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Precambrian Research 117 (2002) 21 – 40
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Proterozoic tectonism of the Arabian Shield
A. Genna a,*, P. Nehlig a, E. Le Goff a, C. Guerrot a, M. Shanti b
a
BRGM, 3, A!enue C. Guillemin, BP 6009, 45060 Orleans Cedex 2, France
b
Saudi Geological Sur!ey, P.O. Box 1492, Jeddah 21431, Saudi Arabia
Received 23 May 2000; accepted 2 May 2002
Abstract
New field and analytical work, together with a new aeromagnetic map and a geologic, structural, geochemical, and
geochronologic synthesis and reappraisal, offer a new view of the anatomy and geologic history of the Arabian Shield.
Although Early Proterozoic rocks have been found in the eastern part of the Shield, the main geologic evolution of
the Shield is limited to a period ranging from 900 to 530 Ma that led to the formation, amalgamation, and final
cratonization of several tectonostratigraphic terranes. The pre-Panafrican structures ( !690 Ma), which are difficult
to decipher due to younger deformation, were essentially the result of the formation, amalgamation, and accretion of
these terranes. The Panafrican tectonism (690–590 Ma) was marked by the formation of the Nabitah Belt and
peripheral ranges punctuated by gneiss domes. Various sedimentary formations contemporaneous with this tectonism
represent foreland or intracontinental molasse basins. After the Panafrican tectonism, widespread extension (590–530
Ma) brought about crustal thinning that generated bimodal magmatism and significant dike swarms; associated
volcanics form the Shammar group. A marine transgression, associated with passive-margin-type structures with tilted
blocks, marked the end of the thinning. The platform facies produced by this transgression correspond to part of the
Jibalah Formation. Other basins formed as deep continental pull-apart basins along transform faults. This updated
view of the Arabian Proterozoic geodynamic evolution provides a framework for reviewing the associated mineralizing events, and places them in a new chronology and structural history. © 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Tectonism; Proterozoic; Saudi Arabia; Panafrican orogeny; Arabian– Nubian Shield
1. Introduction
The present structure of the Arabian Shield
(Fig. 1) is the result of diverse and polyphase
geodynamic events. The first overview of the geology of this region was provided by Karpoff
(1958), who classified the Proterozoic rocks of
* Corresponding author. Tel.: +33-2-38-64-38-96
E-mail address: [email protected] (A. Genna).
Saudi Arabia into two distinct series: an older
Medina series, showing highly variable grades of
metamorphism and cut by intrusives, predating
the discordantly overlying Wadi Fatima series.
No general structural arrangement for the Shield
was proposed at that time.
The Najd Fault system was the subject of the
first tectonic syntheses of the Proterozoic basement (Moore, 1979; Delfour, 1979a, 1980a).
Delfour (1979a) considered the Najd orogenic cy-
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A. Genna et al. / Precambrian Research 117 (2002) 21–40
cle of the Arabian Shield to comprise four structural phases between 800 and 500 Ma. The main
phase, dated at 550 Ma and ending in cratonization
of the Shield, corresponded to the peak of the
Panafrican orogeny. This idea was followed up by
Caby (1982), who compared the geodynamics of
the Arabian Shield to those of the Tuareg Shield,
and considered the tectonic structures of Arabia to
Fig. 1. Geologic map of the Arabian Shield.
A. Genna et al. / Precambrian Research 117 (2002) 21–40
be similar to Andean-type cordilleras. Ophiolite
complexes were recognized by Shanti and
Roobol (1979).
More recently, Kröner (1985) considered the
Arabian Proterozoic basement in terms of an
assemblage of microplates bounded by sutures
that are locally associated with ophiolite formations. This model was developed for the whole
of the Arabian –Nubian Shield by Bentor (1985).
Stoesser and Camp (1985) compiled a tentative
map of these boundaries and their associated
orogenic zones for the Arabian Peninsula, and
Vail (1985) extended this map to cover the entire
Arabian–Nubian Shield.
Along these same lines, Laval and Le Bel
(1986) developed a model for the Al Amar Belt
and the Abt Schist, involving subduction and
collision of volcanic arcs. In order to clarify the
relationships between mineralization (gold and
base metals) and structure, Bokhari and Forster
(1988) suggested a more complex tectonic evolution. They proposed the existence of a prePanafrican oceanic crust (!950 Ma) followed,
from 950 to 720 Ma in a pre-cratonic context,
by the development of volcanic arcs in a marine
environment; the latter period was characterized
by tholeiitic plutonism followed by calc-alkaline
plutonism. Cratonization occurred through accretion of microplates (720–640 Ma). Still according to Bokhari and Forster (1988),
Arabian –Nubian cratonization was followed by
tectonic extension (700– 540 Ma) and marked by
andesitic volcanism leading to molasse sedimentation. They established a parallel with the Tertiary extension of the Basin and Range province
in the USA.
Quick (1991) expanded Kröner’s (1985) model;
he considered the Nabitah Belt to be the main
structure in an assemblage of terranes and proposed extending the model eastward, where he
interpreted the Abt Schist and part of the Murdama formations as an accretionary wedge and
fore-arc basin associated with west-plunging subduction. Windley et al. (1996) identified the
southern extension of the Nabitah Belt in Yemen, where it lies within a succession of arcs
and terranes that correlate with those in Arabia
and the Mozambique Belt.
23
Al-Saleh et al. (1998) held that the eastern
part of the Arabian Shield was subjected to two
major orogenic events, 680 and 600 Ma old,
based on a metamorphic study and Ar40/Ar39
dating. Lescuyer et al. (1994) described a rightlateral ductile fault in the western part of the
Shield, which they attribute to a cratonization
phase.
Numerous radiometric age determinations are
available for rocks and minerals of the Shield.
