How is strain localized in a meta-granitoid, mid-crustal basement section? Spatial distribution of deformation in the central Aar massif (Switzerland)

Abstract

This study investigates strain distribution in granitoid rocks formerly in the middle crust in the Central Aar massif, Switzerland and places the deformation behavior in the tectonic framework of the Alpine orogeny. Strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones (SZ) (>80 SZ/km with SZ thicknesses <10 cm; about 10 SZ/km with SZ thicknesses of 0.5–10 m) separating 3D bodies of low to moderate background strain. Both the degree of background-strain intensity as well as the number of shear zones increases from granitic to granodioritic host rocks and is controlled by primary variations in the mica content between 10 and 15 vol% (granodiorite) and <8 vol% (granite). Shear zones evolved from ductile shearing in granodiorites, whereas they often nucleated from fractures in the stronger granites.The majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 °C in the North to 450 °C in the South of the Aar massif. The shear zones initiated at about 18–20 km depths during a stage of crustal thickening (Handegg phase). Subsequent deformation reactivated some shear zones with a gradual transition from reverse dip-slip over oblique-slip to strike-slip shear zones under local transpressional conditions (Oberaar phase).