Evolution of olivine lattice preferred orientation during simple shear in the mantle

Abstract

Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M- indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models.