In this paper, we show the impact of Thermo-Hydro Mechanical couplings (THM) on the stability of a saturated fault gouge under shear. By resorting to Cosserat continuum mechanics, that allows to take into account rotational degrees of freedom, we regularize the problem of localisation and we predict the thickness of a shear band. A linear stability analysis of the homogeneous state is performed and then the system of equations is integrated using a Finite Element (FE) analysis. These analyses can be used for studying the evolution of the thickness of the principal slip zone in a fault under undrained adiabatic shear. Good agreement is found between theoretical predictions and field observations.