1. Key Points

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A vast area with a high seismic wave velocity was discovered at a depth of 500–600 km beneath the equatorial region of the western Pacific via seafloor earthquake observations.

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This area is considered a remnant of the Pacific plate and is thought to have been subducted approximately 50–25 million years ago.

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An upwelling rising from the lowermost mantle collides with the remnant of the Pacific plate and changes into a sheet-like shape along the volcanic Caroline Islands chain. This was the first time a volcanic chain with this kind of structure has been identified.

2. Overview

Senior Researcher Masayuki Obayashi and other members of the Volcanoes and Earth’s Interior Research Center at the Research Institute for Marine Geodynamics, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC)–in collaboration with the Earthquake Research Institute at the University of Tokyo and Kobe University–investigated the seismic velocity structure of the mantle beneath and in the vicinity of the Ontong Java Plateau (OJP) in the equatorial region of the western Pacific and were the first to discover a vast area with high seismic wave velocity at a depth of 500–600 km beneath the OJP.

The Australian plate is currently being subducted beneath the Pacific plate southwest of the OJP at the plate interface along the Solomon Islands, although the Pacific plate had been subducted until 25 million years ago. The plate discovered in the new research is considered a remnant of the subducted Pacific plate. Beneath the volcanic chain of the Caroline Islands, which extends east and west on the northern side of the OJP, low seismic wave velocity anomalies indicating high temperatures were observed spreading out like a sheet from the edge of the horizontal remnant of the Pacific plate. The anomalies were presumed to be the result of an upwelling caused by the collision of the lower mantle with that horizontal remnant at a depth of 600 km, causing them to spread into a sheet-like structure. This is the first time that a volcanic chain with this kind of structure has been identified.

The findings demonstrated how the interaction between the subducted plate and the mantle upwelling affected surface tectonics such as volcanic activity.

These results will be published in Scientific Reports on October 28th (Japan time). Part of this research was funded by JSPS Grants-in-Aid for Scientific Research (15H03720JP, 18H04373JP).

Title：

Interrelation of the stagnant slab, Ontong Java Plateau, and intraplate volcanism as inferred from seismic tomography
（DOI:10.1038/s41598-021-99833-5）

Authors：

Masayuki Obayashi¹, Junko Yoshimitsu¹, Daisuke Suetsugu¹, Hajime Shiobara², Hiroko Sugioka³, Aki Ito¹, Takehi Isse², Yasushi Ishihara¹, Satoru Tanaka¹, Takeshi Tonegawa¹

Affiliation：

¹ Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, Yokosuka, 237-0061, Japan
² Earthquake Research Institute, The University of Tokyo, Tokyo, 113-0032, Japan
³ Graduate School of Science, Kobe University, Kobe, 657-8501, Japan