The basin is situated within the approximately (ca.) 300 km-long, NW-trending Qazaz Fault Zone of the Najd Fault System. The Thalbah Group consists of three siliciclastic units: Hashim Formation (ca. 1,050–1,300 m thick) and likely coeval Zhufar Formation (ca. 600–1,400 m thick), and the younger Ridam Formation (ca. 1,000 m thick). Recently published U-Pb dating of detrital zircons gave ages of ≤ 596 ± 10 Ma for the Hashim Formation, and ≤ 612 ± 7 Ma for the Zhufar Formation (Bezenjani et al., 2014). The maximum depositional ages of the Hashim and Zhufar formations indicate they are approximately coeval to the lower part of the sedimentary and volcanic rocks of the Jibalah Group (≤ 605 ± 5 and ≥ 525 ± 5 Ma). The latter group was deposited in pull-apart basins along the ca. 600 km-long Rika and several other extensive fault zones of the NW-trending Najd Fault System in the northern and eastern parts of the Arabian Shield. The Qazaz Fault Zone left-laterally dislocated ophiolites of the NE-trending Yanbu Suture Zone (≥ 700 Ma) by about 100 km. The strike of the Qazaz Fault Zone projects into the Rika Fault Zone, along which five major pull-apart basins contain the Jibalah Group. The Rika Fault Zone dislocated by about 100 km the NS-trending ophiolite outcrop belts of the Ad Dafinah and Hulayfah fault zones (sometimes interpreted as parts the Nabitah Suture Zone, 680–640 Ma). Based on the time correlation of the Thalbah and Jibalah groups, and the highlighted structural features, the Rika and Qazaz fault zones are interpreted as a continuous 30 km-wide, 1,200 km-long, N63°W-striking fault zone, the “Rika-Qazaz Fault Zone”, which left-laterally dislocated the Arabian Shield by approximately 100 km after 605 ± 5 Ma and before 525 ± 5 Ma.
... The dome is dominated in its inner core by medium-to high-grade gneisses, flanked by low-grade mylonitic zone, low-grade metapelites, conglomerates, and volcanic rocks of the Thalbah and Bayda Groups. Meyer et al. (2014) studied in some detail the Qazaz metamorphic dome and described a new mechanism whereby core complexes can be exhumed along crustal-scale strike-slip fault systems that accommodated crustal shortening. They indicated that the dome was exhumed along a gently dipping jog in a crustal-scale vertical strike-slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. ...
... Davies' (1982) strain profile across the Ajjaj shear zone yielded 60 ± 20 km sinistral strike-slip displacement and 5 ± 1.5 km vertical displacement. Meyer et al. (2014) concluded that there had been 60 km of sinistral displacement along the Qazaz shear zone, with the first 30 km resulting in a 25 km uplift of a core complex bounded by the elements of the shear zone. Hassan et al. (2016a, b) estimated an exhumation from 58 km to 44 km depth (14 km vertical displacement) of the Hamadat complex along the Ajjaj shear zone during shear zone activity. ...
... Most studies of the ductile dimensions of the NFS have been located in the northwestern shears, in particular, the Qazaz, Ajjaj, and Hanabiq shear zones. The comparative variations in metamorphic grade between inner and outer parts of these Najd shears and their enclosing rocks have been recently reported by Meyer et al. (2014), Abu-Alam et al. (2014), Hassan et al. (2016a, b). Metamorphic grade maps of Ajjaj-Hamadat-Hanabiq by Hassan et al. (2016a, b) show that the least deformed parts of the shear zones are broadly low metamorphic grade, with elongate zones of medium grade gneissic bodies along the shears ( Fig. 15.8). ...
