Pressure- and stress-induced fabric transition in olivine from peridotites in the Western Gneiss Region (Norway): implications for mantle seismic anisotropy

The effects of pressure, water and differential stress on the crystallographic preferred orientation (CPO) of olivine were investigated by an integrated study on 13 peridotite samples from four areas in the Western Gneiss Region (WGR) of the Norwegian Caledonides. The highly depleted dunites are mantle residues after Archean melt extraction, while the garnet peridotites were formed by refertilization of the dunites via melt percolation in the Proterozoic. These orogenic peridotites were tectonically emplaced into Proterozoic gneisses and metasedimentary rocks of the WGR and were recrystallized under highpressure (HP) to ultrahigh-pressure (UHP) conditions when the Baltic Shield was subducted beneath Laurentia during the 420–400 Ma Scandian orogeny. The CPOs of olivine, enstatite and diopside were measured using the electron backscatter diffraction technique. The weakly deformed garnet wehrlites and dunites from the HP central WGR (the Gurskøy, Almklovdalen and Tafjord areas) show the A-type olivine fabric defined by the [100] axis subparallel to the stretching lineation and the (010) plane subparallel to the foliation, which may be a relict fabric formed at high temperature and low strain rate in the mantle wedge prior to subduction. The [001](010) fabric (B-type) is observed in strongly sheared HP garnet peridotites and dunites from the Almklovdalen and Tafjord areas, suggesting the dominant activation of [001](010) slip system at high stress and high strain rate. Recrystallized fine olivine grains may develop a fabric different from porphyroclasts and record a gradual fabric transition. The olivine CPO in garnet lherzolites from Otrøy in the northernmost WGR is characterized by the concentration of [001] axes parallel to the stretching lineation and [100] axes normal to the foliation, indicating a prevailing [001](100) slip (C-type fabric) at P–T conditions of >6 GPa and 850–950 C. Despite very different deformation microstructures, Fourier transform infrared analysis reveals very low water contents (<13 ppm H2O by the Paterson calibration) in olivine for both coarse porphyroclasts and recrystallized small grains, which is consistent with the high-degree partial melting of the peridotites during the Archean. Therefore, a combination of UHP and low temperature plays a more important role than water in promoting the C-type olivine fabric in a dry, cold and deeply subducted continental slab. With increasing subduction depths, the C-type olivine fabric will predominate over the A-type fabric and make the maximum P-wave velocity and shear wave splitting of peridotites normal to the foliation. However, the stress-induced fabric transition in olivine from the A-type to the B-type may be localized in shear zones and not distinguishable in seismic anisotropy observations.