TY - JOUR T1 - Seismological Evidence for Girdled Olivine Lattice-Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading JF - Geochemistry, Geophysics, Geosystems Y1 - 2022 A1 - Russell, J. B. A1 - Gaherty, J. B. A1 - Mark, H. F. A1 - Hirth, G. A1 - Hansen, L. N. A1 - Lizarralde, D. A1 - Collins, J. A. A1 - Evans, R. L. KW - grain-boundary sliding KW - mid-ocean ridge KW - oceanic lithosphere KW - seafloor spreading KW - seismic anisotropy KW - surface waves AB - Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain-scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co-located compressional- and shear-wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil-spreading direction, consistent with corner-flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D-type olivine lattice-preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D-type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain-boundary sliding during dislocation creep relaxes strain compatibility, allowing D-type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation-accommodated grain-boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain-size dependent deformation, in contrast to pure dislocation creep. VL - 23 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2022GC010542 N1 - _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2022GC010542 ER - TY - JOUR T1 - Constraints on the Depth, Thickness, and Strength of the G Discontinuity in the Central Pacific From S Receiver Functions JF - Journal of Geophysical Research: Solid Earth Y1 - 2021 A1 - Mark, H. F. A1 - Collins, J. A. A1 - Lizarralde, D. A1 - Hirth, G. A1 - Gaherty, J. B. A1 - Evans, R. L. A1 - Behn, M. D. AB - The relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere-asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere. VL - 126 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2019JB019256 N1 - _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JB019256 ER - TY - JOUR T1 - Azimuthal Seismic Anisotropy of 70-Ma Pacific-Plate Upper Mantle JF - Journal of Geophysical Research: Solid Earth Y1 - 2019 A1 - Mark, H. F. A1 - Lizarralde, D. A1 - Collins, J. A. A1 - Miller, N. C. A1 - Hirth, G. A1 - Gaherty, J. B. A1 - Evans, R. L. AB - Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2-D corner flow and plate-driven shear near mid-ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70-Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2-D mantle flow near a mid-ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution. VL - 124 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2018JB016451 N1 - _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JB016451 ER -