The Arabian-Nubian Shield (ANS) was assembled from juvenile crust during a three-stage Neoproterozoic tectonic evolution involving: (1) intra-oceanic subduction and arc accretion stage, (2) orogenic extension stage, and (3) post-extensional compressional stage. Stage 1 is manifested by ophiolite-decorated arc-arc high-strain zones (suture zones) and calc-alkaline magmatism. The orogenic extensional tectonic stage generated dyke swarms, bimodal volcanism, molasse basins, A-type granite magmatism, low angle normal faulting (LANFs) and metamorphic gneiss complexes. The geological features attributed to this stage have been interpreted in terms of continental rifting, gravitational collapse, crustal and mantle delamination, transpression, escape/extrusion tectonism, and gravitational uplift. The post-extensional compressional stage is typified by dominantly NW–SE trending folds and thrusts, E–W transpression, and the N–S shortening zones. The Najd Fault System (NFS) (ca. 630–540 Ma) to be described in this chapter is attributed by some workers to the orogenic extension tectonic stage and by others to the post-extensional compressional stage. Earlier interpretation connects the NFS to the Najd Orogeny (570–520 Ma). The NFS is one of the largest transcurrent shear systems worldwide and deciphering its kinematic history adds considerably to our understanding of the cratonization of Gondwana, and specifically to mechanisms of exhumation of metamorphic complexes in the ANS. The NFS extends in a NW–SE direction across the Arabian Shield (e.g., Ajjaj, Qazaz, Ruwah, Ar Rikah, and Halaban strands of the NFS) for more than 1300 km (~400 km wide) and continues beneath Phanerozoic cover in Yemen. The NFS is believed to extend into the Nubian Shield (Egyptian Eastern Desert and Sinai). The dominant sense of shearing along the NW–SE trending Najd megashears is sinistral, however, evidence exists for an earlier phase of dextral slip. NE- (to ENE-) oriented shear zones (e.g. the Ad-Damm, Fatima, Idfu-Mersa Alam, Qena-Safaga shear zones) could be Najd-related conjugates or earlier fault systems. The shear and volume strain aspects of Najd shears are described, as are the stress controls on the brittle evolution of Najd faults. The role of Najd brittle structures in hydrothermal mineral deposits and ground water flow patterns are also covered in this chapter.
... The NW-SE-striking faults, known as the Najd faults are generally viewed as late-stage left-lateral strike-slip faults that are considered as the largest pre-Mesozoic shear zone system on Earth (Moore, 1979;Stern 1985;Meyer et al. 2014). The Najd faults are the result of a two-phase evolution. ...
... The NW-SEstriking faults are considered the pathways for the mineralizing fluids that coincide with the Ruwah fault zone (Fig. 1), generally strike-slip faulting that offsets the northern tip of the Nabitah N-S belt (Quick 1991). Structurally controlled gold mineralization in the Arabian Shield is favored by some authors during the last two decades and the role of the Najd fault system as an expression of exhumation of older Neoproterozoic complexes to form either orogenic or epithermal gold ores connected to syn-to post-collision granitic rocks (Fritz et al. 1996(Fritz et al. , 2002Loizenbauer et al. 2001;Abu-Alam and Stüwe 2009;Abu-Alam et al. 2018;Harbi et al. 2014;Meyer et al. 2014;Abd El-Wahed et al. 2016;Abu El-Enen et al. 2016;Zoheir et al. 2018Zoheir et al. , 2019a. Recently, Abu-Alam et al. (2018) stated that the Najd fault system enables the vertical transport of gold-bearing fluids from the source region to the depositional sites. ...
