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.

The India-Asia continental collision, starting at ~50 Ma, has resulted in about 2000 km crustal shortening to build the Himalaya-Tibetan plateau, which is one of the landmark terrestrial features on the Earth. In this study, using a thin viscous sheet approximation we performed scaled laboratory model experiments to investigate the spatiotemporal variation in the Himalaya-Tibetan tectonics. The experiments allow us to constrain the Tibetan plateau topography as a function of the varying India-Asia convergence rates. Our results suggest two rigid crustal blocks: Tarim and Sichuan basins steered the growth pattern in the Tibetan plateau. Because of the resistance from the rigid Tarim block western Tibet uplifted first relative to eastern part, creating a topographic elevation difference, which directed the crustal flows grossly to NE. We show from experiments the elevated plateau topography underwent gravitational collapse when the indentation velocity dropped to present average of ~3.5 cm/yr at around 18 Ma. This event eventually led to a transition from contraction to extensional tectonics, dominated by east-directed crustal flows in response to the eastward topographic gradient developed during the early stage of fast collision. We compare the present day crustal flow velocity field, strain rates, and topographic variations in the model Tibet with the actual observations in the Himalaya-Tibet Mountain System.