The effect of deformation history on the evolution of olivine CPO

Abstract

Olivine crystallographic preferred orientation (CPO) is the primary cause of seismic anisotropy in the upper mantle. In tectonic environments with complex flow patterns, for example corner flow near mid-ocean ridges or subducting slabs, the interpretation of seismic anisotropy may be complicated by evolving thermochemical deformation conditions and the integrated deformation history. To understand how deformation history influences CPO evolution, deformation experiments were conducted on samples of Åheim dunite, which has a strong pre-existing texture. Experiments were performed in a triaxial geometry using a Griggs apparatus at P=1 GPa, T=1473 K, up to a maximum strain of ∼0.7. To simulate different deformation histories, samples were deformed in three different configurations, with the pre-existing foliation perpendicular, oblique, and parallel to the shortening axis of deformation. Distinct patterns of CPO development are observed for each experimental configuration. Likewise, texture strength, symmetry, and orientation evolved differently in each set of experiments. These data are interpreted as evidence that CPO did not reach steady state and that achieving steady state texture requires larger strains than previously thought. It is concluded that the integrated deformation history plays a significant role in CPO evolution and the consequent interpretation of seismic anisotropy in Earth's mantle.