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 10-15 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° 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).

Permeability and seismic velocity anisotropy across a ductile-brittle fault zone in crystalline rock

“…• C and 150 to 250 MPa peak conditions during Alpine metamorphism (Steck, 1968;Rolland et al, 2009;Wehrens et al, 2016Wehrens et al, , 2017. Our thin section observation shows fracturing of feldspar and undulose extinction along with subgrain boundaires in quartz, which are consistent with the inferred metamorphic temperatures.…”

“…This case study from the GTS is a hybrid between the two end member systems. During the two ductile deformation phases slip was accommodated along localized foliated shear zones that are mylonitic and ultramylonitic (Challandes et al, 2008;Rolland et al, 2009;Belgrano et al, 2016;Wehrens et al, 2016Wehrens et al, , 2017. The transition from highly foliated and extremely recrystallized fault cores towards the host granodiorite represents a strain gradient, which is ∼ 0.5 m thick in this study (Figure 5a).…”

“…Detailed deformation histories of the Grimsel region are available (Challandes et al, 2008;Rolland et al, 2009;Belgrano et al, 2016;Wehrens et al, 2016Wehrens et al, , 2017, which are summarized here. From argon-argon and rubidium-strontium dating, as well as field relations, beginning around 21 Ma and continuing until approximately 10 Ma, ductile deformation resulting from 5 transpression created NNE-SSW, E-W, and NW-SE striking shear zones with steep dips to the south (Figure 1a).…”

“…Recrystallized plagioclase and quartz form rotated clasts within the biotite-quartz foliation. The shear zones in the region are often interpreted as former mafic dykes (e.g., Wehrens et al, 2017). The MFC has a gradient of deformation between the Grimsel granodiorite in the TZ and the MFC, and most notably there is a heterogeneous layering within the ultramylonite with larger grain lenses that are compositionally similar to the granodiorite (e.g., Figure 1d).…”

“…Previous studies from the laboratory (cm) to field outcrop (km) scale have developed into generalized models for the mechanical and Figure 1. a) Geologic map of the Grimsel pass region (after Wehrens et al, 2016Wehrens et al, , 2017, b) borehole orientations (this study -red borehole) with respect to the underground research laboratory (after Krietsch et al, 2017;Amann et al, 2017), c) the borehole in this study depicting the location of the damage zone with coring and stress measurement locations projected along the borehole. The shear zone at the base is divided into three components: 1) transition zone (TZ), 2) mylonitic fault core (MFC), and 3) damage zone (DZ).…”