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Description: Localization processes in the viscous lower crust lead to the formation of deformation zones over a broad range of scales that may affect the mechanical response of faults in the upper crust during the entire seismic cycle. In order to gain detailed insight into the processes involved in strain localization and rheological weakening in viscously deforming rocks we conduct centimeter-scale numerical models. Our 2D Cartesian models are benchmarked to high-temperature and high-pressure torsion experiments on Carrara marble samples containing a single weak Solnhofen limestone inclusion. The numerical models successfully reproduce bulk stress-strain transients and final strain distributions observed in the experiments by applying a simple softening law that mimics rheological weakening. By varying softening parameter values within this modeling framework, we quantify the impact of rheological weakening on localization and shear zone formation. We find that local stress concentrations forming at the inclusion tips initiate strain localization inside the host matrix. Rheological weakening is a precondition for shear zone formation within the matrix. At the tip of the propagating shear zone, weakening occurs within a process zone which expands with time from the inclusion tips towards the matrix. Shear zone width is found to be controlled by the degree of softening. Introducing a second softening step at elevated strain, a high strain layer develops inside the localized shear zone, analogue to the formation of ultramylonite bands in mylonites.