When does slab fragmentation evolve into a STEP fault? The Kefalonia Transform Fault case study

Vertical tears in subducted slabs have been recognized in the majority of global subduction zones. Surprisingly, slab tears evolved into Subduction-Transform-Edge-Propagator (STEP) faults in only very few regions, i.e., the conditions under which STEPs form are special. It is relevant to constrain the conditions that facilitate STEP fault initiation because they leave a clear geological footprint in the overriding plate, whereas vertical tears generally do not. We therefore study a candidate region for STEP fault initiation in the western Hellenic Subduction Zone. We investigate the structure and seismicity of the shallow western Hellenic slab using a recent full-waveform inversion model which both captures details of crustal and upper-mantle structure, yielding constraints in the depth interval from 10-200 km where lithosphere-mantle interactions have tectonic expressions. The western end of the Hellenic slab is fragmented near the Kefalonia transform fault. We identify a separate Epirus lithospheric fragment that is roughly vertical below the southern Albanides. The southern edge of the Epirus fragment correlates with the subducted projection of the Apulia Escarpment. Its western edge correlates with a newly identified major contrast within the lithospheric mantle of the Ionian Ocean basin, which aligns with a gradient in free-air anomalies. In the overriding plate, the fault zone between the dextral Kefalonia and Lefkada faults accommodates strike-slip. Together with the Central Hellenic Shear Zone and the North Aegean Trough, we interpret this fault zone as a STEP fault that formed simultaneously with Pliocene fragmentation of the Epirus fragment. What we learn from the Kefalonia case is that contrasts in convergence velocity of the fragments and the amount of time over which it persists was likely relevant for STEP fault initiation. Mechanical coupling to, and the deformability of, the overriding plate appear to also play a role in the initiation of the STEP fault once a slab is fragmented. We hypothesize that the absence of a contrast in the subduction velocity of slab fragments in particular prevents the development of STEP faults above currently fragmented slabs.