EXTENSION DURING ACTIVE COLLISION IN THIN-SKINNED WEDGES: INSIGHTS FROM LABORATORY EXPERIMENTS

Extension coeval with contraction is not predicted by analytical solutions for the mechanics of purely frictional orogens, yet it is observed at many convergent margins in which deformation is predominately brittle/frictional. We suggest that the solution to this paradoxical extension lies in ductile behavior at depth. We present a series of simple analog experiments, some with purely frictional material (sand) and others with layered frictional-ductile rheologies (sand over a viscous gel). In order to test the idea that ductile rocks at depth might allow extensional deformation to occur in otherwise frictional and contractional orogens, and to isolate the role of rheology in determining the distribution of contraction and extension, we run these models with simple convergence geometries and boundary conditions. By precisely measuring the horizontal deformation field in conjunction with the development of topography and surface structures, we have quantitatively confirmed the qualitative observation that analog frictional wedges do not extend. Our results are consistent with published numerical models, and are in sharp contrast with purely viscous models that commonly display coeval extension with contraction. We also find that layered frictional-ductile models can, during active convergence, result in discrete extensional normal faulting even when only a relatively thin ductile layer is present at the base of the model orogen. The resulting extension, though relatively small in magnitude compared to the active contraction, can greatly modify the final profile of a convergent orogen and can lead to the formation of a broad plateau between the pro-wedge and the retro-wedge.