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The Western Arabia rift system
A. M. Quennell
SUMMARY: The origin of the Dead Sea rift has generally been linked with that of the
Red Sea, the widening of the latter involving left-lateral strike-slip on the rift. Its
extension through the Lebanese fold belt has been denied by some because of the absence
of linear and displacement continuity. Others, however, have been unable to see any other
choice in spite of these difficulties, but a satisfying structural model has not yet been
offered. The present author claims that the rift system is a transform plate boundary
between the Arabian plate and the Sinai-Levant plate, but without uniformity along its
length.
It is here suggested that during the Lower Miocene the Arabian planation surface,
which before the opening of the Red Sea was the probable extension of the African
mid-Tertiary surface, was ruptured by the first phase of movement on the west Arabian
transform fault system, which probably reflected a geosuture zone in the continental plate
and was originally a simple arc. The Arabian plate moved northward with the opening of
the Red Sea and the closing of the Bitlis ocean, leaving behind the Sinai-Levantine part of
the African Plate. Oblique compression on the northern Syria unstable platform across the
East Anatolian transform fault caused dextral distortion on the Lebanon-Palmyra zone to
create mountain ranges of two styles and a translation in the alignment of the rift segment.
This inhibited further uniform strike-slip movement on the rift and displacement on the
Yammoun6 fault was subsequently predominantly vertical. The movements on the rifts to
the south and the north thus became independent. There was Arabian plate lithosphere
consumption north-east of the Houl6 depression to accommodate the second phase of rift
movement. On the basis of this explanation, the vexed question of transmission of left
lateral strike-slip across the Lebanon segment does not arise.
Lartet in 1869 recognised the Red Sea as having
been formed by the separation of Arabia from
Africa, accompanied by complementary shear
movement along the Gulf of Aqaba and the
D e a d Sea rift system. The description accepted
without much question has been an asymmetrical unilateral rift, a left-lateral strike-slip fault
zone. The question of how far north the rift
extended was not given much attention.
Dubertret (1932) accepted Lartet's model as
a working hypothesis. The formation of the
R e d Sea presented no problem to him at this
stage, and he carried the rift northward via the
faults and folds of the Lebanon as far as the
Taurus fold belt. However, with more extensive
mapping of the L e b a n o n and Syria and especially after having accepted the thesis of Drake
& Girdler (1964) that the Red Sea was floored
by continental crust, he abandoned his view
(1970) but realised that there is no simple path
for the Dead Sea rift beyond the Houl6 depression. However, he did propose a possible preCenomanian movement related to the Roum
fault. Dubertret had an intimate and detailed
knowledge of the geology of the Lebanon
second to none.
Quennell (1958, 1959) revived the hypothesis
abandoned by Dubertret and quantified the
major shift along the Dead Sea rift segment of
the rift as having taken place in two stages,
6 2 k m pre-Miocene and 45 km in Pleistocene
times. H e related more than ten h o m o l o g o u s
structural and stratigraphical features, and
noted the displacement of the northern coast of
the R e d Sea and the opening of the Dead Sea as
a rhomb-graben in two stages. He foresaw
difficulty in the disposal of the excess crust
arising from the 107 km displacement.
Dubertret's conclusion regarding extension
through the Lebanese folds was ignored by
Freund et al. (1970) who held that a solution
could be found in accepting the Yammoun6 and
Gharb faults as the northern extension of the
rift system. They matched not only occurrences
and structure as Quennell had done, but also
columnar sections. However, their endeavour
to extend this technique beyond the n o r t h
Jordan valley to match Lebanese features such
as Mt. H e r m o n and to solve the divergences in
direction and anomalies in amount of displacement for the Yammoun6 and the Gharb faults,
is not convicing. To account for the bend of the
Y a m m o u n 6 fault they introduced oblique and
lateral shift of the bordering blocks. They also
proposed to use a suggestion of offset of
ophiolites in the far north to account for some
of the total strike-slip movement.
In his 1958 and 1959 papers, Quennell did no
more than speculate about the northern exten,
sion but assumed as others had done, that the
fault path would be found to be the faults and
folds in the Lebanon.
775
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776
A. M.
The rift system
The rift system extends from the Red Sea
speading zone to the plain of Antioch where it
is accepted as joining the Border Z o n e (Fig. 1).
There are three segments: R e d Sea to Lebanese
border; across the Lebanese fold belt; from the
northern Lebanese border to Turkey.
