Constitutive behavior of olivine gouge across the brittle-ductile transition

The bottom of the lithosphere is characterized by a thermally controlled transition from brittle to ductile deformation. While the mechanical behavior of rocks firmly within the brittle and ductile regimes is relatively well understood, how the transition operates remains elusive. Here, we study the mechanical properties of pure olivine gouge from 100 to 500ºC under 100 MPa pore-fluid pressure in a triaxial deformation apparatus as a proxy for the mechanical properties of the upper mantle across the brittle-ductile transition. We describe the mechanical data with a rate-, state-, and temperature-dependent constitutive law with multiple thermally activated deformation mechanisms. The stress power exponents decrease from 70±10 in the brittle regime to 17±3 and 4±2 in the semi-brittle and ductile regimes, respectively. The mechanical model consistently explains the mechanical behavior of olivine gouge across the brittle-ductile transition, capturing the gradual evolution from cataclasis to crystal plasticity.