A VISCOELASTIC SHEAR ZONE MODEL OF COMPRESSIONAL AND EXTENSIONAL PLATE BOUNDARIES

A model is proposed describing the mechanical evolution of a shear zon e along compressional and extensional plate boundaries, subject to con stant strain rate. The shear zones are assumed as viscoelastic with Ma xwell rheology and with elastic and rheological parameters depending o n temperature and petrology. Stress and strain are computed as functio ns of time and depth. For both kinds of boundaries the model reproduce s the existence of a shallow seismogenic zone, characterized by a stre ss concentration. The thickness of the seismogenic layer is evaluated considering the variations of shear stress and frictional strength on faults embedded in the shear zone. Assuming that a fault dislocation t akes place, the brittle-ductile transition is assumed to occur at the depth at which the time derivative of total shear stress changes from positive to negative values. The effects of different strain rates and geothermal gradients on the depth of the brittle-ductile transition a re studied. The model predictions are consistent with values inferred from seismicity data of different boundary zones.