Remains of an ancient megaplate discovered

The edges of the Pacific Ocean are among the most exciting regions in geophysics - in the truest sense of the word, if you look at the Circumpacific Ring of Fire with its numerous volcanoes and earthquakes alone. However, the complicated tectonics of the various earth plates that border each other here can only be understood if you also know their past. Suzanna van de Lagemaat and Douwe van Hinsbergen from Utrecht University have now been able to reconstruct a crucial part of this history and describe it in "Gondwana Research": the Pontus Plate, which has since disappeared back into the Earth's interior, but which made up a quarter of the Pacific Ocean 160 to 120 million years ago.

The existence of this plate was already predicted in the 2010s based on seismic data from the Earth's mantle, but it was only the discovery of rocks from oceanic crust in the north of the island of Borneo by van de Lagemaat that confirmed Pontus. Based on this data and other rock samples from various Asian mountains, the geologist and her colleague have now been able to reconstruct the entire plate, which at that time extended between the Eurasian Plate in the north-west, various Pacific plates in the east, the Philippine Plate in the south-east and Indian Ocean plates in the south and south-west.

"We thought we were dealing with relics of a past plate that we already knew," says van de Lagemaat: "But our magnetic laboratory tests on these rocks showed that our finds originally came from much further north and must be remnants of another, previously unknown plate." When Pontus existed, there was a huge ocean between Eurasia and Australia, which was then still connected to Antarctica in the supercontinent Pangaea.

As Pangaea broke apart and Australia moved northwards, Pontus slowly disintegrated as the plate was subducted into the depths of the Earth's mantle. At the same time, Borneo and the Philippines drifted with their plates into their current positions. Overall, the greater area today forms one of the most complex plate tectonic regions on Earth.

In contrast to earlier reconstructions of this area, van de Lagemaat and van Hinsbergen decided against using palaeogeomagnetic data to trace the position of the Philippine Plate over time. Such measurements are still scarce in this region, which limits their usefulness. Instead, the duo looked at the entire western Pacific region and its predecessor, the Panthalassa superocean that surrounded Pangaea. They then worked backwards from the current geological arrangement of the plates to the Jurassic era, starting from the simplest plate tectonic scenario consistent with geological observations.

The results point to fragments of an ancient plate that had sunk deep into the Earth's mantle and whose remnants disturb and deflect seismic waves, which emerges as a characteristic pattern in the data. Relics of the Pontus plate were also found on the Philippine island of Palawan and in the South China Sea, which are ultimately linked to the Borneo formation under investigation and complete the picture.

Spectrum of Science

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