Along-strike variations in the fossil subduction zone of the Western Alps revealed by the CIFALPS seismic experiments and their implications for exhumation of (ultra-) high-pressure rocks

Abstract

In complex plate-boundary settings, a reliable 3-D geophysical characterization of the deep tectonic structure is a fundamental starting point for a breakthrough in the analysis of processes controlling plate subduction and (U)HP rock exhumation. The Western Alps host one of the best-studied fossil subduction zones worldwide, with a well-defined deep structure in 2-D based on recent geophysical experiments. However, a full 3-D characterization of its deep tectonic structure is still lacking. Here we present a series of new receiver function cross-sections across the northern and southern Western Alps, validated and complemented by a S-wave velocity model from ambient-noise tomography that provides additional constraints between the profiles. We document a marked change in Moho attitude from the northern Western Alps, where the eastward-dipping European Moho reaches ∼45 km depth beneath the Gran Paradiso dome, to the southern Western Alps, where the European Moho reaches ∼70 km depth beneath the equivalent Dora-Maira dome. This change in Moho attitude takes place over a few tens of kilometers and was likely emphasized by deformation of the slab during subduction. The Western Alps subduction wedge is much thicker in the south than in the north, and the mantle-wedge rocks are deeply involved in orogeny exclusively in the south, where coesite is found in continental (U)HP rocks at several locations. Our detailed information on the 3-D structure of the subduction wedge provides first-order constraints for the next-generation of thermo-mechanical numerical models and may help explain the lateral variations in exhumation style revealed by the geologic record.