Does subduction polarity changes below the Alps? Inferences from analogue modelling

The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the zone separating the domains, and the lithosphere geometry and rheology. The model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates, which have been separated by a narrow transition zone. A wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone. In addition, interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography. The lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains, the rheology of the mantle lithosphere and the amount of bulk shortening. The modelling results have major implications on the interpretation of seismic and tomographic data from the European Alps in terms of the crust and lithosphere geometries. It appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build-up between the Western/Central and Eastern Alps.