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The paper presents new insights to gold mineralization at the Jabal Umm Watirah (JUW) area in western Saudi Arabia using surface and subsurface information from field, mineralogical and geochemical studies. Host rocks are Neoproterozoic serpentinites and arc meta-andesite. Gold in serpentinites is enriched in listwaenite that results from silica and CO2 metasomatism. In the meta-andesite, prominent NW-SE alteration zones follow the Najd Fault system (NFS) along the Ruwah Fault Zone (RFZ). These alterations are sulphide-rich and show sericitization, chloritization, silicification and hematitization. The auriferous pyrite-rich listwaenite contains relict of Cr-spinel of the ophiolitic serpentinite precursor. Anomalous gold up to 10 g/t in listwaenite is encountered in one borehole whereas the rest assay up to 0.27 g/t with the exception of silicified serpentinite and quartz-carbonate rocks with up to 0.59 and 0.57 g/t gold, respectively. Hydrothermally altered arc meta-andesite contains up to ~25% sulphides (mostly pyrite, arsenopyrite and chalcopyrite) with up to 1.62 wt% As2O5 and up to ~21 g/t gold. The results indicate that arsenic is a pathfinder for the studied gold mineralization. On the other hand, fresh meta-andesite is very poor in gold, up to 0.05 g/t only. Remobilized visible gold ore can be seen in some quartz veins and stringers with 150-170 μm wide gold along fractures. Gold enrichment is confirmed in the meta-andesite from boreholes up to ~250 m deep that are dug beneath ancient workings and dumps at the two sectors in the JUW prospect. Gold mineralization in the JUW prospect is structurally controlled orogenic gold mineralization because of the following reasons: proximity to the major first order structure, 2) host rocks are metamorphosed in the greenschist facies conditions, 3) Listwaenitization by carbonic and silicic metasomatism of the ophiolitic serpentinite.
... The ANS has been affected by the Najd fault system (NFS), which is a system of NWtrending, crustal-scale, sinistral strike-slip faults, and ductile shear zones formed during the collision between east and west Gondwana (Figure 1b) [37,39,40]. The NFS extends for 2000 km, is over 400 km wide, and is partially covered by Cenozoic lavas and alluvium [32,41]. ...
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Saudi Arabia covers most of the Arabian Peninsula and is characterized by tectonic regimes ranging from Precambrian to Recent. Using gravity data to produce the lateral boundaries of subsurface density bodies, and edge detection of potential field data, a new subsurface structural map was created to decipher the structural framework controls on the distribution of gold deposits in Saudi Arabia. Moreover, we detected the relationships between major structures and mineral accumulations, thereby simultaneously solving the problem of edge detectors over complex tectonic patterns for both deeper and shallower origins. Analytic signal (ASg), theta map (TM), TDX, and softsign function (SF) filters were applied to gravity data of Saudi Arabia. The results unveil low connectivity along the Najd fault system (NFS) with depth, except perhaps for the central zones along each segment. The central zones are the location of significant gold mineralization, i.e., Fawa-rah, Gariat Avala, Hamdah, and Ghadarah. Moreover, major fault zones parallel to the Red Sea extend northward from the south, and their connectivity increases with depth and controls numerous gold mines, i.e., Jadmah, Wadi Bidah, Mamilah, and Wadi Leif. These fault zones intersect the NFS in the Midyan Terrane at the northern part of the AS, and their conjugation is suggested to be favorable for gold mineralization. The SF maps revealed the boundary between the Arabian Shield and Arabian Shelf, which comprises major shear zones, implying that most known mineralization sites are linked to post-accretionary structures and are not limited to the Najd fault system (NFS).
... For the ANS, late-orogenic ANS granitoids are related to subduction-unrelated I-type and A-type melts connected with post-collisional mantle processes (Stein 2003;Kröner and Stern 2004;Avigad and Gvirtzman 2009;Be'eri-Shlevin et al. 2009;Stern et al. 2010;Eyal et al. 2010;Johnson et al. 2011;Ali et al. 2012Ali et al. , 2013Ali et al. , 2016. This stage could have been spanned the period of the onset of Najd Fault System in the Arabian Shield and the equivalent Najd-related shear zones (e.g., Kröner and Stern 2004;Fritz et al. 2013;Meyer et al. 2014;Hassan et al. 2016;Zoheir et al. 2020Zoheir et al. , 2021. ...
