Inversion of extensional basins parallel and oblique to their boundaries: Inferences from analogue models and field observations from the Dolomites Indenter, eastern Southern Alps

Abstract. Polyphase deformation of continental crust is analysed through physical analogue models for settings where platform-basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle-ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength differences between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel to oblique (10 to 20 degrees) with respect to the basin axes has been applied leading to the inversion of earlier formed basins. The experiments show that the simple presence of an inherited platform-basin configuration controls the overall style of compressional deformation, no matter of including frictional or viscous basal décollements, of varying the rheology of the basin fill, or of changing platform-basin thickness ratios. Orientations of thrust faults change laterally across inherited platform-basin transitions throughout all experiments; higher obliquity of basin inversion leading to stronger curvature of thrusts with respect to the pre-existing rift axes. Variations in the strike of thrust fronts are accompanied by changes of the shortening direction along one single fault and time step. Furthermore, our models support localisation of deformation in areas of lateral strength contrasts, as platform-basin transitions represent. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform-basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Both parallel and oblique inversion experiments can be applied to polyphase deformed continental crust, as, e.g., the Dolomites Indenter of the eastern Southern Alps. Our models involving two phases of deformation, suggest that the whole tectonic evolution of the Dolomites Indenter is controlled by inherited features. Fault slip data and shortening directions from fold axes from our field case study along the western segment of the Belluno thrust of the Valsugana fault system support variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along strike) along one single fault. Based on our modelling results, we infer that this variability of shortening directions depends on inherited structures and do not necessarily reflect different deformation phases.