Gulf of Aqaba, Wadi Araba and Jordan
Valley
The first segment is d e t e r m i n e d by the circular arc of the boundary between the Arabian
and Sinai-Levant plates. The reality of this
arcuate trace of a generally concealed leftlateral strike-slip fault is well established: by
photogeology and field geology in West Araba
and the Jordan valley (Quennell, 1955--6,
1959); in Wadi Araba by mapping of sediments
(Bender, 1974b, 1982); and from Landsat imagery (Fig. 2). There is rarely more than minor
Quennell
departure (1 or 2 k m ) from the arc which was
described by trial and error originally on the
scale of 1:250,000. The pole of rotation was
fixed at Lat. N. 33; Long. E. 24 (Drake &
Girdler, 1982).
A later contribution detailing the pattern of
the Wadi Araba segment has been made by
Garfunkel et al. (1981). This can be accepted as
the definitive description. In considering irregularities in the course of the fault trace (named
the er Risha fault by Quennell, 1959) it is
recognised that the fault 'plane' above the plate
margin was in sedimentary fill, generally unconsolidated.
The Gulf of Aqaba (Fig. 1-inset) has been
described by B e n - A v r a h a m et al. (1979) and by
Garfunkel (1981), but without recognition that
the circular arc of the plate boundary was
projected to the south and that the gulf was
oblique to the arc. In Fig. 1-inset the process of
initiation of the gulf and its history in terms of
that of the Rift system (Quennell, 1959) are set
FI6. 1. Geology and structure (simplified) of the West Arabian rift system from the Red Sea to the
Eastern Taurus, comprising: Aqaba (Elat) Gulf; Dead Sea transform; Yammoun6 fault zone; Gharb
fault which appears to meet the Amanos Fault of the Border Zone fault system. The Arabian plate
comprises: the Arabian stable shelf; Lebanon--Palmyra fold belt; and the Syrian unstable shelf. On
the west of the rift system is the Sinai--Levantine plate, an extension of the Nubian plate. The
Arabian Rift system is the inter-plate boundary, while the southern Palmyra fault zone ~and possibly
also the northern fault zone are intra-plate boundaries. Geology is after: Dubertret (1955),
Ponikarov (1964), Wolfart (1967), Picard (1959), Quennell (1959), Bender (1968, 1974) and Ben
Avraham et al. (1979). Both north and south of the Dead Sea rhomb-graben are the circular arcs of
the plate boundary. The location of the pole of rotation was geometrically determined (Quennell
1959; Drake & Girdler, 1982). The Lake Tiberias rhomb-graben (Garfunkel 1981) is terminated
against the Lebanon-Palmyra fold belt. The Dead Sea transform has a total sinistrai displacement of
107 km. The Yammoun6 fault zone (Hancock & Atiya, 1979) extends across the belt, and is the only
fault to do so. Sinistral displacement is of the order of 7-10 km. The el Gharb sinistral fault system
containing the Gharb rhomb-graben may extend the Yammoun6 and is assumed to join the sinistral
Amanos fault beneath the alluvial plains north of Antioch. It may have a sinistral displacement of
50 km (based on evidence of the rhomb-graben). The el Gharb fault separates the northern extension
of the faulted and folded Sinai-Levantine plate from the Syrian unstable platform. The Precambrian
crystalline continental crust disappears northward near Elat on the west of the rift, and on the east of
the rift near the Dead Sea. Exposed cover formations, Palaeozoic to Neogene, generally marine,
decrease in age northwards. The Harrat ash Shamah flood basalts, Miocene to Recent, are
noteworthy.
FIG. 1 (inset). Development of the Gulf of Aqaba (Elat) and its prolongation northward as the Wadi
Araba. The mechanism is the anti-clockwise rotation of the stable shelf part of the Arabian plate
along the circular arc transform interplate boundary (Quennell, 1959; and Garfunkel, 1981). A zone
of fracturing, stage A and a-a 1, probably extensional and normal, resulted from pre-Neogene
tectonism with the formation of the Raham conglomerate (Garfunkel et al. 1974) and preceded the
initiation of the transform interplate boundary. Rotation of the southern Arabian plate by 62 km
length on the circumference was to position, stage B, b-b1, in the early Neogene. This same
movement was resumed by 45 km to reach stage C, c-c1, and is continuing. Sedimentation probably
kept pace during first phase with enlargement of the Gulf to stage B, but it has lagged behind the
tectonic lengthening and widening of the gulf between stages B & C and extensional basins or
rhomb-graben were formed (see figure and reference, Ben Avraham et al. 1979). The last phase
lengthened the W. Araba to c 1. The head of the Gulf is a prograding beach. Restoration to stage A
leaves the opposite Gulf margins of crystalline basement clear of each other. Impingement and
overlapping of margins and submarine contours of the floor of the Gulf only involves younger
sediments (see also Fig. lb in Freund et al. 1970).
