Shear localisation and strain distribution during tectonic faulting - new insights from granular-flow experiments and high-resolution optical image correlation techniques

Brittle faults reflect a complex strain history that emerges from contrasting modes of distributed and localised deformation, and their interaction on various spatial and temporal scales. To better understand this process, we monitor the displacement field in scaled tectonic model experiments using high-resolution optical image correlation techniques (particle imaging velocimetry, PIV). 2D and 3D surface displacement data of extensional and contractional sandbox experiments show that the mode, pattern, and temporal variation of strain accumulation are strongly dependent on the non-linear strain-dependent frictional strength of granular model materials similar to natural deformation processes in brittle rocks. Strain hardening and softening control the shear zone formation. The pattern of localised deformation is established much earlier than is visible from visual inspection of the experiment. Evolution of distributed strain in the surrounding material of the shear zones and discontinuous shear re-localisation control later stages irrespective of the kinematic boundary conditions. High-resolution optical strain monitoring quantifies the spatial and temporal patterns of strain accumulation in our model experiments with unprecedented detail. Together with detailed characterisation of the deformation behaviour of the model materials, our experiments will help to re-evaluate important scaling issues, and allow accurate comparison of analogue experiments with numerical simulations. (c) 2004 Elsevier Ltd. All rights reserved.