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We analyze peridotites from a wide range of tectonic settings to investigate relationships between olivine crystallographic preferred orientation (CPO) and deformation conditions in naturally deformed rocks. These samples preserve the five olivine CPO 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 CPO types are able to form at very low water contents and stresses and that there is no clear relationship between water content and CPO type. This implies that at the low stresses typical of deformation in the mantle, CPO type more commonly varies as a function of strain geometry. Because olivine CPO is primarily responsible for seismic anisotropy in the upper mantle, the results of this study have several implications. These include (1) the many olivine CPO 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 CPO—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 CPO in naturally deformed peridotites to infer deformation conditions.

We use xenoliths from young cinder cones in the eastern Mojave region of Southern California to investigate deformation fabrics and their implications for strain localization, lithospheric viscosity, and controls on mineral lattice‐preferred orientations (LPOs) and seismic anisotropy at Moho depths. Lower crustal gabbros and upper mantle peridotites were collected from two areas separated by ∼80 km—the Cima and Deadman Lake Volcanic Fields. At both localities, mantle peridotites exhibit solid‐state deformation fabrics that show strong LPOs and other evidence for dislocation creep as the dominant deformation mechanism. The preservation of microstructures ranging from annealed, granular, to mylonitic indicates that there is substantial strain localization in the Mojave mantle near the Moho. Paleopiezometry conducted on subgrains in olivine indicates that stress magnitudes during deformation were 12–26 MPa. Calculations using olivine flow laws yield strain rates on the order of ∼10−12/s and an associated viscosity of ∼1019 Pa s, consistent with estimates of mantle lid viscosity from postseismic relaxation studies. Two types of olivine LPOs were observed in the peridotites: A‐type and E‐type fabrics, both of which predict seismic fast axes that align parallel to the lineation within the foliation plane. The occurrences of the two fabric types appear to correlate with strain magnitude but not with water contents measured using Fourier transform infrared spectroscopy. Lower crustal fabrics are dominated by magmatic foliations and LPOs in plagioclase, which produce seismic fast axes oriented perpendicular to the foliation plane. This suggests that even where mantle and lower crustal fabrics are kinematically linked, their seismic fast axes may appear anticorrelated across the Moho.

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Removal of mantle lithosphere by Rayleigh-Taylor (R-T) instabilities is invoked to explain the formation of high plateaus and mountain ranges. Here we report geochemical and microstructural observations from mantle xenoliths from Lunar Crater volcanic field, central Nevada, which we interpret to directly sample a R-T instability beneath the Basin and Range. The xenoliths comprise a suite of mylonitic and granular peridotites with fertile and refractory major and trace element compositions, suggesting a mantle lithospheric origin. Temperatures calculated using several geothermometers are 1,200-1,300°C, in contrast to xenoliths from other localities in the Basin and Range (typically~1,000°C). High Lunar Crater temperatures suggest the xenoliths originate from the base of the mantle lithosphere. The mylonitic peridotites exhibit olivine deformation microstructures characteristic of deformation in the dislocation creep regime; orthopyroxenes experienced brittle deformation. Recrystallized grain sizes (~80 μm) suggest the mylonites deformed at~50 MPa. Recrystallized olivines demonstrate isochemical deformation, suggesting the~50-MPa differential stresses were achieved at~1,200°C, implying strain rates of 2 × 10 −9 to 4 × 10 −7 /s, and corresponding to effective viscosities of 8.9 × 10 13 to 1.4 × 10 16 Pa/s. The most plausible mechanism for producing such conditions in the deep lithosphere beneath central Nevada is extreme strain localization within an actively deforming R-T instability. The most highly strained samples have relatively low effective viscosities, suggesting that strain is preferentially partitioned into weaker rocks within the deforming lithosphere. This study highlights the importance of strain localization and associated weakening as mechanisms for facilitating lithospheric R-T instabilities.

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