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The Western Arabia rift system
777
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778
A. M. Quennell
E
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. ~ aZ
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The Western A r a b i a rift s y s t e m
out. The initial form of the gulf (Stage A) has
some agreement with that portrayed by Freund
et al. (1970).
The influence of strike-slip movement on
supposedly anastomozing faults in the Sinai
block is important only if such faults can be
seen to add materially to or subtract from total
movement. This does not appear to be the case
(Eyal et al. 1981; Bartov et al. 1980).
Extension of the gulf northward up the Wadi
Araba involves a maximum depth of infilling
sediment of perhaps 600 m between the plate
margins (see also Quennell 1958). The prograding beach at the head of the gulf indicates the
process by which the Wadi Araba sedimentation proceeds at the rate of approx 0.2m per
year.
The flanks of the Wadi Araba show great
contrast but it should be recognized that at any
one locality they were originally separated by
107km. For example the steep but dissected
eastern flank north of Aqaba was formerly the
eastern coast of the gulf about half way down
and opposite the western coast, while the subdued western flank of dissected Mesozoic sediments with escarpments stood opposite the
dissected flank a short distance south of the
Dead Sea.
The Dead Sea as a rhomb-graben has been
described initially by Quennell (1959) and later
by others (Freund & Garfunkel, 1981).
The northern part of the Jordan valley and its
relationship to the Lebanon-Palmyra fold belt is
of greatest interest. The existence of a rhombgraben containing part of the Jordan valley and
L. Tiberias as well as the Houl6 depression, was
suggested by Quennell (1959). Freund et al.
(1970) appeared to accept this. The main
Jordan valley rift fault, concealed beneath lava,
forms its eastern margin and the probable
southern extension of the Yammoun6 fault
appears to form the western margin, also concealed beneath lava flows and a very young
sedimentary cover. Movement, all strike-slip,
has taken place on the eastern marginal fault
which carries northward the whole flank which
here is the basalt lava flow probably hundreds
of metres thick, resting on Cretaceous and
Eocene marine sediments. These are seen 5 km
to the south. Much of the sedimentary succession from further south no doubt extends northward beneath the lava (Wolfart, 1967).
There need have been no displacement on
the southward prolongation of the Yammoun6
fault along the west of the rhomb-graben. This
borders the passive block, the continuous SinaiLevant plate. However, on the east is the
Arabian platform. This moved 62km northward and then a further 45 km, following uplift
779
of the Mt Hermon massif. For the movement to
have continued, the geometry of the rhombgraben would require it to be transferred to the
western fault, the Yammoun6 or the Roum.
What then has happened to 107 km of crust?
This can best be considered in the context of the
marked change in tectonic style when we encounter the Lebanon-Palmyra fold belt.
The Lebanon Segment
This segment can be described as the
traversing of the Lebanon-Palmyra fold belt by
the rift system. It is not simply to be equated
with the Yammoun6 fault zone.
The northern end of the Houl6 depression is
a focus of great significance (Figs 1, 2, 3 & 4).
Radiating from this focal area are various elements. Southward is the rhomb-graben which
may extend as much as 100 km southwards. It is
marked by a gravity low which may indicate a
great thickness of infilling sediments. The
Roum fault ends 40 km to the north where it
disappears beneath a mask of Cenomanian near
the southern end of the Lebanon range; the
Yammoun6 fault follows a course partially
masked by young sediments to form the west
flanks of the Litani and Bekaa valleys. Two
faults, the Rachaya and the Serrhaya, terminate
northwards within the Mt. Hermon massif and
the Anti Lebanon range. Mt. Hermon is bordered by fault scarps against which lava flows
and young sediments abut. The vertical displacement is many thousands of metres. An
estimation based on a stratigraphic column
arrived at from a number of sources (Freund et
al. 1970; Wetzel & Morton 1959; Parker 1969)
suggests that the top of the Precambrian south
of Mt. Hermon lies at 3000-4000m and the
Jurassic seen in Mt. Hermon could be at say,
2000m below. On the west there is no such
displacement feature. The sandstones of Triassic and Palaeozoic age, if they extend so far
north, could be at no great depth beneath Mt.
