Separation of Fiji and Vanuatu depends on Samoan seamounts

The islands of Fiji and Vanuatu rise from the tropical waters of the South Pacific in one of the world’s most tectonically active and geologically complex regions. A new volcanism study in this area sheds light on the ancient rupture of a long island arc that split apart like a “double hall door”. Fiji and Vanuatu started off as close neighbors and ended up 800 miles apart in what was once an unbroken arc.

Island arcs form where a plate of oceanic crust sinks under an adjacent plate and occurs in a process known as subduction, giving rise to a volcanic belt parallel to the trench where the descending plate bends downward. The islands are the highest peaks of vast underwater mountain ranges formed by volcanic activity in the subduction zone. Such a range now stretches from New Zealand to Tonga, then curves westward to Fiji. Another stretches from New Guinea to Vanuatu.

“They were all connected and then separated in the geological past,” said James Gill, professor of Earth and planetary sciences at UC Santa Cruz and first author of the new paper. “This article attributes the breakup to the sinking of the Samoan Seamount Chain.”

Samoa, like Hawaii, is part of a linear chain of volcanic seamounts formed by the movement of oceanic crust over a “hot spot” in the Earth’s mantle, causing a series of volcanoes to grow above that point. A long chain of seamounts extends to the west of the Samoan Islands.

“When this chain of seamounts was pushed towards Earth under the island arc, it caused indigestion at the subduction zone, which eventually broke it up,” Gill said. Said.

In addition to the seamounts suspended in the subduction zone, other complex processes were at work along the island arc, including reversing the direction of subduction along part of the arc, turning different sections and opening rifts. the spreading of the seafloor creates new oceanic crust. The Vanuatu Arc rotated clockwise, while the piece of crust carrying Fiji rotated counterclockwise.

These events (called “double hall door tectonics” by geologist Keith Martin in 2013) began about 10 million years ago and progressed slowly over millions of years to the present configuration of the islands.

Gill and his co-authors explored this history by analyzing samples of igneous rock collected at sites in the region in the 1980s by Gill and Peter Whelan, then UCSC graduate students, who conducted the first analyzes of the samples. For the new work, Gill received funding from the Humboldt Research Award to work with researchers at the GEOMAR Helmholz Center for Ocean Studies in Germany, who are performing modern geochemical analyzes to determine the isotopic and elemental composition of the samples.

“These analyzes allow us to use isotopes as long-lived tracers to find out what melts to produce magma erupting from a particular volcano,” Gill said. “In this case, we can see that the Samoan seamounts are the best match for the rocks that erupted in Fiji when this island arc broke apart.”

Gill has been studying the geology of the South Pacific islands for over 50 years, collecting hundreds of volcanic rock samples from Fiji, eastern Indonesia, and remote islands in the Marianas, as well as other places around the Pacific “ring of fire.” Many of these specimens are now in collections at the Smithsonian Institution and the American Museum of Natural History.

“When I was hired 50 years ago, UCSC was establishing a Center for South Pacific Studies, which is one of the reasons the Arboretum has so many plants from New Zealand and Australia,” Gill said. “This article is part of my career-long efforts to understand the geological evolution of Fiji and connects the histories of Fiji, Vanuatu, and Samoa.”

The December issue featured the article “Breaking Up Is Hard To Do: Oceanic Arc Breakup, Subduction Reversal, and Cessation During Magmatism.” Geochemistry, Geophysics, Geosystems.