Accommodation of deformation in Earth's lithosphere typically results in an heterogeneous distribution of strain, which is a function of material properties, environmental conditions and imposed mechanical boundary conditions at different structural levels. Beneath the pressure-dependent rheology of the brittle upper crust, deformation is dominated by thermally activated viscous flow in the lower crust and in the upper mantle. At the transition between these two rheological end-members, a combination of both transient, potentially seismogenic, brittle failure and continuous, aseismic, steady-state viscous flow occurs. This 'brittle-ductile transition zone' represents the zone of maximum crustal strength and favours the accommodation of strain along layered rheological instabilities that develop into wider ductile shear zones in the lower crust and upper mantle. These structures are crucial for the generation of plate tectonics from mantle convection. The understanding of the modes of strain localization and how deformation partitions between the two rheological end-members (brittle and/or ductile) is therefore of paramount importance in developing adequate rheological models and numerical simulations of the strength of the Earth's lithosphere.

We invite contributions that address strain localization at depths ranging from the brittle-ductile transition to the upper mantle. We welcome experimental, microstructural and field studies, as well as numerical modelling of relevant deformation mechanisms and macroscopic strain localization. We aim to advance in the understanding of the role of fracturing, viscous flow, fluid-rock interaction and thermo-mechanical properties in strain localization and their potential implications for the strength distribution throughout mid-lower crust and upper mantle, as well as their role in (de)coupling crust-mantle in large-scale tectonics.