Numerical modelling of strain localisation during extension of the continental lithosphere

Dipping reflectors in the upper mantle are observed on reflection seismic d ata and, in some cases, have been interpreted to represent mantle shear zon es. It is not clear, however, how these shear zones develop and under what circumstances. Previous attempts to model strain localisation by strain sof tening in the ductile regime have failed to show the development of mantle shear zones. We present here results from a numerical model of lithospheric extension. The model uses an elasto-visco-plastic rheology dependent on te mperature, pressure, deviatoric stress and composition. Strain softening is incorporated into the mechanical model via a simple ad hoc strain-viscosit y relationship. The results demonstrate that strain localisation can domina te lithospheric deformation in an extensional regime. We illustrate the eff ects of varying the parameters, which define the strain-viscosity relations hip on the geometry of lithospheric-scale shear zones and the rift basin. W e also investigate the effects on shear zone development of introducing a v ariety of pre-extension weak zones in the crust and/or mantle. Our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones, there is a need to review our conceptual models of strain/strength evolution during the de velopment of continental rifts. (C) 2001 Elsevier Science B.V. All rights r eserved.