Johnson et al. (1993) made a first comprehensive
review of these data, and stratigraphic conclusions were drawn later (Johnson, 1996).
The Proterozoic sedimentary formations of the
Arabian Shield have been the subject of many
studies and classifications (Jackson and Ramsay,
1980; Delfour, 1980a). Hadley and Schmidt
(1980) separated them into three depositional
phases; a first phase composed only of volcanic
formations and containing no plutons; a second
phase consisting of sandstone, polygenetic conglomerate, meta-greywacke and marble; and a
third phase of terrigeneous and siliciclastic formations, as well as stromatolitic limestone and
dolomite. Phase 1 is represented by the Baish,
Baha and Jiddah groups, Phase 2 by the Ablah,
Halaban and Murdama groups, and Phase 3 by
the Shammar and Jubaylah groups.
In the context of mineral exploration, numerous gold and base-metal prospects have been the
subject of detailed structural analysis (Sahl,
1993; Koch-Mathian et al., 1994; Genna, 1996;
Récoché et al., 1998a,b). Two mapping and
structural syntheses have been completed on the
Bidah and Shwas volcano-sedimentary belts
(Donzeau and Béziat, 1989; Béziat and
Donzeau, 1989), and a composite metallogenic
map was compiled by Béziat and Bache (1995).
These studies also show local structural evolutions, which are addressed here.
In general, the previous studies dealt primarily
with the ancient pre-Panafrican structures, and
paid little attention to more recent geodynamic
events, contemporary with or later than the formation of the Nabitah Belt. Our study focuses
on these later events. Fig. 2 summarizes the
main stages of the Shield’s structural development, as established from our results.
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A. Genna et al. / Precambrian Research 117 (2002) 21–40
Fig. 2. Chronology of tectonic events in the Arabian Shield,
relationships with the main types of structures.
2. Pre-Panafrican events (!690 Ma)
very thick turbidite succession (Delfour, 1979b;
Delfour et al., 1982). The rocks are more distal
and siliciclastic than the schist in other basins,
which is commonly composed of tuff and cinerite.
This difference led some authors to interpret the
Abt Schist as a product of accretionary wedges
(Laval and Le Bel, 1986).
Volcanic arcs and ophiolitic assemblages typical
of a subduction context indicate that convergence
occurred in an oceanic environment (Camp, 1984;
Laval and Le Bel, 1986; Pallister et al., 1987).
However, few kinematic elements are known for
this first period, and they cannot be extrapolated
to the scale of the whole Shield. The earliest faults
of regional importance seem to be younger than
the basins, and are probably indicative of an
ocean-closure mechanism.
2.1. Basins exhibiting marine affinity
Before formation of the Panafrican Belt, Saudi
Arabia was an area of mostly volcano-sedimentary formations (Fig. 1) deposited in an oceanic
or marginal marine environment associated with
subduction zones, and intruded by granite and
diorite. These are represented by various groups,
the main ones being the Al Ays, Abt, Halaban,
Jiddah, Bahah, Baish, and Hali groups. Several
authors (e.g. Hadley and Schmidt, 1980; Jackson
and Ramsay, 1980; Delfour, 1980a) proposed
classifications that assume two or three major
tectonic events during this first long period. The
basement underlying these early sedimentary formations is either poorly or not at all defined.
Stacey and Hedge (1984) called it Early Proterozoic, but other authors merely considered it as an
‘older basement’ of gneiss and migmatite or ophiolites (e.g. Delfour, 1980a; Jackson and Ramsay,
1980).
The pre-Panafrican formations were subjected
to highly varied degrees of deformation before
deposition of the Panafrican molasse. For example, the Al Ays Formation in the northwest
(Kemp, 1981) was not deformed, whereas the
Jiddah Formation in the south was intensely foliated before the Panafrican Basin molasse was
deposited.
Among the Panafrican Basin deposits, the Abt
Schist constitutes a unit that is exceptional for its
2.2. Closure of the oceanic domain ( " 690 Ma)
The pre-cratonic elements were brought together during a collision that occurred about 690
Ma ago (Johnson, 1996; Fig. 3). The ocean-closure mechanisms are still poorly understood, but
Fig. 3. The Shield divided into terranes according to Johnson
(1998).
A. Genna et al. / Precambrian Research 117 (2002) 21–40
25
same family. Overall, we can recognize two types
of major fault: the first demarcate large blocks
and correspond to major sutures; the second are
faults that developed within a tectonic block, like
those in the Al Amar Belt, and are accompanied
by folds with vertical axes (Ad Dafinah Fault),
boudin shaped quartz veins (Nabitah Fault), or
tension gashes that are variably boudinaged (Abt
Schist). This first deformation was also accompanied by a remobilization of sulphide mineralization along shear zones in the Al Amar Belt, as
seen in the Khnaiguiyah prospect; structural analysis of this prospect provides an example of the
elementary kinematics operating in these shear
zones (Fig. 5).
Fig. 4. Structural pattern of the currently recognized major
faults attributable to the collisional phase in the Arabian
Shield. (a) Chronology of the elementary deformation: the first
phase 1 – corresponds to vertical stretching, the second 2 –
corresponds to dextral strike-slip.
the resultant sutures are marked by ophiolite formations such as Jabal Ess (Shanti and Roobol,
1979), or by ultramafic rocks that are not clearly
defined as ophiolitic. Close analysis of these sutures has provided a few keys to understanding
the phenomenon.
The first deformation to affect the basin sedimentary series was characterized by a very steeply
plunging lineation concentrated in kilometre- to
multi-kilometre-wide corridors whose regional extent is not everywhere clearly defined (Fig. 4).
