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

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