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Syncline-toppedanticlinalridgesfromthe
HighAtlas:AMoroccanconundrum,and
inspiringstructuresfromtheSyrianArc,Israel
ArticleinTerraNova·August2011
DOI:10.1111/j.1365-3121.2011.01016.
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Syncline-topped anticlinal ridges from the High Atlas: a Moroccan
conundrum, and inspiring structures from the Syrian Arc, Israel
A. Michard,1 H. Ibouh2 and A. Charrière3
1
10, rue des Jeûneurs, 75002 Paris, France; 2Faculté des Sciences et Techniques, De´partement de Ge´ologie, Bd A. Khattabi BP 549, 40000
Gue´liz Marrakech, Morocco; 326 rue J.-P. Chabrol, 34740 Vendargues, France
ABSTRACT
We question for the first time the origin of enigmatic structures,
herein termed syncline-topped anticlinal ridges (STARs). In the
Central High Atlas (CHA) of Morocco, small synclines of Upper
Palaeocene-? Eocene deposits are only preserved on top of
Triassic–Jurassic anticlinal ridges. We explain these peculiar
structures through a three-step evolution: (i) early halokinetic
evolution leading to the formation of elongated diapiric ridges
over basement faults, ending with magmatic intrusions and
enhanced diapiric ascent up to the surface; (ii) Palaeocene-?
Introduction
Fold-and-thrust belts usually result
from superimposed folding episodes.
Classical examples are those of the
External Western Alps (Lemoine,
1972) and Provence areas in SE
France (Andreani et al., 2010; with
references therein), where the early,
Late Cretaceous–Eocene folds were
partly eroded before being reactivated
by the Neogene shortening event. In
such classical examples, early anticlines are converted into tighter anticlines or thrusts, whereas late
synclines develop above early ones.
Alonso (1989) demonstrated theoretically that the reactivation of flexural
folds and flexural-slip folds below an
unconformity produces folds of the
same sense of curvature in layers
above and below the unconformity.
In contrast, the High Atlas Belt offers
three stunning examples of perched
synclines developed on top of narrow
anticline hinges in the Imilchil area
east of Marrakech (Fig. 1). All these
perched synclines correspond to
Upper Palaeocene-? Lower Eocene
formations overlying tight, elongated
anticlinal ridges made up of Mesozoic
rocks (Fig. 2). These anomalous structures, termed herein Ôsyncline-topped
anticlinal ridgesÕ (STARs) depend
clearly on superimposed folding
Correspondence: Em. Pr. André Michard,
10 rue des Jeûneurs, 75002 Paris, France.
Tel.: +33(0)142360483; e-mail: andremichard@
orange.fr
2011 Blackwell Publishing Ltd
Eocene unconformable sedimentation; (iii) Late Eocene–
Quaternary shortening phases, which resulted in the erosion
of the Palaeocene-? Early Eocene deposits, except in the
breached anticlinal axes. The comparison with the breached
valley (‘makhteshim’) of the Syrian Arc in the Negev Desert
allows us to emphasize the role of the early diapiric evolution
of the CHA domain in the genesis of the STAR structure.
Terra Nova, 00, 1–10, 2011
events as the layers forming the
perched synclines overlie unconformably the oldest formations of the
anticline. Why the Tertiary deposits
do not occur within the wide, low
elevation synclines next to the tight,
culminating anticlines, but only occur
over the eroded crest of the latter?
This enigma has been never addressed
up to now.
Geological setting
The Moroccan High Atlas is the most
elevated part of the intracontinental
belts developed in northern Africa
(Fig. 1, insert) through inversion of
former Triassic–Liassic rift basins
during the Africa–Eurasia convergence (Frizon de Lamotte et al.,
2008; with references therein). Its high
elevation depends on the hot mantle
anomaly developed under northern
Morocco during the Neogene (Teixell
et al., 2005; Missenard et al., 2006;
Fullea et al., 2010). The Central High
Atlas (CHA) corresponds to the deepest part of the former Atlas basin,
underlain by a necked continental
crust (Gouiza et al., 2010). In contrast
with the other segments of the Atlas,
the CHA is characterized by alkalinetransitional gabbroic magmatism emplaced in the form of sills, dykes and
outpours (Fig. 2; Saidi, 1992; Zayane,
1992; Armando, 1999; Lhachmi et al.,
2001; Ibouh et al., 2002; Zayane et al.,
2002). This event occurred shortly
after the Bathonian emersion of the
Atlas domain (Fig. 3), during the
latest Dogger and Late Jurassic (Hailwood and Mitchell, 1971; Armando,
1999;
Haddoumi
et al.,
2010)
although the latest basalts emplaced
during the Barremian, coeval with the
youngest red beds (Haddoumi et al.,
2010).