Hermon which has a fault relationship with all
adjacent formations to the west and south-east.
It may be significant that Mt. Hermon is the site
of a gravity high which may indicate a rise in the
denser layers (Fig. 4).
The Yammoun6 fault zone is well described
by Hancock and Atiya (1979) (see also
Garfunkel et al. 1981). Evidence for any strikeslip movement is described as slickenside
grooves and other features. They quantify the
left-lateral slip as 7 k m based on the rhombgraben. At its northern emergence from the
Lebanon range there is an offset of about
10 kin. It is obvious that this fault zone, despite
being the only such feature completely
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C
/
I
N
P
C
8
Haouran
P
Lake
B
0
I
i
2Km
I
FI6. 3. Structural relations of the Dead Sea transform to faulting in Lebanon-Palmyra fold belt.
Geology and faulting after Dubertret (1945; 1947; 1955), Garfunkel et al. (1981); Landsat image
(1972). The Houl6 depression is the northern part of the L. Tiberias rhomb-graben and ends beneath
a basalt flow (B). The course of the Yammoun6 fault southward is not clear but it is accepted as being
probably continuous with the west flank of the rhomb-graben. The branching Roum fault ends
northward where it passes under Cretaceous marine beds (C) and there may be stream erosion
simulating strike-slip movement. Faults within the Anti-Lebanon range die out, but the NE-trending
fault on the east of the range continues to the NE and separates the Palmyra from the Lebanon style
of folds. The South Palmyra fault-zone (Ponikarov 1964) may be an intra-plate boundary. The main
Dead Sea transform fault apparently ends as the eastern flank of the Houl6 rhomb-graben, but this is
obscured by young basalt flows (B).
J = Jurassic sediments; P = Pleistocene deposits.
FI6.4. Seismic and gravity data related to structure.
Gravity data from sources listed varies in reliability and value. The isogals at 10mgal interval
generally confirm disposition of crustal masses and thicknesses expected from the geology. Note: (a)
the steep gradient acrosss zone west of Dead Sea transform; (b) the less steep zone across the
Yammoun6 fault zone and Lebanon fold-belt; (c) contrast between areas of Lebanon and Palmyra
styles of folding; (d) undisturbed area between Palmyra fold belt and Turkish Border Folds; (e)
gravity information from Jordan could not be used but is probably as for (d). Earthquake data: Note
(a) important epicentre localities are located at both ends of Dead Sea rhomb-graben; (b) virtual
absence of macroseisms along Wadi Araba, but microseisms lie west of or on the plate margin of
Sinai (Araba fault trace). Seven fault plane solutions (to 1976) are recorded in the Palmyra belt or
close to the Dead Sea transform (Ben Menahem et al. 1976; Ben Menahem & Aboodi 1981). All are
left-lateral strike slip.
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The Western Arabia rift system
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Microseisms and fault plane solutions, Ben Menahem et. al., 1976
II AMMAN
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Knopoff and Belshe, i966;
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A. M. Quennell
782
traversing the-Lebanon fold belt, cannot be an
adequate representative of the Dead Sea rift
zone, as it has a trend of about 30 ~ from that of
the north Jordan and a totally inadequate net
slip.
South Palmyra zone of faulting
Along this zone the Anti Lebanon open folds
give way near Damascus to Jura-type folds
trending north-east. The folds are partially
overturned to the south-east and this flank is
generally faulted (Fig. 1). The zone extends
beyond Palmyra. The basalt flows and young
masking sediments abut against the folds for
200 km along the zone where intensity of folding begins to decrease until at about 400 km the
fold belt disappears.
It appears that a corner of the crustal slab, a
triangle, measuring 45 km along its western side
and as much as 400 km along its northern edge
(Fig. 1) has been underthrust and consumed in
some manner.
Lebanon-Palmyra fold belt
Probably accompanying the underthrusting
by the Arabian platform, the Lebanon-Palmyra
fold belt, confined between the Arabian and
north Syrian platforms, was apparently subject
to a clockwise rotational couple (pure shear)
producing a strain rhomb (Fig. 5). There has
also been compression from south to north
when the movement of the Arabian plate was
checked by meeting the Van plate. The axes of
folds would be re-oriented clockwise and they
would be lengthened and narrowed. The contrast between the Lebanon and Palmyra folds is
probably related to depth to basement (Fig. 4).
