Assessment of relative tectonic activity in the Trichonis Lake graben (Western Greece) a combined approach using geomorphometry and Self Organising Maps

The geomorphometric analysis of mountain fronts, drainage networks and catchments in tectonically active areas can provide valuable information about the spatial distribution of relative tectonic activity. The aim of this study is to investigate the contribution of neotectonics to the development of the fluvial landscape of the broader Trichonis Lake area (located in western continental Greece) through quantitative geomorphological analysis. The Trichonis Lake graben is a is a late Plio-Quaternary extensional basin, which cuts across the early Tertiary NW-SE fold and thrust structures of the Pindos Mountain belt. It strikes WNW–ESE for a distance of 32 km and has a width of 10 km. The graben at the north and south flanks of the lake is bounded by E-W and NW-SE trending faults. Recent seismic activity (a shallow earthquake sequence in 1975 and a 2007 earthquake swarm) showed the existence of a NNW-SSE normal fault that dips to the NE and bounds the SE shore of the lake. The studied catchments have developed on the hanging walls of these active normal faults. To evaluate the relative tectonic activity in the study area, various morphometric indices including stream length-gradient index, drainage basin asymmetry factor, hypsometric integral, valley floor width-valley height ratio, drainage basin shape, drainage basin slope, relief ratio and Melton's ruggedness number were measured for 35 catchments. In addition, mountain-front sinuosity index has been estimated for 20 mountain fronts around the lake. For the measurement of the geomorphometric variables a digital elevation model (with 5 m cell size), derived from topographic maps at 1:5000 scale (with 4 m contour lines), was used. The catchments of the study area were classified into three classes based on the morphometric variables and a series of maps showing their spatial distribution were produced by applying GIS techniques. The combination of these morphometric variables yielded two new indices of relative tectonic activity (named IRTA: Index of Relative Tectonic Activity and IAT: Index of Active Tectonics). Based on the values of IRTA and IAT, the area was classified into three classes of relative tectonic activity (low, medium, and high). To evaluate the landscape around Lake Trichonis in terms of neotectonic activity, we identified clusters of drainage networks and catchments according to their geomorphic characteristics (expressed by the geomorphometric indices) using self-organizing map, which is a type of unsupervised artificial neural network (ANN). The analysis showed that the development of the landscape of the central part of the northern flanks of the graben has been highly influenced by the tectonic uplift since these catchments are characterized by high IRTA and IAT values which correspond to high relative tectonic activity. The study of these fault-generated mountain fronts showed that although both graben flanks have low sinuosity index values, the central segment of the north flank of the graben has slightly lower values which are indicative of active tectonics.