Slab Tearing, Fluid Pathways, and Seismic Segmentation in the Hellenic–Aegean Subduction Zone Revealed by Receiver Functions and OBS Tomography

The Hellenic–Aegean subduction zone is a key natural laboratory for studying convergent margin dynamics, with well-documented surface deformation, upper-crustal geology, and deep mantle processes such as slab rollback. The architecture of the subduction system at intermediate depths (∼50–150 km), however, still remains insufficiently resolved.

Using receiver-function analyses from a dense seismic network deployed across the Peloponnesus and central Greece within the EU-funded THALES WAS RIGHT project, we have resolved the three-dimensional geometry of the subducting slab Moho in unprecedented detail. These studies revealed a systematic segmentation of the Ionian oceanic lithosphere by nine trench-normal, subvertical fault zones that remain seismically active at intermediate depths beneath the entire Peloponnesus and the marine forearc domain. This fault-controlled architecture provided compelling evidence for slab tearing and highlights the role of internal slab deformation. Clustered seismicity in the mantle wedge above the tear faults suggests their potential role as pathways for fluid migration.

These slab faults appear to influence seismicity up to the forearc backstop. New results from ocean-bottom seismometer local tomography in the forearc domain further illuminate upper plate structural segmentation. We image a strongly imbricated upper-crustal wedge composed of blocks with contrasted P-wave velocities overlying the megathrust down to ~30 km depth. These blocks likely correspond to accreted terranes previously inferred from geological reconstructions but never imaged seismically. Beyond their geodynamic significance, this segmentation may modulate megathrust slip behaviour, as illustrated by our study of the Methoni earthquake. We propose that in the southwestern Hellenic subduction zone, megathrust rupture propagation is limited by the combined effects of small-scale upper-plate discontinuities and larger-scale lower-plate segmentation associated with slab tearing.

Complementary receiver-function results reveal a low-velocity layer -over 200km wide- located within the mantle wedge, below the shallow Aegean Moho and above the slab top at depths of ~50–70 km. Owing to the dense 2-D profile coverage, we resolve that this layer is segmented into distinct panels that closely mirror the along-strike segmentation of the retreating slab. This layer may represent inherited underplated material accreted during earlier subduction episodes, in a process analogous to the accretion of the Hellenic tectonostratigraphic terranes. Our observation of slab-parallel segmentation provides a key constraint on mantle wedge rheology, implying that slab faulting not only governs slab dynamics, associated upper plate deformation and fluid flow pathways but also structurally organizes the mantle wedge. Future finer scale imaging derived from multiscale analysis methods and synthetic modelling are planned to better constrain the nature of this layer and its role in fluid transfer and mantle wedge seismicity.