Where thickness of competent formations is
greatest, the folds would have greatest amplitude and underthrusting would be the manner
:t Anatolian Fault
o
,
.
,
,
Border Folds
Syrian Platform
.r
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f'l
/
\\
9
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,
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Arabian Stable
Platform
s'~"
FIG. 5. Model for the generation of the Lebanon-Palmyra fold belt by oblique compression between
the two platforms. Eastward movement of Syrian platform (note possible involvement of plate
movement along East Anatolian transform) results in a rotational couple (pure shear) with NE-SW
folds. Contrast in thickness of cover beds between Lebanon-Anti-Lebanon and Palmyra may
account for differing styles and scales of folds. Superficial east-west right lateral strike-slip faults in
Lebanese segment of Sinai-Levantine plate suggest simple shear west of Yammoun6 zone.
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783
T h e W e s t e r n A r a b i a rift s y s t e m
of disposal of the excess crust. Where the cover
beds are thin-skinned and the basement shallower, detachment (d6collement) would operate. Further, in this case the width of crust to be
absorbed decreases to the north-east. The dividing line appears to be the fault east of the
Anti Lebanon.
The segment of the Sinai-Levant plate, lying
west of the Yammoun6 fault has apparently had
imposed upon it, probably in a later phase,
simple shear producing latitudinally orientated
shear planes now reflected in the E-W faulting
and other fold and fault patterns decribed by
Hancock & Atiya (1979) (see also Palaeomagnetism below).
El Gharb segment
Where the Yammoun6 fault emerges from
the Lebanon, it is deflected to the west. It is
suggested that this may be a weak sigmoidal
pattern caused by drag as the clockwise rotation
(Fig. 5) of the fold belt took place. Here the
fault zone disappears beneath a young
sedimentary cover but it may link with the E1
Gharb transform fault which resumes the original trend.
The E1 Gharb fault described by Dubertret
(1967) and by Ponikarov (1967) first traverses
the Homs basalt flow of Pliocene age. The
apparent strike-slip dislocation of the flow by
about 20 km is post-Pliocene if the basalt is of
that age (Ponikarov 1967).
The fault traverses Mesozoic formations,
Jurassic on the west (the Jebel Alaouite) and
Cretaceous on the east. The E1 Gharb depression (Dubertret 1967) commences on the northern flank of J. Alaouite. It has a length between
the southern fault in Jurassic to the basalt flow
and Palaeogene sediments in the north of about
50km and a width of 10km. The northern
termination is ill-defined. Beyond this the el
Gharb fault, continuing along the western flank
of the depression, disappears beneath the
Quaternary sediments of the plain of Antioch.
There can be no simple junction with the
Amanos (East Anatolian) fault.
Freund et al. (1970) attributed a sinistral
movement of 70 km to the El Gharb fault on
what they regarded as evidence from displacement of the ophiolite massifs, the Kurd Dagh
on the east and the Brier and Bassit on the west
(Fig. 1).
The ophiolite masses, the 'roches vertes' of
Dubertret, which are pre-Maastrichtian in age,
belong to the Anatolian plate margin (see later)
and lie within a zone containing these obducted
bodies (Dewey et al. 1973). If indeed the El
Gharb fault continues into this zone which is
improbable, the Kurd Dagh lies well beyond its
end and the relationship of the two masses
cannot be used as Freund et al. have done to
determine a displacement on the El Gharb fault
of 70 km. This is therefore discounted.
Volcanics
The flood basalt volcanic activity of the Arabian
stable shelf (Ponikarov, 1967; Barberi et al.
1980; Wolfart 1967; Burdon 1959) was chiefly
centred on Jebel Druze 35 km south of Damascus (Fig. 1). The general name of the plateau is
Ash Shamah while the northern part is the
Haouran. The flows form a wide zone of length
360kms extending to the southeast, with a
width of 160km reducing to 50km. This zone
lies to the northeast of the parallel Sirhan
depression both features manifesting NW-SE
tensional fractures.
