A new method is introduced for quantifying the scale and the intensity of strain localization from maps of natural shear zones. The method employs autocorrelation functions to determine local areal scales of geometric homogeneity. These homogenization scales are used to calculate scale-dependent localization fractions of deformed rock. The strain localization intensity is quantified from measurements of mean relative to maximum shear strain. This approach is used to analyze shear zones on different scales from an exposure (Cap de Creus, Spain) of the fossil brittle-to-viscous transition (BVT). Changes in the scaling characteristics of shear zones are interpreted to reflect a time sequence of localization during the evolution of the continental BVT. We show that shear zone scaling is related to inherited anisotropies (older schistosity, lithological layering, pegmatite bodies) and to the predominant mode of deformation (brittle, viscous). The length-to-width ratio of shear zones increases with their length up to the meter scale and decreases for larger length scales as they evolve from isolated shear fractures to interconnected mylonitic shear zones. Variations in strain localization intensity calculated along a single shear zone indicate that such shear zones weakened from their brittle tips to their mylonitic centers, thus driving their propagation and growth to larger scales. Our results imply that the BVT evolves by “network widening,” a process whereby strain localizes on progressively larger scales until a dense network of weak, mylonitic layers tens to hundreds of meters wide and hundreds to thousands of meters long forms subparallel to the regional shearing plane.