Several faults are attributable to this deformation
phase, such as the Ad Dafinah Fault, the fault
bounding the Abt Schist to the east, the Nabitah
Fault, and various transverse faults in the Al
Amar Belt; many other shear zones that are visible on satellite photos probably belong to the
Fig. 5. The Khnaiguiyah prospect (Cu, Zn). (a) Location; (b)
General structural setting of the study zone, showing the
mineralized zone and foliation traces; (c) Stereographic projection of the stretching lineation (Schmidt, lower hemisphere);
(d) Kinematic interpretation.
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A. Genna et al. / Precambrian Research 117 (2002) 21–40
Fig. 6. Main structures attributable to the Nabitah deformation phase, and molasse basins.
3. The Panafrican orogeny
In Saudi Arabia, the Panafrican orogeny is
represented by a complex web of orogenic zones
arranged in an anastomosing network of primarily strike-slip faults. It includes the Nabitah Belt
(Quick, 1991), which is the central part of the
structure, and several peripheral ranges. Note that
this distribution (‘Orogenic belts’ in Fig. 6) does
not correspond completely to the configuration of
orogenic zones proposed by Stoesser and Camp
(1985), based on the distribution of ultramafic
complexes.
3.1. The Nabitah Belt
The Nabitah Belt (Quick, 1991) is oriented
generally north–south (Fig. 6) and divides the
Shield in two. For descriptive purposes, the belt
can be divided into an inner zone and an outer
zone, both with distinct structural features. The
inner zone, characterized by sigmoid fish-shaped
shear zones whose geometry is clearly visible on
satellite photos, is composed of large, pre-existing
intrusions and sedimentary formations that have
undergone compressional deformation. The outer
zone is characterized by lateral slip along the
margin, reflected by the presence of transpressional gneiss domes.
The intrusive complexes of the inner zone of the
Nabitah Belt are primarily batholiths that acquired a sigmoidal shape during Panafrican deformation. The most striking on the satellite images
are the Furayhah Batholith (Kemp et al., 1982)
and the Al Bara Batholith (Letalenet, 1979), both
in the central part of the Nabitah Belt. The An
Nimas Batholith in the southern part of the Shield
(Prinz, 1983; Greenwood et al., 1986) shows a
more complex evolution, with subsequent
deformation.
The large Murdama Basin, located east of the
Nabitah Belt (Letalenet, 1979; Delfour, 1979b),
displays increasing degrees of deformation and
metamorphism from east to west. To the west, it
is bounded by gneiss domes with foliation folds
that have curved axes and merge progressively
southwestward into the Nabitah structures and
northeastward into the folds of the sedimentary
basin. One of the domes, Jabal Kirsh, has been
extensively studied from the standpoint of both
geologic mapping (Delfour, 1979b, 1980b) and
mineral exploration (Genna, 1996). Fig. 7 shows
the general structure of the dome. The geometric
and kinematic relations between shearing (C), foliation/schistosity/cleavage (S), and stretching lineation (L) are illustrated in Fig. 7f; they are valid
for our entire study. Boudinaging at all scales in
the gneiss is shown in Fig. 8.
Many faults cut the Nabitah Belt, the largest
being the Jabal Tin Fault (Fig. 9). This is marked
by a band of NW– SE-trending gneiss whose
width varies from 25 km in the northwest to 10
km in the southeast. The Jabal Tin exposures
(Fig. 9b–e) illustrate the transpressional context
of the deformation, with partial melting and the
emplacement of leucogranite veins as shear planes
during the last stages of deformation. The shear
A. Genna et al. / Precambrian Research 117 (2002) 21–40
direction is indicated by the late deformation of
cleavage seen in the horizontal (Fig. 9d) and
vertical (Fig. 9e) planes. As in the northwestern
part of the Shield, the relationships between foliation and microshearing indicate a left-lateral sense
of shear for the whole of the Tin Complex.
3.2. The peripheral ranges
The Panafrican chain of Saudi Arabia is composed of an axial part, the Nabitah Belt, and
several parallel features called the peripheral
ranges. The latter are punctuated with gneiss
domes, which are common in the Proterozoic
basement of Saudi Arabia; for example, Johnson
27
(1998) shows six major gneiss structures in his
tectonic map of Saudi Arabia. The domes exhibit
left-lateral transcurrent tectonism along the socalled ‘Najd’ faults and right-lateral movement
along conjugate structures. The whole unit forms
a network of anastomosing faults within the major orogenic zone known as the Nabitah Belt
(Quick, 1991) and its adjacent structures.
Analysis of several anticlinal gneiss structures
shows that they are composed of ortho- or paraderived formations. A thoroughly studied sector is
the northwestern part of the Shield (Grainger and
Rashad Hanif, 1989; Davies, 1985; Hadley, 1987;
Pellaton, 1982; Kemp, 1981; Pellaton, 1979; Fig.
10), where satellite imagery reveals a unique struc-
Fig. 7. The Jabal Kirsh gneiss dome. (a) and (b) Location; (c) NE –SW section; (d) Stereographic projection of S/C (foliation/shear)
relationships in the Jabal Kirsh gneiss (Schmidt, lower hemisphere); (e) General organization of second-order folds in the axial part
of the dome near the Jabal Kirsh prospect (kyanite); 1 – Murdama Basin, 2 – foliation folds, 3 – Nabitah Belt, 4 – anticlinal axis,
5 – synclinal axis; white background: gneiss of the Kirsh dome; (f) Block diagram showing the relationship between foliation (S),
shearing (C) and stretching lineation (L) for the Jabal Kirsh dome.
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A. Genna et al. / Precambrian Research 117 (2002) 21–40
lineation parallel to the fold axes; it is subhorizontal over most of the dome and plunges south in
the southern part, at the same angle as the dome
itself. Kinematic deformation criteria, based on
S/C plane relationships, indicate left-lateral shear.
