CYCLIC NON-STEADY STATE DEFORMATION RECORDED BY PERIDOTITES OF THE FINERO COMPLEX (IVREA ZONE, WESTERN ALPS)

The microfabrics of deformed peridotites of the Finero complex (Ivrea Zone, western European Alps) are investigated by optical and electron microscopic techniques (SEM/EBSD, TEM). The rocks are lherzolites and contain minor amounts of amphibole and/or phlogopite. They have undergone multiple stages of deformation and metamorphism.

The SPO of large elongate olivine grains (long axis up to 15 mm) defines the foliation of the peridotite. These grains are inhomogeneously deformed, containing abundant kink bands and deformation bands sub parallel to (100). Kink bands are characterized by discrete boundaries with a misorientation of up to 35°. Kink band boundaries and grain boundaries are sutured indicating strain-induced grain boundary migration after inhomogeneous deformation. Deformation bands are defined by sub parallel, more or less straight but diffuse low angle grain boundaries. Their orientation sub parallel (100) indicates the activation of a glide system with [100] as glide direction, which is reported to be characteristic for high temperature deformation. Locally, kink band boundaries cut deformation bands at a low angle; otherwise both features are sub parallel.

In zones of localized deformation, olivine microstructures reveal at least two stages of recrystallization. An earlier generation of recrystallized grains, 0.1-0.3 mm in diameter, shows a foam structure, which implies annealing at low differential stress. A later generation of recrystallized grains, less than about 0.05 mm in diameter, is confined to shear zones about parallel to the foliation. These grains show an irregular grain shape and a weak CPO, indicating deformation in the dislocation creep regime. These shear zones (mm-cm wide) appear to have been repeatedly reactivated. The latest deformation stage is represented by ultrafine grained, cataclastic and metasomatized domains containing serpentine, talc and dolomite.

The microstructures in the Finero peridotites indicate episodic, markedly non-steady state deformation, recording several cycles with initially high differential stress and subsequent stress relaxation. The stress cycles which occurred under distinct conditions are attributed to stresses transferred to the deeper lithosphere during major earthquakes, presumably on faults in the overlying brittle crust.