subduzione pacifico nuovo.

The remains of a seabed that sank 250 million years ago have been discovered under the Pacific Ocean

A team of researchers fromUniversity of Marylandcoordinated by the geologist Jingchuan Wangdiscovered i remains of the oceanic crust which constituted the seabed of theSoutheast Pacific Ocean when the Earth was populated by dinosaurs. About 250 millions of years agoin the Mesozoic, this portion of crust sank into the Earth’s mantle, east of the Eastern Pacific Ridgean arch in the ocean floor that separates some tectonic plates from each other. The remains of the ancient seabed form ahuge structure in the mantlebetween 400 and 700 km deep. The researchers detected it by studying the speed of seismic waves inside the Earth. The discovery is important because it allows us to understand how the internal dynamics of the Earth influence the movements of the plates.

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The area where the ancient ocean floor has been discovered. Credit: Jingchuan Wang

The discovery of the enormous structure and the remains of the ancient seabed

The researchers identified the buried structure thanks to theseismic imaginga geophysical method that allows you to obtain images of the interior of the Earth just as a CT scan (computerized axial tomography) allows you to “see” the inside of the human body. The technique is based on the analysis of speed with which seismic waves generated by earthquakes or artificially they propagate within the Earthwhich varies depending on the physical and chemical characteristics of the materials passed through. It was thus possible to obtain a detailed map of the crust and mantle under the Eastern Pacific Ridge, showing areas of different temperatures.

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Seismic imaging of the studied area. Credit: Jingchuan Wang

The ridge separates the Pacific plate from other plates, including that one of Nazca. The latter is currently going subducting under South America. Right under the Nazca plate one has been identified colder, denser and thicker structure compared to the surrounding areas, corresponding to the so-called transition zone present between the upper and lower mantle (between approximately 400 and 700 km depth). This structure is what remains of the portion of the oceanic plate that constituted the ancient seabed and which 250 million years ago sank into the mantle by subduction. Over time, subducting plates melt and are reabsorbed into the mantle. In this case the temperature of the collapsed portion of the crust shows that it is not yet completely melted. Moreover, it turned out that it is sinking much more slowly of what usually happens.

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The reconstruction of a possible dynamics of the subduction of the ancient seabed. Credit: Jingchuan Wang

The importance of the discovery of the ancient seabed

The slow speed with which the edge of the oceanic plate sinks”suggests that the mantle transition zone may act as a barrier and slow the movement of material across the Earth”, explains Wang.

Furthermore, seismic imaging allows us to “witness” the subduction processof which the only evidence is found on the earth’s surface, for example in the form of volcanoes. “This thickened area is like a fossilized fingerprint of an ancient piece of sea floor that subducted into the Earth about 250 million years ago“, describes Wang. “It’s giving us a glimpse into Earth’s past that we’ve never had before“. The reconstruction of these processes allows geologists to better understand how the internal dynamics of the planet influence the movements of the tectonic plates. In particular, according to the researchers, the mechanisms identified under the eastern Pacific ridge would be linked to the unusually high speed (7.5 cm/year) with which the plates move apart along it. Further investigations will be needed to better understand this link.

In the future, the researchers intend to extend their studies to other areas of the Pacific Ocean to create a complete map of ancient subduction zones.