The impact of analogue material properties on the geometry, kinematics, and dynamics of convergent sand wedges

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Simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks. We present data on the frictional strength of different sand types employing static and dynamic shear tests. The sand types analysed are characterised by an elastic/frictional plastic mechanical behaviour with a transient strain-hardening and strain-softening phase prior to transition to stable sliding. This is in conflict with the standard assumption of an ideal cohesionless Coulomb-material with constant frictional properties. The influence of the identified transient material properties on the kinematics, growth mechanisms, and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary—depending on material compaction—between wedges with well defined segments (i.e. frontal-deformation zone, frontal-imbrication zone and internal-accumulation zone) with straight slopes and wedges with a continuous convex topographic profile. For most materials, only the frontal part of the wedge is critical during experimental runs. Taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults. Wedge segmentation is controlled by a bulk-strength increase toward the rear of the wedge due to fault rotation in mechanically less-favourable orientations and plastic material hardening. The limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs. On this basis, we suggest that critical-taper analysis of wedges must be restricted to specific kinematic segments. Comparison of the experimental results with the Nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges. Moreover, we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges.