Lateral Spreading Above Volcanic Tephra as a Potential Geohazard in the Epidavros Basin (Saronic Gulf, Greece)

The Epidavros Basin in the Saronic Gulf is located in close proximity to active volcanic centers, including the Pausanias volcanic field. The Saronic Gulf is affected by extensional back-arc tectonism predominantly oriented N–S, while evidence for older E–W-directed rifting is also preserved. The Epidavros Basin is bounded to the north and south by NW–SE-striking fault systems. Previous studies have suggested the presence of additional NW–SE-striking fault patterns within the basin interior, which have been mapped and interpreted in differing ways. Within the framework of the MULTI-MAREX project, the MSM135 expedition aboard RV MARIA S. MERIAN in spring 2025 acquired the first high-resolution multichannel seismic reflection data covering the entire basin, enabling a reassessment of the intrabasinal deformation mechanisms and their relevance for submarine geohazards.

The time-migrated seismic data reveal two deformation zones comprising complex extensional fault systems, including listric normal faults, rotational fault blocks, and synthetic and antithetic connecting faults. Prolonged or recurrent growth faulting and recent activity are indicated by an increase in vertical fault displacement with depth, and by faults reaching the seafloor.

Such fault patterns are commonly associated with transtensional deformation and the development of negative flower structures. However, this interpretation is inconsistent with both the regional tectonic framework and the absence of seismological evidence within the Epidavros Basin. The observed fault architecture is consistent with lateral spreading above a mechanically weak detachment layer. We propose Early Pleistocene tephra deposits from explosive Methana volcanism as the primary detachment horizon. Chaotic seismic reflection patterns beneath the faulted sedimentary cover, comparable to tephra facies documented during IODP Expedition 398, support this interpretation. Lateral spreading is likely facilitated by regional NE–SW extension and could promote submarine slope instability, fault-controlled seafloor deformation, and localized mass wasting.

Amplitude anomalies associated with near-vertical pipe structures and laterally confined chaotic zones in the overlying sediments are interpreted as tephra injections, some of which likely extruded at the paleo-seafloor. These features indicate fluid- and sediment-mobilization processes that may further weaken the basin fill.

Due to the presence of a mechanically weak décollement, lateral spreading can be initiated not only by large-scale basement extension but also by earthquake activity, volcanic eruptions, or fluid migration into the weak zone. Our results suggest that lateral spreading above volcanic tephra may represent a previously unrecognized geohazard in the Saronic Gulf, particularly in settings where mechanically competent lava flows overlie mechanically weak tephra deposits. This may be particularly relevant for populated coastal regions located in close proximity to volcanic flanks.