Marine sedimentation resumed during the Aptian, extending all over
Morocco during the Cenomanian–
Turonian high stand. Regression began during the Late Cretaceous, but
the earliest unconformable, continental red beds are those of the Imilchil
region (herein discussed). They were
previously regarded as Jurassic or
Cretaceous (e.g. Laville and Piqué,
1992) or possibly Triassic (Barbero
et al., 2007), but are now dated from
the Thanetian-?Ypresian (Charrière
et al., 2009). In both the North and
South Sub-Atlas Zones, superimposed
folding events are dated from the Late
Eocene, Early Miocene and Pliocene–
Quaternary (Frizon de Lamotte et al.,
2000; El Harfi et al., 2001; Teson and
Teixell, 2008; Frizon de Lamotte
et al., 2008). Within the mountain
belt, narrow, faulted anticlinal ridges
cored by Triassic argillites and magmatic rocks extend between wide open
synclines (Fig. 2).
Syncline-topped anticlinal ridges
from Central High Atlas
In the Imilchil area (Fig. 4), the Tassent and Tasraft Ridges north of the
Plateau des Lacs Syncline offer the
best examples of STAR. A third
1
Synclinal-topped anticlinal ridges • A. Michard et al.
Terra Nova, Vol 00, No. 0, 1–10
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Fig. 1 Schematic structural map of the High Atlas Mountains (after Teixell et al., 2003; modified) with location of Figs 2 and 4.
Amg, Amagmag; Azk, Azourki; Tml, Tioumliline; TTZr, Talmest-Tazoult Ridge; Tz, Tizal.
Fig. 2 Generalized cross-section of the Central High Atlas showing the Tasraft, Tassent and Amagmag anticlinal ridges topped by
small synclines of unconformable Upper Palaeocene-? Lower Eocene deposits. At Tioumliline and Izerki, the Upper Triassic
diapirs contain halite (Teixell et al., 2003; Ettaki et al., 2007). See Fig. 1 for location.
example is the Amagmag Ridge
(Fig. 2), east of the Ait Ali-Ou-Ikkou
(AAK) syncline. The intervening
AAK-Msadrid Ridge is devoid of
any Tertiary perched syncline.
2
In every case, the ridge axis consists
of a somehow chaotic association
involving: (i) Triassic reddish, gypsiferous argillites and green, spilitic basalts of the CAMP event (Verati et al.,
2007); (ii) slivers of recrystallized
Lower Liassic carbonates, sometime
deformed into upright isoclinal folds,
and (iii) Late Jurassic plutonic rocks,
including stratified troctolites and
2011 Blackwell Publishing Ltd
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A. Michard et al. • Synclinal-topped anticlinal ridges
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Fig. 3 Columnar section of the Mesozoic–Cenozoic series of the Central High
Atlas, after Frizon de Lamotte et al.
(2008) and Charrière et al. (2009), modified. The left part of the column summarizes the sequence of the Imilchil
ridge axes, the right part that of the
adjoining areas (CHA main synclines
and southern Sub-Atlas Zone, respectively).
gabbros, pneumatolytic gabbros (ÔdioritesÕ) and cross-cutting syenites
dykes (Fig. 4). The sandy-conglomer 2011 Blackwell Publishing Ltd
atic layers that occur elsewhere at the
bottom of the Triassic sequence in the
Atlas and High Moulouya domains
(Figs 1 and 3) are lacking, suggesting
a décollement on top of the Palaeozoic
– Early to Middle Triassic basement.
The ÔchaoticÕ assemblage of every
ridge axis is bounded by faults
(Ibouh, 1995; Fadile, 2003; Ibouh,
2004). In the main part of the ridge,
the bounding faults are steeply dipping (e.g. Tassent Ridge, Fig. 5),
whereas they display shallow dipping
and pericline geometry at the end of
the ridges (e.g. Tasraft Ridge, Fig. 6).
