Using plane-strain compression experiments this article elucidates the competing mechanisms of shear band formation (spatially distributed narrow, sharp shear bands: NBs versus localized composite shear bands: CBs) in heterogeneous elastoplastic solids as a function of pre-existing weak flaws. Homogeneous representative models without pre-existing flaws produced uniformly distributed, closely spaced NBs in conjugate sets, symmetrically oriented at angles of 41°-44° to the compression axis. Heterogeneous models, in contrast, formed CBs at an angle of 46°-49°, localized preferentially against the flaws, leaving the host almost free from any band growth. With increasing finite strains (6% to 18%) the CBs grew to a characteristic wide-band structure, typically comprising a core of densely packed band-parallel sharp secondary bands, flanked by linear regions (transition zone) of closely spaced, across-band NBs. We provide a band density analysis to show the distinctive shear-band characteristics for the homogeneous and heterogeneous models. This study also investigates the effects of global strain-rate (ε′) on the band localization mechanism in heterogeneous solids. Decreasing ε′ (3 x 10-5 sec-1 to 2 x 10-5 sec-1) is found to transform the composite bands into well-defined homogeneous shear bands (HBs) that contain a homogeneously sheared core, flanked by narrow zones of gradational shear into completely unstrained walls. We support our experimental findings with numerical model simulations in the framework of visco-elasto-plastic rheology. The article finally presents a set of geological examples to discuss various types of shear band structures in the light of heterogeneous model findings.

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.