Fig. 8. Modes of deformation in the Jabal Kirsh Dome
(boudinage). (a) Outcrop drawing; (b) Composite block diagram at the scale of a second-order fold; (c) Stereographic
projection of the stretching lineation (Schmidt, lower hemisphere).
tural arrangement comprising an anastomosing
network of planar structures that demarcate large
fish-shaped units (Fig. 10b) that are outlined by
metamorphic foliation. Measurements revealed
the existence of a network of ductile left-lateral
transcurrent faults with occasional right-lateral
faults. Numerous domes, four of which have been
studied in detail, mark the network.
The Hamadat anticlinorium is a huge gneiss
dome (Pellaton, 1982; Kemp, 1981; Fig. 10)
stretching 100 km in a NW– SE direction. It
consists of folds that are strongly pinched to the
north and more open to the south. The entire
structure is marked by a well-developed stretching
Fig. 9. Jabal Tin example. (a) Location; (b) General structure
of the Tin Complex; (c) E – W section through the Tin Complex; (d) Outcrop drawing (plan view showing left-lateral
shearing of foliation, contemporaneous with emplacement of
leucogranite veins); (e) Outcrop drawing (cross section, showing compressional aspect of the Jabal Tin Dome); (f) Composite stereograph of the S/C (Foliation/Shear) relationships
and the stretching lineation (L) in the Jabal Tin Dome
(Schmidt, lower-hemisphere); 1 – Tertiary basalt, 2 – Bani
Ghayy group, 3 – gneiss of the Tin Complex, 4 – pre-Bani
Ghayy formations, 5 – fault, 6 – thrust, 7 – cleavage.
A. Genna et al. / Precambrian Research 117 (2002) 21–40
29
Fig. 10. Panafrican structural development in the northwestern part of the Shield. (a) General structural pattern; (b) Satellite image
survey of the metamorphic and kinematic foliation trace of the major faults (stereographic projection of S/C relationships, Schmidt,
lower hemisphere); 1 – Paleozoic cover, 2 – molasse basin, 3 – wedge effect in the Al Ays region, 4 – synclinal axial trace, 5 –
anticlinal axial trace.
The Wajiyah anticlinorium (Kemp, 1981; Fig.
10) bounds the Hadiyah Basin to the north. It
exhibits the same structural characteristics as the
Hamadat anticlinorium, but with less clear-cut
boundaries and no obvious direction of plunge.
The shear direction is left-lateral. Relationships
with the sedimentary formations of the Hadiyah
Basin are clear and it is possible to determine that
the intense, constrictive deformation advanced
through the basin from north to south. This analysis is based on observations of the stretching
lineation intensity in both sedimentary and intrusive rocks.
The Baladiyah Complex, located southeast of
the Thalbah Basin (Davies, 1985; Fig. 10), is
exposed over 20 km along a north– south axis. It
has a very regular geometry that is particularly
apparent on aerial photos. A subvertical to very
steeply dipping foliation, which, at its northern
end, closes into a north-plunging dome axis,
marks most of the structure. The complex also
displays a well-developed subhorizontal stretching
lineation parallel to the dome axis, again northplunging at the northern end of the structure.
Amphibolite-facies metamorphism affected the
central part of the dome (Davies, 1985), which
also exhibits a loss of foliation that expresses
planar deformation, in favour of a more developed lineation, indicating a primarily constrictive
environment of the deformation.
The Qazaz Complex, located east of the Thalbah Basin (Davies, 1985; Fig. 10) is generally
antiformal with several second-order folds. In the
northern part, the foliation forms an arch with a
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A. Genna et al. / Precambrian Research 117 (2002) 21–40
NW – SE axis. The upper part of the structure
consists of orthogneiss marked by a well-developed stretching lineation parallel to the dome
axis. The core of the structure comprises mica
schist (amphibolite facies) with two foliations: a
first foliation is folded isoclinally along subhorizontal axial planes and a second one forms the
anticlinal geometry of the dome. At outcrop scale,
the second foliation forms the axial plane of the
first-foliation folds, but at the scale of the dome, it
forms the general anticlinal geometry. The northern ‘tail’ of the structure consists of amphibolitefacies gneiss (Davies, 1985) marked by left-lateral
shear.
A comparative analysis of these domes, which
reflect transpression in a domain of ductile deformation, has enabled us to develop a general genetic model for this type of structure, as
summarized in Fig. 11. In this model, the lower
part of the structure would show subvertical
metamorphic foliation with a slight horizontal
lineation, the central part would be a zone of
intense constriction, and the top would show subFig. 12. The molasse basins of the Nabitah orogeny. (a)
Location; (b) Cross section of the Hadiyah Basin; (c) Cross
section of the northern Thalbah Basin; (d) Cross section of the
southern Thalbah Basin; (e) E – W cross section of the Murdama formations in the Junaynah Belt; (f) Cross section of the
main Murdama Basin. 1 – molasse basin, 2 – granite, 3 –
gneiss-dome foliation
horizontal foliation and a poorly developed subhorizontal lineation. The consequences of this
model are, for example, that the kyanite-bearing
quartzite at Jabal Kirsh represents the exhumation of deep metamorphic facies and that the
Hamadat anticlinorium represents the onset of
gravitational processes lateral to the dome.
4. The molasse basins
Fig. 11. Theoretical block diagram showing the evolution of
the strain ellipsoid at the core of a gneiss dome. The transpressive setting causes simple-shear deformation to dominate over
pure-shear deformation.
Several molasse basins collected the sediments
derived from the progressive erosion of the
Nabitah Belt (Fig. 12). Among these are the large
foreland basin and intramontane basins associated with the peripheral ranges.
