Shear bands critically govern the shear failure processes and associated many geophysical phenomena, e.g., faulting, in Earth's crust. Earlier investigations on homogeneous materials recognized temperature and strain rate as the principal factors controlling the band growth. However, how inherent mechanical heterogeneities can influence their growth mechanism and internal structures remained unexplored. From plane-strain compression experiments on rock analogue models the present article addresses this issue. It is demonstrated from these experiments that mechanical heterogeneities dictate the failure to develop wide bands, localized preferentially in their neighborhood, unlike uniformly distributed, conjugate sets of closely spaced narrow bands in homogeneous models. The wide bands eventually attain a composite structure with a core of densely packed band-parallel sharp secondary bands, flanked by a linear zone of closely spaced, narrow bands. This study also reveals the effects of global strain-rate on the band evolution in heterogeneous systems. Decreasing strain rates replace the composite bands by well-defined homogeneous shear bands, containing a core of uniform shear, bordered by narrow zones of weak shear, grading into completely unstrained walls. Numerical modelling in a visco-elasto-plastic rheological framework under geological strain-rates is used to quantify the strain partitioning patterns in the heterogeneity-controlled transformation of spatially distributed narrow to localized, wide composite shear bands. The model simulations also reveal that increasing strain rates facilitates the bands to form clusters, instead of single wide bands. The article finally provides a set of field observations to demonstrate the importance of heterogeneity-driven band mechanics in interpreting macro-scale shear zones in geological terrains.