From Peri-Tethys to Paratethys: Basin restriction and anoxia in central Eurasia linked to volcanic belts in Iran

Abstract

From the Alps in western Europe to the great steppes of Kazakhstan, the former Paratethys Sea once covered a vast area of our planet, of which the Black Sea, Caspian Sea and Aral lake are today’s remnants. The Paratethys formed as the Tethys ocean gradually closed, and was known as ‘Peri-Tethys’ until the late Eocene. The Peri-Tethys transformed into the Paratethys after it became an isolated basin from the Oligocene onwards. Initially, Laskarev (1924) coined the term Paratethys, describing the environment of Miocene sediments, containing peculiar fossils that developed in this isolated environment. Later, also Oligocene strata were included, which show the first isolation from the Tethys ocean. The birth of the Paratethys is characterised by a change in sedimentation reflecting well-oxygenated bottom waters during the Eocene, towards low-oxygen environments in the Oligocene (Maikop Series). In my thesis, we set out to unravel the roles of tectonic and eustatic mechanisms on the transition from Peri-Tethys to Paratethys and onset of oxygen-poor conditions. We focus on sections that show the change from the Eocene open configuration of the Peri-Tethys to the enclosed setting of the Paratethys, and on the progressively closing marine connections of the Paratethys. We investigate Maikop equivalent sediments in southern Germany (North Alpine Foreland basin; NAFB), and the Maikop and underlying sediments at its type section (Belaya river, Russia). My thesis covers millions of years, and a vast geographic area, to better understand the timing and paleogeographic changes in the basin. This large-scale approach has enabled us to shed light on many different aspects that were important for the evolution from Peri-Tethys to Paratethys. We show that the connection of the Paratethys trough the NAFB closes at 33.15 Ma, millions of years earlier than the previously estimated 28 Ma. In Russia, we show that oxygen-depleted episodes already occurred in the Kuma formation (middle Eocene) that coincide with the Middle Eocene Climatic Optimum (MECO; at 40.5 Ma). In Azerbaijan, we show that volcanism was active around 40.5 Ma, and that overlying sediments that are mapped as ‘Maikop Series’ are of Late Eocene age. We show that the geochemical signature of the volcanic rocks is that of a continental arc, which is in contrast to previous studies that interpreted them as back-arc basin volcanics. We suggest that there is one volcanic belt running from the Talysh in the south of Azerbaijan through Iran towards Bazman (southeast Iran). In the Alborz Mountains (Iran), we use vertical axis rotations derived from paleomagnetic data combined with literature data of the Pontides (Turkey) and the Lesser Caucasus (Georgia and Armenia) to calculate the amount of convergence that is taken up in the Greater Caucasus. We present new Ar-Ar ages of volcanic rocks in Azerbaijan and Iran, and compile ages of volcanic rocks throughout Iran from literature. We show that volcanism in Iran was very active around the MECO, and hypothesize that there is a relationship between the increase in arc volcanism and global climate.