Effects of crystal preferred orientation on upper-mantle flow near plate boundaries: rheologic feedbacks and seismic anisotropy

Abstract

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High‐Resolution Constraints on Pacific Upper Mantle Petrofabric Inferred From Surface‐Wave Anisotropy

“…We observe ξ > 1 as well as G and E directions parallel to FSD and FSD+45°, respectively, which are all consistent with horizontal lithospheric fabric that formed due to corner flow at the ridge. Furthermore, our observations of a positive gradient in G from the Moho to ∼30 km depth is consistent with numerical flow models of passive spreading that predict a positive gradient in LPO strength within the upper 20–80 km of the mantle (Blackman et al, , ; Blackman & Kendall, ). The depth‐dependent LPO strength predicted by flow models is a result of the positive temperature gradient, which reduces viscosities leading to higher strain and enhanced fabric alignment with depth.…”

“…The PN78 harzburgite is an example of such a fabric with [100] rotated ∼ 20° out of the foliation plane and agrees with the radial anisotropy that we observe quite well, although E / N is overestimated. Such rotated fabrics are commonly observed in natural (Warren et al, ; Webber et al, ) and laboratory (Skemer et al, ; Zhang & Karato, ) olivine samples as well as in numerical models of fabric development (Blackman & Kendall, , ; Blackman et al, , Kaminski & Ribe, ) and may be linked to deformation history or preexisting LPO fabrics (Skemer et al, ). Forward calculations suggest that BIM98 fabric with the fast direction rotated ∼25° from the horizontal plane produces azimuthal and radial anisotropy that are very similar to the NoMelt model.…”

“…In the oldest reaches of the Pacific, the lithospheric anisotropy varies over relatively short length scales and does not always correlate with the direction of seafloor spreading (e.g., Shintaku et al, ; Takeo et al, , ). These observations suggest a rich diversity of dynamic processes beneath MORs that go well beyond the simple symmetrical‐spreading models explored to date (e.g., Blackman et al, ; Blackman & Kendall, ; Blackman et al, ).…”

“…This interpretation is consistent with numerical models of such buoyancy‐driven upwelling at slow‐spreading ridges, which predict primarily vertical off‐axis fabrics associated with the downgoing limb of cooler mantle material, extending from the Moho down to 40–50 km depth (Blackman et al, ; Blackman & Kendall, ). Conversely, numerical flow models of passive upwelling at fast‐spreading ridges produce primarily horizontal lithospheric fabrics that are oriented in the direction of spreading and increase in strength with depth in the lithosphere (Blackman et al, , ; Blackman & Kendall, , ).…”

Dynamics of the lithosphere and upper mantle in light of seismic anisotropy

“…Self-consistent modeling of both seismic and viscous anisotropy was implemented by Chastel et al [1993] for an idealized convective cell, and more recently by Blackman et al [2017] for a more complete convection model of a spreading center. Models that include the LPO feedback show generally similar flow patterns than simpler models, but there can be up to a factor ∼ 2 enhancement of predicted surface wave azimuthal anisotropy close to the ridge because of increased strain-rates, and the transverse isotropy symmetry axes are more horizontally aligned [Blackman et al, 2017] than those of earlier one-way LPO predictions . It remains to be seen if such effects of viscous anisotropy feedback are relevant for the interpretation of regional or global convection, or if other uncertainties such as the effects of temperature, composition, and volatile anomalies on isotropic olivine rheology swamp the signal.…”