Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

Magnetostratigraphy of Miocene–Pliocene Zagros foreland
deposits in the front of the Push-e Kush Arc
(Lurestan Province, Iran)
Stéphane Homke a, *, Jaume Vergés a , Miguel Garcés b , Hadi Emami a , Ridvan Karpuz c
a Group of Dynamics of the Lithosphere, Institute of Earth Sciences bJaume AlmeraQ, CSIC, Lluís Solé i Sabarís, s/n, 08028 Barcelona, Spain
b Group of Geodynamics and Basin Analysis, Facultat de Geología, University of Barcelona, Campus de Pedralbes, 08028 Barcelona, Spain
c Hydro Zagros Oil and Gas, Bokharest Bld., Bokharest St., Tehran, 15137 Iran
Abstract
Earth and Planetary Science Letters 225 (2004) 397–410
Received 8 December 2003; received in revised form 8 May 2004; accepted 2 July 2004
Available online 10 August 2004
Editor: V. Courtillot
The timing of the deformation in the Zagros Simply Folded Belt is poorly constrained because of the lack of an accurate
absolute chronology of the syntectonic sedimentary sequences. The foreland basin infill at the front of the Push-e-Kush Arc is
composed of fine-grained fluvial plain deposits (Agha Jari Fm.) and coarse conglomerates at the top of the section (Bakhtyari
Fm.). A magnetostratigraphic study was carried out in a composite section spanning about 2800 m in order to date growth
strata, and to constrain the timing of the deformation in the Mountain Front Flexure (MFF). Magnetostratigraphic correlation of
the base of the Agha Jari Fm. with chron C5A yields an age of 12.8 to 12.3 Ma for this base. The transition to the conglomerates
of the Bakhtyari Fm. correlates with the upper Gauss chron C2An at approximately 3 Ma. The deposition age of the top of the
preserved Bakhtyari Fm. is extrapolated around the Pliocene–Pleistocene boundary.
The base of the Agha Jari Fm. growth strata, and thus the beginning of the deformation in the front of the Push-e Kush Arc,
is dated at 8.1–7.2 Ma. The topmost preserved Bakhtyari is folded in the NE flank of the Changuleh anticline and is
unconformably overlying the SW flank of the Anaran anticline. This indicates that the tectonic deformation in the front of the
Push-e-Kush Arc was active at least during 5 My. The MFF is a relatively long-lived structure active from 8.1 to 7.2 Ma to
about the Pliocene–Pleistocene boundary, partly synchronous with the Changuleh anticline to the foreland. After MFF tectonic
cessation, only the Changuleh anticline remained active.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Zagros Simply Folded Belt; Lurestan province; Push-e Kush Arc; magnetostratigraphy; timing of deformation; Neogene foreland
basin
* Corresponding author. Tel.: +34 93 409 54 10; fax: +34 93 411 00 12.
E-mail address: shomke@ija.csic.es (S. Homke).
0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2004.07.002
www.elsevier.com/locate/epsl

398
1. Introduction
S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410
Dating of syntectonic foreland basin deposits plays
a central role in determining the timing of the
deformation of a fold-and-thrust system in an orogenic
belt. Magnetostratigraphy provides the most
suitable tool for dating continental sedimentary
sequences in foreland basins. Magnetostratigraphy
has been successfully applied to several foreland
basins in the Himalayas [1], the Andes [2,3], the Alps
[4] and the Pyrenees [5]. Contrastingly, foreland basin
syntectonic deposits in the front of the Zagros have
never been accurately dated.
The SE–NW trending Zagros belt is the result of
the collision between the Arabian and the Persian
Plates. The shortening is accommodated in the thick
sedimentary cover with a deformation decreasing
from the Suture Zone to the present deformation
front, near the Iraq border [6,7]. The belt is divided
from the NE to the SW into four zones (Fig. 1), i.e. the
Sanandaj–Sirjan metamorphic zone, the Imbricated
Belt dominated by thrusting, the Simply Folded Belt
characterized by folding, and the Mesopotamian
foreland basin with buried folds, which extends to
the SE into the Persian Gulf [8,9]. The Mountain
Front Flexure (MFF) [10], characterized by high
structural relief and high topographic altitudes, is the
boundary between the Folded Belt and the foreland
basin. It presents an irregular geometry showing
salients and embayments (Fig. 1): the Coastal Fars
Arc to the SE and the Push-e Kush Arc to the NW,
separated by the Dezful Embayment [7,11–13]
(Fig. 1).
The beginning of compression in the Zagros Belt is
poorly dated. The initial Arabian–Central Iranian
continental collision is considered to be Late Cretaceous
[7,14,15], Eocene–Oligocene [12], Oligocene–
Miocene [16] or Late Miocene in age [17,18]. The
most spectacular deformation took place in the Simply
Folded domain displaying the well-known whaleback
anticlines, which has been dated using both plate
motions and foreland unconformities. Wells [19]
suggested that bthe mountain building in the SW
IranQ is related to the movement of the Arabian Plate
due to the opening of the Red Sea, which began
during the Miocene. Berberian and King [15] pro-
Fig. 1. Localization of the study sections. Boxes delimit areas displayed in Figs. 3 and 4. Abbreviations used in the legend are the following:
Bk=Bakhtyari Fm., Aj=Agha Jari Fm., Gs=Gachsaran Fm., As =Asmari Fm., Pd=Pabdeh Fm., Gu=Gurpi Fm., Bgp=Bangestan Group.

