Strain localization as a key to reconciling experimentally derived flow-law data with dynamic models of continental collision

Published strength profiles predict strength discontinuities within and/or at the base of continental crust during compression. We use finite element models to investigate the effect of strength discontinuities on continental collision dynamics. The style of deformation in model crust during continued subduction of underlying mantle lithosphere is controlled by: (1) experimental flow-law data; (2) the crustal geotherm; (3) strain localization by erosion; (4) strain-softening and other localization effects. In the absence of erosion and other factors causing strain localization, numerical models with typical geothermal gradients and frictional/ductile theologies predict diffuse crustal deformation with whole-scale detachment of crust from mantle lithosphere. This prediction is at odds with earlier model studies that only considered frictional crustal theologies and showed asymmetric, focused crustal rheologies deformation. Without localization, model deformation is not consistent with that observed in small collisional orogens such as the Swiss Alps. This suggests that strain localization by a combination of erosion and theological effects such as strain softening must play a major role in focusing deformation, and that strength profiles derived under constant strain rates and uniform material properties cannot be used to infer crustal strength during collision dynamics.