There are NW-SE eruptive fissures marked
by vents. They have been well mapped in Syria
and NE Jordan. In the northwest, close to the
Houl6 focus of structural features (Fig. 3),
detailed mapping (Ponikarov 1966) reveals a
number of flows of basalt, the earliest of which
is mid-Miocene in age and the youngest are
Recent (ages from Barberi et al.). The earliest
flows were erupted on the 'mid-Tertiary'
Arabian planation surface (later). The fissures
have a general bearing 340 ~ nearly normal to
the south Palmyra zone of faulting. All volcanic
rocks surveyed south of the Syrian border
(Barberi et al. 1980) belong to the alkalic clan,
generally undersaturated. They fall within the
definition of alkali basalt, basanite, nepheline
basanite and hawaiite.
There is no recorded volcanic activity within
the Lebanon-Palmyra fold belt. It ends abruptly
at the Palmyra fault zone. However, the trend
of the fissures has considerable significance in
relation to the directions of shear, tension and
compression of the strain ellipse for the region
(Quennell 1951, 1959; Burdon 1959).
The loading on the crust is considerable, the
thickness at Jebel Druze being more than
1500m, and this may have played a part in
encouraging underthrusting.
The other basalt flow of significance is that
lying across the Lebanon-Syrian frontier west of
Horns (also known as the Shin field). It is
approximately 45 x 5 5 k m and the basalt is
chiefly Pliocene but there is an early small flow
of Upper Miocene (Ponikarov 1966).
The El Gharb fault, here N-S, commences
south of the flow and with left-lateral strike-slip
movement displaces the flow. Fault movement
of about 20 km appears to post-date the volcanics, i.e. to be post-Pliocene. Although
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784
A. M. Quennell
obscured by recent sediments this fault appears
to extend the Yammoun6 fault.
Across the northern end of the E1 Gharb
rhomb-graben, i.e. 50km distant from the
southern cross fault, is a flow of Pliocene basalt.
This is not intersected by the eastern fault nor
are the Palaeogene sediments. It appears the
fault had ended.
This Homs basalt could have erupted from
tension fissures. It lies in the area where the
N30~ trend of the Lebanon anticlinorium
changes to the N-S trend of the northern end of
the Sinai-Lebanon plate (Fig. 5), the E1 Gharb
fault making the same change. This would
favour tensile stress and the formation of tension fissures.
Palaeomagnetic research
This has been carried out by Van Dongen et al.
(1967) on Mesozoic formations of the Lebanon
Mountains west of the Yammoun6 fault zone
and northeast of Beirut; and on Pliocene basalt
flows near Tartous on the Syrian coast 40km
north of the Lebanon frontier. This was followed by a survey by Gregor et al. (1974) on
adjacent but more restricted areas in the Lebanon mountains on Mesozoic formations, and on
Upper Pliocene Homs basalt north of L e b a n o n
and west of the Yammoun6 zone, as well as on
basalt flow remnants in the south close to Mt.
Hermon and east of the Yammoun6 fault.
Sampling was therefore from four structurally
distinctive areas of contrasting tectonic histories
only the last of which belongs to the Arabian
plate. The others are all on the Lebanese
segment of the Sinai-Levantine plate.
For our purpose it is convenient to consider
only the Mesozoic sites. Gregor et al. (1974)
state that their results ' . . . indicate a progressive anticlockwise rotation of Lebanon during
late Mesozoic . . . times'. As this appears on
geological and tectonic evidence highly improbable, testing by a separate approach is necessary.
If the mean directions for sites in Lebanon
can be made by rotation to coincide with directions at sites in the Sinai-Levant (Africa) plate,
then Central Lebanon can be restored to its
original orientation (Fig. 5).
Relevant work was done by Helsley & Nur
(1970) on samples from the Ramon asymmetric
andc|~ne in the Negev (folded since magnetization) and trom near Mt. Carmel; and by Freund
& Tarling (1978) from Ramon and near Jerusalem.
Results from these formations which are
comparable stratigraphically give no rational
solution. On first inspection for Lower Cre'
taceous declinations there would have been
clockwise rotation of Central Lebanon from
125~ to at least 155~ and for Jurassic from 95~ to
172~, an improbable situation. If the plate tectonic model proposed (Fig. 5) is accepted then the
palaeomagnetic data require a different interpretation from that of Gregor et al.
