Present-day upper mantle structure of the Alps: insights from data-driven dynamic modelling

Present-day surface deformation in the Central Alps, that is, uplift and upper-crustal level seismicity in contrast to its northern and southern forelands, has been attributed to surface (i.e., climatic) and tectonic processes (i.e., subduction, slab detachment/break-off, mantle flow). Understanding the relative contribution of these processes is fundamental to understanding their coupling and role in mountain building. The present-day 3D architecture of the lithosphere (i.e., lateral variations of crustal layers and lithospheric mantle thickness) and asthenosphere (i.e., subducted slabs, attached or detached to the orogenic lithosphere) resulting from tectonic processes operating at geologic time scale serve as a boundary condition to test the contribution of surface processes. While the crustal structure in the Alps is well constrained by seismic and gravity data, the upper mantle (i.e., lithospheric mantle and asthenosphere) structure differs from that due to the diversity and subjective interpretation of seismic tomography models. We convert the results of regional shear-wave seismic tomography models to temperature models using the Gibbs-free energy minimization algorithm to define the base of the lithosphere and the position of slabs in the asthenosphere. Our results show that the shallow/attached slab in the Northern Apennines is a common feature in different tomography models, but there are differences in the Alps area. We statistically cluster tomography models into three end-members corresponding to the mean and 67% confidence intervals to address these differences objectively. These end-members represent scenarios ranging from shallow/attached slabs to almost no slabs in the Northern Apennines and Alps. The three end-member scenarios are then used as an input to model the topography and velocities by solving the buoyancy-forces driven instantaneous flow, subject to the first-order rheological structure of the lithosphere-asthenosphere system. Modelled topography and velocities are compared to the first-order patterns of observed topography and GPS derived vertical velocities to discern among the end-member scenarios. Our preliminary results suggest that the lithospheric slab subducting beneath the Northern Apennines should be connected to the overlying lithosphere, whereas it appears to be detached along most of the Alps. The sensitivity of results to the viscosity structure of the crust, lithosphere, and asthenosphere will be discussed.