The lithosphere is mechanically and compositionally heterogeneous across virtually all spatial scales. Competence contrasts between minerals and lithologies, inherited shear zones and sutures, reaction fronts, fractures, and variations in hydraulic properties can redistribute stress, strain and mass, exerting a complex control on the effective rheology of deforming rocks. At subduction interfaces, heterogeneous assemblages of blocks, slivers and weak or strong metamorphic rocks may govern mechanical coupling; in the lower crust, deformation is commonly partitioned into weak shear zones surrounding stronger domains; and at the lithospheric scale, inherited sutures and weak orogenic belts can guide deformation around mechanically stronger cratonic terranes. Thus, such anisotropy influences deformation across the entire temporal spectrum, from long-term ductile flow and strain localization to short-term mechanical instabilities with direct implications for earthquake rupture and natural hazards.

This session aims to determine how mechanical, compositional and hydraulic heterogeneities arise, evolve and govern deformation and fluid–rock interaction across the lithosphere, from microstructures to plate scales and across tectonic environments. We encourage contributions combining field and microstructural observations, petrological and geochemical constraints, rock mechanics, geodesy and geophysical imaging, and numerical modelling. By integrating these perspectives, we aim to uncover how contrasts in composition, strength and hydraulic properties give rise to emergent patterns of strain localization, fluid and mass transfer, and bulk rheological behaviour, linking long-term tectonic evolution with transient mechanical processes.