Within the Upper Liassic–Dogger
series adjacent to the anticlinal ridges,
varied stratigraphic observations suggest a coeval growth of the ridges,
namely, fanned sedimentary bedding,
frequent hydroplastic slump structures (Ibouh, 1995, 2004) and unconformities (e.g. in the AAK syncline at
the bottom of the Bathonian red beds;
Fadile, 2003).
The Lower Tertiary continental formations are particularly well preserved on top of the Tassent Ridge,
in correspondence with a saddle of the
ridge axis (Fig. 4). There, the continental sequence includes marly sandy
red beds and an interbedded basalt
outpour. The uppermost red beds on
top of the basalts (Fig. 5C,D) yielded
Ostracods and Charophytes from the
Thanetian–Lower Ypresian (Charrière et al., 2009). The whole continental series displays synformal
geometry (Fig. 5). This perched syncline overlies the varied rock units of
the ridge axis; it does not extend onto
the northern envelope of the ridge,
whereas the southern flank of the
ridge (Bajocian ÔCalcaires-cornichesÕ)
overhangs the Palaeocene syncline by
more than 150 m.
Unconformable Upper Palaeocene
deposits are also well preserved in the
eastern pericline of the Tasraft Ridge
(Fig. 4). There, the Bajocian limestones surround and slightly overhang
both the ridge core and the Palaeocene
syncline (Fig. 6). The Palaeocene succession, well dated from the Thanetian
(Fig. 6B; Charrière et al., 2009) differs
from the Tassent one as it includes
several layers of lacustrine to lagoonal
limestones interbedded with red marls,
sandstones and conglomerates. The
basal conglomerates (Fig. 6C) show
the characters of a proximal piedmont
deposit. The geometry of these Upper
Palaeocene deposits corresponds to a
faulted (Fig. 6D), asymmetric open
syncline, within which fanned bedding
(see Charrière et al., 2009, their fig. 2)
suggests synsedimentary collapse of
the substratum.
The Amagmag marly sandy red
beds form a perched Palaeocene syncline on top of the Amagmag Ridge
(Fig. 2 for location). This structure
compares with the Tassent one, except
for the lack of interbedded basalt.
Discussion
Formation of the anticlinal ridges: salt
diapirism and magmatic intrusions
Addressing the origin of the Imilchil
STARs implies first of all to clarify
the origin of the anticlinal ridges
themselves. The CHA ridges were
ascribed to different processes: (i)
Jurassic compressional folding associated with reverse faults (Studer and
du Dresnay, 1980; Studer, 1987); (ii)
Jurassic emplacement of magmatic
intrusions (Schaer and Persoz, 1976;
Laville and Harmand, 1982); (iii)
Jurassic transpression along NEtrending sinistral strike-slip faults
associated with en-échelon opening
sites (Laville, 1985; Laville et al.,
1991; Laville and Piqué, 1992), or
(iv) uplifted borders of tilted blocks
created by synsedimentary extensional faulting (Jenny et al., 1981;
Monbaron, 1982; Poisson et al.,
1998). Gomez et al. (2002), Frizon
de Lamotte et al. (2000, 2008)
and Barbero et al. (2007) emphasized
the lack of regional Jurassic compression.
The idea that diapirism could explain the early development of some
ridges southwest of the Imilchil area
(Talmest-Tazoult and Ikerzi; Fig. 1)
was proposed by Bouchouata et al.
(1995) and Ettaki et al. (2007), respectively. Halite is actually mined in the
Ikerzi Ridge. In line with these previous proposals, we consider that the
Imilchil anticlinal ridges developed as
Ôsalt wallsÕ during the Liassic–Middle
Jurassic interval above the dominant,
NE-trending basement faults, based
on the following evidences: (i) the
ridge cores include large amounts of
Triassic gypsum-bearing argillites
associated with vertical slivers of
Lower Liassic limestones; (ii) the
ridges are bounded by subtractive
3
Synclinal-topped anticlinal ridges • A. Michard et al.
