Evaluation of stresses in two geodynamically different areas: Stable foreland and extensional backarc

Abstract

Areas which are geodynamically different have different behaviors both in their thermal regime and seismic activity. A stable area has a geotherm which can be considered as standard, extensional and compressional areas have, respectively, high and low temperature gradients. The Italian region includes different geodynamical areas and all such situations are present. We consider the Apulian platform as an example of a stable area and the Tuscany-Latium as an example of an extensional area. For both of them the present geotherms are calculated, taking into account, for the Tuscany-Latium, its thermal history. Assuming that each region is subject to a constant strain rate, the stresses are calculated as functions of depth and time. The rheological behavior is assumed to be linear viscoelastic, with viscosity dependent on temperature and elastic parameters dependent on lithology. The geothermal profile and the rheological structure of the lithosphere remarkably affect the processes of stress accumulation which control the distribution of seismic activity. The abrupt decrease of the temperature gradient at the Moho produces considerably higher stress values with respect to the case of uniform gradient, thus favoring subcrustal seismicity. In the case of a standard temperature gradient, subcrustal seismicity is predicted and a gap in seismicity, indicating a soft intracrustal layer, exists if there is a discontinuity in rheology. By contrast, in the case of a high-temperature gradient, subcrustal seismicity is not to be expected, even in the presence of a discontinuity in rheology, since subcrustal temperatures are already too high to permit a sufficient stress accumluation.