Research Article
Timing and forcing mechanism of the final Neotethys seawater retreat from Central Iran in response to the Arabia-Asia collision in the late early Miocene

https://doi.org/10.1016/j.gloplacha.2020.103395Get rights and content

Highlights

  • New chronology of the Upper Red Formation in Central Iran.

  • Final marine regression from Central Iran at ca. 16.8 Ma.

  • Final seawater retreat controlled by the Arabia-Asia plate collision.

  • Enhanced aridification in Central Iran since 13 Ma.

Abstract

The Iranian Plateau lies in the middle of the Neotethys tectonic domain; to the east it connects with the Tibetan Plateau, and to the west with the Anatolian Plateau and the Alpine orogenic belts. In the Cenozoic it underwent land/sea changes and tectonic uplift in response to the Arabia-Asia plate collision. One of the most prominent geological consequences of the collision was the disappearance of an epicontinental sea in Central Iran, which was a northeastern branch of the Neotethys seaway in the Oligocene-early Miocene. The timing of the final seawater retreat from Central Iran and its forcing mechanism are important for understanding the effects of Arabia-Asia plate collision as well as global eustatic sea-level changes. In this paper we present new magnetostratigraphy, Usingle bondPb ages of one tuffaceous bed, and stable isotopic records of carbonates. The results of this multidisciplinary study indicate that a shallow open sea in the Qom back-arc basin ended at 17 Ma, but the final seawater retreat from a restricted marine environment was at 16.8 Ma. The final regression was just before the Middle Miocene Optimum, implying that it was not related to a climatic factor, but driven by the Arabia-Asia plate collision. Moreover, the δ18O record of fine-grained lacustrine carbonates indicates an isotopic shift towards more positive isotopes after 13 Ma, suggesting an enhanced aridification in Central Iran. This climatic deterioration was a response to reduced transport of moisture by westerlies from a retreating Neotethys Sea driven by a global eustatic sea-level drop in response to the East Antarctica ice-sheet expansion after the middle Miocene Optimum.

Introduction

During the early Cenozoic the Neotethys Sea extended from Europe to Central Asia, connecting with the Atlantic Ocean to the west and the Indian Ocean to the southeast (e.g., Rögl, 1999; Popov et al., 2004). The diachronous closure of the Neotethys Sea is important for evaluating the effects of plate convergence between the India, Africa, and Arabia and the Eurasia, as well as global eustatic sea-level fluctuations (Rögl, 1998, Rögl, 1999).

One of the most important questions about the history of the Neotethys Sea is the time and cause of the closure of the Tethyan Seaway between Arabia and Asia. Although the continental collision between Arabia and Asia might be as early as 35 to 20 Ma (McQuarrie et al., 2003; Allen and Armstrong, 2008; Agard et al., 2011; McQuarrie and van Hinsbergen, 2013; Saura et al., 2015; Pirouz et al., 2017), a wide epicontinental sea, as the northeastern branch of the Tethyan seaway, was still in existence in Central Iran in the Oligocene-early Miocene (e.g., Popov et al., 2004; Reuter et al., 2009), and a 150–500 km-wide Tethyan seaway connecting the Indian Ocean and the western Neotethys lasted to the middle Miocene (Popov et al., 2004; Pirouz et al., 2017).

To date, there have been different views about the timing of the final seawater retreat in different regions of the Iranian Plateau. Homke et al. (2004) suggested a late Miocene age of 12.8 to 12.3 Ma for the topmost marine sedimentation in the northwest part of the Zagros foreland basin, whereas Pirouz et al. (2015) proposed a diachronous marine-continental transition from 14 to 12 Ma in the western Zagros foreland and from 8 to 1 Ma in its eastern sector.

