Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410
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