Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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408 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
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. References S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 409 [1] D.W. Burbank, R.G.H. Raynolds, Stratigraphic keys to the timing of thrusting in terrestrial foreland basins: applications to the Northwestern Himalaya, in: K.L. Kleinspehn, C. Paola (Eds.), New Perspectives in Basin Analysis, Springer-Verlag, New York, 1988, pp. 331–351. [2] J.H. Reynolds, T.E. Jordan, N.M. Johnson, J.F. Damanti, K.D. Tabbutt, Neogene deformation of the flat-subduction segment of the Argentine–Chilean Andes: magnetostratigraphic constraints from Las Juntas, La Rioja province, Argentina, Geological Society of America Bulletin 102 (1990) 1607–1622. [3] T.E. 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