S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 399
posed then that folding in the Zagros fold-and-thrust
belt started around 5 Ma, coinciding with the second
phase of extension in the Red Sea and Gulf of Aden.
Falcon [10] argued in 1961 that the rapid change to
evaporitic conditions at the end of deposition of the
Asmari Fm. (Fig. 2), during the Lower Miocene,
indicated the onset of tectonic activity in the Folded
Belt. He suggested then, based on the unconformity
between the Agha Jari Fm. and the Bakhtyari Fm.
(Fig. 2), that the deformation was initiated at the early
Pliocene [9]. It is nevertheless now accepted that the
maximum deformation episode occurred during the
late Pliocene, before this major unconformity [14,20–
22]. More recently, Hamzepour et al. [23] suggested
from sedimentological and structural observations that
the folding in the MFF occurred during the late
Pliocene. Hessami et al. [24] proposed then on the
basis of several unconformities at different stratigraphic
levels, a deformation occurring by pulses
since the end of the Eocene, and reaching the front of
the folded belt during an end-Pliocene phase. All
these estimations are based on ages of unconformities
and sediment formations mostly provided in 1965 by
James and Wynd [20]. Documented Holocene anticline
growth [25,26] and recent seismicity [27]
indicate that the deformation in the Zagros belt is
still active, especially at deep crustal levels.
Despite these studies, the timing of the deformation
in the Zagros Simply Folded Belt remains poorly
Fig. 2. Schematic stratigraphic column of the Mesozoic–Cenozoic
cover rocks of the Zagros Fold Belt.
constrained because of the lack of precise dating of
syntectonic sediments. In this paper, we provide for
the first time an absolute dating of growth strata in the
front of the Push-e Kush Arc (Fig. 1), by means of a
magnetostratigraphic study. The timing of the growth
of the frontal folds of the Push-e Kush Arc as well as
its duration is thus directly constrained.
2. Upper Miocene to Pliocene foreland
stratigraphy
The uppermost part of the 10–12-km-thick sedimentary
pile in the Zagros corresponds to foreland
basin sediments, deposited on top of passive margin
sedimentary rocks ranging from upper Paleozoic to
Late Cretaceous [8,20]. Above the Asmari Fm.
(Fig. 2), the top of the Gachsaran evaporites forms
the base of the sampled succession, which encompasses
the complete thick fluvial Agha Jari Fm. and
the lowermost part of the alluvial conglomerates of
the Bakhtyari Fm.
Two main sections were sampled in order to obtain
a complete succession of the Agha Jari Fm. The first
one was sampled along the southern flank of the
Zarrinabad syncline, which is located to the NE of the
Anaran anticline (and the MFF), in the front of the
Push-e Kush Arc (Fig. 1). The Zarrinabad syncline
forms a closed double plunging structure, with fairly
constant thickness of the outcropping Agha Jari Fm.
around the structure (Fig. 3). The studied section
encompasses the upper 45 m of the Gachsaran Fm. as
well as 780 m of Agha Jari Fm. (Fig. 3). The second
main section is sampled along the common flank of
the Changuleh anticline–syncline pair, to the SW of
the Anaran anticline (and the MFF). At the SE end of
the structure, the moderate NW plunge of the
Changuleh syncline axis permits to observe clear
growth geometry, with the upper part of the Agha Jari
Fm. thinning and onlapping the pre-growth Agha Jari
deposits (Fig. 4). The sampled section is apparently
complete, with a progressive dip decrease toward the
NE. It encompasses the upper 815 m of the Lower
Agha Jari Fm., the 835 m of the Upper Agha Jari Fm.
(Lahbari Mb.), and 200 m of Bakhtyari Fm. (Fig. 4).
On the NE flank of the Changuleh growth syncline,
the near horizontal top of the preserved Bakhtyari
conglomerates overly unconformably the southwest