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Although formed after subduction tectonics have already waned and terrane accretion has concluded, numerous late-orogenic granitoid phases in the Neoproterozoic Arabian-Nubian Shield (ANS) hold geochemical characteristics typical of the volcanic-arc granites (VAG). The latter are accentuated by the high abundances of LILE and LREE relative to the HFSE and HREE, and thus high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios along with fractionated REE patterns. Synthesis of bulk-rock geochemical and zircon Hf isotope data of the ANS late-orogenic granitoids, exclusively dated as ~640-600 Ma, is employed herein to reveal the origin of the VAG signature in many of these rocks. Data of widespread granitoid intrusions reveal that these rocks span typically metaluminous to peraluminous, calc-alkalic I-type to alkali-calcic ferroan A-type granite compositions. Mild-to-strong negative Eu anomalies generally correspond to increasing magmatic differentiation, and transition from hybrid to evolved granites is envisaged for the younger phases, irrespective of their distribution. The available bulk-rock geochemical data and zircon Hf isotope compositions weigh with parental magmas produced by high-temperature melting of subduction-related/subduction-metasomatised lower crustal rocks (e.g., TTGs, high-K mafic rocks, and metasediments) and mixing with mantle-derived sanukitoid-like melts. These magmas then experienced plagioclase-rich/poor fractionation (revealed by variable negative Eu anomalies) prior to crystallization and fractionation toward A-type/ferroan granite geochemistry. The heat source and melting trigger were most likely asthenospheric mantle upwelling during the onset of extensional collapse of the ANS. The widespread distribution of the post-collisional granitic intrusions in most of the shield area can be explained by lithospheric delamination generally results in voluminous rather than restricted linear magmatism. The commonly described VAG signature in some ANS late-orogenic granitoids (<640 Ma) does not, therefore, depict the factual tectonic setting of these rocks, but rather reproduces their source materials and petrogenesis.
... La présence de dômes gneissiques dans le désert oriental égyptien n'est pas tant associée aux failles du système de Najd qu'à la présence de détachements de grande échelle et de complexes à noyau métamorphique, communément nommés metamorphic core complexes (e.g., Fritz et al., 1996 ;Blasband et al., 1997Blasband et al., , 2000Neumayr et al., 1998 ;Bregar et al., 2002 ;Abd El-Naby et al., 2008 ;Meyer et al., 2014). La chronologie relative des zones de cisaillement les bordant est donc discutée ; sont-elles synchrones de l'exhumation de ces dômes et ont elles accommodé une déformation dans un régime transtensif (Fritz et al., 1996 ou sont-elles des reliques de chevauchements nordouest -sud-est ayant rejoué (Andresen et al., 2010) Le magmatisme tardi-orogénique à anorogénique a également été particulièrement important dans cette partie du BAN (Fig. 0-10 ;e.g., Lundmark et al., 2012 ;Abu El-Enen et al., 2016). ...
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Gold is a siderophile element preferentially concentrated in the Earth’s mantle and core. The understanding of mechanisms leading to its transfer towards the crust requires the study of the geodynamical evolution of juvenile crust portions, i.e., crystallized from melt directly extracted from the mantle. The gold mineral system thus combines the deciphering of crustal fertilization geodynamical processes and subsequent transient tectono-metamorphic setting(s) favorable for remobilization of this metal-enriched reservoir and formation of mineralized occurrences. This approach is applied to the Keraf and Atmur-Delgo sutures which are part of the Arabian-Nubian shield, the world-largest track of Neoproterozoic juvenile crust and one of the main Pan-African gold provinces. Although these structures are considered as insightful windows into the assembly of Gondwana in Sudan and host major gold deposits, their coupled geodynamical-metallogenic evolution remains poorly understood. We report the existence of two magmatic flare-ups with suprachondritic Hf and Nd signals, recording the build-up and maturation of island arcs along these suture zones between 840-810 Ma and 760-655 Ma. The ~185 Myr-long lifespan of the island arc described along the Keraf suture and its predominant juvenile nature likely accounted for crustal gold fertilization at the regional scale. The earliest crustal growth stage, only recorded along the Keraf suture, is coeval to a minor magmatic-hydrothermal gold event. Both the Keraf and Atmur-Delgo sutures keep record of the second island arc flare-up. The matching between field and laboratory data exemplifies the district- to microscopic-scale spatial continuity of structural control on later gold-bearing structures described in this study. On one hand, a gold event occurred between 755-725 Ma, during high-strain progressive deformation expressed by sheath folding and the formation of linear ore shoots under lower amphibolite facies metamorphism. It likely relates to