Earth mantle convection, resulting from the transport of heat from the interior of the Earth, involves the deformation of solid mantle rocks over billions of years. The lower mantle of the Earth is primarily composed of iron-bearing bridgmanite MgSiO3 and approximately 25% volume periclase MgO (also containing some iron). Significant advancements have been made in recent years to study lower mantle assemblages under relevant pressure and temperature conditions, which have confirmed the usual view that ferropericlase is weaker than bridgmanite. However, natural strain rates are 8 to 10 orders of magnitude lower than those observed in the laboratory, and remain inaccessible to us. Once the physical mechanisms of the deformation of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modeling, which allows for the determination of rheological properties for inaccessible strain rates. This presentation will demonstrate how this theoretical approach can be used to describe the elementary deformation mechanisms of bridgmanite and periclase. The comparison of theoritical approach to experimental results demonstates the validity of our approach. In a second step, the impact of very slow strain rates on the activation of the aforementioned mechanisms is evaluated. Our findings indicate that significant alterations in deformation mechanisms can occur in response to changes in strain rate. Consequently, we propose the necessity of developing a novel approach that integrates physics-based modeling informed by data obtained from high-pressure, high-temperature experiments.