Quaternary transtensional stretching thinned the Hellenic forearc of Greece and uplifted several islands (e.g. Karpathos, Crete, Rhodes), yet produced the Karpathos Basin, a deep forearc basin with water depths >2.5 km. The Lefkos Basin on the west coast of Karpathos island offers onshore exposure of the adjacent offshore Karpathos Basin and insight into its neotectonic and kinematic history.

The Lefkos Basin, originally thought to be a Pliocene simple half-graben, is instead a complex forearc basin of Pleistocene age based on foraminifera, macrofauna and Sr/Sr dating. The base of the basin is not observed, but two phases of basin fill are exposed. The older basin fill thickens southeastward, records SSE-directed sediment transport below marine wave base and displays syn-depositional normal faults suggesting the initial basin was controlled by NE-SW faults. These marine marls, mass flows and calcareous turbidites onlap Hellenide metamorphic basement on both the NW and SE basin margins with no observed basin-bounding faults. The final phase of basin filling comprises a conglomeratic calcareous fan delta recording subaerial exposure of Karpathos to the E and prograding westward into water depths of 15-20 m. These marine deposits now reside at elevations ≤140 m.

This entire Lefkos basin fill is dissected and displaced by a younger fault system in which >45% of the faults dip ≥80°, some of which record reverse displacements. Such steep faults suggest wrench-fault dominance coupled with subordinate normal faulting, stretching that is clearly displayed at the outcrop scale. NNW faults (350-010 degrees) record the youngest displacements and control modern topography. Submergence of Byzantine structures and clay potshards caught in these shear zones indicate that this youngest faulting post-dates Byzantine occupation (i.e. post-mid-6th century). Because these faults project offshore through the SE margin of the seismically active Karpathos Basin, their reactivation may pose a seismic and/or tsunami hazard. Overall, the basins of western Karpathos likely represent a stepover between the sinistral Cretan and Nisyros basins along faults that root in a mid-crustal transtensional detachment. Quaternary kinematics are consistent with anticlockwise rotation during increasing curvature of the Hellenic forearc.