tectonic accretion of the sedimentary wedge at the Atmur-Delgo intra-oceanic subduction site. At the microscopic scale, the ore formation involved the syn-metamorphic remobilization of the gold budget contained in earlier sulfide generations. On another hand, several orogenic gold deposits display intrusion-hosted mineralized extension veins, highlighting a strong rheological control on ore deposition. The fault-valve mechanism induced the formation of mineralized veins, triggered by episodic and localized reactivation of crustal strike-slip shears within the Keraf suture zone until 550 Ma. Gold events described along the Atmur-Delgo and Keraf sutures are the oldest vein-type gold and youngest orogenic gold episodes reported throughout the Arabian- Nubian shield to this day, respectively. The mineral system approach therefore enables district-scale ore targeting by translating magmatic processes and tectono-metamorphic settings into environments and geological features (e.g., relics of an island arc and/or an accretionary wedge, presence of late collisional strike-slip shears) favoring the formation of gold occurrences with very distinct structural, geochemical and timing characteristics in the western Arabian-Nubian shield. This alternative view of ore deposits sharply contrasts with the typological strategy which targets a single class of ore occurrence with an effectiveness limited to the deposit scale.
... In the northern terranes, the displacements associated with these shear zones have led to the exhumation of high-grade rocks of the arc roots, forming metamorphic core complexes (Meyer et al., 2014). They were also responsible for the development of unconformable ''molassic" basins (starting at c. 630 Ma, Bezenjani et al., 2014), as well as of volcanic activity (Dokhan ignimbrite 610-590 Ma: Hamimi et al., 2019). ...
Gold in the Ariab district (Red Sea Hills, Sudan) is mined in gossans developed over volcanogenic massive sulphide (VHMS) deposits formed in the context of a Tonian ensimatic arc of the Haiya terrane, in the Neoproterozoic Arabo-Nubian Shield. All the primary gold occurrences of the Ariab belt, whatever their style (VHMS-, quartz lode- or shear-zone-hosted), appear, however, to have been syn-to late-kinematic, i.e., formed in the context of the final Pan-African collision leading to the Arabian-Nubian Shield (D3 event), and therefore are post-VHMS. Gold is thus found associated with either galena or bismuth telluride in late syn- to post-metamorphic sets of (micro)cracks, mostly within earlier pyrite or arsenopyrite. Gold deposition as native gold or electrum occurred when an intense fluid circulation of metamorphic CO2-H2O rich fluids underwent significant decompression from lithostatic to hydrostatic pressures at temperatures decreasing from 400-430°C at around 8 km depth down to 300°C. Although no clear evidence of volatile unmixing was observed, the decompression, together with a temperature decrease due to dilution by aqueous rich fluids, favoured gold deposition. The late-D3 gold event in the Ariab belt has all the makings of an orogenic gold system, notably similar to the Late Carboniferous gold event in the West European Variscan belt. Thus, it is possible to propose that, at the end of the collision events, fluids released from the newly formed terrane root were channelled towards the upper crust through the major D3 shear zones such as the Oko shear zone. The fluids outpouring from these major drains were then conveyed through fault plays and damaged zones, up to specific chemical traps within massive sulphide ore bodies or quartz veins systems. Thus, the Ariab gold district could be classified as a representative of the class of orogenic gold deposits.
The Central Asir Shear Zone (CASZ) is a N- to NNE-oriented inhomogeneous strain zone parallel to the Nabitah shear belt in southern Saudi Arabia. It is ∼200 km long and 20 km wide, and traverses the Asir terrane. The deformed terrains in the vicinity of the CASZ have experienced a prolonged history involving three successive phases of deformation: arc-collision, dextral transpression and sinistral strike-slip faulting. Remote-sensing data analysis and field investigation provide insights into the geological and structural features along the CASZ. Image transformation and enhancement reveal faults and folds, and discriminate between granitoids, metavolcanics, gabbros and younger (Ablah Group) sediments. The obtained evidence points to polyphase deformation of the area, characterized by narrow zones of high strain, folding and shearing enclosing slices of less deformed rocks. Lineament analysis shows predominantly NNE, NNW, N and NE trends of folds and shear zones, and NW, NE and E fracture trends. The CASZ has a > 100 m.y. long deformation history, including a change from dextral to sinistral shearing. Both shearing senses are found on the older N-S and younger NW-SE fault systems. N-S oriented shearing was accompanied by intrusion of granitoids and localized hydrothermal alteration along N to NNE trending shear zones. Strain analysis shows low strain in the Ablah basin sediments, consistent with Ablah deposition after the tectonic climax in the region.
