Dynamic rheology and strength of the lithosphere

The strength of the lithosphere is the integrated force required to cause deformation at a given rate. It has previously been assumed that continents subject to deformation are weaker when they are hotter. We argue that it is not the steady-state heat flow of continents that control their strength, but time-dependent feedback effects, triggered by shear heating and thermal expansion. These effects localize strain into weak shear zones which control the time-dependent strength of continents. Here we present numerical results showing that a cold and strong continent is substantially weakened by development of intensely localized shear zones. In contrast, weakening effects are less efficient in an initially warmer continent where shear zones are more diffuse. This leads to self-organization of the dissipative structures, i.e. the width, length, distribution and heat generation of shear zones. As a result, regardless of initial temperature profiles and crustal thicknesses, all modelled continents yield similar time-dependent strengths that follow similar temporal evolution defining a lithospheric strength attractor. An important implication is that even cold and deeply-rooted Archaean cratons may be vulnerable to deformation as evidenced by a number of cratons that were rifted apart during the break-up of Gondwana