Terra Nova, Vol 00, No. 0, 1–10
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5•• 40'
Ta
dge
ft Ri
sra
5•• 30'
Fig. 6
2412
Fig. 5B T
R
t
s e n
s
a
i
e
g
d
Tassent
Bab n’ Ouayad
2804
Sy
Tislit
Lake
de
P
la
a
te
e
Isli
Lake
3059
u
s
a
d
ri
dge
Ri
s
M
32••10'
s
c
La
n
in
cl
d
2440
2151
AAK
IMILCHIL
c
e
A
g
R
A
id
N
A
A
K
2804
n
n
K
e
S
y
li
1
2
11
3
12
4
13
5
14
4 km
6
15
7
16
8
17
9
18
10
19
Fig. 4 Structural map of the Imilchil area, after Ibouh (1995), modified. See Fig. 1 for location and Fig. 3 for stratigraphic terms.
AAK, Ait Ali-Ou Ikkou. 1. Triassic green basalts and argilittes; 2. Lower Liassic limestones; 3. Ag1 Fm.; 4. Ag2 Fm (ÔCalcairescornichesÕ); 5. Ag3 Fm; 6. Lower Anemzi Fm; 7. Upper Anemzi Fm; 8. Thanetian–Ypresian red marls and sandstones (Tassent);
9. Upper Palaeocene basalts; 10. Thanetian lacustrine-lagoonal limestones, marls, red sandstones and conglomerates (Tasraft);
11. Troctolites; 12. Gabbros; 13. ÔDioritesÕ or pneumatolytic gabbros; 14. Syenites; 15. Doleritic ⁄ undetermined dykes; 16. Faults;
17. Anticlinal axis; 18. Synclinal axis; 19. Quaternary deposits.
faults, with stratigraphic omission
more and more important from the
root zone to the hinge zone; (iii) the
progressive growth of the ridge is
recorded by fanned bedding, slump
structures and unconformities in the
Upper Liassic–Middle Jurassic series
adjacent to the ridges (Ibouh, 2004).
Such characters are usually associated
with salt tectonics (e.g. Fossen, 2010).
Indeed, beautiful examples of diapiric
4
structures are well known in the
western High Atlas, either onshore
or offshore (Cochet et al., 1970;
Hafid, 2006; Hafid et al., 2008), and
alike the western High Atlas, the
CHA rift basin was filled with
argillaceous–evaporitic deposits during the Carnian–Norian (Oujidi et al.,
2000).
Magmatic intrusions also played an
important role in the genesis of the
CHA ridges (Ettaki et al., 2007). The
largest intrusions emplaced as sills
within the sedimentary cover, particularly beneath the competent Liassic
carbonates (Fig. 2). We may admit
that magma ascent was favoured
within the pre-existing diapirs (this is
supported by the ellipsoidal geometry
of the individual gabbro massifs described by Bougadir, 1991 and Rahimi
et al., 1991). As a feedback, magmatic
2011 Blackwell Publishing Ltd
A. Michard et al. • Synclinal-topped anticlinal ridges
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N
LL
S
J. Bab n'Ouayad
Tassent perched
syncline:
basalts
red beds
F
Ag2
F
LL
LL
UTa
Anticline core
LL
UTb
Ag1
LL
UJg
UJg
F
Ag1
(A)
N
Dogger
(Ag 2)
C
T
to Tislit
lake
F
D
Liassic slivers
Upper Liassic
(Ag 1)
S
J. Bab n'Ouayad
T: Upper Paleocene - ?Ypresian fossils
F
to Tassent
Triassic core
?
Upper Paleocene
- ? Eocene (peC)
Basalt
Sandy-argillaceous
red beds
Unconformity
Jurassic intrusion
Jurassic
b
0
500 m
Dogger limestones/marls
Green spilitic basalts
(B)
Violin gypsum argillites
W
NW
E
Upper Liassic marly lmst.
