Structures and kinematics of the Cephalonia-Lefkada Transform Fault zone

The Cephalonia-Lekfada Transform Fault Zone (CLTFZ) lies at the transition between the continental subduction of Adria microplate underneath Eurasia to the north, and the oceanic subduction of the Nubian plate along the Hellenic Arc to the south.

Since its onset, estimated around the Late Miocene-early Pliocene, the CLTFZ is considered to have accumulated between 40 and ~80 km right-lateral displacement, with most of the offset occurring in the last ~5 Ma. Currently, the intense crustal deformation characterising the area results in high seismicity affecting the western part of Lefkada and Cephalonia islands, as demonstrated by several Mw>6 earthquakes that struck this narrow region in the last two decades (Mw 6.2 Lefkada, 2003; Mw 6.1 and Mw 5.9 Cephalonia, 2014; Mw 6.4 Lefkada, 2015).

We use conventional structural mapping and geomorphic analysis to identify and measure the main onshore faults and their kinematic evolution along the western coast of Cephalonia and Lefkada islands.

We observe that the topography of the islands is mostly tectonically controlled: right-lateral transpression is expressed as elongated ridges, trending ~N15 and delimited by thrusts. Transtensive, NE-SW-trending en échelon faults develop on the ridge crests, where measured fault throw indicates up to 1.5 km NW-SE-directed extension on a single ridge. Similar structures have been recently observed in analogue modelling experiments, forming as Riedel-shears in the later stages of the transpressional system evolution.

Inactive, and hence older faults seem to have formed as opening fractures in an extensional regime (~W-E). The fractures are filled with calcite veins displaying multiple growth generations, which suggests formation in a fluid-rich environment. Active faults display at least two generations of striae indicating pure dip-slip and strike-slip/oblique movement. Where visible, the cross-cutting relationship between the two generations indicates a first phase of normal faulting followed by more recent transcurrent faulting. This is in agreement with geodetic data, reporting a dominant horizontal component of movement during the recent earthquakes.

The overall fault analysis indicates an important strain partitioning along the CLTFZ, providing relevant constraints to the seismotectonic pattern of the region.