Analysis of natural tectonic systems coupled with numerical modelling of the polycyclic continental lithosphere of the Alps

Subduction–collision zones are characterized by the amalgamation and disaggregation of lithospheric slices; such processes work in competition in constructing the tectonic architecture of metamorphic belts. Determination of contours for thermally and struc- turally characterized units is crucial to define the variations in sizes of such slices involved in the dynamic evolution of an active margin. The dimensions of these entities change over time and must be reconstructed using the structural and metamorphic evolution of the basement rocks as tracers, rather than by simply relying on lithologic associations. They constitute tectono-metamorphic units (TMUs) and represent discrete portions of the orogenic crust influenced by a sequence of metamorphic and textural changes. Their translational trajectories and shape changes during deformation cannot simply be derived from the analysis of the geometries and kinematics of tectonic units but from a joint reconstruction of quantitative P-T-d-t paths. The TMU investigation tool bears a marked thermo-tectonic connotation and, through modelling, offers the opportunity to test the physical compatibilities of interconnected variables, such as density, viscosity, and heat transfer, with the interpretative geologic history. Comparison between modelling predictions and natural data obtained by this analytical approach has helped solve longstanding ambiguities on the pre-Alpine and Alpine geodynamic evolution of the different continental units of the Central and Western Alps and explore the crustal levels of protolith derivation. Three-dimensional estimation of structurally and chemically re-equilibrated volumes aids in the evaluation of physical parameters chosen for the numerical modelling.