The Hellenic Subduction System: A revised view of its structure and kinematics

The Hellenic forearc is one of the least understood forearc systems globally due to limited availability of high-resolution imagery of its deep structure, especially landward of the Mediterranean Ridge. This has resulted to ambiguity about the origin of its key structural and morphotectonic features, the location of the active subduction trench, the relationship between different fault types within its forearc and to whether this system is capable of generating large (M>8) subduction earthquakes and associated tsunamis. Here, we combine widely spaced high-resolution multichannel seismic-reflection profiles with seafloor morpho-bathymetric analysis and earthquake moment-tensors to investigate the structure and post-Messinian (0–5.9 Ma) fault kinematics in the Hellenic forearc. Our work provides, for the first time, strong evidence for the presence of active thrust faults along the inner forearc, from the backstop of the Mediterranean Ridge to the Hellenic Trough. Many thrusts are imaged to splay from the subduction plate-interface, at depths of 6–8 s (TWT), while normal and strike-slip faults commonly form in the upper-crust landward of the 20 km slab-isodepth, and abut against thrust hanging-walls. Observed fault patterns are supported by seabed fault-scarp analysis and are consistent with the distribution and kinematics of earthquake moment-tensors. Analysis of fault-intersections at depth suggests that forearc kinematics are characterized by a fault hierarchy, in which normal and strike-slip faults commonly form as secondary structures above active thrusts, accommodating oblique plate-convergence. Our analysis also highlights a structural division of the forearc into landward- and seaward-verging thrusts, similar to that recorded along the Cascadia and Sumatra margin, with the Hellenic troughs accommodating their geometric transition. Thrust vergence variability likely results from the northward steepening of the underlying plate-interface and marks the across-forearc transition from aseismic to seismic-slip. These significant revisions in understanding of the Hellenic Subduction System and its upper-plate structures are expected to flow into future geodynamic, hydrocarbon-exploration and earthquake hazard models.