Seismic imaging in northeast China revealed both the upper (X1) and lower (X2) boundaries of a tectonic plate (blue) that was formerly at the bottom of the Pacific Ocean and is drawn into the transition zone of the mantle between 254 and 410 lies miles (410-660 kilometers) below the earth’s surface. Photo credit: Courtesy of F. Niu / Rice University
The study provides clues about the fate of tectonic plates that sink deep into the earth’s mantle.
In a study that gave a new meaning to the term “low point”, seismic researchers discovered the underside of a rocky plate of the Earth’s surface layer, or lithosphere, drawn more than 400 miles below northeast China by the process of tectonic subduction.
The study, published by a team of Chinese and American researchers in Natural geosciencesprovides new evidence of what happens to water-rich oceanic tectonic plates as they are dragged through the Earth’s mantle under continents.
Rice University seismologist Fenglin Niu, a co-author, said the study provides the first high-resolution seismic images of the upper and lower boundaries of a rocky or lithospheric tectonic plate within a key region known as the mantle transition zone, which begins approximately 410 kilometers below Earth’s surface and extends to about 660 kilometers.
“Many studies suggest that the plate actually deforms a lot in the mantle transition zone, that it becomes soft and therefore deforms easily,” said Niu. How much the plate deforms or maintains its shape is important to explain whether and how it mixes with the jacket and what kind of cooling effect it has.
Fenglin Niu is Professor of Earth, Environmental, and Planetary Sciences at Rice University. Photo credit: Courtesy of Rice University
The earth’s mantle convects like heat in a furnace. The heat from the Earth’s core rises through the mantle in the center of the oceans, where tectonic plates form. From there, heat flows through the mantle and cools down on continents, where it falls back to the core to collect more heat, ascend and complete the convection cycle.
Previous studies have explored the limits of plate subduction in the mantle, but few have looked deeper than 200 kilometers and none with the resolution of the current study, which took more than 67,000 measurements from 313 regional seismic stations in northeast China. This work, carried out in collaboration with the China Earthquake Administration, was led by co-author Qi-Fu Chen of the Chinese Academy of Sciences.
The research examines fundamental questions about the processes that have shaped the earth’s surface over billions of years. Mantle convection drives the movements of the Earth’s tectonic plates, rigid interlocking parts of the Earth’s surface that are in constant motion as they float on the asthenosphere, the topmost mantle, and the most fluid part of the inner planet.
Where tectonic plates meet, they crowd and drag together, releasing seismic energy. In extreme cases, this can cause devastating earthquakes and tsunamis, but most seismic movements are too weak for humans to feel without instruments. With the help of seismometers, scientists can measure the size and location of seismic disturbances. And because seismic waves get faster in some rock types and slower in others, scientists can use them to create images of the Earth’s interior, much like a doctor could use ultrasound to map inside a patient.
Niu, professor of earth, environmental, and planetary sciences at Rice University, has been a leader in seismic imaging for more than two decades. When he completed his Ph.D. More than 20 years ago, researchers trained in Japan and used dense networks of seismic stations to collect some of the first detailed images of the submerged plate boundaries of the Pacific plate.
“Japan is roughly where the Pacific plate reaches a depth of about 100 kilometers,” Niu said. “There is a lot of water in this plate and there is a lot of partial melt. This creates arc volcanoes that contributed to the creation of Japan. However, we are still debating whether this water will be fully released at this depth. There is increasing evidence that some of the water remains in the plate to go much, much deeper. ”
Northeast China offers one of the best vantage points to investigate if this is the case. The region is about 1,000 kilometers from the Japanese Trench, where the Pacific plate begins its leap back into the interior of the planet. In 2009, Niu and scientists from the University of Texas at Austin, the China Earthquake Administration, the University of Tokyo’s Earthquake Research Institute, and the Earthquake and Volcanic Eruption Prediction Research Center in Tohoku, Japan, were funded by the National Science Foundation and others The University began with the installation of broadband seismometers in the region.
“We set up 140 stations there, and the more stations, the better for the resolution,” said Niu. “The Chinese Academy of Sciences has set up additional stations to give them a finer, more detailed picture.”
In the new study, data from the stations revealed both the upper and lower boundaries of the Pacific plate sloping at an angle of 25 degrees within the mantle transition zone. Placement within this zone is important for studying mantle convection, as the transition zone lies below the asthenosphere at depths where increased pressure causes certain mantle minerals to experience dramatic phase changes. These phases of the minerals behave very differently in seismic profiles, just like liquid water and solid ice behave very differently, although they consist of identical molecules. Because phase changes occur in the mantle transition zone at certain pressures and temperatures, geoscientists can use them like a thermometer to measure the temperature in the mantle.
Niu said the fact that both the top and bottom of the plate are visible is evidence that the plate has not completely mixed with the surrounding mantle. He said that heat signatures from partially melted portions of the mantle under the plate also provide indirect evidence that the plate was transporting some of its water into the transition zone.
“The problem is to explain how these hot materials can be dropped into the deeper part of the mantle,” Niu said. “It’s still a question. Because they are hot, they can swim. ”
This buoyancy should act like a lifebuoy, pushing the underside of the sinking plate upwards. Niu said the answer to this question could be that holes have appeared in the deforming plate so that the hot melt can rise as the plate sinks.
“If you have a hole, the melt will come out,” he said. “That’s why we think the record can go deeper.”
Holes could also explain the occurrence of volcanoes such as Changbaishan on the border between China and North Korea.
“It’s 1,000 kilometers from the plate boundary,” said Niu. “We don’t really understand the mechanism of this type of volcano. But melt rising from holes in the plate could be a possible explanation. ”
Reference: “Different plate interfaces that are mapped within the mantle transition zone” by Xin Wang, Qi-Fu Chen, Fenglin Niu, Shengji Wei, Jieyuan Ning, Juan Li, Weijun Wang, Johannes Buchen and Lijun Liu, November 9, 2020, Natural geosciences.
DOI: 10.1038 / s41561-020-00653-5
The study co-authors include Xin Wang and Juan Li, both from the Chinese Academy of Sciences, Shengji Wei from Nanyang Technological University in Singapore, Weijun Wang from the China Earthquake Administration, Johannes Buchen from the California Institute of Technology, and Lijun Liu from the University of Illinois at Urbana-Champaign. The research was funded by the Chinese Academy of Sciences (XDB18000000) and the National Natural Science Foundation of China (91958209, 41974057, 41130316).