A. Genna et al. / Precambrian Research 117 (2002) 21–40
4.1. The foreland basin
The foreland basin is the large north– southtrending Murdama Basin to the east of the
Nabitah Belt (Letalenet, 1979; Delfour, 1979b;
Fig. 12f), of which only the marginal sedimentary
deposits were caught up in the deformation. The
basin is cut by granitic intrusions whose ages vary
from 650 to 530 Ma (Béziat and Bache, 1995).
The sedimentary formations of the Murdama
group consist primarily of sandstone from the
underlying Afif Formation. At the basin’s western
margin (Letalenet, 1979), the base of the series is
marked locally by rhyolite flows and polygenetic
conglomerate; argillaceous or conglomeratic intercalations and greywacke are present in the succession. At the eastern margin (Delfour, 1979b), the
series begins with a conglomerate, more than 200
m thick (the Hibshi Formation) overlain by a
complex formation (the Farida Formation) of
carbonates, sandstone, conglomerate, and stromatolitic rocks. A 10 000-m-thick sandstone formation occupies the central part of the basin.
4.2. Intramontane basins associated with the
peripheral ranges
The Thalbah Basin (Davies, 1985) consists of
two main structural units in the south (Fig. 12d)
and a single basin in the north (Fig. 12c) that are
filled with deposits of three distinct formations
(Hashim, Ridam and Zhufar) separated by discontinuities. The oldest of these formations, the
Hashim Formation, comprises a 50– 300-m-thick
basal conglomerate, overlain by a 1000-m-thick
succession of sandstone, siltstone, and intraformational conglomerate. This was transgressed from
east to west by the Ridam Formation that, in the
centre of the basin, is as much as 1000 m thick
with a thick basal conglomerate. The Zhufar Formation lies discordant on the Ridam Formation
and ranges in thickness from 600 m in the west to
1400 m in the east.
These sedimentary formations also show significant structural and metamorphic variation from
west to east. In the west, the succession shows
little folding and is monoclinal with an eastward
dip of 5 – 15°. In the east, in contact with the
31
gneiss, the succession is intensely folded, almost
vertical, and displays a subhorizontal stretching
lineation; it also shows significant vertical variation in its deformation. A left-lateral shear zone
with characteristics identical to those of the
Panafrican shear zones is observed in the basement rocks of the southeastern part of the basin
(Johnson and Offield, 1994). A simple fold in the
Thalbah sedimentary formations prolongs this
fault.
The Hadiyah Basin (Pellaton, 1979; Kemp,
1981), at the eastern margin of the Al Ays range
(Fig. 10), consists of a single main structural basin
that is folded and overturned to the east (Fig.
12b). The fill comprises three elementary megasequences, the first (Siqam Formation) being primarily volcanic and the other two (the Tura’ah
and Aghrad formations) being entirely sedimentary. They each have a thick basal conglomerate
and show significant variations in thickness; the
maximum thickness of the three formations taken
together is about 7000 m. As with the Thalbah
Basin, the deposits of the Hadiyah Basin show
significant variations in deformation style and
metamorphic grade. In the west, folds are upright
and fairly compressed, whereas in the east they
are less closely spaced and east-verging. Similarly
from south to north, the sediments pass from
undeformed, with perfectly preserved sedimentary
structures, to intensely deformed in the greenschist facies, with a well-developed subhorizontal
stretching lineation identical to that found in the
neighbouring gneiss.
The molasse basins in the northwestern part of
the Shield show varying degrees of deformation.
Those to the west reveal no significant deformation and were never buried (no vertical or burial
foliation, and incomplete lithification in places).
Those to the east are metamorphosed (greenschist
facies or higher), intensely deformed, and display
the same well-developed horizontal lineation as
the basement and nearby gneiss domes. These
basins thus appear to be post-tectonic in the west
and pre-tectonic in the east. This observation,
which is supported by a geometric analysis of
their sedimentary fill, leads us to consider them as
syntectonic and as reflecting the topographic variations that accompanied this stage of deforma-
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A. Genna et al. / Precambrian Research 117 (2002) 21–40
tion. Several sedimentary megasequences thus
show unconformable relationships, induced by
subsidence that was the result of crustal flexuring.
The southwestern part of the Shield is characterized by the sedimentary molasse formations of
the Ablah Basin (Greenwood, 1985a,b; Prinz,
1983; Carter and Johnson, 1987), a deformed
sedimentary basin between the granites of the
Shwas Pluton and the Thurat Batholith. In its
northern part, the basin is generally monoclinal
with an easterly dip, and has a fold train parallel
to its axis; it shows little deformation and no
metamorphism. In the east, the basal deposits
comprise a coarse conglomerate, several hundred
metres thick and in places dipping almost vertically along a subvertical boundary fault. In the
west, the basal series is composed of gently dipping tuff and basic lava lying discordantly on the
Thurat Pluton. The asymmetry marked by the
difference in basal facies to the east and west
indicates that the basin was initiated by an eastward tilt of its substratum.
Above the basal deposits, the fill consists of
siltstone, sandstone, and conglomerate, with interbedded volcanic flow deposits. The folded
rocks show a general eastward dip, reflecting the
progressive tilting of the basin during its formation. This configuration is similar to that of the
Thalbah Basin (Davies, 1985), which developed
along a left-lateral strike-slip fault of the Najd
system during the Panafrican tectonism in the
northwestern part of the Arabian Shield (Delfour,
1979a).
Deformation in the Ablah Basin intensifies
southward as the basin deposits thicken. A vertical foliation appears in the fold hinges, at first in
the carbonate layers at the top of the succession
and then in the other rock types. The folds become tighter and their axes become sigmoidal.
The movement of these fold axes in a transpressive and rotational setting has caused a new deformation with a sigmoidal aspect. Still farther
south, the deformation intensifies and the metamorphic grade reaches amphibolite-facies conditions (Greenwood, 1985a,b), so it is difficult to
determine the initial structure of the sedimentary
rocks.
