The competing effects of olivine and orthopyroxene CPO on seismic anisotropy

Bernard, Rachel E.; Schulte-Pelkum, V.

doi:10.1016/j.tecto.2021.228954

Cited by 16 publications

(15 citation statements)

References 71 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Compared to the pure olivine estimates, the mixture systematically decreases the strength of anisotropy due to the weaker single-crystal anisotropy and LPO of orthopyroxene compared to olivine (Mainprice & Silver, 1993), while fast azimuths are largely unaffected. This observation is consistent with the recent findings of Bernard et al (2021), who investigated orthopyroxene effects on seismic anisotropy using a large suite of upper mantle xenoliths and found that the fast azimuth of anisotropy is not affected by orthopyroxene LPO for samples with >60% olivine.…”

Section: Quantification Of In Situ Strainsupporting

confidence: 92%

“…Secondary phases other than olivine in a given sample act to reduce the bulk strength of seismic anisotropy (Bernard et al., 2021; Mainprice & Silver, 1993). Yet, the laboratory and natural petrofabrics used in this study consider aggregates of pure olivine.…”

Section: Methodsmentioning

confidence: 99%

“…We do not account for more complex structures, such as pyroxene‐rich bands, which may develop in realistic polyphase aggregates with higher modal fractions of pyroxene. This treatment of the influence of orthopyroxene acts to reduce the overall strength of the fabric without having a large effect on the fast propagation azimuth (Bernard et al., 2021).…”

Section: Methodsmentioning

confidence: 99%

See 2 more Smart Citations

Seismological Evidence for Girdled Olivine Lattice‐Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading

Russell

Gaherty

Mark

et al. 2022

Geochem Geophys Geosyst

View full text Add to dashboard Cite

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.Plain Language Summary Earth's upper mantle is composed primarily of the mineral olivine, which responds to deformation by organizing its seismically fast axis in the flow direction. During seafloor spreading, olivine crystals align with the spreading direction and become frozen into the lithosphere preserving the near-ridge deformation history. The resulting rock fabric can be observed in place via the directional dependence of seismic wavespeeds (seismic anisotropy) as well as in hand-sample rocks collected from the field. However, interpreting seismic anisotropy observations in the context of laboratory and field data remains a challenge, due to large differences in length scale and incomplete seismic constraints. Here, we bridge this observational gap by incorporating multiple data types to solve for the complete anisotropic structure of oceanic lithosphere that formed ∼70 Myr ago. By comparing our model to laboratory data, we infer the magnitude of shear strain and style of olivine deformation during seafloor spreading for the first time. Our results indicate large shear strains and an olivine fabric type different to that typically assumed, consistent with deformation that is sensitive to the presence of grain boundari...

show abstract

Section: Quantification Of In Situ Strainsupporting

confidence: 92%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Seismological Evidence for Girdled Olivine Lattice‐Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading

Russell

Gaherty

Mark

et al. 2022

Geochem Geophys Geosyst

View full text Add to dashboard Cite

show abstract

“…A CPO type is chosen as input to the calculation for D-Rex by assigning reference dimensionless resolved shear stresses (RSS) (Table S2 in Supporting Information S1). Multiple slip systems have been reported for enstatite (Bernard et al, 2021;Jung et al, 2010). If more than 40% of the aggregate is olivine, the effect of enstatite on the orientation of aggregates' anisotropy is relatively small while it may impact the strength of anisotropy (Bernard et al, 2021).…”

Section: Methodsmentioning

confidence: 99%

“…Multiple slip systems have been reported for enstatite (Bernard et al, 2021;Jung et al, 2010). If more than 40% of the aggregate is olivine, the effect of enstatite on the orientation of aggregates' anisotropy is relatively small while it may impact the strength of anisotropy (Bernard et al, 2021). For simplicity, we choose to use one CPO for enstatite for all calculations and neglect the variation in the strength of anisotropy due to the choice of enstatite CPO.…”

Section: Methodsmentioning

confidence: 99%

Shear‐Wave Splitting in the Mantle Wedge: Role of Elastic Tensor Symmetry of Olivine Aggregates

Kenyon

Wada

2022

Geophysical Research Letters

View full text Add to dashboard Cite

Using a 3‐D mantle wedge flow field for a generic oblique subduction system, we calculate elastic tensors of mineral aggregates in the mantle wedge for A‐, B‐, C‐, and E‐type olivine crystal preferred orientations (CPO) and apply the calculated elastic tensor in the forward calculation of shear‐wave splitting (SWS) through the mantle wedge. We find that the hexagonal approximation of the full tensor does not affect the SWS parameters (the fast direction and the delay time) significantly for all CPO types except that the delay time for C‐type CPO becomes shorter. Additionally, we find that despite the 3‐D mantle flow field that results from oblique subduction, the fast direction is margin‐normal for A‐, C‐ and E‐type CPOs and margin‐parallel for B‐type CPO.

show abstract

A Reformulation of the Browaeys and Chevrot Decomposition of Elastic Maps

Tape,

Tape

2024

J Elast

View full text Add to dashboard Cite

An elastic map $\mathbf{T}$ T associates stress with strain in some material. A symmetry of $\mathbf{T}$ T is a rotation of the material that leaves $\mathbf{T}$ T unchanged, and the symmetry group of $\mathbf{T}$ T consists of all such rotations. The symmetry class of $\mathbf{T}$ T describes the symmetry group but without the orientation information. With an eye toward geophysical applications, Browaeys & Chevrot developed a theory which, for any elastic map $\mathbf{T}$ T and for each of six symmetry classes $\Sigma $ Σ , computes the “$\Sigma $ Σ -percentage” of $\mathbf{T}$ T . The theory also finds a “hexagonal approximation”—an approximation to $\mathbf{T}$ T whose symmetry class is at least transverse isotropic. We reexamine their theory and recommend that the $\Sigma $ Σ -percentages be abandoned. We also recommend that the hexagonal approximations to $\mathbf{T}$ T be replaced with the closest transverse isotropic maps to $\mathbf{T}$ T .

show abstract

The competing effects of olivine and orthopyroxene CPO on seismic anisotropy

Cited by 16 publications

References 71 publications

Seismological Evidence for Girdled Olivine Lattice‐Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading

Seismological Evidence for Girdled Olivine Lattice‐Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading

Shear‐Wave Splitting in the Mantle Wedge: Role of Elastic Tensor Symmetry of Olivine Aggregates

A Reformulation of the Browaeys and Chevrot Decomposition of Elastic Maps

Contact Info

Product

Resources

About