Hidden 100-million-year-old volcanic system discovered in northwestern Iran

An international team led by Istanbul Technical University has identified remnants of an ancient continental magmatic arc that had previously gone unrecognized. Named the Azerbaijan Continental Magmatic Arc, the system formed during the Late Cretaceous period in a region that is now part of the Arabia-Eurasia collision zone.

The research, published in Geoscience Frontiers, indicates that the volcanic system was active approximately 101 to 97 million years ago, when the ancient Neo-Tethys Ocean was undergoing major geological changes. Over time, tectonic deformation, erosion, burial beneath sedimentary basins, and a younger episode of volcanic activity during the Eocene obscured much of the evidence of its existence.

The researchers examined volcanic and plutonic rocks collected from multiple locations across northwestern Iran, focusing much of their investigation on formations along the Siah Cheshmeh-Khoy-Misho-Tabriz Fault. They used zircon U-Pb geochronology to establish the rocks' ages, Hf isotope analyses to investigate their sources, and whole-rock geochemistry to examine their chemical composition. The results revealed signatures consistent with continental arc magmatism, a geological process that occurs when one tectonic plate sinks beneath another, generating magma that can rise through the overlying continental crust.

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

The international research effort also involved scientists from the Institute of Tibetan Plateau Research and Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences; ETH Zurich, Switzerland; Karadeniz Technical University, Türkiye; and Damghan University, Iran.

Previous reconstructions of Iran's ancient magmatic activity have largely centered on two major geological features: the Sananda-Sirjan Magmatic Arc and the Urumieh-Dokhtar Magmatic Arc. The newly identified Azerbaijan Continental Magmatic Arc adds another piece to that history, suggesting that northwestern Iran experienced a separate episode of continental arc activity during the Late Cretaceous.

The findings also point to a potentially important role for the Siah Cheshmeh-Khoy-Misho-Tabriz Fault. Researchers suggest that this fault may mark a major tectonic boundary connected to changes in the ancient Neo-Tethys Ocean system, possibly including slab rollback, a process in which a descending tectonic plate retreats as it sinks into Earth's interior, as well as the migration of volcanic arcs to new locations. Such movements could help explain why this particular episode of magma formation lasted for a relatively short period.

Beyond Iran, the discovery shows how ancient magmatic arcs do not always survive as continuous, easily recognizable formations, since tectonic movements can break them apart, erosion can remove exposed rocks, and sedimentation can bury what remains. By recognizing surviving fragments, scientists can better reconstruct how tectonic plates descended beneath one another, how volcanic arcs shifted, how continental crust grew, and how once-vast oceans eventually closed. The Azerbaijan Continental Magmatic Arc offers new evidence for understanding the Late Cretaceous evolution of northwestern Iran and the larger Neo-Tethyan region.