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S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410
Fig. 3. Geological map showing the location of the Zarrinabad syncline section (modified from the National Iranian Oil Company geological
map of Iran at scale 1:100,000, satellite images and field data). The stratigraphic column of the section is displayed.
dipping Agha Jari Fm. A third and very short section
(150 m) was sampled on the NE flank of the
Changuleh syncline, across the Gachsaran–Agha Jari
contact (Fig. 4). Structural cross sections across this
area indicate that overlap between the Zarrinabad and
Changuleh sections is of about 45 m, assuming
constant thickness for pre-growth strata in both areas.
The two sections provide thus a complete record of
the continental sediment succession at the front of the
Push-e Kush Arc.
The uppermost part of the Gachsaran Fm. in both
the Zarrinabad and Changuleh synclines consists of
well-bedded evaporites, metric blue to red silty-clay
beds, and green to brown sandstones with a thickness
ranging between 10 cm and 2 m, which
parallel the Gachsaran–Agha Jari contact (Fig. 3).
The Agha Jari Fm. is made up of brown to red
silty-clay layers intercalated with grey to brown
sandstone beds (Fig. 3), which form channels with
variable extensions. In the basal 50 m of the
formation, sandstone beds are generally of decimetric
scale, and are separated by decimetric to metric
brown silt layers. They exhibit at the top some
ripple marks that are mainly symmetrical. Gypsum
veins are common. Towards the middle of the
section, well-consolidated sandstone beds suddenly
become more spaced and thicker, with metric
thickness and extension ranging from several tens
to several hundreds of meters. They display
numerous well-developed cross bedding structures.
In the upper part of the Lower Agha Jari Fm.,
observed on the Changuleh anticline, sandstone
layers also present paleoflow indications, principally
oriented toward the SE (Fig. 4). Recurrence,

S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 401
Fig. 4. Geological map showing the locations of the Changuleh anticline and Changuleh syncline sections, respectively, on the left and on the
right of the map (modified from the National Iranian Oil Company geological map of Iran at scale 1:100,000, satellite images and field data).
The stratigraphic columns of the sections are displayed.
dimensions and granulometry of sandstone layers
increase however significantly in the 130 m
preceding the transition to the Lahbari Mb. Some
conglomeratic levels, characterized by thin gravels,
appear indeed at the base of sandstones. Sandstones
of the Lahbari Mb. are less consolidated than those
of the Lower Agha Jari Fm. A large part of
sandstone layers, generally thinner than 5 m, are
covered by mud, conferring a relatively smooth
topography that contrasts clearly with the relief of
the lower part of the formation. The beige colour of
the mud is equally a distinctive characteristic. Near
the top of the formation, several sandstone beds
contain thin dark gravels with pebbles smaller than
5 cm. The conglomerates of the Bakhtyari Fm. have
a characteristic rough topography which is often
difficult to access. They consist of conglomerates
mostly made of grey carbonatic and dark brown
clasts with a varying amount of interbedded fine
brown sandstones (Fig. 4). The clast size increases
toward the top of the formation.
Ages of the foreland sediments of the Zagros belt
have been constrained by biostratigraphic studies,
compiled in 1965 by James and Wynd [20]. According
to James and Wynd [20], the Gachsaran Fm. was
dated in the Lurestan and the Khuzestan provinces as
late early Miocene. A Miocene to Pliocene age is
given for the Agha Jari Fm. It was not specified in

