Dislocation dynamics modelling of the power-law breakdown in olivine single crystals: Toward a unified creep law for the upper mantle

In the present work, we use a numerical modelling approach based on 2.5-dimensional dislocation dynamics simulations to investigate the transition between the power and the exponential laws in olivine for temperatures ranging between 800 K and 1700 K and stresses between 100 and 500 MPa. We model the deformation of an olivine crystal by the interplay of glide and climb of dislocations. Plastic strain is produced by glide, the amount of gliding dislocations being controlled by climb acting as a recovery mechanism. Within this framework, and without the need of introducing any other mechanism, our model reproduces a power law breakdown above 200 MPa. Consequently, we conclude that the use of two rheological laws to describe the creep of olivine can be motivated by convenience, but is not imposed by theoretical needs. Alternatively a unified creep law can be proposed to describe the rheology of olivine in a wide range of temperature relevant for the upper mantle. This flow law may have an exponential form and describe the entire range of experimental data, from room temperature to 1800 K at both low and high stresses, using a single adjusting parameter, the so-called mechanical resistance (sigma) over tilde. We also propose an alternative mathematical expression based on a sigmoid function, which is more suitable for implementation in geodynamical models