Extrusion vs. accretion at the frictional–viscous de´collement transition in experimental thrust wedges: the role of convergence velocity

In many cases, thrust wedges accreted at shallow crustal levels show an across-strike rheological variability along the basal de´ collement, notably from brittle to ductile behaviour. In this paper, we illustrate the results of sandbox analogue modelling research devoted to studying the influence of convergence velocity on wedge architecture when laterally juxtaposed frictional and viscous materials occur along the basal de´ collement of accreting thrust wedges. Our results show that slow convergence favours a near symmetrical distribution of thrust vergence within wedge sectors accreted above viscous de´ collement material, whereas fast convergence favours vergence asymmetry. In particular, at fast convergence rates the hinterlandward extrusion of viscous de´ collement material at the toe of the frictional wedge is favoured and contributed to accommodate a significant amount of the total contraction.