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S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410
which region the study was performed, except for the
Lahbari Mb., which was dated as Pliocene in the
Khuzestan Province. No diagnostic fossils have been
found in the conglomeratic Bakhtyari Fm., but it is
however considered to be late Pliocene or younger in
age [20].
3. Magnetostratigraphy
3.1. Sampling strategy
A total of 149 sites were sampled along 2580 m of
total section, every 15 m although due to the logistic
Fig. 5. Graphics (A) to (F) are representative NRM demagnetization diagrams. Black (white) points represent horizontal (vertical) projections of
the NRM vector end-points during demagnetization. Graphics (A) to (C) are related to samples from the Changuleh anticline, and graphics (D)
to (F) are related to the Zarrinabad syncline.

problems of this area close to the Iran–Iraq Border,
some of the sampling intervals were up to 30 m.
Overall, 46 sites were sampled in the Zarrinabad
syncline, 94 on the Changuleh anticline, and 9 in the
Changuleh syncline with two cores per site.
3.2. Paleomagnetic analysis
S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 403
Paleomagnetic samples were analysed at the Laboratory
of Paleomagnetism of the CSIC-University of
Barcelona at the Institute of Earth Sciences bJaume
AlmeraQ in Barcelona (Spain). The Natural Remanent
Magnetization (NRM) was measured in a three axes
superconducting rock magnetometer (2G Enterprises)
and stepwise thermal demagnetization was applied to
all samples up to complete removal of the NRM. This
allowed us to isolate the different paleomagnetic
components and to interpret the demagnetization data
from the vector endpoint diagrams (Fig. 5) [28].
Stereographic projections of stable components are
shown in Fig. 6. A limited number of representative
samples were demagnetised using a tumbling AF
demagnetizer (Fig. 5C). These same samples were
then subjected to stepwise IRM acquisition in order to
estimate the remanence carrying mineralogy (Fig. 7A).
The average NRM intensity was of the order of 10 3
A/m, ranging from 10 4 to 0.14 A/m. In most of the
samples, thermal treatment revealed the presence of a
low-temperature component, which parallels the
present north-directed ambient field. This recent over-
print was removed after moderate heating to 250–350
8C (Fig. 5B,D,E,F) or by applying a low alternating
field of 5 mT (Fig. 5C). A stable characteristic
remanent magnetization (ChRM) showing either a
normal (Figs. 5A,D and 6) or reverse polarity (Figs.
5B,C,E and 6) was commonly observed, representing
more than 50% of the initial NRM.
Demagnetization of the ChRM revealed a linear
trend towards the origin with maximum unblocking
temperatures typical of hematite, ranging from 630 to
650 8C (Figs. 5A,D and 7A). Some samples, however,
showed a significant decay of the remanence from 500
to 600 8C (Figs. 5B and 7A), which suggests the
additional occurrence of magnetite. This is supported
by the low coercivity of the ChRM component
revealed by AF demagnetization of some samples
(Fig. 5C). This is also in agreement with IRM
acquisition experiments, which often reveal steep
acquisition curves at fields lower than 0.1 T (Fig.
7B). In many samples, most of the IRM is acquired at
relatively low fields (b0.3 T), but remain unsaturated
at fields up to 1.0 T (Fig. 7B). All the observations are
in agreement with a magnetic mineralogy consisting
of a mixture of both magnetite and hematites in
varying proportions.
A small number of samples exhibited a complex
demagnetization trend (Fig. 5F), with the presence of
an intermediate temperature component (B component).
The B component is easily identifiable when
recording a magnetization opposed to both the ChRM
Fig. 6. Equal-area stereographic projections of characteristic directions (normal and reversed polarities). Mean directions with 95% confidence
limit are displayed. N = number of directions; dec=declination; inc=inclination; k = precision parameter; a95 =confidence limit.

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S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410
Fig. 7. (A) Representative normalized NRM decay curves. (B) Representative normalized IRM acquisition curves.
and the low-temperature recent overprint. The progressive
demagnetization of all three components
confers a distinctive bSQ shape to the Zijderveld plots
(Fig. 5F). IRM experiments do not provide evidence
for a distinct magnetic mineralogy associated with the
B component. Given that its occurrence is limited to a
few sites, the origin of the B component is likely
related to an early post-depositional magnetization
and not to a late widespread remagnetization event.
Delayed magnetization can lead to the record of two
successive polarities when a geomagnetic reversal
occurs soon after deposition. Some samples with a
reversed B component occur in the middle of a long
normal polarity magnetozone (Figs. 5F and 8). These
are interpreted as recording very short geomagnetic
events (cryptochrons) occurring in the middle of
larger chrons.
The bulk susceptibility of the samples was
measured after each thermal demagnetization step
using a KLY-2 susceptibility bridge. Most of samples
present a similar rapid increase of susceptibility at
about 400 8C, which is probably the result of
magnetite formation upon heating. No directional
changes or remanence intensity peaks correlate with
the temperature intervals of increasing susceptibility,
indicating that the new forming magnetic minerals are
not contributing to the net remanence of the samples.
Samples are divided into three classes reflecting
the quality of the demagnetization analysis (Fig. 8).
The first class contains samples with an ideal
demagnetization pattern (Fig. 5A to F). Samples of
second class present poorer quality demagnetization
curves, but the polarity of the Virtual Geomagnetic
Pole (VGP) is clearly identifiable from their ChRM
(Fig. 5G,H). The third class includes samples with an
unclear demagnetization pattern (Fig. 5I). Results are
very good, with 78.5% of the sampled sites represented
by at least one first-class sample, 18.1%
represented by second-class samples, and only 3.4%
only represented by third-class samples (Fig. 8).
Normal and reverse mean directions in both
sections passed the reversal test with class C [29],
indicating that the ChRM was successfully isolated.
The mean inclinations of about 358 are significantly
lower than that expected from the geocentric axial
dipole model at the site (528). Such a discrepancy can
be attributed to an inclination error induced upon
deposition and early compaction of the sediments,
which is often observed in red alluvial sediments
carrying a detrital remanence [30,31]. The mean
paleomagnetic declinations do not provide evidence
for statistically significant vertical axis rotation of the
rocks after magnetization. The mean direction of the
Changuleh anticline Section is rotated about 78
clockwise, a variation which is within the angular
error of the mean (Fig. 6).
The ChRM directions were used for calculation of
the Virtual Geomagnetic Pole (VGP) latitude at each

S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 405
Fig. 8. Stratigraphic columns, magnetic polarity sequences and plots of stratigraphic position vs. Virtual Geomagnetic Pole Latitude for the three
studied sections. Black zones represent normal polarities, while white zones represent reversal polarities. Positions of samples located in Figs. 3
and 4 are indicated. See the text for the sample class characteristics.
stratigraphic level. Positive and negative VGP latitudes
were interpreted as normal and reverse polarity,
respectively, in order to construct a local magnetic
polarity stratigraphy for each of the three sections
studied (Fig. 8). Most of the magnetozones were
determined by two or more consecutive sites, averaging
3.5 sites per magnetozone in the Changuleh
anticline and 2.5 in the Zarrinabad syncline, but,
respectively, 6 and 7 magnetozones (23% and 39% of
the total) were represented by only one site. This
indicates that the recovered magnetostratigraphic
record may be incomplete, and that the chances of
missing chrons of short duration (under 10 5 year) of
the GPTS may be significant. However, as we will see
below, except for the lowermost part of the series
which is problematic, the good match of the polarity
stratigraphy with the GPTS suggests a sufficient
recovering of true reversal magnetic sequence.

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S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410
4. Correlation to the geomagnetic polarity time
scale (GPTS)
The weak absolute age constraints in the studied
magnetostratigraphic sections have lead to a mostly
independent correlation with the GPTS, largely
based on a best-fit solution. The correlation of the
upper part of the local magnetic polarity sequence
with the GPTS [32,33] is based on the pattern of
magnetozones recorded between 1326 and 2214 m,
which matches the sequence of reversals of the
lower Pliocene Gilbert chron (C2Ar, C3n, C3r), the
C3An chron, and the C3Ar chron (Fig. 9). This is
coherent with biostratigraphic data, which indicates
a Miocene to Pliocene age for the Agha Jari Fm.
[20]. Above and below this interval, the correlation
becomes less evident. But even if the detailed
polarity pattern is not clearly represented, some
Fig. 9. Correlation of the magnetic polarity sequences of the studied section to the GPTS. Sedimentation rates are given in cm/ka. Approximate
positions of the base of the growth strata, calculated in different localities of the Changuleh region, are displayed.