Freund & Tarling extend the discussion
beyond our present objective. They criticize the
sampling plan in Central Lebanon and note that
results appear to group themselves according to
locality. There is not the expected consistency
from sites close to each other. They suggest that
these inconsistencies in palaeomagnetic results
' . . . may indeed record the structural deformation of this country.' They suggest that the
present orientation of declinations may be
linked with Freund's model of internal rotation of faulted blocks which is integral with his
model for the Lebanon segment of the rift
system (Freund et al. 1970). The system of
latitudinally-trending faults, most of them
dextrally strike-slip as mapped by Dubertret
(1944) and Hancock & Atiya (1979), has been
stated by the latter to have been accompanied
by clockwise rotation. The latter give a description of northern Lebanon, especially of the
areas sampled by Van Dongen et al. and by
Gregor et al. They state ' . . the Mount
Lebanon anticlinorium is divided into several
compartments separated by narrow zones of
E-W trending, nearly vertical, dextral f a u l t s ' . . .
'A conjugate set of NW-SE f a u l t s . . , resulted
in a clockwise sense of rotation.' The value of
palaeomagnetic results on samples from what is
a unique structural and meso-structural area are
limited, and cannot support the view that
Lebanon west of the Yammoun6 fault could be
a microplate which behaved as did those of the
Western Mediterranean.
Palaeomagnetic results from the Ash Shamah
volcanic plateau in NE Jordan and from W. el
Mojib, east of the Dead Sea, are reported in
~Barberi et al. (1980). There is a marked difference between results for the two sites but this is
attributed to different positions on the plate.
The Ash Shamah samples give palaeomagnetic
pole results close to those for the Pliocene
sampling in the Lebanon and also to results for
Aden (Tarling et al. 1967). These results
confirm the comments above on the Lebansese
survey regarding value of the Mesozoic sampling without reference to a structural framework.
Geomorphology and tectonics
A single planation surface of mid-Tertiary age
which is reasonably well preserved and identi-
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on March 5, 2016
The Western Arabia rift system
fiable on the Arabian stable platform east of the
Dead Sea rift (the Arabian surface, Quennell
1958) and on the unstable shelf of N W and SE
Syria (Syrian plateau) can be recognised
(Burdon 1959; Wolfart 1967). It has suffered
distortion and erosion on the west and northeast. There are few residuals other than inselbergs and monadnocks, which occur mostly in
the south-east. The fold mountains of the Lebanon-Palmyra fold belt and the Judean and
Ajlun arches are not residual but younger tectonic landforms.
West of the rift on the Sinai-Levantine plate,
the surface can be reconstructed. Although it
has been folded and faulted and warped into
arches, there is summit accordance and some
remnants remain as in Sinai, Galilee, and the
Tyre-Nabatiye Plateau.
Before the tectonic disturbance of these surfaces they were co-extensive across the site of
Levantine
,v,ntine Jt //
0....
.,g l ,
/.!
.
#/
~e
I
",;
.. .,.~>~" ~ i
I
~ Stable Platfo,m
.~ I
an .'7~ i '.
t~
II/
.
.
ltlr
~ ~ ~
A,abian
# I
k
-'2
=
SYRIA
~ ~~ ~'~/.~i=
. . . . ","
.
.
Arabian
Plate
J?!
/
P,ate
the rift system. They are the same altitude
across the Gulf of Aqaba (QuenneU 1958).
An oldest age for the surface is Upper Oligocene, the cycle having been initiated in the
Middle or U p p e r Eocene. This is based on
stratigraphic evidence from Sinai. The upper
age limit is that of the earliest flood basalt flows,
the Haouran, of Helvetian age.
The diastrophic episode which ended the
erosion cycle was probably coincident with the
opening of the R e d Sea, the Sinai-Levantine
plate lagging behind as part of the African plate
(there is not sufficient recorded field evidence
that the Gulf of Suez is an inter-plate boundary). The rift was apparently initiated as the
terminating cross fault of the Red Sea spreading
zone, its northerly course being influenced by
(a) preferred trends in the Precambrian
basememt (Lenze et al. 1972; Bender 1982) and
(b) the rotation of the Arabian platform on a
~
~ IJ
Unstable
Y~ /
785
""
,7:,;|~us,
J
t
~" .~" rl
.
.obia k
7~p~rO~f~
~"
~-'"
ddcollement
~, z
V
Fit. 6. Plate tectonics west Arabian rift system related to east Mediterranean region (after Dewey &
Sengor 1979).
Pre- and L. Miocene: Situation prior to separation of Arabia from Africa across early tensional
zone of the Red Sea. Arabian plate has freedom of movement northwards along the geosuture.
