Tectonic Analysis Vs Numerical Modelling: Insight from the Polycyclic Continental Lithosphere of the Alps

Subduction-collision zones are characterised by coupling and decoupling of lithospheric slices, which work in competition building up the tectonic units of metamorphic belts. During plate convergence, contours of these units are transient and can be investigated integrating structural and petrologic analysis. Structural and metamorphic evolutions of basement rocks, rather than purely lithologic associations, are tracers of their transit throughout different levels of the lithosphere and sublithospheric mantle. Individuation of contours of thermally-characterized and structurally distinct units (= tectono-metamorphic units = TMU; SPALLA et al., 2005) is crucial to define the variation in size of such lithospheric slices, involved in the dynamic of an active margin: the fundamental tool is the reconstruction of quality P-T-d-t paths, implying exploitation of the full structural and metamorphic “rock memory”. Size definition of TMUs is critical to infer geological processes as tectonic erosion or accretion at the trench margins, continental collision or deep subduction of continental crust (ablative subduction) or exhumation velocity variation and its influence on rapid and effective meta-stabilisation of HP- and UHP-LT assemblages (e.g. CLOOS 1993; ENGLAND & THOMPSON 1984; ERNST 2001; LALLEMAND 1999; TAO & O'CONNEL 1992). The TMU investigation tool bears a marked thermo-tectonic connotation and offers possibilities to test by numerical modelling the physical compatibilities of some interconnected variables (e.g. gravity, plasticity, heath transfer) with the interpretative geologic history. Comparison between modelling predictions and natural data obtained by this analytical approach helped to solve standing ambiguities on the pre-Alpine and Alpine geodynamic evolution of different continental units of Central and western Alps and to explore the crustal level of protoliths derivation. Computer-aided 3D estimate of structurally and chemically “reactive” volumes, helps to evaluate their potential influence on the choice of the physical parameters for numerical modeling.