TECTONIC EVOLUTION OF NORTHEASTERN IRAN DEDUCED BY PALEOMAGNETIC INVESTIGATIONS

Abstract

In this thesis, an extensive paleomagnetic sampling (70 sites) was carried out in north-eastern Iran along the Allah Dagh, Binalud and Kopeh Dagh mountain belts, with the aim of reconstructing the rotation history and evaluating oroclinal processes of this portion of the Eurasia-Arabia collision area, as well as assessing whether the studied sediments have a magnetic fabric dominated by sedimentary or tectonic processes. I sampled the red beds units from the Lower Cretaceous Shurijeh Fm. and from the Middle-Upper Miocene Upper Red Fm (URF). The intracontinental deformation history of Northern Iran, was dominated by the long-standing convergence history between Eurasia and Perigondwanan terranes. Within this scenario Kopeh Dagh region is a crucial area, which today represents the north-eastern side of the Arabia-Eurasia collision zone, where the convergence is divided into compressional and strike-slip components. This scenario is largely debated by several authors, who mainly focused on active tectonics in the central and eastern part of the chain. This study area had represented a good target for understanding the Post Cimmerian evolution of north-eastern part of the iranian region. AMS data has been integrated with paleomagnetic results, confirming the predominance of Arabia Eurasia collision process as fundamental tectonic element which drove all the magnetic fabric configuration detected on the red beds units. The interpretation of magnetic fabric and paleomagnetic data in the Ala-Dagh, Binalud and Kopeh Dagh mountains, along the north-eastern margin of Iran investigated in this PhD thesis, allowed to further contribute to clarifying the tectonic evolution of this region and its role in the accommodation of the deformation related to the Arabia Eurasia collision. The integration with previous paleomagnetic results and GPS, seismological, geomorphological and structural data available in the area, allowed to propose a hypothesis of tectonic evolution of the north Iran-Caspian Sea area, from Middle-Late Miocene to Present. Knowing how the present-day kinematics is not able to explain the curvature of the northern Iran mountain belts, which has been acquired throughout oroclinal bending in a different tectonic regime (Mattei et al., 2017), our investigations confirm that this region represent a key area for defining and for understanding the timing of the tectonic processes which have occurred at the boundary between the Iranian deforming zone and the southern margin of stable Eurasia. Paleomagnetic results show a homogeneous amount of CW rotations measured in red bed units of both the Lower Cretaceous Shurijeh Fm. and Middle-Late Miocene URF., suggesting that the oroclinal bending process, which occurred in North Iran after the Middle-Late Miocene, also extended to the north-eastern border of the Arabia-Eurasia deforming zone in Iran. The curvature of the Northern Iran mountain belts is due to the collision of Central Iran with two rigid blocks, the oceanic South Caspian Sea and the continental Turan platform, which represent the southern margin of Eurasia in the area. Taking together all the data collected in the region (from this work, Bazhenov, 1987; Mattei et al., 2017) it can be affirmed that Eastern Alborz, Binalud-Fariman Mts. and Kopeh Dagh underwent comparable amount of CW rotations in both Cretaceous, Paleocene and Neogene deposits. The integrations of paleomagnetic analysis with AMS analysis allow to define the tectonic origin of the magnetic lineation in the sampled Formations. In fact, showing the same NW-SE orientation of magnetic lineation, both for Cretaceous and Neogene units, the magnetic fabric investigation confirm how the deformation in the area was dominated mainly by the Arabia-Eurasia collision. It is proposed that the measured pattern of paleomagnetic rotations has occurred in a tectonic regime different from the present-day one, and that the tectonic reorganization of north-eastern Iran is relatively young and therefore its rotational pattern cannot be detected using paleomagnetism. Paleomagnetic rotations occurred between ≈6-4 Ma and ≈2Ma, before the beginning of the westward extrusion of the South Caspian Block caused a drastic change in the tectonic regime of the area. In this reconstruction, the initiation of the northward subduction of the South Caspian Sea underneath the Aspheron Sill (≈ 6-4 Ma) preceded the initiation of strike-slip tectonics in north eastern Iran (≈ 2Ma), which accompanies the westward movement of the South Caspian block. Previous paleomagnetic results in northern Iran (Ballato et al., 2008; Cifelli et al., 2015; Mattei et al., 2017) which showed a 15–20° CW rotation for the WNW-ESE oriented western Alborz and Rivand and Samghan fold belts and 20°CCW rotation for the WSW-ENE oriented central Alborz Mts. , has been interpreted as the result of an oroclinal bending process that occurred in the Alborz Mts. after Late Miocene. Our paleomagnetic results from the Kopeh Dagh, Allah Dagh and Binalud-Fariman ranges confirm that the WNW-ESE oriented structure of north-eastern Iran underwent CW rotations after Late Miocene, supporting the oroclinal bending hypothesis of the whole mountain range. Furthermore, the absence of differential rotations in the Shurijeh Fm. and URF excludes that the oroclinal bending processes started earlier than Late Miocene. Our paleomagnetic data also prove that oroclinal bending and CW rotation in north-eastern Iran are not limited to the Alborz Mts. but also extend to the southeast (Binalud Mts.) and to the outer border of the deformation structures related to the Arabia-Eurasia convergence (Kopeh Dagh Mts.) Another important contribution from this work is represented by the reinterpretation of the previous paleomagnetic data collected by Bazhenov (1987) in the Turkmenistan portion of the Kopeh-Dagh Mountains as part of a new tectonic framework. No relationships were found between the fold axes orientation and the amount of vertical axis rotations; that absence is interpreted not as a primary curvature of the fold axes but as the consequence of the different tectonic location of the sampled sites. The sites with no or limited amount of CW rotation are located in a restraining bend due to interaction of two segments of the right-lateral Ashkhabad fault, whereas larger CW rotations have been interpreted as the results of block rotations of a left-lateral strike-slip fault system which accommodate the N-S convergence between Iran and Eurasia and the E-W elongation due to the westward extrusion of the South Caspian Block.