Stream piracy and tectonic control on the evolution of drainage networks: A case study from the Island of Evia, Greece.

The drainage networks are very sensitive to changes caused by the interactions between tectonic activity, climate, and earth’s surface processes and thus contain useful information on the landscape development. Drainage networks and catchments located on the footwall of normal faults are excellent archives for understanding and quantifying the active deformation processes. This study aims to identify stream piracies due to tectonic activity, to record the main factors that facilitated river captures, as well as to identify and interpret lateral tilting of river valleys. The study area includes 3 drainage networks and their corresponding catchments (ranging in area between 15.2 km² and 29 km²), located at the eastern slopes of the Dirfis Mountain range, in Evia Island, Central Greece. The rough landscape of the area and the evolution of the drainage networks seem to be strongly controlled by the neotectonic activity of the offshore Dirfis normal fault zone. Selected morphotectonic parameters including Asymmetry factor (Af), Transverse topographic symmetry factor (T), Mountain front sinuosity (Smf), and Valley height-width ratio (Vf) were estimated for the drainage networks and their catchments, whereas the longitudinal profiles of the main streams’ channels were constructed and analyzed. All three streams are characterized by active headward erosion and intense incision. Along the main streams’ channels a large number of knickpoints, mainly of tectonic origin, were detected. The results suggest that the evolution, the shaping up and the patterns of the drainage networks along with the morphological characteristics of the valleys and the stream longitudinal profiles anomalies are controlled by neotectonics. The stream piracies in the study area (network adjustments, abrupt changes in flow direction, elbow of capture), result mainly from changes in response to the activity of the local fault zones of the eastern slopes of Dirfis Mountain range and reveal the effect of both tectonic uplift and tilting.