Geological and structural study of the recent and active brittle deformation of the Neogene-Quaternary basins of Thessaly (Central Greece)

The present research have been carried out to study in detail the tectonic structures and kinematics of the Neogene and Quaternary brittle deformation of Thessaly basins (western of Karditsa, eastern of Larissa and southern of Almiros). For the geological mapping, especially of the Neogene and Quaternary deposits, and their lithostratigraphic sequence, has been employed the already existing information (mainly the geological maps of the I.G.M.E., besides other works), but it has been modified, completed and generally improved. While the original mapping has been carried out at the scale 1:50.000, in the present work is presented a geological map at the scale 1:250.000 (encl. 1). From the study of the sedimentary deposits of western Thessaly has been realized the lithostratigraphic column of the Karditsa Sequence (Oligocene-Burdigalian), while in the Central Hills have been separated and distinguished the Pliocene fluvio-lacustrine deposits from the more characteristic Pleistocene Red Beds (chapter 2). Most of the field work and particularly of the tectonic analysis has been spent studying the brittle structures that affect both the boundaries of the basins and the infilling sediments. Consequently, neotectonic faults, dislocated pebbles and extensional joints have been quantitatively analysed to estimate the three principal axes of the stress field (sigma1, sigma2, sigma3) of each separate tectonic phase. To compute the principal directions of the neotectonic stress fields from shear planes (striated fault planes and dislocated pebbles) have been used either already existing methodologies (right dihedrons and mean stress tensor) but mainly the mathematical procedure proposed by Caputo and Caputo (1988a). On the other hand, the joint systems have been elaborated separately applying the physical model and the mathematical procedure proposed by Caputo and Caputo (1988b), to estimate the three principal stress axes. All the data site by site, the geology of each area of measurement and the quantitative analysis are analitically presented and discussed in chapter 3. The result of the above mentioned tectonic analysis are summarized and shown in the structural map (encl. 2), where are represented all the stations of measurement (separately for faults, dislocated pebbles and joints). For each site are shown the direction of the sigma3 (minimum principal stress) and of the sigma1 (maximum principal stress) only when subhorizontal. The main conclusions carried out from the structural analysis are the existence of two main neotectonic extensional phases and an older compressional one and the knowledge of their relative and absolute chronology (fig. 6.1). 1) The first phase (fi1), the older one and less constrained, has an ENE-WSW direction of compression (average sigma1 N82°); it is post-Burdigalian because it affects sediments of that age and has been related to the alpide paroxistic phase of Middle-Late Miocene occurred in the External Hellenides. 2) The second phase (fi2), is characterized by a NE-SW direction of extension (average sigma3 N51°); it does not affect Quaternary sediments but it is well recorded in Pliocene deposits and is the first neotectonic extensional phase of Latest Miocene-Pliocene. This event generates the main actual morphology of the basins, with a lengthened shape trending NW-SE, partly inherited (Karditsa Basin) and partly of new formation (Central Hills and Larissa Basin). 3) The third phase (sigma3), is also characterized by extension but with a N-S direction (average sigma3 N9°); it is the most recent (Quaternary) and more recognizable of the three. To better understand and for a more complete study of this phase (sigma3) and to correlate it with the active deformation, as it is clear from the seismicity of the area, in chapter 4 are reported and discussed all the historical and recent seismic data regarding the strongest earthquakes occurred in the area while a seismotectonic study is carried out. In this chapter are also discussed the geometry (mainly E-W), the kinematics and the criteria to characterize the active faults of the study area, as that of Nea Anchialos-Volos, in the Almiros Basin, that of the Chalkodoni Mt, in the southern Central Hills, and its possible relation with the Velestino earthquake (1957), and expecially the Tirnavos Fault, in the northern Larissa Basin, that shows a very recent, probably historical, reactivation. Consequentely, this third tectonic phase is considered still active in the study area. Furthermore, have been used and described, in chapter 5, geomorphological data as the topography of the basins (encl. 3), the lineaments obtained from the topographic maps (encl. 4) and those obtained from the satellite photographs (encl. 5) to better understand the geometrical evolution of the basins. They show a first NW-SE direction of lengthening that is followed by an E-W trending geometry which is superimposed disrupting the previous one. In the last chapter (6) of tectonics and geodynamics, are summarized and discussed the late-Alpide (sigma1), the post-Alpide (sigma2) and active events (sigma3). The latter two extesional phases distinguished in Thessaly are also compared with the results obtained by other researchers in surrounding areas of the Aegean Region.