Constraining deformation stages in brittle–ductile shear zones from combined field mapping and 40Ar/39Ar dating: The structural evolution of the Grimsel Pass area (Aar Massif, Swiss Alps)

40Ar/39Ar dating of syn-kinematic white mica (phengite) and biotite is used to constrain the age of shear zones within the granodiorites of the Grimsel Pass area (Aar Massif, Central Alps). Three sets of steeply dipping shear zones have been mapped. Stage 1 shear zones are very broad (∼200 m wide) zones of biotite-bearing rocks, with a progressive increase in intensity of ductile deformation from rim to core, and have dip-slip kinematics. Subsequent Stage 2 shear zones are narrower, contain phengite-rich mylonites, and show a gradual change from dextral strike-slip to dip-slip kinematics across strike. Strain localisation during the formation of Stage 2 shear zones is interpreted to have been aided by reaction-weakening during extensive fluid–rock interaction. Brittle precursors of Stage 2 shear zones are marked by biotite-rich cataclasites preserved along the northern contact of the overprinting main Stage 2 shear zone.Biotite from a Stage 1 shear zone yields an age of 21.1 ± 0.2 Ma. Phengites in Stage 2 mylonites in the Grimsel Pass area have a very narrow age range (13.8–12.2 Ma), which is interpreted to bracket the duration of ductile deformation during Stage 2 deformation at mid-crustal levels. However, the biotite-rich cataclasites at the margin of a Stage 2 shear zone may have localised fluid flow up to ∼2 Ma prior to the onset of main Stage 2 ductile deformation. Stage 3 shear zones are brittle strike-slip faults containing cataclasites, breccias and clay fault gouges. Earlier studies have established that these late faults formed at <9 Ma and were active until at least 3 Ma.