Intrusion‐Related Gold Deposits (IRGDs) occur in the Eastern Desert (ED) of Egypt within magmatic districts worked out for tungsten and tin mineralization. IRGDs and Intrusion‐Related Rare Metal Deposits (IRRMDs) are almost invariably linked with the late to post collisional Younger Granites (YGs) that have three successive phases (I, II and III). At (∼635‐630 Ma), ED underwent a transition in the deformation style from compression to extension and a switching from subduction and crustal thickening to delamination and crustal thinning. This transition was concomitant with the emplacement of the short magmatic pulse (∼635–∼630 Ma) that represents a transition between orogenic gold deposits and IRGDs. K‐rich calc alkaline granites (phase I and II of the YGs) hosting IRGDs like Hangalia deposit were emplaced at the time span (630–610 Ma). Alkaline magmatism started at 610 Ma, and coexisted with the K‐rich calc‐alkaline magmatism at the time span (610–590 Ma), where Fawakhir (598± 3 Ma) and Um Had (596 ± 2 Ma) granites hosting IRGDs were emplaced. By time, the alkaline magmatism changed to more alkaline giving rise to the phase III of the YGs that hosts IRRMDs. A distinct metallogenic epoch comprising both IRGDs and IRRMDs was extremely growing at (∼ 600 Ma).
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Well-known controversy over the genesis of orogenic gold mineralization in granites has been a subject of research worldwide. Hamash gold mine is considered one of the pharaohs mines in the South Eastern Desert of Egypt, where gold mineralization is hosted by granites. In this study, we utilized field, ore microscopy, and geochemical data to determine the genesis of granites and the associated gold deposits. We found that the granites, intruded into island-arc metavolcanics and metavolcanoclastic assemblages, are characterized by variable major and trace elements, calc-alkaline, and I-type granitoids that reflect a fractional crystallization. Their trace and rare earth elements reflect magma that is related to subduction tectonic setting. The gold mineralization is confined to quartz veins that cut through granitic rocks, extend in NW-SE and N-S directions, and are formed during shear-extensional tectonics. The gold is found with sulfides in quartz veins and wallrock alteration. Silicification and sericitization are the most hydrothermal alteration zones associated the ore formation, comprise an inner quartz-sericite-carbonate-chlorite-sulfide assemblage. The results of chemical analysis reveal an increase of SiO2, Fe2O3, S, and L.O.I. concentrations and moderate depletion in K2O, Na2O, and LILE suggesting that they were affected by mineral replacement reactions and sulfidation in the altered wallrocks close to Au-quartz veins. The gold is represented by tiny inclusions, with minor chalcopyrite, galena, and arsenopyrite that mainly fill the microfracture in quartz and pyrite grains. Furthermore, a general model of gold mineralization suggests that the gold was contemporaneously formed during syn-orogenic stage, transpressional deformation assigned to D2 deformation, and subduction-related thermal processes followed by gold-bearing fluids that fill the fractures, forming veins.
