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An ancient fragment of the Pacific Ocean found buried 400 miles below China



Scientists have identified an old piece of the Pacific Ocean – the ancient remains of its ancient seabed – stretching hundreds of miles below China as it is pulled down into the transitional zone of the Earth’s mantle.

This rocky plate, which once lined the bottom of the Pacific Ocean, is a relic of the oceanic lithosphere, the outermost layer of the earth’s surface, composed of the crust and hard outer parts of the upper mantle.

However, the lithosphere is not always designed to enjoy the views from above. The upper surface layer is composed of several fragmented tectonic plates that move slowly and shift to the surface, colliding with each other from time to time.

During these collisions, a geological process called subduction can occur, in which one plate is forced beneath the other into the subduction zones and eventually sinks deeper and deeper into the planet.

In a new study, scientists from China and the United States are already witnessing this epic phenomenon, which occurs at greater depths than ever observed.

Previously, scientists have recorded subduction plates exploring the boundaries at a depth of about 200 kilometers (about 1

25 miles).

Now, thanks to a giant network of more than 300 seismic stations spread around northeast China, researchers have been able to see the event at a much lower point, depicting parts of the tectonic plate that previously lay beneath the Pacific Ocean pushed into the middle of the mantle plain transition zone. , at depths between 410–660 kilometers (254–410 miles) below the Earth’s surface.

Screenshot 2020 11 17 at 5.44.54 PM (Christoph Burgstedt / Science Photo Library)

To interpret the sinking slab, the team identified two seismic velocity interruptions, regions far underground where seismic waves encounter anomalies. In this case, two anomalies were found, which according to the team are related to both the upper and lower side of the immersion plate.

“Based on detailed seismological analyzes, the above break was interpreted as a Moho break on the subterranean slab,” said geophysicist Ki-Fu Chen of the Chinese Academy of Sciences.

“The lower interruption is probably caused by the partial melting of the asthenosphere of the under-roof slab under hydrate conditions in the marine part of the slab.”

While the subduction of the plate can be seen in a process below China, the subduction zone itself lies far to the east, with the plate at an angle at a relatively shallow angle of 25 degrees down.

“Japan is where the Pacific plate reaches a depth of about 100 kilometers,” said seismologist Fenglin Niu of Rice University.

Thanks to the new image, scientists get a better idea of ​​what happens to a subducted plate when it reaches that part of the transition zone, including how deformed it becomes and how much water it loses from its oceanic crust.

“A lot of research shows that the plate actually deforms a lot in the transition zone of the mantle, that it becomes soft, so it deforms easily,” says Niu.

“We are still debating whether this water separates completely at this depth. There is growing evidence that some of the water remains inside the plate to go much, much deeper.”

The findings were reported in Nature Geoscience.


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