Mesozoic extension controlling the Southern Alps thrust front geometry under the Po Plain, Italy: insights from sandbox models.

In the Southern Alps fold and thrust belt, east-west Mesozoic rifting controlled the development of extensional faults, the deposition of the sedimentary sequences and the thickness and distribution of the main detachment levels. Vertical and lateral heterogeneities strongly influenced the structural style of the subsequent north-south contractional phase, as revealed by surface mapping and by subsurface maps derived from seismic data. Along the strike of the chain, lateral terminations of thrust faults occur at syn-rift extensional faults, trending normal with respect to the contractional structures, From west to east, the thrust sheet number decreases and the order of structures and their wavelength changes. On a regional scale the effects produced by pre-existing heterogeneities on the development of contractional Alpine tectonic structures in the external area of the Southern Alps were evaluated by means of scaled sandbox analogue models. Modelled lateral heterogeneities produced different wavelength thrust sheets, salients and recesses, whereas vertical heterogeneities facilitated the development of structures of different orders. First-order structures detached at the base of the model and second-order structures detached at the intermediate relatively weak layers. Comparison of analogue model results with nature showed good geometrical similarities.