Lower Liassic limestones
SE
Basalts
T
peC
b: metagabbro
a a: olivine gabbro
Triassic
T
peC
(C)
Qc
T
Qc
(D)
Fig. 5 Tassent Ridge. (A) View of the natural cross-section looking eastward from the Imilchil-Tassent road. (B) Geological crosssection, after Chèvremont (1975), Ibouh (1995) and Charrière et al. (2009); the detail cross-section (uppermost Tertiary layers) is
located approximately 2 km east of the main section. (C and D) Close views (approximately located in B) of the supra-basaltic red
beds; (C) shows the uppermost, fossiliferous part of the syncline; (D) shows the conformable contact on top of the basalts. Ag1,
Toarcian–Aalenian; Ag2, Bajocian limestones; LL, Lower Liassic limestones; peC, Upper Palaeocene-?Early Eocene continental
red beds; Qc, Quaternary calcrete and slope breccias; UJg, Upper Jurassic gabbros; UTa ⁄ b, Upper Triassic argillites ⁄ basalts.
2011 Blackwell Publishing Ltd
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Synclinal-topped anticlinal ridges • A. Michard et al.
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(A)
(B)
(C)
(D)
Fig. 6 Tasraft Ridge eastern pericline and Thanetian perched syncline. (A) Geological interpretation of the Google earth satellite
image. (B) Terrestrial view (photograph by H. Haddoumi) from the northern end of the track shown in (A). (C) Palaeocene coarse
basal conglomerates (located in B) with limestone, basalt and gabbro pebbles or boulders. (D) Verticalized gypsum layers next to
the north faulted border of the diapiric ridge.
6
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heating increased halokinetic ascent in
the ridges, which could explain the
ÔchaoticÕ structure of the ridge core
and the coexistence of both hot and
cold contacts around the magmatic
bodies (Ibouh, 1995; Armando, 1999).
It is worth noting that the apatite
fission track results obtained from
these magmatic rocks suggest a steady
cooling from 140 to 50 Ma (Barbero
et al., 2007), which is consistent with
our interpretation.
Tertiary evolution
From the latest Cretaceous to the
Middle Eocene, the CHA and possibly
the entire High Atlas axis emerged
between two shallow marine gulfs
connected with the Atlantic Ocean
(Frizon de Lamotte et al., 2008; fig.
4.19). The palaeomorphology of the
Imilchil area before the Upper Palaeocene subaerial sedimentation likely
included almost undeformed plateaus
and breached diapiric–magmatic
ridges as: (i) if we subtract the effects
of the Neogene Atlas shortening from
the present-day cross-section, the
wide, open synclines between the
anticlinal ridges become virtually
horizontal; (ii) the Miocene–Pliocene
conglomerates exceptionally preserved
north of the Talmest-Tazoult Ridge
overlie the Middle Jurassic series
with a very shallow unconformity
angle (Bouchouata et al., 1995); (iii)
the preserved Palaeocene formations
(A)
unconformably overlie the ridge cores.
The Upper Palaeocene-? Eocene red
beds likely extended widely over the
CHA up to the Sub-Atlas Zones, but
the tiny preserved samples suggest the
occurrence of compartments with varied stratigraphic successions.
During the Late Eocene–Pliocene
shortening of the Atlas Belt (Frizon
de Lamotte et al., 2000; El Harfi
et al., 2001; Teson and Teixell,
2008), the pre-existing anticlinal
ridges were reactivated as anticlines
with diapiric core. The Upper Palaeocene-? Lower Eocene formations
included between the carbonate rims
of the anticlinal ridges were slightly
deformed into narrow, open synclines. Due to the successive episodes
of shortening and erosion, most of
the Upper Palaeocene-? Eocene
deposits disappeared from the major
synclines between the anticlinal
ridges. Only tiny recalls of these
subaerial deposits were preserved
within the ÔguttersÕ of the breached
ridges likely thanks to: (i) the sheltering effect of the carbonate rims
surrounding the gutters, and (ii) a
particular thickness of the Lower
Tertiary deposits due to the collapse
of their substratum by karstic dissolution of the Triassic evaporites (cf.
Fossen, 2010, fig. GR22). Moreover,
the Tasraft and Tassent deposits were
protected by the presence of interbedded limestones or basalts, respectively.
