To tear or not to tear? A comparison between analogue modelling and field observations along the Kefalonia Transform Fault System

The Central Mediterranean is a great natural laboratory for many processes related to subduction. Along the Dinaric-Hellenic margin, the rigid Adria microplate indents the Eastern Alps and Dinarides in the north, while the southern part subducts beneath the advancing Hellenides. The Kefalonia Transform Fault System (KTFS) marks the current position of this unique transition between subduction of more buoyant continental lithosphere and less buoyant oceanic lithosphere. The resulting differential convergence is thought to have caused vertical tearing or bending of the subducting slab, although the lack of detailed seismological investigations leaves an open question concerning this geometry.

Slab tears have a significant role in surface evolution around subduction zones. They affect mantle flow, stress propagation within the subducting plate, as well as dynamic topography and volcanism on the surface. However, most models of slab tears investigate their evolution by pre-cutting the subducting lithosphere. We investigated the mechanisms underlying the dynamic formation of a vertical slab tear to interpret geodetic, tomographic, and tectonic observations from around the KTFS. To achieve this, we built a setup with a geometry inspired by the natural subduction system, varied the continental domain's rheology, and introduced an ocean-continent transition zone composed of non-Newtonian analogue materials that allow for strain localisation and slab detachment.

In particular, we wanted to: i) explore how the subducting plate deforms when a tear is forming; ii) observe how the mantle flow reacts to such changes in subduction dynamics; iii) estimate what are the resulting effects on the stress distribution and surface strain on the overriding plate.

We analysed two experimental end-members (i.e., model (A) ocean and continent in lateral contact Vs model (B) separated by non-Newtonian, transitional material) and compared them with the natural observations and the geometry of the subduction system. In both models the rigidity of the continental segment has a critical role in the type of deformation we observe during continental subduction, and controls the amount of stretching, rotation, and continental subduction. The transition zone in model (B) localises deformation, minimising shear and extensional deformation of the continent.

At the end of the experiment, the subduction front geometry of model (B) better reproduces the actual eastern Adriatic margin in correspondence of the KTFS, and the deformation observed on the continental plate is consistent with the structures observed on the field, indicating a certain level of coupling between slab and overriding plate. This similarity without achieving slab tearing suggests that a slab bend may be sufficient to reach the present natural configuration. Consequently, a slab tear may be absent or its extent be limited to a deeper section of the slab.