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Detrital zircon U–Pb SIMS dating is used to evaluate the provenance of two correlative basins in the Arabian-Nubian Shield (ANS). The Wadi Igla Formation in the Central Eastern Desert (CED) of Egypt and the Thalbah Group in the Midyan Terrane (MT) of NW Saudi Arabia are considered to be post-amalgamation terrestrial basins, developed during closure of the Mozambique Ocean and amalgamation of the ANS in Cryogenian-early Ediacaran time. The analytical results indicate that the upper-part of the Wadi Igla Formation has a maximum depositional age of 628 ± 6 Ma, contains 98% Neoproterozoic zircon with ages between 815 and 628 Ma, and has two distinct peaks at 690 Ma and 652 Ma. A rhyolite clast from the upper-part of the Wadi Igla Formation gives a U–Pb age of 700 ± 6 Ma. This age significantly pre-dates Dokhan volcanism, indicating that the dominant rhyolitic clasts within the Wadi Igla Formation are not from the Dokhan Volcanics, as previously believed. Analytical results from the Thalbah Group suggest multiphase basin formation and development. The lower part of the Thalbah Group is intruded by monzogranites of the Liban complex, has a minimum depositional age of 635 ± 5 Ma, resembling that of the Wadi Igla Formation. Its middle part has a maximum age of 612 ± 7 Ma and is comprised of 90% Neoproterozoic zircon with ages ranging from 820 to 612 Ma. The upper part of the Thalbah Group has a maximum age of 596 ± 10 Ma and contains a wider range of Neoproterozoic detritus with ages between 985 and 596 Ma. The basement of the Thalbah Group is represented by metasediments and metavolcanics of the Zaam Group. The sample collected from the uppermost part of the Zaam Group (Um Ashsh Formation) contains zircon of mostly Cryogenian age (ca. 812–697 Ma) and has a maximum age of 700 ± 4 Ma, suggesting that the Zaam Group might be correlative with the subduction-related metavolcanic and metasedimentary rocks that are overlain unconformably by the Wadi Igla Formation in the CED.
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The Nigde Massif, a migmatite-cored structural dome in south central Turkey, was metamorphosed, deformed, intruded, and exhumed in an oblique tectonic regime involving a switch from transpression (prograde metamorphism) to transtension (exhumation). An 80 Ma history of transpression and transtension is recorded in metamorphic rocks of the massif, and the last ~50 Ma are recorded in both basement and overlying basin deposits. Nigde metamorphic rocks record a high-temperature, top-to-north, syntranspression ductile fabric that was overprinted by lower-grade, top-to-south, syntranstension ductile shear zones. The 40Ar/39Ar data for hornblende, muscovite, and biotite document rapid Late Cretaceous cooling, with the oldest Ar hornblende ages (88-79 Ma) overlapping U-Pb ages in monazite and zircon (92-78 Ma). Muscovite Ar ages are dominantly 81-79 Ma, suggesting a short time between crystallization of high-temperature metamorphic rocks and their cooling. Biotite Ar ages are 79-74 Ma, with the exception of disturbed samples from the eastern margin of the massif and from Eocene granitic stocks in the dome core. The eastern margin, adjacent to the sinistral Central Anatolian Fault Zone, records another cycle of burial and exhumation that overprinted earlier fabrics and disturbed isotopic systems. These two cycles of burial-exhumation took place repeatedly along the same zone; this may be characteristic of highly oblique tectonic regimes. In the Nigde Massif, burial and exhumation became more localized and of lower amplitude over time, suggesting stabilization of the crust.
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During the late Cryogenian–Ediacaran (650–542 Ma), the Arabian–Nubian Shield (ANS) underwent final assembly and accretion to the Saharan Metacraton concurrent with the assembly of eastern and western Gondwana. At the end of the Precambrian it lay at one end of the East African Orogen, with its northern margin (present coordinates) forming a low-relief stable shelf facing an open ocean; to the south the ANS transitioned into the Mozambique Belt. The geologic history of the ANS during this period provides insight into the closing developmental stages of one of the world’s largest accretionary orogens. Following a 680–640 Ma orogenic event reflecting amalgamation of a core grouping of island-arc terranes (the proto-Arabian–Nubian Shield; pANS), the region underwent extensive exhumation, erosion, and subsidence. Depositional basins formed in the northern and eastern pANS, with those in the east below sea level and connected to an ocean. Periodic basin closure and formation of new basins in other parts of the ANS followed. Many basins were filled by terrestrial, molasse-type sediments interfingering with subordinate to predominant amounts of volcanic rocks. Magmatism was extensive throughout the period, initially characterized by tonalite–trondhjemite–granodiorite (TTG) and granite (monzogranite, syenogranite), but also characterized, from ∼610 Ma on, by increasing amounts of alkali-feldspar granite and alkali granite. The plutons are largely undeformed, except where cut by brittle–ductile shear zones. The magma sources of the late Cryogenian–Ediacaran granitoids were dominated by juvenile crust and(or) depleted mantle and magmas mostly originated in anorogenic, post-collisional, commonly extensional, settings. They were derived by melting and fractionation of anhydrous high-grade metamorphosed lower crust, mafic- to intermediate calc-alkaline crust, and(or) subduction-modified mantle wedges associated with slab break-off or delamination.
