Volcano-Tectonic Evolution of the Eastern Christiana Basin (South Aegean Volcanic Arc): Insights from the MULTI-MAREX cruise 2

The South Aegean Volcanic Arc (Greece) is among the most active volcanic systems in Europe and poses an ideal natural laboratory to study the interplay of volcanism and tectonics as drivers of explosive eruptions, earthquakes, submarine landslides and tsunamis. This study focuses on a structurally independent sub-basin in the eastern Christiana Basin, located between the Christiana and Santorini island groups and southeast of the regionally significant Christiana Fault. Although the Christiana-Santorini-Kolumbo volcanic field has been extensively investigated, this basin has not yet been specifically targeted in a comprehensive study.

During the MULTI-MAREX research cruise 2 (MSM135), nearly 640 km of hydroacoustic and 2D multi-channel seismic reflection data were acquired across the eastern Christiana Basin. The MSM135 seismic grid provides increased profile density and signal penetration and establishes a connection with the IODP 398 sites U1591 and U1598. Using the prominent Archaeos Tuff (765 ka) as a marker unit, we updated and harmonised the regional seismostratigraphic model. We refine the estimated volume of the Archaeos Tuff, and map deposits of the Poseidon eruption, providing an initial minimum bulk-volume estimate of 9 km³.

We discovered a syncline, measuring around 8 km in diameter, beneath the almost flat seafloor. The Archaeos Tuff drapes a pre-existing central cone in a W-shaped geometry and reaches a maximum thickness of almost 200 m near the central cone. The syncline accommodates an additional 500 m of post-Archaeos deposits, primarily the Thera Pyroclastic Formation. The infill transitions quickly from an undulating W-shape to a horizontal stratification, indicating short-lived sag-style subsidence. To the northwest, the syncline is bounded by a major fault system, dubbed Thera Fault System, that strikes parallel to the Christiana Fault exhibiting vertical offsets of up to 160 m. Like the Christiana Fault, the Thera Fault System is likely a continuation of the normal faults northeast of Santorini.

The seismostratigraphic model constrains the timing of eruptive and tectonic events, assembled in a comprehensive timeline. We date the activity of at least 10 previously little-considered volcanic cones near the margin of the basin to the Late Pleistocene, based on their relative position between known stratigraphic units. Our findings imply a slow, continuous down-faulting at the Christiana Fault, likely related to the rift extension in the region, whereas the Thera Fault System faulted in two stages of shorter duration. The timing of the subsidence coincides approximately with the first explosive eruption cycle on Santorini.