Multidisciplinary investigations on the relationships between deformation and metamorphism across crustal rheological boundaries

The rheology of crustal rocks plays a key role in lithosphere dynamics, influencing the orogenic cycle and how plate tectonics evolve. Lithosphere deformation involves a complex interplay between metamorphic and deformation processes, whose effects tend to be enhanced in domains characterized by variable composition or inherited petrological and structural history. These variations may produce heterogeneous rheological behaviours, which allow concentrating stress or strain, finally resulting in tectonic structures, such as shear zones. Given the complexity of the relationships between deformation and metamorphism, reconstructing the reasons controlling the birth and the evolution of tectono-metamorphic structures is challenging, but of fundamental importance for understanding the initiation of geodynamic processes. The use of multidisciplinary approach is fundamental to isolate the contribution of each of the several parameters controlling the rock rheology. Rheology is expected to significantly change in correspondence of major variations in rock composition, therefore the boundaries between contrasting lithologies are the preferential location for study the role of inherited heterogeneities in the interlinked evolution of deformation and metamorphism. In this thesis, the interplay between metamorphism and deformation is explored throughout the investigation on how different paired rock types may: 1) record contrasting metamorphic conditions despite a structural coherence, and 2) drive strain localization leading to shear zone nucleation. These purposes are addressed to two case studies from the Alps, whose topics are highly debated by the geological community as they are representative of significant processes occurring in the middle to lower crust during opposite regional tectonic regime (compressive and extensional): 1) the Alpine HP/HT occurrence of Cima di Gagnone (Central Alps) and 2) the Tethyan rift-related extensional shear zone (Anzola shear zone, Ivrea-Verbano zone), respectively. The Cima di Gagnone area (Case study 1) belongs to the Cima Lunga unit, which is part of the southern sector of the Lepontine dome of the Central Alps. It represents an example of ultrahigh-pressure and high-temperature ultramafic lenses enveloped within amphibolite-facies metasedimentary rocks as the result of the Alpine subduction/collision deformation phases. For this case study, investigations focused on the metamorphic and deformation evolution of the metasediments, which received minor attention respect with the ultramafic lenses. For this case study, the aims consisted to i) constrain the P-T-D-t path of micaschists in relation with the ultramafics, ii) to investigate the geochemical exchange at the boundary of these compositionally different rock types.The Anzola shear zone (Case study 2) represents a major extensional structure from one of the best-preserved cross-sections through the middle to lower continental crust, the Ivrea-Verbano Zone (Southern Alps, Italy). It is interpreted as one of the main rift-related structure of the Late Triassic-Jurassic deformation in the lower crust of the Adriatic margin. Nevertheless, the timing of its activity is still poorly constrained. Furthermore, the Anzola shear zone is believed to have developed within a rheologically hard and isotropic mafic body rather than in the surrounding weaker and anisotropic metamorphic sequence. However, a detailed characterization at the meso-microscale of its compositional and structural features is still lacking. For this case study, investigations focused on the i) characterization of compositional and structural features and ii) identification of the protolith of the shear zone rocks, with the aim to decipher the role of inherited rock heterogeneities as drivers of weakening and strain localization at middle/lower crustal layers.In both case studies, pre-existing rock heterogeneities played an important role on their rheological behaviour and evolution