The Ablah Basin sediments are dated at 642# 1
Ma (Genna et al., 1999), i.e. contemporaneous
with the Murdama basins in the north of the
Shield, and lie discordantly over the Jiddah group
that was affected by subvertical foliation before
their deposition. According to Donzeau and
Béziat (1989), the basin underwent two phases of
deformation, the first with right-lateral shearing,
and the second with left-lateral shearing.
The Junaynah Fault corresponds to an intramontane trough in our model. The trough corresponds to a synclinal structure in the
anticline– syncline succession. It is marked by the
sedimentary formations of the Junaynah Belt
(Greene, 1993), which are of Murdama age (Simons, 1988). Most likely highly eroded, they were
folded and foliated (Fig. 12e) during a single
phase of deformation. These formations display a
subvertical foliation that is axial planar with respect to the single large crustal fold that forms
each part of the basin. Relationships between the
foliation planes and shear planes clearly indicate a
left-lateral sense of shear with a subhorizontal
stretching lineation. Note that this shear occurs
on a north– south fault, and that faults with this
orientation in the Shield are generally right-lateral. We can thus assume that the regional direction of shortening either varied from north to
south in the belt, or varied over time. If it varied
over time, the question is whether the deformation propagated from north to south or from
south to north.
The Murdama formations of the Junaynah
Basin are discordant over the volcanic and volcano-sedimentary succession of the Halaban
group to the east and also over some older
granitic formations. The basal basin fill is a conglomerate that varies from a few to 300 m in
thickness and that generally contains clasts of its
nearby substratum (Halaban group and granitic
formations), providing evidence of correlative tilting of the basement during the filling of the basin.
The entire succession can be up to 800 m thick
and consists primarily of sandstone and siltstone,
in sequences several metres thick, with thin intermediate arkose and conglomerate. An east– west
cross section through the west branch of the basin
(Fig. 12) reveals a very asymmetric syncline.
A. Genna et al. / Precambrian Research 117 (2002) 21–40
There is no boundary fault to the west, and the
faults along the eastern margin are reverse strikeslip. Deformation was single phased. The presence
of a subhorizontal stretching lineation suggests, as
with the Murdama basins in the northern part of
the Shield, a left-lateral transpressional mode during basin development.
5. Formation kinematics of the Panafrican
structures
The gneiss domes described above fit into a
left-lateral kinematic context of deformation
(Figs. 7–10). They are contained within the
Panafrican Belt in Saudi Arabia, whose axial zone
is the Nabitah Belt, consisting primarily of granite
and granodiorite batholiths (Johnson, 1983)
whose geometry, generally sigmoidal, is consistent
with the formation kinematics of the domes.
The left-lateral faults of this belt are oriented
NW–SE, with conjugate N– S to NNE – SSW
right-lateral faults also being observed. In the
northwest of the Shield, a gneiss shear zone with
a well-developed stretching lineation (Fig. 10) has
kinematic criteria of the S/C planes that indicate
right-lateral movement. In the Samran Belt
(Genna, 1994; Bellivier et al., 1998), right-lateral
faults that developed in the ductile–brittle transition were synchronous with the emplacement of
gold mineralization in the chronology of regional
tectonic deformation; in the Nabitah Belt, the
north–south trend of the pre-Murdama Batholith
margins correlates with the same structural position. In the southwest of the Shield, the main
ancient volcano-sedimentary belts and the large
batholiths underwent right-lateral transcurrent
tectonism, the sedimentary markers of which are
the Ablah formations (Donzeau and Béziat,
1989).
Obvious relationships exist between the gneiss
domes described above and the adjacent sedimentary formations of Murdama age. Sedimentological and structural analyses of these basins
(Davies, 1985; Kemp, 1981; Pellaton, 1979; Camp,
1986) reveal that they were formed during an
active tectonic period, with the basins occupying
the synclinal depressions and the domes represent-
33
ing the anticlines of the same fold train. The
synclines thus formed were filled by discontinuous
detrital sedimentation, consisting of several
megasequences, each with a base of thick conglomerate grading upward into sandstone and
siltstone. Several unconformities, caused by
synsedimentary tectonics, separate these megasequences. The basal conglomerates of each megasequence thus cover schistose and deformed rocks
of the preceding one. The total thickness of the
basin sediments was about 10 000 m.
6. The post-Nabitah tectonism
The end-Proterozoic formations in Saudi Arabia indicate that the crustal thickening brought
about by the formation of the Panafrican Belt was
followed by an episode of crustal thinning. This
was expressed by extensional deformation with
contemporaneous bimodal magmatism, later followed by a return to marine sedimentation (Fig.
Fig. 13. Composite structural framework of post-Nabitah
extension in the Arabian Shield. T: axis of the Tabuk Basin, J:
substratum axis of the Jeddah Basin, W: axis of the Widyan
Basin, RK: axis of the Rub al Khali Basin. 1 – normal fault,
2 – strike-slip fault, 3 – gravitational slide, 4 – presumed
boundary of the basin substratum, 5 – axis of the inferred
basin substratum.
34
A. Genna et al. / Precambrian Research 117 (2002) 21–40
13), after a long post-Panafrican period of nondeposition. The volcanic activity was associated
with various intrusive complexes and dike swarms
dated between 530 and 590 Ma. The extrusive
formations are known collectively as the Shammar group. Listric or gently dipping faults are
interpreted as the traces of gravitational sliding in
the uppermost part of the crust. The geometry of
these normal faults and dike swarms indicates
extension in multiple directions. The subsidence is
also marked by more complex geometry of inferred basements of the intracontinental or marine
basins induced by this deformation.