S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 407
intervals with dominant polarities can be correlated
with the GPTS. Consequently, the reversed magnetozone
at the top of the series should correspond to
the C2r chron, and the normal polarity interval
recorded around 850 m should correlate with chron
C4An (Fig. 9). The reliability of this part of the
correlation is further supported by the high quality
of the sample demagnetization data. Only 3.2% of
the samples yielded class 3 directions (Fig. 8), while
87.2% were of class 1.
The lower part of the magnetic polarity sequence
presents a large normal polarity interval, recorded over
265 m of sediments. Since biostratigraphic ages
constrains the correlation to the late Miocene and the
Pliocene [20], this large normal polarity interval must
be correlated with chron C5n (Fig. 9). Consequently,
the reverse polarity recorded around 800 m must be
related to the uppermost sub-chron of the C4Ar chron.
Four samples collected between 400 and 600 m
present an intermediate magnetic component with
reversed polarity. They could represent the record of
cryptochrons occurring during this long normal period,
i.e. the C5n.2n-1, C5n.2n-2 and C5n.2n-3 cryptochrons,
and the C5n1r subchron.
Despite the good quality of the samples from the
Zarrinabad syncline (none of the sites is represented by
only third-class samples) (Fig. 8), the correlation of the
lowermost part of the section is uncertain. The
assumed early Miocene age of the Gachsaran Fm.
based on regional biostratigraphic data [20] suggests a
hiatus of more than 3 My at the base of the section. If
we assume, however, a continuous sedimentation, the
normal polarity recorded between 115 and 178 m
could be correlated with chron C5An (Fig. 9),
providing the youngest possible correlation for the
base of the section. The magnetic polarity sequence
obtained in the Changuleh syncline does not provide
better constraints. Within the assumption of a continuous
sedimentation, the youngest possible correlation
to the GPTS can only be based on the geometric
relationship with the Changuleh anticline section (Fig.
9), because of the insufficient thickness of the sampled
section and the poor quality of paleomagnetic analyses,
22.2% of analysed sites being represented by
third-class samples (Fig. 8).
Thus, except for the lowermost part of the studied
stratigraphic column, the magnetostratigraphy study
provides a very reliable dating for the Agha Jari Fm.
5. Results
5.1. Age of Agha Jari Fm.
The age of the base of the fluvial Agha Jari Fm. is
little constrained. If the sedimentation has been
continue through the deposition of the top of the
Gachsaran Fm. and the Agha Jari Fm., the transition
between the two formations would be dated around
12.8 Ma in the Zarrinabad syncline and at 12.3 Ma in
the Changuleh syncline (Fig. 9), both in the middle
part of the Serravalian (late middle Miocene). The
transition could however take place as soon as the
early Miocene, as suggested by biostratigraphic age of
the Gachsaran Fm. [20], implying the presence of a
hiatus at the base of the series. The deposition of the
Lahbari Mb. begins in the Changuleh region at 5.5
Ma, at the end of the Messinian (latest Miocene)
(Fig. 9). This fine-grained member is followed by
deposition of conglomerates of the Bakhtyari Fm.,
starting at 3 Ma. The last age control is located 50 m
above the base of the Bakhtyari Fm., which is dated at
2.5 Ma (Fig. 9).
The long-term sedimentation derived from magnetostratigraphy
averages 26 cm/ky, which is well in
the range of the Miocene alluvial sedimentation in
the Himalayan foreland [34]. Short-term sedimentation
trends are smooth and vary gradually during the
deposition of the Agha Jari Fm. The sediment
accumulation rate does not vary greatly during the
deposition of the Agha Jari Fm (Fig. 9). At the base
of the series, the deposition rate of the lowermost
Agha Jari Fm. is about 19.5 cm/ka. The rate
increased then regularly, reaching 30.5 cm/ka around
700 m, and slowed down to 19.5 cm/ka after the
deposition of the lower part of the Agha Jari growth
strata. The rate starts again to increase few tens of
meters below the top of the Lower Agha Jari Fm.,
until reaching about 33 cm/ka in the upper part of
the Lahbari Mb. and in the lower 50 m of the
Bakhtyari Fm. (Fig. 9). In the Zarrinabad syncline
section, sedimentation increases gradually from
bottom (19.5 cm/ky) to top (30.5 cm/ky).
If we assume constant rates of deposition through
the Bakhtyari conglomerates, the extrapolated age for
the youngest preserved strata in the Changuleh
syncline is close to the Pliocene–Pleistocene boundary
(Fig. 9).

