Knickpoint extraction for the evaluation of active tectonics: The case of Evia Ιsland, Greece

Τhis study aims to investigate to what extent the drainage systems of the northern part of Evia Island, in Central Greece, reflect the contemporary tectonic regime of the area. The effects of tectonic activity have been detected and described by performing a landscape morphological analysis of the drainage systems, followed by a field survey for validating the results. The Relative Declivity Extension (RDE) index, which is based on the Stream Length (SL) gradient index, was calculated along the channels of 189 drainage basins of the study area, using the Knickpoint Finder tool integrated into ArcGIS desktop. The Hack RDE index is defined by the ratio of the RDEs index (which refers to a stretch) and the RDEt index (which refers to the total length of a river) was applied, and its calculation identifies the knickpoints based on anomaly values detection along the longitudinal profiles of the rivers. Furthermore, knickpoint analysis was carried out along the channels of the drainage networks of the northern part of the Evia Island and the corresponding anomaly maps were designed. A total of 2,486 knickpoints were identified and the greatest anomalies (139 points), named first-order anomalies (RDE>10), seem to correspond to a recently shaped or rejuvenated landscape, indicative of intense neotectonic activity. In some cases, the spatial distribution of knickpoints delineates a NW-SE trending yielding the structural control of the study area. The comparison of the results with the lithological map of the area showed that about 30% of the knickpoints are lithologically controlled. The identification of zones of neotectonic activity and consequently the existence of active faults is highly correlated with the distribution of knickpoints and their core density, the drainage density, the drainage asymmetries, and other morphometric indices such as the hypsometric integral, along with the earthquake epicentres, and the morpho-lineament density. The overall analysis showed a positive correlation of the concentration of the derived knickpoints with both active structures and tectonic activity rates. The results were consistent with field observations, which were mainly focused on waterfall landforms. Our study proves that the landscape evolution of the northern part of the Evia Island is considerably affected by the recent activity of normal fault zones, whereas the drainage systems react to the changes of base-level providing insights on active tectonics.