No abstract

Experiments comprising deformation at 600°C and annealing at 700-1000°C were performed on natural peridotite to examine the microfabric evolution at conditions corresponding to those prevailing just below the seismogenic zone in suboceanic upper mantle. The found olivine microstructures indicate that deformation occurs by low-temperature plasticity. At low and high annealing temperatures, zones of high strain are replaced by subgrains and recrystallized grains respectively. The microstructures after annealing at 1000°C resemble`coreand-mantle structures´from shear zone peridotites, often interpreted as imprint of steady-state dislocation creep. Our study shows that such structures can form by a sequence of low-temperature plasticity at high stress and subsequent recrystallization at low stress, as corresponding to coseismic deformation and post-seismic creep. In this case, palaeopiezometers are not applicable and a local CPO in aggregates of new grains does not necessarily indicate the activated glide system.

Mylonitic peridotites from the Finero complex are investigated to detect characteristic olivine microfabrics that can resolve separate deformation cycles at different metamorphic conditions. The heterogeneous olivine microstructures are characterized by deformed porphyroclasts surrounded by varying amounts of recrystallized grains. A well-developed but only locally preserved foam structure is present in recrystallized grain aggregates. This indicates an early stage of dynamic recrystallization and subsequent recovery and recrystallization at quasi-static stress conditions, where the strain energy was reduced such that a reduction in surface energy controlled grain boundary migration. Ultramylonites record a renewed stage of localized deformation and recrystallization by a second generation of recrystallized grains that do not show a foam structure. This second generation of recrystallized grains as well as sutured grain and kink band boundaries of porphyroclasts indicate that these microstructures developed during a stage of localized deformation after development of the foam structure. The heterogeneity of the microfabrics is interpreted to represent several (at least two) cycles of localized deformation separated by a marked hiatus with quasi-static recrystallization and recovery and eventually grain growth. The second deformation cycle did not only result in reactivation of preexisting shear zones but instead also locally affected the host rock that was not deformed in the first stage. Such stress cycles can result from sudden increases in differential stress imposed by seismic events, i.e., high stress-loading rates, during exhumation of the Finero complex.