The ÔMakhteshimÕ of the Syrian Arc in
Israel, an inspiring comparison
In the Negev Desert, several anticlines
of the Syrian Arc (Fig. 7A) offer a
peculiar morphology of erosion cirques or breached valleys termed ÔmakhteshimÕ, sing. ÔmakhteshÕ, crater(s) in
Hebrew (Zilberman, 2000). It is
worthwhile comparing the ÔmakhteshimÕ evolution with that of the Atlas
STARs as: (i) they are both located in
the south-Tethyan continental margin, with noticeable similarities in
their Mesozoic sedimentary evolution
(Hirsch, 1990; Segev et al., 2006; Frizon de Lamotte et al., 2011); (ii)
alkaline magmatism occurred in both
areas with two main episodes, Late
Jurassic and Early Cretaceous, respectively (see above, and for the Negev:
Garfunkel and Derin, 1988), associated with uplift, erosion and red bed
accumulation; and (iii) both areas
suffered superimposed folding before
and after the Palaeocene–Eocene.
Zilberman (2000) recognized two
main periods in the ÔmakhteshimÕ folding evolution, pre- and post-Middle–
Late Eocene, respectively, separated
by the sedimentation of thin, unconformable shallow water series (Fig. 8
A). The CHA anticlinal ridges also
involve two main periods of deformation (Fig. 8B), almost coeval with that
of the Syrian Arc. However, in the
Negev ÔmakhteshimÕ, remains of the
intermediate, unconformable series are
(B)
Fig. 7 Makhtesh Ramon, a polyphase breached anticline from the Syrian Arc, Negev Desert, Israel. (A) Location map. (B)
Geological interpretation of the satellite view (Google earth).
2011 Blackwell Publishing Ltd
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Synclinal-topped anticlinal ridges • A. Michard et al.
Terra Nova, Vol 00, No. 0, 1–10
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(A)
(B)
Fig. 8 Comparing the evolution of the Negev ÔmakhteshimÕ (A, after Zilberman, 2000; slightly modified) with that of the Imilchil
STARs (B, this work). In (A), Late Jurassic–Early Cretaceous magmatism is featured with grey signature after Garfunkel and
Derin (1988). Early Cretaceous basalts also occur in Central High Atlas, but are not exposed around Imilchil.
only preserved outside the anticline
axis (Fig. 8A, stage d), and not in
Ôperched synclinesÕ on top of the anticline core. The peculiarity of the Atlas
STARs relies mainly on the early
diapiric origin of the CHA anticlines
contrasting with the late, open folds of
the Syrian Arc. This permitted, first,
the development of early breached
anticlines, and second, a thicker accumulation of unconformable sediments
onto the collapsing diapir cores (Fig. 8
8
B, stage b). Moreover, the stronger
shortening, uplift and consecutive erosion of the Atlas belt during the last
folding events explain why the Palaeocene–Eocene sequences are not preserved in the synclines next to the
ridges, contrary to the Negev case.
Conclusions
At least three of eight major anticlinal
ridges from the CHA have preserved
synclines of unconformable Thanetian-?Ypresian formations perched
onto the eroded ridge axes. To our
knowledge, they are the unique case of
ÔSTARsÕ with such geological and
geometrical characters (chaotic-intrusive core, faulted flanks) in the world.
The interpretation of these unique
structures is addressed herein for the
first time, based on field observations
and detail mapping, and on a comparison Ôall else equalÕ with the anti 2011 Blackwell Publishing Ltd
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A. Michard et al. • Synclinal-topped anticlinal ridges
.............................................................................................................................................................
clines from the Negev Desert. Our
conclusions are as follows:
1
2
3
4
A primary condition for STAR
development is halokinetic formation of an early anticlinal ridge.
This is the reason why they escape
the AlonsoÕs rule quoted above.
The diapiric ridge has to burst
through the surface and be deeply
eroded, which is favoured by
magmatic activity and associated
regional uplift.
Following the sedimentation of
an unconformable sedimentary
sequence, regional shortening will
form STARs that will be preserved
provided that potentially protecting competent lithologies do occur
in the underlying stratigraphic
column.
The isolation of the STARs with
respect to other coeval, classical
syncline occurrences depends on
an important regional uplift and
erosion.
Acknowledgements
Thanks are due to E.C. Rjimati for assistance and stimulating discussions in the
field, and to D. Avigad for documentation
on the Negev geology. A. M. acknowledges
a grant from the Direction du Développement minier (Dr A. Charik) of the Moroccan Ministry of Energy, Mines, Water and
Environment. Thorough reviewings by
Philippe Olivier and Antonio Teixell are
warmly acknowledged.
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Received 5 April 2011; revised version
accepted 30 June 2011
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