Magnetic Anomaly of the Anhydros Horst (Southern Aegean Volcanic Arc): Diking or Ophiolites?

Several recent studies interpret the earthquake swarm observed in early 2025 on the Anhydros Horst in the South Aegean Volcanic Arc as the result of magma-filled dike intrusion. Magnetic data acquired in 2015 during the PROTEUS cruise revealed that the part of the Anhydros Horst where earthquake hypocenters were shallowest below the seafloor (Isken et al., 2025) occurred northwest of a pronounced magnetic anomaly. This led to the hypothesis that the anomaly reflects cooled magmatic material and that the 2025 seismic crisis was associated with renewed magma accumulation.Here, we present a joint interpretation of the 2015 magnetic dataset and newly acquired marine magnetic and 2D multichannel seismic reflection data collected during MULTI-MAREX research cruise 2 (MSM135) aboard RV MARIA S. MERIAN in 2025. The renewed magnetic survey of the Anhydros Horst aimed to better constrain the location and geometry of the inferred dike by comparing magnetic anomalies measured in 2015 and 2025.All magnetic data were processed using a standardized Python-based workflow including IGRF removal, diurnal variation correction, and bandpass filtering. Although differences between the two magnetic datasets are observed, they are best explained by variations in acquisition geometry and instrumentation rather than temporal changes in subsurface magnetization. Forward modeling demonstrates that the proposed dike width of 3–5 m would be insufficient to generate a detectable magnetic anomaly at the seafloor.Integrated interpretation of the magnetic data with multichannel seismic profiles from the University of Hamburg and constraints from Site U1600 from IODP Expedition 398 (Kutterolf et al., 2024), suggests that the magnetic anomaly is instead generated by ultramafic basement located only a few hundred meters below the seafloor. The top of this body is marked by strong seismic reflection amplitudes. We interpret the ultramafic basement as part of an ophiolite complex. While ophiolites are documented on the Greek mainland and several Aegean islands, submarine ophiolitic occurrences within the Aegean Sea have not previously been described. Generally, the emplacement of the ophiolitic body has been interpreted as related to subduction processes during the closure of the Vardar Ocean.This study demonstrates that marine magnetic data, when jointly interpreted with seismic observations and seafloor sampling, provide important constraints on crustal composition and significantly contribute to the reconstruction of plate-tectonic evolution in complex volcanic arc settings. Isken, M.P., Karstens, J., Nomikou, P. et al. Volcanic crisis reveals coupled magma system at Santorini and Kolumbo. Nature 645, 939–945 (2025). https://doi.org/10.1038/s41586-025-09525-7Kutterolf, S., Druitt, T. H., Ronge, T. A., Beethe, S., Bernard, A., Berthod, C., ... & Yamamoto, Y. (2024). Site U1600. Proceedings of the International Ocean Discovery Program Expedition reports, 398(114).