Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

摘要: Abstract In the Neves area, pre-Alpine intrusive protolith of Zentralgneise unit (Tauern window, Eastern Alps) is well preserved in a kilometric-scale low-strain domain without pervasive Alpine deformation. It compositionally heterogeneous, consisting predominantly granodiorites, with lesser leucocratic granites, and different generations lamprophyres aplites. The rocks are crosscut by fractures that were locally infiltrated fluids surrounded alteration haloes. Incipient amphibolite facies ductile deformation strongly localized on these precursor lithological planar heterogeneities, resulting development several types shear zones. Fractures haloes initially accommodate slip entirely fracture itself. With increasing deformation, foliation progressively developed adjacent host rock, eventually producing single heterogeneous “ductile” zone typical sigmoidal pattern. Strong layers (aplite dykes bleached to either side fractures) localize their boundaries produce characteristic paired Shearing more evenly distributed within weak (lamprophyres quartz veins), marked discontinuity strain against adjacent, little deformed granodiorite. Shear was accompanied formation new quartz-rich veins which turn also shear. Magmatic contacts, mostly steeply dipping effectively span complete range strike orientations. kinematics overprinting (strike-slip) zones determined orientation initial discontinuities relative local principal compressive stress axis σ1 (here oriented ca. 345°). Discontinuities almost all orientations show reactivation, even case very low resolved stress, indicating an overall viscous response system specific yield stress. geometry network suggest domains close coaxial. During along network, compatibility maintained fracturing (with veins) especially contractional at intersections. Deformation never propagates into undeformed homogeneous granodiorite as discrete but limited original extent discontinuities. intact rock always preceded fracturing, localizes subsequent reactivation.