Structural Evolution of Periadriatic Plutons and its implications on solid-state deformation of granitoid rocks

Exhumed granitoid plutons are an ideal natural research target for studying the processes of nucleation and evolution of ductile and brittle deformation structures. Granitoid plutons, unaffected by later tectono-metamorphic cycles, preserve pristine deformation structures developed during cooling from magmatic to host rock ambient temperatures, that can be assumed as representatives of structures that at different structural levels of the continental crust. The main focus of this Ph.D. project is the analysis of deformation structures of the Rieserferner pluton – one of the major Periadriatic intrusions. The aim of the study is two-fold: (i) reconstruct the tectonic framework during the different stages of the pluton structural evolution, and (ii) determine the processes controlling localized ductile strain at different scales and the environmental conditions at which they occurred. The structural evolution during pluton cooling consists of 5 main deformation stages, that have been bracketed in time and thermal conditions according to microstructural and textural analysis, literature and field data: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins and associated ductile shear zones; (ii) shallowly dipping joints with associated epidote and quartz veins and ductile shear zones; (iii) steeply dipping mafic dykes and calcite-white mica-bearing brittle-ductile faults; (iv) steeply dipping pseudotachylyte-bearing cataclastic faults; and (v) zeolite-bearing faults. Integrating new field, microstructural and geothermo-chronological data with published data we have related the deformation sequence to the Tertiary tectonics of the Eastern Alps. (i) three main ductile deformation stages developed during Oligocene, followed by two brittle deformation stages during Miocene; (ii) paleostress inversion from kinematic analyses suggest a complex stress field variation during the structural evolution mainly due to switch in relative magnitudes of principal stress components; (iii) the described paleostress evolution reflect the sequence of tectonic processes occurred during Oligocene and Miocene at the scale of the Eastern Alps, from slab break-off to indentation and lateral escape tectonics. Microstructural investigations were mainly focused on the analysis of ductile shear zones exploiting epidote- and quartz-rich veins. Softening and localization in quartz veins was mainly controlled by grain size reduction by recrystallization. EBSD mapping and image analyses have shown that different crystallographic orientations of quartz vein crystals controlled the evolution of microstructures and crystallographic preferred orientations (CPO) during vein-parallel simple shear up to high shear strains ( ≈ 10). Recrystallization by Subgrain Rotation (SGR) lead to the development of fine-grained ultramylonitic quartz veins, in which, the observed CPO banding have been inherited from the original crystallographic orientation of the vein crystals. Localization of ductile strain within heterogeneous shear zones exploiting epidote veins was mainly obtained through myrmekite development and following shearing. EBSD investigations suggest that myrmekite induced a switch in the dominant deformation mechanism, from dynamic recrystallization by SGR to diffusion-assisted grain boundary sliding (GBS) during shearing of plagioclase + quartz aggregates. Thermodynamic modelling was aimed to define the temperature-pressure-fluid conditions under which deformation these processes occurred. Pseudosections computed for the chemical systems NaCaKFMASHO and MnNaCaKFMASHO suggest that: (i) the epidote-veining event in the RFP likely occurred at temperatures between 520°C and 490°C at water-saturated conditions; (ii) the main deformation phase likely occurred at 460±40°C and 0.35 ± 0.05 GPa, lasting probably during pluton cooling down to 350°C at slightly under-saturated water-conditions