A previously unknown tectonic plate that was once one-quarter the size of the Pacific Ocean was ... [+]
Utrecht University PhD candidate Suzanna van de Lagemaat has reconstructed a massive and previously unknown tectonic plate that was once one-quarter the size of the Pacific Ocean. She reconstructed the lost plate through computer simulations and field research in the mountain belts of Japan, New Zealand, New Guinea, Borneo and the Philippines.
Earth's rigid outer shell is divided into a dozen large tectonic plates and microplates, but their number varied over geological time. Tectonic plates consisting mostly of heavy oceanic crust have disappeared into the Earth's mantle by means of subduction. Only fragments thrusted into mountain belts survived this process.
"The Philippines is located at a complex junction of different plate systems. The region almost entirely consists of oceanic crust, but some pieces are raised above sea level, and show rocks of very different ages," explained de Lagemaat.
Van de Lagemaat first reconstructed the movements of the current plates in the region between Japan and New Zealand over the past 150 million years. The reconstruction showed a gap opening between the Australian Plate in the south, the Eurasian Plate in the north, the Indian Plate in the west and the Pacific Plate in the east. So she started to look for tangible evidence of a missing plate fitting this gap.
"We also conducted field work on northern Borneo, where we found the most important piece of the puzzle. We thought we were dealing with relicts of a lost plate that we already knew about. But our magnetic lab research on those rocks indicated that our finds were originally from much farther north, and had to be remnants of a different, previously unknown plate."
Tectonic plates sink only slowly into Earth's mantle. The Farallon Plate, part of the Pacific Ocean subducted over 30 million years ago beneath the North American continent, is still visible in seismic surveys. Geologists already assumed that a now vanished plate existed in the western Pacific region, named Pontus after the Greek deity of the sea, but they weren't sure where.
"Eleven years ago, we thought that the remnants of Pontus might lie in northern Japan, but we'd since refuted that theory," said Douwe van Hinsbergen, Van de Lagemaat's Ph.D. supervisor. "It was only after Suzanna had systematically reconstructed half of the Ring of Fire mountain belts from Japan, through New Guinea, to New Zealand that the proposed Pontus plate revealed itself, and it included the rocks we studied on Borneo."
The relics of Pontus are not only located on northern Borneo, but also on Palawan, an island in the Western Philippines, and in the South China Sea. Together with the position of former tectonic borders, this discovery allowed to reconstruct the full extent of the Pontus Plate. About 150 million years ago it was at least one-quarter the size of the modern Pacific Ocean. As the expanding paleo-Pacific pushed westwards, the Pontus Plate was eventually subducted beneath the Eurasian Plate. Fragments of the seafloor were thrusted onto land and incorporated in the rising mountains as the plates were squeezed together, where they were eventually discovered by de Lagemaat.
The geological history of the Pontus oceanic plate as reconstructed by Suzanna van de Lagemaat.
This video animation shows the entire geological history of the Pontus oceanic plate as reconstructed by Suzanna van de Lagemaat:
The study "Plate tectonic cross-roads: Reconstructing the Panthalassa-Neotethys Junction Region from Philippine Sea Plate and Australasian oceans and orogens" was published in the journal Gondwana Research (2023). Additional material and interviews provided by Utrecht University.
