Hidden 100-Million-Year-Old Volcanic Arc Discovered in Iran

They applied three analytical techniques:

• Zircon U-Pb geochronology, which measures radioactive decay in the mineral zircon to date rock formation with precision
• Hf isotope analysis, which traces where the original magma came from within the Earth
• Whole-rock geochemistry, which charts the chemical composition of the samples

Together, the results showed signatures consistent with continental arc magmatism — the process by which a sinking tectonic plate generates magma that rises through the overlying continental crust.

"This study shows that the magmatic history of northwestern Iran is more complex than previously recognized," said Dr. Ali Mohammadi of Istanbul Technical University, the study's corresponding author. "By integrating geochronological, isotopic, and geochemical evidence, we identified a Late Cretaceous continental arc that was subsequently obscured by later tectonic processes."

The team also included scientists from the Institute of Tibetan Plateau Research and the Xinjiang Institute of Ecology and Geography (Chinese Academy of Sciences), ETH Zurich in Switzerland, Karadeniz Technical University in Türkiye, and Damghan University in Iran.

What does the discovery change?

Previous reconstructions of Iran's ancient magmatic activity rested largely on two features: the Sananda-Sirjan Magmatic Arc and the Urumieh-Dokhtar Magmatic Arc. The newly identified arc means northwestern Iran experienced a separate episode of continental arc volcanism during the Late Cretaceous, expanding the known picture of how the region developed.

The findings also elevate the status of the Siah Cheshmeh-Khoy-Misho-Tabriz Fault. The researchers suggest it may mark a major tectonic boundary tied to changes in the Neo-Tethys Ocean system.

Those changes may have included slab rollback — a process in which a descending tectonic plate retreats as it sinks into Earth's interior — along with the migration of volcanic arcs to new locations. Such movements, the authors propose, could explain why this magma-forming episode lasted only about four million years, a relatively short window in geological terms.

What does this mean for practitioners?

For geologists working in the region, the message is cautionary: ancient magmatic arcs do not always survive as continuous, easily recognizable formations. Recognizing surviving fragments allows scientists to reconstruct how plates descended beneath one another, how volcanic arcs shifted, and how continental crust grew.

The study carries the usual caveats of a single-region reconstruction, and the slab-rollback interpretation remains a proposal rather than a settled conclusion. Even so, the arc offers fresh constraints for models of the Late Cretaceous Neo-Tethyan region — an area shaped by repeated episodes of ocean closure and continental collision.

A volcanic system erased from the record for 100 million years has now been read back out of the rocks.