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Channel incisions and other paleocurrent indicators
in the Lower Agha Jari Fm. yield paleoflow directions
mainly oriented towards the SE (Fig. 4), sub-parallel to
the axis of the foreland basin. This longitudinal
paleoflow for Agha Jari deposits is similar to present
Tigris and Euphrates fluvial systems in the Mesopotamian
foreland basin. The pre-growth Agha Jari
paleo-fluvial system was shifted to the SW during
the growth of individual structures and uplift of the
Push-e Kush Arc.
5.2. Timing of the deformation of the Main Front
Flexure
The magnetostratigraphic dating of the pre-growth
and growth foreland sediments in the front of the
Push-e Kush Arc constrains the growth of frontal
structures like the Anaran and Changuleh anticlines
(Fig. 4). According to these ages, the onset of folding
in the frontal structure occurred after the deposition of
1100–1300 m of the Agha Jari fluvial deposits with an
age of 8.1–7.2 Ma, corresponding to the Tortonian
(Fig. 9).
On the NE flank of the Changuleh syncline, the
uppermost preserved part of the Bakhtyari Fm. is subhorizontal
and overlies the steep SW-dipping beds of
the Agha Jari Fm. Tectonic relationships together with
the assumed ages for the Bakhtyari Fm. indicate that
the growth of the Mountain Front Flexure ended
around 2.5 Ma, after a relatively long period of
tectonic activity of about 5 My or longer (Fig. 9). The
Bakhtyari Fm. appears however slightly tilted and
concordant with the Lahbari Mb. on the NE flank of
the Changuleh anticline, indicating that this foreland
structure continued its growth after the cessation of
the MFF.
These observations point out, at the scale of the
studied zone, a progression of the deformation
toward the SW, coherent with the extended assumption
of a progression of the deformation front in this
direction since the initiation of the Zagros orogen
[7,24]. If the deformation migrated following a
foreland sequence, the deformation in the hinterland
of the Simply Folded Belt must have started before
8.1–7.2 Ma, i.e. earlier than the beginning of the
second phase of extension in the Red Sea [15] and
well before the usually proposed late Pliocene
maximum folding phase [14,20–23].
A different possibility, however, could be that
folding did not follow a simple foreland sequence but
a more complex one. Supplementary absolute dating
of the tectonic activity in the hinterland of the belt is
nevertheless necessary to determine the mode of
progression of the deformation in the Zagros fold
and thrust belt.
6. Conclusions
Magnetostratigraphy of the complete section of the
pre-growth and growth Agha Jari deposits on the front
of the Push-e Kush Arc accounts for the timing of the
deformation on the Mountain Front Flexure. The
correlation of magnetic polarity sequences to the
GPTS indicates that the deposition of these continental
sediments started not later than 12.8 Ma in the
Zarrinabad syncline and 12.3 Ma in the Changuleh
region (middle part of the Serravalian). The Lahbari
Mb. of the Agha Jari Fm. was deposited between 5.5
Ma (uppermost Messinian) and 3 Ma (late Pliocene).
The deposition of conglomerates of the Bakhtyari Fm.
started after 3 Ma. The sediment accumulation rates of
the Agha Jari Fm. globally increased from 19.5 cm/ka
at the base to 33 cm/ka at the top of the formation.
The onset of the deformation in the front of the
Push-e Kush Arc, related to the base of the growth
strata observed in the NE flank of the Changuleh
syncline, is dated between 8.1 and 7.2 Ma, during the
Tortonian. Moreover, the fossilization of the folding
by the top of the preserved Bakhtyari Fm. indicates
that the growth of the frontal anticline ended after 2.5
Ma, around the Pliocene–Pleistocene boundary. The
deformation in the Mountain Front Flexure lasted for
at least 5 My. The Bakhtyari Fm. appears however
tilted on the NE flank of the Changuleh anticline,
indicating a progression of the deformation toward the
SW at the scale of our study area.
Acknowledgments
This study has been financed by a collaborative
project between the Institute of Earth Sciences bJaume
AlmeraQ, CSIC of Barcelona (Spain) and the Norsk
Hydro Research Centre of Bergen (Norway), with the
partial support of project 2001 SGR 00339 Grup

d’Estructura i Processos Litosfèrics. We also thank the
support in the field of Hydro Zagros Oil and Gas
Tehran and NPA people, and the National Iranian Oil
Company (NIOC) for their collaboration during this
project. We thank finally W. Lowrie and an anonymous
reviewer for their constructive remarks and
suggestions.
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