Miocene (mid): Arabian plate movement northward on circular arc transform closes Bitlis ocean
and suture zone, with compression of Border Folds. Sinai-Levant (Africa) plate lags. This is the
62 km movement phase on the transform, with opening of the ,Dead Sea to the south by formation of
a rhomb-graben. Arabian-Sinai planation surface deformed and faulted.
U. Pliocene to Present: With movement on the East Anatolian transform, westward translation of
Turkey (Anatolian plate) is matched by eastward movement of north Arabia platform. Combination
of latter movement and narrowing of north Arabia platform (with compression by Arabian plate on
Bitlis Zone and Border folding) creates the dextral Lebanon-Palmyra fold-belt with translational
kink in the geosuture (Fig. 5). Southern margin, the south Palmyra fault zone (Ponikarov 1966), is
underthrust by excess lithosphere by movement of the Arabian plate from south to north by 45 km
on the Dead Sea rift transform. Amount of underthrusting reduces from 45 km to nil at 400 km to
NE. Gulf of Aqaba and Dead Sea lengthen. Sinai-Levant plate segment opposite fold belt suffers
simple shearing and translation as does the geosuture and Yammoun6 fault.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on March 5, 2016
786
A . M . QuenneH
small circle transform (Figs 5 & 6). The preexisting interplate boundary between SinaiLevant and Arabia from the Dead Sea northward, was followed.
The transform margin of the Arabian continental plate is well illustrated in the Bouguer
anomaly map (Fig. 4) which also confirms the
Lebanon segment as being the Yammoun6 fault
(Fig. 5). This movement took place well before
the Lebanon-Palmyra folding. Probably the
whole of the 62 km first phase of movement was
completed before the northward movement of
Arabia closed the Bitlis ocean giving rise to a
pause and the initiation of the East Anatolian
transform separating the Anatolian and north
Arabian plates,
The Lebanon-Palmyra fold belt appears to
have formed in the weakest zone in the Arabian
plate lying as it does between the stable shelf of
south Arabia with its sedimentary cover on a
sialic base, and the unstable shelf of Syria with
its thicker sedimentary cover lying possibly on
the oceanic part of the plate. It suffered a
dextral distortion which apparently acted as a
barrier to further northward strike-slip movement on the rift transform (Fig. 6). This must
have happened after the Bitlis ocean closed
(Dewey & Seng6r 1979) and the northern free
margin of the Arabian plate which previously
was only impeded in the NW, collided with the
Anatolian and Van plates. The obliquity of the
East Anatolian transform could have caused
the eastward migration of the unstable shelf of
Syria in the same manner as the Anatolian plate
was being expelled westward with dextral
movement on the Lebanon-Palmyra fold belt
(Fig. 6).
The Arabian platform south of the fold belt
resumed its northward migration on the Dead
Sea transform in Plio-Plestocene times and has
moved 45 km and is continuing. It is only this
excess lithosphere, and not the total of 107 km,
which has to be consumed by the process of
underthrusting (at Mt. Hermon) or beneath a
detachment surface (below the Palmyra
sedimentary cover).
Conclusion
The conclusion is that the West Arabian rift
system, originally continuous from the midMiocene, was divided by the Upper Pliocene
into: a southern transform system, Red Sea to
the Lebanese frontier; and a northern transform system from the Syrian frontier to the East
Anatolian transform boundary. They were both
sinistral strike-slip and acted independently
with differing displacements. They were
separted by the belt of oblique folding, faulting
and thrusting. It is probable that the Yammoun6 fault, a young feature, lies above the
geosuture.
This explanation obviates the necessity for
involved hypotheses for the passage of the
faulting through the Lebanon folds and faults,
with changes in strike and displacement,
which Dubertret had come to deny. It also leads
to reconciliation with the kinematics of the
eastern Mediterranean and Aegean plates:
ACKNOWLEDGEMENTS;The author wishes to record his
gratitude to the late Dr Louis Dubertret with whose
classical mapping and writings, study of Levant geology has been enriched; to Dr. D. J. Burdon, late
FAO; Dr. Paul Hancock of Bristol University; Prof.
N. Ambraseys who assisted in the early study of the
geophysics; the Global Seismology Unit of IGS; and
the Jordanian Geologists' Association. The paper was
reviewed by D. Neev, F. Mirsch & J. E. Dixon.
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A. M. QUENNELL, Department of Geology, University of Bristol, Queen's
Building, University Walk, Bristol, BS8 1TR.