Different from the above studies about chronology of the Cenozoic marine-terrestrial sequences in the Zagros foreland basin, our study focused on the timing of final marine regression in the Qom back-arc basin in Central Iran. Although there have been many studies about the age of the last marine regression in Central Iran, they were largely based on biostratigraphic data, including benthic/planktic foraminifera, nannofossils, ostracods, gastropoda, and corals (e.g., Khaksar and Moghadam, 2007; Zhu et al., 2007; Reuter et al., 2009; Behforouzi and Safari, 2011; Yazdi-Moghadam, 2011; Mohammadi et al., 2013, Mohammadi et al., 2015; Daneshian and Ramezani Dana, 2018; Mohammadi, 2020) and mammalian fossil (Abbassi et al., 2016). Therefore, there is still no high precision magnetostratigraphic age control of the Miocene strata in Central Iran, which could delimit precisely the age of the final Neotethys seawater retreat from this region.

A number of questions remain unanswered. What is the age of the final marine regression in Central Iran? What was the forcing mechanism of this seawater retreat, tectonic or global climatic control? How did the paleoenvironment change in this region after the final marine regression?

The aims of this paper are: (1) to provide a new magnetostratigraphic constraint for the Miocene marine-terrestrial sequence in the Qom back-arc basin, (2) to constrain the timing of the final seawater retreat from Central Iran, (3) to discuss the forcing mechanism of this marine regression, and (4) to use oxygen isotope records to reveal paleoenvironmental changes.

Section snippets

Geological setting and lithostratigraphy

The geo-tectonic domains of Iran from west to east are: the Zagros fold-thrust belt, the Sanandaj-Sirjan Zone (SSZ) metamorphic–magmatic belt, the Urumieh–Dokhtar magmatic arc (UDMA), and the Central Iran block (Fig. 1a). In this study, we focus on Miocene strata in the Qom back-arc basin in Central Iran.

Our sections are located about 18 km to the west of Qom City (Fig. 1b) in the type area of the Qom Formation, as established by Furrer and Soder (1955). The marine sedimentation of the Qom

Usingle bondPb age analyses of a tuffaceous bed

In sedimentary basins the Usingle bondPb ages of detrital zircons can be used not only for tracing sedimentary provenance, but also for limiting the maximum deposition age of the sedimentary horizon. For this study, one sample of fine-grained tuffaceous sandstone was collected from the lowest part of the Upper Red Formation (see Fig. 3 for location). Zircon grains were separated by standard heavy liquid and magnetic techniques, and then randomly picked, mounted in epoxy resin, and coated with ~100 Å of

Usingle bondPb zircon ages of the tuffaceous sandstone

The Usingle bondPb zircon ages of the tuffaceous sandstone sample are listed in Table S1 in Supplementary Data and Fig. 5a, b. Among all the Usingle bondPb ages of zircons, only four are in the age range of Late Cretaceous to Eocene; the remainder have a remarkable early Miocene age peak (Fig. 5a). According to Dickinson and Gehrels (2009), the youngest single grain age is a potentially misleading measure because any single data point in any analytical matrix may be spurious and not reproducible. The weighted mean

Magnetostratigraphic correlation and chronology of the Qom Section

In order to have a chronology of the Qom Section, the two overlapped paleomagnetic polarities were correlated with a standard Geomagnetic Polarity Time Scale (GPTS) (Gradstein et al., 2012, GPTS2012) (Fig. 8). To do this, we integrated three different stratigraphic constraints: biostratigraphy, Usingle bondPb ages of the tuffaceous sandstone, and magnetostratigraphy.

Firstly, although our marine bivalve fossil Spondylus decussatus and the echinoid fossil Clypeaster intermedius from the topmost bed of the

Conclusions

The Arabia–Eurasia collision caused not only the uplift of the Iranian-Anatolian Plateau, but also the diachronous closure of the Tethyan seaway. The timing and forcing mechanism of the final seawater retreat from different tectonic domains of Iran provide important information for understanding the tectonic processes of plate collision, as well as the effects on paleoenvironmental changes. Our work from the Qom back-arc basin in Central Iran offers the following conclusions.

Based on two

Declaration of Competing Interest

The authors declare that they have no competing financial interests or personal relationships that could have influenced the work reported in this paper.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (41888101 and 41672168), and the Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070202). We thank Mahmoud Navvar Noveiri for the help in field sampling. We are also grateful for the detailed and helpful comments from Jamshid Hassanzadeh and two other anonymous reviewers as well as the Editor, Liviu Matenco, which contributed greatly to the improvement of this paper.

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