Ancient Pacific Ocean fragment found buried 400 miles below China

Scientists have identified an old piece of the Pacific Ocean – the ancient remnants of its long-ago seabed – that extends hundreds of miles below China as it is dragged down into the transition zone of the Earth’s mantle.

This rock slab that flanked the Pacific floor is a relic of the oceanic lithosphere, the outermost layer of the earth’s surface, composed of the crust and the outermost solid parts of the upper mantle.

However, the lithosphere isn’t always meant to enjoy the view from above. The upper surface layer is composed of several fragmented tectonic plates, slowly moving and moving on the surface, occasionally meeting in each other.

During these collisions, a geological process called subduction can occur, in which one plate is forced under the other in the subduction zones and ends up being pushed deeper and deeper into the planet.

In a new study, scientists from China and the United States have now witnessed this epic phenomenon taking place at greater depths than has ever been observed before.

Prior to this, scientists had recorded subduction plates probing boundaries at depths of about 200 kilometers (about 125 miles).

Now, thanks to the gigantic network of more than 300 seismic stations spread across northeastern China, the researchers were able to see the event at a much lower point, imagining parts of the tectonic plate that lay beneath the Pacific Ocean that they were pushed into the level transition zone, at depths between 410 and 660 kilometers (254-410 miles) below the earth’s surface.

(Christoph Burgstedt / Scientific Photographic Library)

To interpret the sinking slab, the team identified two seismic velocity discontinuities, very underground regions where seismic waves encounter anomalies. In this case, two anomalies were found, which the team says related to both the top and bottom sides of the plunger plate.

“Based on detailed seismological analyzes, the upper discontinuity was interpreted as the Moho discontinuity of the subducted slab,” says geophysicist Qi-Fu Chen of the Chinese Academy of Sciences.

“The minor discontinuity is probably caused by the partial dissolution of the asthenosphere under the plate under water conditions in the seaward part of the plate”.

While plate subduction can be seen going on under China, the subduction zone itself lies far east, with the plate tilted at a relatively low angle of 25 degrees downward.

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

Thanks to the new image, scientists are getting a better idea of ​​what happens to a subducted plate when it reaches this part of the transition zone, including how much it deforms and how much water it loses from its ocean crust.

“Many studies suggest that the plate actually deforms a lot in the transition zone of the mantle, which becomes soft, so it deforms easily,” says Niu.

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

The results are reported in Nature Geoscience.