An invisible seam stretches from eastern Turkey to the Persian Gulf – the border between the Arabian and Eurasian tectonic plates. Until the Cretaceous period, around 60 million years ago, the two were separated by an ocean, the Neotethys . But the northward migration of Africa caused a collision. The oceanic part of the Arabian plate – the bottom of the Neotethys Sea – was pushed under Eurasia and sank into the Earth’s mantle.
From Subduction to Collision
But around 25 million years ago, this subduction came to a halt: the oceanic Neotethys plate piece was now completely subducted. “The continental part of the Arabian plate now reached the subduction zone and collided with the Eurasian plate,” explain Renas Koshnaw from the University of Göttingen and his colleagues. Because the continental crust is lighter and thicker, it did not sink but rather compressed – a mountain range towered up from eastern Turkey to the Persian Gulf.
This is where the study by Koshnaw and his team comes in. Geological and seismic analyses were used to investigate what is happening beneath this interface between the Arabian and Eurasian plates. The focus was on the foreland of the Zagros Mountains in northwestern Iraq. There, the enormous weight of the piled-up mountains has created an elongated depression. It is three to four kilometers deep and almost completely filled with sediment.
Neotethys remnant tugs at the crust
The strange thing, however, is that this depression in the Zagros Mountains is deeper than it should actually be, as seismic measurements have shown. “Given the relatively low altitude of the northwestern Zagros Mountains, it was surprising that so much sediment had accumulated in our study area. This means that the subsidence of the earth’s crust there is greater than can be explained by the weight of the Zagros Mountains,” says Koshnaw.
But what is the reason for this? As the geologists discovered, the old Neotethys plate piece is behind this subsidence: deep beneath the Zagros Mountains, this heavy, oceanic plate piece is still attached to the Arabian Plate. Its weight pulls the earth’s crust downwards in the Zagros foreland, thus promoting excessive subsidence.
Plate rupture under Turkey
But that is not the case everywhere: In the northern part of this collision zone – beneath Anatolia – a different picture emerged. “Towards Turkey, the depression becomes much flatter,” report the geologists. Instead of sinking further, the subsoil there has been rising again for around 15 million years. Seismological data also showed that the upper mantle beneath Anatolia is significantly hotter and softer than beneath the Zagros Mountains.
“This indicates that the oceanic plate has already broken away in this area and its pulling force has decreased,” says Koshnaw. This means that the Arabian plate has broken through beneath Turkey. The submerged oceanic part has separated from the continental part. “This crack appears to be spreading from Turkey towards Iraq, similar to a page being torn off a calendar,” explains Koshnaw.
As Below, So Above
These findings not only shed light on what is happening geologically beneath Asia Minor and the Middle East. They also illustrate how strongly processes deep inside our planet control the development of the Earth’s surface: the shape of the landscape reflects the hidden subterranean processes. “This research helps us understand how the Earth’s rigid outer shell works,” says Koshnaw’s colleague Jonas Kley.
But the geodynamic model developed by the researchers could also have a very practical use: “Such fundamental investigations pave the way for practical applications, for example the search for ore deposits and geothermal energy or to assess earthquake risks,” says Kley. (Solid Earth, 2024; doi: 10.5194/se-15-1365-2024 )
Source: Solid Earth, Georg-August-University Göttingen