Deformation Partitioning and Fluid-Rock Interaction in the Southwestern Tauern Window.

The study of my master thesis had manifold goals with the aim of learning different research approaches, techniques and expertises at this stage of my career. The studied area is the southwestern portion of the Tauern Window of the Eastern Alps exposing the metagranitoids and the derived mylonites forming the core of the Penninic basement nappe exposed beneath the Austroalpine unit. The study of the area has addressed different topics ranging from the regional tectonics of this part of the Eastern Alps to more process-oriented issues such as the nucleation of ductile shear zones and the processes of fluid-rock interaction during deformation. The thesis has involved 2 campaigns of field work addressed to the structural analysis in several selected areas and the characterization of samples in the lab by different methods. The Tauern Window is a major tectonic feature of Eastern Alps where the Alpine Pennidic tectonic unit (nappe) is exposed beneath the Austroalpine nappe. The sequence of Alpine deformation events should be recorded in the field by a series of overprinting fabrics including mylonites and faults with a different kinematics: (i) thrusting associated with the stage of nappe stacking; (ii) strike-slip associated with the component of lateral escape, and (iii) normal “faulting” associated with the unroofing of the exhuming nappe pile and with the activity of the Brenner Fault system (Goldny et al., 2008). The structural field work was mainly addressed to identify and map the structures associated with the different tectonic components described above in the southwestern border of the Tauern Window. The structural study followed the reference scheme given by previous works of Mancktelow and Pennacchioni (2005) and Pennacchioni and Mancktelow (2007) for the Neves area was extended during my master thesis (i) N-S to cover a complete traverse across the tectonic unit (from the Neves area to the Zillertal Valley) and (ii) E-W over a distance of ca. 8 km (from the Moosboden glacial cirque to the Hochfeiler area). The structural sequence includes a phase of ductile deformation, syn-kinematic to amphibolite facies conditions, and a later overprint by cataclastic faults. The ductile deformation is heterogeneously distributed and this partitioning is scale-independent. Within the low strain domain the ductile deformation is localized at the outcrop scale to a network of discrete (of as much as a few meters thick) shear zones. This structural evolution indicates a NNWSSE to almost N-S shortening during the Alpine deformation from the synmetamorphic (amphibolite facies: 550–600°C, 0.4-0.7 GPa) mylonites to the brittle faulting during exhumation, respectively. Both the mylonites and the faults of the Neves area have a strike-slip kinematics. Both the ductile and the brittle deformation occurred under hydrous conditions as witnessed by the common occurrence of veins associated with deformation structures. An episode of fluidrock interaction postdated the main phase of faulting and is associated with the local development of episyenites. Structures linked to thrust and exhumation tectonics are missing from our are. Thrust structures are likely to have been overprinted by the following strike-slip lateral escape tectonics. Exhumation structures instead seems to be confined to a narrow deformation zone along the Brenner Line. The western zone have a homogeneous distribution of the strain, and the comparison between homogeneous deformation structures and partitioned structures has allowed us to define the zone as a purely constrictional transpression zone (Fossen & Tikoff, 1998). However, the partitioning in strain intensity over the region is not associated with a partitioning of components of thrusting, lateral escape and detachment in no stage of deformation, as is instead observed in other geological contexts (Fossen et al., 1994). The study area provides an ideal natural laboratory for the study of the process of ductile shear zone nucleation. It provides spectacular glacier-polished outcrops within metagranitoids that preserve all stages of incremental development of structures in a relatively simple and “isotropic” material. The field observations indicates: In contrast with numerical and rock-analogue models (e.g. Mancktelow, 2002; Mancktelow and Pennacchioni, 2014) shear zone nucleation never occurred within homogeneous metagranodiorites Almost any structural or compositional (rheological) surface heterogeneity (e.g. fractures, dykes, veins and lithological contacts) was capable of been exploited by localized shear deformation. The type of exploited precursor determined the type of shear zone and geometrical characters: (i) weak precursors compared to the host metagranodiorites (“unfilled” fractures; quartz veins; biotite-rich basic dykes) localized strain: shear zones have commonly a rather homogeneous strain distribution and a sharp boundary to almost undeformed host rock; (ii) strong layers (aplite dykes; alteration haloes surrounding veins) localized the deformation at their boundaries forming paired shear zoned at their selvages mainly developed within the metagranodiorite. Reactivation of original ductile shear zones can explain the controversial sense of shear of some paired shear zones exploiting the alteration halos surrounding epidote-filled veins. This contrasting sense of shears is interpreted as due to Alpine reactivation of original ductile shear zones developed in the pre-Alpine granitoid protolith as result of the original evolution during pluton cooling similar to what is described for some intrusions elsewhere (e.g. Pennacchioni, 2005; Pennacchioni and Zucchi, 2013). The latest episode of fluid-rock interaction, which has not been previously described, is responsible for the formation of local episyenites which postdated the main episode of brittle faulting. Episyenites are quartz-depleted, alkalimetasomatized granitoids (Cathelineau, 1986), strictly connected to either brittle or ductile shear zones (Rossi et al. 2005), and characterized by a conspiquous porosity. The field study has indicated that episyenites are associated to cataclastic faults but do not pervasively exploited the fault structure; fluid flow and alteration were not influenced by compositional and/or textural anisotropies of the host rock; no clear relation between fault slip and episyenite volume was observed. The composition of the host rock influence the spatial development, indeed quartz-poor lithologies commonly limit the diffusion of the fluid and of the alteration; in a similar way ductile textures of the host rock may limit or restrain the fluid diffusion. Mass-balance calculation from XRF bulk chemistry, aided with μ-CT volume evaluation, suggest that episyenites are due to a Na-metasomatism of parent rock and that have a particular paragnesis including: Plagioclase (Plg2:Ab99 that overgrows Plg1:Ab85) + Vermicular Chlorite + Adularia + Calcite ± Anatase ± Hematite ± Apatite. Albitization and dequarzification are the two main mineralogical changes (Feldspar increase of ca 35 wt%, Quartz decrease by a 25%). SEM BSE and μ-CT images shows that porosity develops also at the grain scale as the result of Albitization process of the parent Plagioclase (Oligoclase), but most of the macro-porosity is due to Quartz dissolution. Stable Isotope (δ18O and δ13C) analysis on calcite precipitated in episyenite indicate a possible contribution of a meteoric fluid to the episyenitization process.