Abstract Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.

Abstract The 3D reconstruction of geological bodies is an excellent tool for the representation of crustal structures and is applied here to understand related heterogeneities in the grain-scale fabrics; the western portion of the Languard–Tonale Alpine tectono-metamorphic unit (Austroalpine domain, Central Alps) allows evaluation of the per cent volume of textural reworking during polyphase pre-Alpine and Alpine deformations. The structural and metamorphic overprinting during the last deformation imprint involved less than 50% of rock volume; this estimate is obtained by discriminating domains that homogeneously recorded structural and metamorphic re-equilibration during crenulation–decrenulation cycles. These domains are reconstructed using a geograhpical information system (GIS) to manipulate field data and interpretative cross-sections as a means to constrain their 3D volumes. The degree of fabric evolution is integrated at the microscale with the estimate of the reactants/products ratio to infer the progress of metamorphic transformation related to advancing degree of mechanical reactivation. The correlation between degree of fabric evolution and progress of synkinematic metamorphic reactions shows that differences between pristine mineral assemblages v. pre-existing fabrics influence the rate of reaction accomplishment. Fabric evolution and degree of metamorphic transformation increase proportionally once above the threshold value of 60% of volume affected by fabric rejuvenation; metamorphic degree also influences the progress of metamorphic reactions.

Abstract Feedback relations between deformation and metamorphic mineral reactions, derived using the principles of non-equilibrium thermodynamics, indicate that mineral reactions progress to completion in high-strain areas, driven by energy dissipated from inelastic deformation. These processes, in common with other time-dependent geological processes, lead to both strain, and strain-rate, hardening/softening in rate-dependent materials. In particular, strain-rate softening leads to the formation of shear zones, folds and boudins by non-Biot mechanisms. Strain-softening alone does not produce folding or boudinage and results in low-strain shear zones; strain-rate softening is necessary to produce realistic strains and structures. Reaction–mechanical feedback relations operating at the scale of 10–100 m produce structures similar to those that arise from thermal–mechanical feedback relations at coarser (kilometre) scales and reaction–diffusion–mechanical feedback relations at finer (millimetre) scales. The dominance of specific processes at various length scales but the development of similar structures by all coupled processes leads to scale invariance. The concept of non-equilibrium mineral stability diagrams is introduced. In principle, deformation influences the position of mineral stability fields relative to equilibrium stability fields; the effect is negligible for the quartz → coesite reaction but may be important for others. Application of these results to the development of structures and mineral reactions in the Italian Alps is discussed.

Abstract When continental rifting does not develop on a stable continental lithosphere, geodynamic interpretation of igneous and metamorphic records, as well as structural and sedimentary imprints of rifting-related lithospheric extension, can be highly ambiguous since different mechanisms can be responsible for regional HT–LP metamorphism. This is the case of the European Alps, where the exposure of Variscan structural and metamorphic imprints within the present-day Alpine structural domains indicates that before the Pangaea break-up, the continental lithosphere was thermally and mechanically perturbed by Variscan subduction and collision. To reduce this ambiguity, we use finite-element techniques to implement numerical geodynamic models for analysing the effects of active extension during the Permian–Triassic period (from 300 to 220 Ma), overprinting a previous history of Variscan subduction-collision up to 300 Ma. The lithosphere is compositionally stratified in crust and mantle and its rheological behaviour is that of an incompressible viscous fluid controlled by a power law. Model predictions of lithospheric thermal state and strain localization are compared with metamorphic data, time interval of plutonic and volcanic activity and coeval onset of sedimentary environments. Our analysis confirms that the integrated use of geological data and numerical modelling is a valuable key for inferring the pre-orogenic rifting evolution of a fossil passive margin. In the specific case of the European Alps, we show that a relative high rate of active extension is required, associated for example with a far extensional field, to achieve the fit with the maximal number of tectonic units. Furthermore, in this case only, thermal conditions allowing partial melting of the crust accompanying gabbroic intrusions and HT–LP metamorphism are generated. The concordant set of geological events that took place from Permian to Triassic times in the natural Alpine case is justified by the model and is coherent with the progression of lithospheric thinning, later evolving into the appearance of oceanic crust.

Abstract A correlation procedure of scattered tectonic and metamorphic imprints in the reactivated crust is elaborated from recent analytical work in three Alpine metamorphic complexes. It consists of: interpretation of the time-sequence of tectonic fabrics and test of their kinematic coherence; determination of paragenetic compatibility among the mineralogical support of mesoscopic fabrics; cross-validation of mineral transformation over-prints; construction of P-T-d-t paths using a time-sequence of parageneses. The representation of structural and metamorphic information conveys the full tectono-metamorphic history on maps displaying combined tectonic and metamorphic effects. Shape and size definition of metamorphic units, now individuated mainly using their lithological homogeneity and dominant metamorphic imprint, is improved. The analysis of interaction between fabric and metamorphic imprint distributions, proposed in three Alpine examples, shows that the dominant metamorphic imprint does not coincide with T max - P T max of each inferred P-T-d-t loop; the dominant metamorphic imprint is that given by the mineralogical support of the most pervasive fabric. Different metamorphic imprints may dominate in adjacent areas of a single tectono-metamorphic unit (TMU), or equivalent metamorphic imprints may dominate in different TMUs. Therefore, lithostratigraphic setting and dominant metamorphic imprint are inefficient to contour TMUs in terrains with polyphase deformation and metamorphism, without considering multiscale heterogeneity of superposed synmetamorphic fabrics.