This volume, dedicated to Peter Misch, includes 18 contributions (separately abstracted in the Petrology section), grouped into four parts as follows: 1) Introduction (by P. J. Coney) and Overview, pp 3-31, 2) Southern Basin and Range, pp 35-283, 3) Eastern Great Basin, pp 287-423, 4) The Northwest, pp 427-490. A separate list of references cited is provided with each contribution; there is no index. -A.P.
A synkinematic sample of magnesian hornblende from a Svecofennian ultramafic body in southwestern Finland, metamorphosed under conditions of the upper amphibolite - lower granulite facies, has been structurally refined to R = 0.027 for 1082 reflections. The distribution of cations in M(1), M(2) and M(3) is inferred from site-scattering refinement and the relations between mean bond-length and ionic radius. The Fe2+-ordering pattern is M(1) > M(3) ≫ M(2). Fe2+-Mg partitioning between the M(1) and M(3) sites (KM(1)-M(3)D = 0.85) is consistent with high-temperature hornblende (800° < T < 900°C), whereas Fe2+-Mg distribution between the M(1) and M(2) sites (KM(1)-M(2)D = 0.45) is in better agreement with a lower inferred peak of metamorphism (T = 700°C). Owing to uncertainties in the Mg(M4) estimates, the Fe2+-Mg partitioning between M(2) and M(4), (KM(2)-M(4)D = 4.7 · 10-2) corresponds to an unrealistically high temperature of equilibration (800° < T < 900°C). Compared with other structurally refined calcic amphiboles from ultramafic parageneses, the extent of solid solution toward cummingtonite, 9.8%, is consistent with pressure and temperature being the main controlling parameters. The refined distribution of cations correlates with the physicochemical conditions of peak and post-peak metamorphic equilibration. Thus: (1) the low contents of M4Na and AlT, which indicate a low pressure (<3 kbar), combined with the high temperatures (680° < T < 720°C) required by the amphibole-plagioclase exchange equilibrium, are consistent with a period of decompression and uplift accompanied by heating; (2) the low oxidation ratio, the absence of detectable Cl, and the limited extent of F-for-OH substitution, imply that hydration of the ultramafic body resulted from hydrothermal fluids of low f(O2), near the magnetite-wüstite buffer, and low fugacity of halogens, and was therefore likely unrelated to dehydration of a downgoing slab.
The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40Ar/39Ar, and fission- track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40Ar/39Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40Ar/39Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°-110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.
The Whipple and South mountains of the southwestern United States have undergone a strikingly similar sequence of deformations. In both ranges, gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low-angle detachment fault. Kinematic indicators reveal that the mylonitic rocks, brittle structures, and detachment faults are kinematically coordinated and were all formed by top-to-the-northeast shear. The structural evolution of both areas can be explained in terms of major, shallow-dipping shear zones that accommodated Tertiary crustal extension. We suggest that detachment faults and associated zones of brecciation, cataclasis, and seismic slip were originally continuous downdip along the low-angle shear zones into mylonitic gneisses formed below or near the ductile-brittle transition. As the mylonites were drawn out from beneath the brittlely extending upper plate, they were progressively uplifted above the ductile-brittle transition and were overprinted by successively more brittle structures.