Structural and kinematic controls on basement-influenced salt diapir geometries in the central Adriatic: Insights from 2D seismic profiles.

The interpretation of deep 2D seismic profiles from the central Adriatic foreland of the External Dinarides in the area of the islands of Vis and Jabuka (Croatia), reveals a complex Mesozoic platform-to-basin architecture, and Cenozoic structural and sedimentary system developed on top of the central part of the Adriatic microplate (Adria). Tectonic subsidence and thick Paleogene to Neogene sedimentary loading in the latest Dinaric foredeep probably initially mobilized buried Middle Triassic evaporites from the proximal to distal foreland. Miocene tectonic is characterized by basement-rooted positive flower structures, pop-up blocks, and upward-diverging fault splays, diagnostic of a transpressional tectonic regime.

The crustal-scale Quaternary subvertical faults without apparent vertical throw are associated with positive and negative structures along the strike. In the overlying sedimentary cover, localized normal faulting and extensional arrays overprint transpressional structures, interpreted as gravitational collapse above pop-up blocks, roof collapse above ascending diapirs, and lateral collapse within a mechanically decoupled cover. The positive structures are associated with the Quaternary salt diapirs, some of which are still active. However, it is not clear which faults are inducing regional seismicity.

Instrumental seismicity is moderate to strong (up to M>5), shallow (≈5–15 km) and spatially clustered around the diapiric structures. Focal‑mechanism solutions predominantly indicate reverse to reverse–oblique faulting, yet the nodal planes do not clearly coincide with any single reverse fault imaged on 2D profiles, and many hypocenters project within or immediately above active salt diapirs. These observations suggest that salt diapirs act as mechanical and geometric controllers that focus stress and localize brittle failure on surrounding basement‑rooted faults, rather than being the primary source of seismic energy, which is difficult to reconcile with the seismic moment of M>5 events if salt flow alone were responsible.

Active salt structures are characterized by long stems and relatively small surface expressions that are aligned along Quaternary faults. Their geometry, disconnection with original depth of the Triassic evaporites, and limited lateral extent, indicate tectonic extrusion of deep evaporites. Variations and segmentation along strike, suggest localized strain and strong structural control on diapir rise. Overall, these observations indicate that diapir growth and surface expression are controlled by the interaction between deep shear zones, active faulting, and a mechanically decoupled overburden. Within this framework, seismicity reflects the interaction between deep shear zones, evaporite mobilization and upper‑crustal faulting, highlighting the need to re‑evaluate focal mechanisms with improved 3D velocity models and to explicitly incorporate salt‑controlled structures into seismic‑hazard assessments for the region.

“This work was supported by Croatian Science Foundation project SALTECTA (HRZZ-IP-2024-05-2957).”

Keywords: Central Adriatic Sea, 2D seismic profiles, Transpressional deformation, Salt diapirs, Active tectonics, Seismicity.