Thinning was governed by a system of transform faults, known as the ‘Najd faults’, which
also controlled the formation of the Jibalah basins
in which the Shammar generally makes up the
basal formations of the sedimentary fill. This extensional episode induced a marine transgression,
evidenced by the carbonate platforms of the
Jibalah basins at various places in the Shield.
Continuation of the thinning process may explain
the deposition of the marine formations of the
Palaeozoic cover. The Late-Proterozoic crustal
thinning caused crustal heating through the emplacement of intrusions and a concomitant rise of
the isotherms. This crustal warming instigated a
major metallogenic event in the Arabian– Nubian
Shield during the Late-Proterozoic extension.
Our results are consistent with recent studies
proposing Late-Panafrican extension mechanisms
in the Arabian–Nubian Shield of the Sinai Peninsula (Blasband, 1999; Blasband et al., 2000) and
Egypt (Renno and Stanek, 1999a,b). Their models
are based primarily on studies of magmatic core
complexes and of the structure of the terranes.
A significant phase of peneplanation then
eroded the belt, revealing a variety of structural
layers before deposition of the discordant Ar
Rayyan and Jurdhawiah conglomerates.
6.1. Magmatism associated with crustal thinning
The post-Panafrican intrusive complexes show
a great variety of composition and shape. Their
detailed geometry is presented in a recent aeromagnetic synthesis (Asfirane et al., 1999). Felsic
intrusions are circular, elongate, or raindrop-
Fig. 14. General distribution of dike swarms and late intrusions in the Shield. 1 – Recent formations, 2 – Tertiary basalt,
3 – intrusive rock, 4 – fault, 5 – dike swarm.
shaped, whereas the mafic intrusions generally
form sills. Moreover, the presence of concealed
batholiths is shown by the dike swarms and ring
dikes they produced, which give an indication of
their general shape. Ring complexes and calderas
are commonly associated with the batholiths.
This magmatic activity was bimodal, being represented mainly by granite and gabbro in the
plutonic bodies, and by rhyolite and basalt in the
extrusive formations of the Shammar group. The
mafic igneous rocks are mainly concentrated in
the northern part of the Shield. In the northwest
(Pellaton, 1979; Kemp, 1981), gabbro and diorite
cut the Hadiyah group molasse deposits, which
are the local representatives of the Panafrican
molasse deposition. In the northeast, gabbro cuts
the Jurdhawiyah Formation (Cole, 1988) and listwaenite is present in the thrusts that we believe
correspond to listric gravitational-detachment
faults that were tilted by late isostatic phenomena,
as in the model set forth by Lister and Davis
(1989) for the Tertiary structures of the USA.
A. Genna et al. / Precambrian Research 117 (2002) 21–40
Multiple generations of dikes (Fig. 14) were
emplaced in the Arabian Shield after the
Panafrican compression. They tend to occur in
linear or curved swarms and vary from a few
centimetres to several tens of metres in thickness
and from several metres to several tens of kilometres in length. They can also form unidirectional
or bidirectional (perpendicular) swarms. In the
latter scenario, the two directions correspond either to the same lithology or to different lithologies. They can also have a ‘cracked glass’
appearance, and some swarms are temporally superposed. The distribution of the dikes relative to
the fault systems is not random. We have observed that they tend to be parallel to the late
faults and in places indicate deviatoric stresses in
bifurcations or fault relay zones. They also indicate the directions of opening in fault networks
with horsetail arrays. They form a circular pattern
around late intrusions, calderas, and the Shammar basins in the northern part of the Shield.
For our structural analysis, we drew up a composite map of the dikes and used them to reconstruct the stress states during the phase of
extension, knowing that the orientation of the
opening plane indicates the minor principal stress
component sigma 3. Other structural elements
thought to be contemporaneous with the dikes,
such as the displacement of the associated faults
and the geometries of the correlative extrusive
formations, were also used to constrain the basic
kinematic processes.
The dikes are generally subvertical with very
diverse orientations. They are commonly rhyolitic, and are also found in swarms of bimodal
composition (Camp, 1986). Their role in the successive tectonic events has never been clearly
stated, even though they were very thoroughly
surveyed during mapping for the 1:250 000-scale
coverage of the Shield. However, the dikes that
were mapped are generally the youngest ones
(530–590 Ma), as they are not folded, are less
weathered, do not blend with the country rocks
through metamorphism, and are more easily discernable on aerial photos. The oldest proposed
ages tend to depend on the age of the country
rocks, but direct dating has also been done. For
example, in the southern part of the Shield, the
35
dikes were emplaced between 519 and 587 Ma
(Greenwood, 1985a,b).
6.2. Proposed model for the post-Nabitah
extension
Our observations on the Arabian Shield must
be compared with the studies made on the evolution of the deformation and metamorphic conditions in the Proterozoic formations of the Sinai
(Blasband, 1999; Brooijmans, 1999; Blasband et
al., 2000), which comprise the northern extension
of the Arabian–Nubian Shield. These studies revealed a NW–SE extension (Blasband, 1999) associated with granites and dikes (590– 530 Ma)
and HT-LP metamorphism (Brooijmans, 1999)
between 600 and 530 Ma. Farther west, in the
Egyptian desert near Marsa Alam (Renno and
Stanek, 1999a,b), circular post-orogenic intrusive
complexes exhibit bimodal compositions in a
structural compartment bounded by two of the
Najd faults.
A simple geologic evolution model can be developed that accounts for all the structural elements described above. In this model, summarized
in Fig. 15, thinning was penetrative on a crustal
scale and was accompanied by bimodal magmatism instigated by crustal melting and influxes of
mantle-derived material that could have been
transported via major faults. This resulted in the
emplacement of complex intrusive suites and associated dike swarms. Magmatism was controlled
by subvertical transform faults (Najd faults) that
initiated the formation of narrow, deep basins,
Fig. 15. Composite model of post-Panafrican crustal thinning
in the Arabian –Nubian Shield. 1 – Mafic igneous rocks, 2 –
felsic igneous rocks, 3 – dikes, 4 – syn-rift deposits, 5 –
post-rift deposits.
