We analyze peridotites from a wide range of tectonic settings to investigate relationships between olivine lattice preferred orientation (LPO) and deformation conditions in naturally deformed rocks. These samples preserve the five olivine LPO types (A through E-type) that rock deformation experiments have suggested are controlled by water content, temperature, stress magnitude, and pressure. The naturally deformed specimens newly investigated here (65 samples) and compiled from an extensive literature review (445 samples) reveal that these factors may matter less than deformation history and/or geometry. Some trends support those predicted by experimentally determined parametric dependence, but several observations disagree — namely that all LPO types are able to form at very low water contents and stresses, and that there is no clear relationship between water content and LPO type. This implies that at the low stresses typical of the mantle, LPO type more often varies as a function of strain geometry. Because olivine LPO is primarily responsible for seismic anisotropy in the upper mantle, the results of this study have several implications. These include (1) the many olivine LPO types recorded in samples from individual localities may explain some of the complex seismic anisotropy patterns observed in the continental mantle, and (2) B-type LPO – where olivine’s “fast axes” align perpendicular to flow direction – occurs under many more conditions than traditionally thought. This study highlights the need for more experiments, and the difficulty in using olivine LPO in naturally-deformed peridotites to infer deformation conditions.

We examine mid-lower crustal xenoliths erupted in Late Pleistocene to Holocene alkali basalt at San Quintin, Baja California, Mexico to investigate the role of partial melt in deep lithospheric deformation. Using major and trace element mineral chemistry data obtained from LA-ICP-MS and microstructural data obtained from EBSD, we cluster the crustal xenoliths into two groups based on textural and geochemical characteristics: Group (1) largely unfoliated two-pyroxene gabbros with minor amounts of olivine, and Group (2) gabbros with strong compositional foliation and up to 30% modal olivine. We interpret Group 1 xenoliths as representing underplated cumulates from basaltic magmas, whereas petrographic and quantitative microstructural data suggest Group 2 xenoliths record melt infiltration of pre-existing gabbros coeval with deformation, perhaps in a deep crustal shear zone. We propose this shear zone represents a deep lithospheric expression of the active transform plate boundary along the Baja California margin.