GNGTS 2022 - Atti del 40° Convegno Nazionale

158 GNGTS 2022 Sessione 1.3 by knowing the average strain rate of an area, variations in Q s can provide a useful attribute to identify the domain within the crust, where the deformation transitions from a brittle dominated to a more ductile deformation mode. In addition, this extrapolation is subjected to less uncertainties than other proxies in terms of variations of P - T conditions and flow law parameters. The results of the lab experiments show that the BDT transition is characterized by the increase of crack-density with a progressive rate reduction. At the same time, both the seismic velocity and energy significantly decrease during the first phase of deformation (brittle regime) and tend towards an asymptotic value, when the sample approaches the ductile deformation (Fig. 3). We interpret the absence of an increase of energy loss at the BDT, as due to the persistent effect of the microfracturation. The last one usually accompanies the deformation mechanisms that occur at the BDT (e.g., pressure solution, twinning), masking the expected increase of attenuation at the beginning of the ductile conditions. This is a matter that still needs to be investigated. To this purpose, we should also consider that in the field fracturing causes an increase in the bulk rock permeability, favoring the influx of fluids, and precipitation of minerals (e.g., phyllosilicate), having a much weaker rheology than the rock matrix (e.g., Collettini et al. , 2019). Therefore, the BDT should rather identify a broader zone of finite thickness, in which both frictional and viscous deformation mechanisms are active, rather than a specific depth level. References Bürgmann R., Dresen G., 2008. Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations . Annu. Rev. Earth Planet. Sci. 36, 531–567. Burov, E.B., 2011. Rheology and strength of the lithosphere . Mar. Pet. Geol. 28, 1402-1443. Byerlee J., 1978. Friction of Rocks. In: Byerlee J.D., Wyss M. (eds) Rock Friction and Earthquake Prediction. Contributions to Current Research in Geophysics (CCRG) 6. Birkhäuser, Basel. Collettini C., et al. , 2019. Beyond Byerlee friction, weak faults and implications for slip behavior. Earth Planet. Sci. Lett., 519, 245–263. Farina, B., et al. , 2019. Seismic properties in conductive and convective hot and super-hot geothermal systems. Geothermics 82, 16-33. Goes, S. et al. , 2020. Continental lithospheric temperatures: A review. Phys. Earth Planet. Int. 306, 106509. Goetze, C., B. Evans, 1979. Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics . Geophys. J. R. Astron. Soc. 59, 463–478. Hasterok, D., Chapman, D.., 2011. Heat production and geotherms for the continental lithosphere . Earth Planet. Sci. Lett. 307, 59-70. Karato, S. 2008.  Deformation of Earth Materials: An Introduction to the Rheology of Solid Earth. Cambridge: Cambridge University Press. Kreemer, et al. , 2014. A geodetic plate motion and global strain rate model. Geochem. Geophys. Geosyst., 15, 3849– 3889. Maggi, A., et al. , 2000. A re-assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes occur in the continental mantle? Geophys. J. Int 143, 629-661.