GNGTS 2018 - 37° Convegno Nazionale

192 GNGTS 2018 S essione 1.2 strength and taking into account that semi-brittle behaviours primarily modify the maximum shear strength (usually decreasing it) rather than the BDT vertical position, we considered the two rheological behaviours simplified approach, though somehow approximated, to be accurate enough for and, at the same time, more functional to our purposes. Secondly, the constitutive equations of the semi-brittle rheologies are primarily empirical and their range of applicability is seriously limited by their relative experimental characteristics, such as the (small) sample of tested lithologies and ambient conditions. Given the exponential dependence of the power-law creep equation on the temperature at depth, we firstly focused on properly calculating the geothermal gradients for the two transects. To do so we used the temperature depth distribution equation by Cermak (1982) which takes into account the variable radiogenic heat production rate and the thermal conductivity of the different layers that make up the considered lithospheric column. Both the geothermal and the rheological constitutive equations imply the use of several input parameters (surface heat flow, thermal conductivity, strain rate, activation energy, friction coefficient, pore fluid pressure etc.) whose values should be carefully selected in order to obtain a reliable strength profile. Accordingly, we collected as many quality literature data as possible in order to define the range of the most likely values for each input parameter, always trying, when possible, to consider literature data based on different methods of measurements, observations and surveys and constraining the values with geological, tectonic and geodynamic considerations. Following this approach, as regards the depth of the Moho, we averaged between results from receiver functions studies (Sodoudi et al. , 2006) and gravimetric inversions constrained by seismic profiles (Makris et al. , 2013); in a similar manner, we used data from local and regional-scale maps (Fytikas and Kolios, 1979) as well as from continental-scale ones (Hurter and Haenel, 2002; Cloetingh et al ., 2010) to obtain a coherent map for the value of the surface heat flow for the whole Aegean Region. As concerns the lithotypes of the sedimentary cover and the upper crust layers, which play a crucial role in determining the resulting rheological properties, we mainly based our choices upon surface geology observations (especially for the sedimentary layer) together with geodynamic evolution and paleogeographic reconstructions (Doutsos et al ., 2006; Mountrakis, 2006; Papanikolaou, 2013 among others) of the Aegean Region, in order to select the upper crust most likely composition. Accordingly, the western segment of the WSW-ENE trending transect, mainly belonging to the Ionian domain, is characterized by a mostly carbonate sedimentary cover and a quartzitic basement, while the central-eastern segment around Thessaloniki has been associated with metasediments in the sedimentary layer and granitic-granodioritic intrusions for the upper crust. The NNW-SSE trending transect lies instead between the Pindos and the Pelagonian isopic zones and it is therefore characterized by a metasedimentary cover and a quartzitic upper crust. Once defined the values for all the input parameters, as described above, we realized the two transects, consisting of 57 1D strength profiles each. We interpolated the 1D profiles applying a local polynomial algorithm for the maximum shear strength depth distribution and a kriging technique for the temperature one, while the BDT depth trends along the transects have been traced following and connecting the BDTs from the densely spaced 1D profiles. Results and seismotectonic applications. TheWSW-ENE trending transect is characterized by a decreasing shear strength and a shallowing BDT depth towards the northeast Fig. 2). More precisely, the strength values are comprised between 550 MPa in the region around Corfu and the western Ionian coast and 90 MPa in southern Thrace, while the BDT depth reaches a maximum depth of 20-22 km in the axial sector of the Hellenides and a minimum of ca. 10 km towards the northeastern end of the transect. Such variations are accompanied and most likely caused by a progressive increase of the geothermal gradient from the WSW to the ENE, which is thought to be related to the greater amount of extension occurred in the (backarc) northeastern Aegean Region and the consequent crustal thinning, rising of the isotherms and overall rheological weakening.