Quantifying active faulting using marine terraces, Kythira island, Greece

The coastal morphology of islands may furnish valuable information regarding deformation rates, their controlling mechanisms and the dynamics of the upper crust in offshore areas along subduction zones. Here we study active deformation and faulting at glacial-cycle time scales in the Kythira island, located at the western part of the Hellenic subduction zone, between Crete Island and the Peloponnese. The island exposes an outstanding sequence of more than twelve successive levels of marine terraces that depict the Pleistocene active uplift of the island. The marine terraces are offset by several NNW-SSE and NNE-SSW active faults. We use high-resolution topography combined with morphometric analysis to map the sequence of marine terraces and active faults. We divide the marine terrace sequence into two groups, the higher marine terraces (260 – 480 masl) include polygenic rasa surfaces, the lower terraces (20 – 220 masl) are characterized by staircase morphologies. Based on a proposed correlation with sea level curves, we estimated ages ranging between MIS 17 and MIS 22 (712 – 1000 ka) for the higher terraces and between MIS 5 and MIS 15 (125 – 620 ka) for the lower terraces. We focus on the two main faults of the island, defined as F1 and F2, they display right- and left-lateral and dip slip displacements, offsetting the marine terrace risers and treads and producing local drainage anomalies. Based on the proposed terrace ages we derived preliminary heave rates between 0.3 and 0.5 m/ka for the right-lateral fault F1 and between 0.8 and 1 m/ka for the left-lateral fault F2. Mean throw rates vary between 0.01 m/ka and 0.03 m/ka for F1 and F2 respectively. We link the activity of these faults with the occurrence of intermediate-depth and strong magnitude earthquakes such as the Mw 6.6 and 6.7 occurred in the area of Kythira in 1903 and 2006, respectively. Further dating of marine terrace deposits and surfaces, and structural analysis will be carried soon to refine our preliminary estimates. Our work emphasizes on the importance of studying islands to elucidate vertical and horizontal deformation rates in offshore areas of subduction zones.