GNGTS 2018 - 37° Convegno Nazionale

206 GNGTS 2018 S essione 1.2 increase of seismogenic layer thickness with convergence rate applies also to megathrusts of subduction zones characterized by intermediate (5–7 cm/yr, i.e. Cascadia, Sumatra and Chile) and fast (up to 10 cm/yr, i.e. Japan) convergence. The relationship between convergence rate and BDT deepening is here investigated by 2D thermo-mechanical models of thrust faults. Searching for the BDT depth values (defined as the point of the model that records the maximum value of differential stress) and considering homogeneous lithologies, the brittle-ductile behavior of rocks is evaluated depending on applied convergence. In addition to the control provided by convergence rate, the effects of other parameters, i.e. the thermal structure of the domain and the viscous limit on strength are tested as well. The 2D numerical model was created using the thermo-mechanical code LAPEX-2D (Babeyko et al. , 2002). This software permits modelling of realistic temperature- and stress-dependent viscoelastic rheology combined with Mohr–Coulomb plasticity. At each calculation step the algorithm selects the more appropriate rheology (elasto-plastic or nonlinear visco-elastic) at the current conditions of temperature and strain rate. The model evaluates the rheological profile through the variation of convergence rates as initial boundary condition. Modelled geometries describe a 2D portion of wet-quartzitic upper crust 250 km wide and 40 km thick, cut by a fault with different dip values in the range of 15–40 degrees. Grid resolution is 1 x 1 km. The initial temperature distribution of the reference model grows from 0 °C at the surface up to 500 °C at the bottom of the domain (40 km depth) following a steady state continental geotherm. Results show that the faster the convergence, the deeper the BDT (Fig. 2). A ten times faster shortening (from 1 to 10 cm/yr) generates a doubling of the BDT depth (from 10 to 20 km). This range of variation is very sensitive to the fault geometry (being larger for shallow dip faults), to the initial temperature profile (shallower BDTs are obtained assuming cooler crust) and to the assumed friction angle (narrower and shallower BDT depth for increasing angle) assigned to the fault especially at slow convergence rates. These values are consistent with observations from fold-and-thrust belts and intraplate contractional areas. As examples, in regions characterized by convergence rates of few mm/yr like Emilia (Italy) or the New Madrid seismic zone, brittle deformation is confined at depths of 10–15 km. This value fits the depth of the decollement constrained by geological data. Where the India plate converges with Eurasia at rates of 4–6 cm/yr, the BDT is well defined at depth of 15–16 km. Intraplate earthquakes around Fig. 1 - The maximum seismogenic depth increases from 15 km at slowly converging regions (1–4 cm/yr) in fold and thrust belts (yellow diamonds) down to 25 km along faster converging boundaries (5–10 cm/yr) at subduction boundaries (blue diamonds).