The brittle-ductile transition in tectonic boundary zones

Observations at the Earth's surface show that tectonic boundary zones are intensively deformed belts with horizontal widths of tens or hundreds of kilometres. Most seismic activity is concentrated in a shallow «seismogenic» layer, suggesting that at larger depths the relative plate displacements are accommodated through ductile, aseismic deformation. This is confirmed by laboratory experiments indicating that, at sufficiently high temperatures and pressures, brittle behaviour in crystalline materials gives way to ductile behaviour. At the very low strain rates typical of the Earth's tectonics, ductility is a long-term behaviour and lithospheric rocks can be modelled as viscoelastic materials, with effective viscosity dependent on depth through temperature and pressure. Theoretical models of tectonic boundary zones are able to reproduce the brittle-ductile transition and give an insight in the relationships between state of stress, rheological properties and seismic activity.