36
A. Genna et al. / Precambrian Research 117 (2002) 21–40
and which were also injected by dikes. The dikes
fed the flows that were deposited in the bottom of
the basins. Dikes and mafic sills were emplaced at
various levels in the structure and initiated or
intruded local gravitational slides. The generally
NE– SW opening direction of the dykes is compatible with the sliding direction of gravity units,
as deduced from micro-tectonic analysis.
Note that these phenomena, which continued
up until 530 Ma ago (i.e. into the Paleozoic), led
to the transgression of the Jibalah basins. Sedimentary carbonate platforms (Basahel et al.,
1984) were undoubtedly the precursors of the
initial geometry of the cover basins. Three large
structures were preserved through the Paleozoic
and may represent the continuation of this extensional event: the Tabuk and Widyan basins in the
north, with Saq Sandstone at the base, and the
Rub al Khali Basin to the south that contains
Wajid Sandstone and Eocambrian salt formations
(Faqira and Al-Hauwaj, 1998). The ‘Jeddah
Basin’ (Fig. 13) probably is another unit of this
assemblage, marked by the Fatima carbonate formation (Basahel et al., 1984).
7. Metallogenic implications
This new description of the chronology of geodynamic events in the Arabian Shield has significant implications for our understanding of the
mineralizing events that produced the gold and
base-metal prospects and deposits in the Shield.
We can distinguish three types of mineralization,
corresponding to three main phases of structural
development: (i) mineralization emplaced before
the formation of the Nabitah Belt, (ii) mineralization contemporaneous with the Nabitah Fault
activity, and (iii) mineralization postdating the
belt that was emplaced during crustal thinning.
Fig. 16 gives an example of each type of
mineralization.
8. Chronology of structural events and conclusions
A structural analysis of Saudi Arabia’s various
Proterozoic sedimentary formations and their
Fig. 16. Examples of gold and base-metal mineralization emplaced during the main structural events of the Arabian Shield.
(a) Rabathan sedimentary massive sulphides, emplaced prior
to formation of the Nabitah Belt (Wadi Bidah Belt; KochMathian et al. 1994). (b) Shayban hydrothermal prospect,
emplaced during the Panafrican orogeny (Samran Belt; Genna
1994); gold is part of the hydrothermal mineralization emplaced along the faults. (c) Gold mineralization emplaced in
subhorizontal faults of the Mohsiniyah prospect during crustal
thinning (Silsilah District; Récoché et al., 1998a,b, Schmidt,
lower hemisphere).
substrata, provides a new tectonic synthesis whose
principal events are outlined in Table 1.
The first discernable events in the Shield were
the development of marginal basins and volcanic
arcs typical of the pre-Panafrican environment,
but the exact kinematic processes involved in their
formation cannot be discerned at present because
A. Genna et al. / Precambrian Research 117 (2002) 21–40
37
Table 1
Composite table showing relationships between the tectonic phases and sedimentary formations
Tectonic phase
Primary structures
Sedimentary formation
Structural
context
Crustal thinning, 590–530 Ma
Najd (transform) faults, normal
faults, tilt blocks, associated basins
Domes and basins, right- and
left-lateral shear zones
Shear zones, ophiolitic sutures
Shammar, Jibalah, Ar Rayyan,
Jurdhawiah, Fatima
Murdama, Hadiyah, Thalbah,
Ablah, Furayh, Ghamr, Junaynah
Intracontinental
extension
Convergence
Marginal basins, volcanic arcs
Al Ays, Halaban, Jiddah, Bahah,
Baish
Nabitah orogeny, 690–590 Ma
Closing of the oceanic domain,
around 690 Ma
Pre-Nabitah structures, before
690 Ma
of their deformation; moreover, the mechanisms
that closed the oceanic domains remain unknown.
It is thought that these events were followed by a
phase of erosion that probably was not homogeneous throughout the Shield. Next came the
Panafrican tectonism, the major cratonization
event of the Shield that produced gneiss domes
and molasse basins over a system of intracontinental shear zones. The resultant belt was then
subjected to gravitational events (late-orogenic extension and crustal thinning) controlled by the
Najd transform faults that gave rise to the Shammar volcanic formations and the Jibalah detrital
formations. Widespread erosion caused gradual
peneplanation of the Shield and crustal thinning
instigated a late marine transgression represented
by the Jibalah carbonate platforms.
This synthesis, which uses all currently available geologic data, is a new overall vision of the
tectonic evolution of the Arabian Shield. It has
direct implications for metallogenic models dealing with the emplacement of the stratiform or
discordant sulphide mineralization (gold and base
metals), because it proposes a new chronology of
mineralizing events.
The consequences we foresee from this study
are primarily a better understanding of the genesis
of the gneiss domes and the exhumation of highgrade metamorphic facies, as well as of the associated intracontinental sedimentary basins. This
paper is a contribution to the overall understanding of the Arabian– Nubian Shield and the
Panafrican Belt.
Collision,
obduction?
Oceanization,
subduction
Acknowledgements
The authors are grateful to Dr M.A. Tawfiq,
Chairman (Acting), Saudi Geological Survey, and
F. Le Lann (Director, BRGM Saudi Arabia), for
their help and permission to publish this paper.
We thank M. Sahl, J.M. Eberlé, J.M. Leistel, I.
Salpeteur, and D. Thiéblemont for reviews and
constructive discussions, and the BRGM Translation Service for translation and editing. This is
BRGM Scientific Contribution No. 42.
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