Mineralogically triggered strain localization: inferences from ductile paired shear zones (Tauern Window, Eastern Alps)

International audienceLithosphere is mainly constituted by polyphase rocks whose mineralogical, structural and textural characteristicscontrol the spatial distribution of strain, and so, its effective mechanical strength. Variations in local mineralogicalcompositions may lead to drastic changes in the local microstructures and texture leading, by mechanical feedbackprocesses, to strain hardening or weakening. Understanding these small-scale relations between the petrologicalcharacteristics, the rheological properties and the development of progressive deformation is thus of fundamentalimportance for understanding the strength of rocks at large-scale, especially the mechanical behavior of platesboundaries.We acknowledge the importance of brittle precursors for the localization of strain in the viscous deformingpart of the crust. In this study, we focus in centimeter-wide paired ductile shear zones shaped nearby alongon both sides of ep-grt-qtz veins within a late Variscan metagranodiorite of the “Zentralgneis” in the TauernWindow (Berlinerhütte, Zillertal, Austria). The paired shear zones, underlined by biotite bands, localized at somecentimeters away from the veins associated with a metasomatic domain, within the relatively undeformed hostrock. According to their spatial orientations, they exhibit different intensities of shearing (from incipient linking ofbiotites to anastomosing ultra-mylonitic bands) and thus can be explored as successive strain domains of a shearzone developing in space and time.Here, we present a combination of high-resolution petro-chemical section across the paired shear zoneswith microstructural and textural measurements in order to constrain mineral reactions and deformation processesassociated with the development of localized shear zones. Whole rock chemical analyzes combined with continuousmineralogical mapping revealed small chemical variations induced by fluid-rock interactions in the vicinityof the veins. Although macroscopically indistinguishable, these small chemical variations cause rheologicalheterogeneities, which in turn control the initialization of the localized strain pattern. Electron backscatterdiffraction data constrain the texture of the different phases helping to determine deformation mechanisms andmapping of relative strain intensities. The present study highlights the importance of combining high-resolutionpetrological, microstructural and textural data in order to understand the complex feedbacks between chemicalreactions and localizing strain at micro- and macro-scale