TY - JOUR
T1 - High-Resolution Constraints on Pacific Upper Mantle Petrofabric Inferred From Surface-Wave Anisotropy
AU - Russell, Joshua B.
AU - Gaherty, James B.
AU - Lin, Pei Ying Patty
AU - Lizarralde, Daniel
AU - Collins, John A.
AU - Hirth, Greg
AU - Evans, Rob L.
N1 - Funding Information:
We thank the captain, crew, and engineers of the R/V Marcus G. Langseth for making the data collection possible. OBS were provided by Scripps Institution of Oceanography via the Ocean Bottom Seismograph Instrument Pool (http://www.obsip.org), which is funded by the National Science Foundation. All waveform data used in this study are archived at the IRIS Data Management Center (http://www.iris.edu) with network code ZA for 2011–2013, and all OBS orientations are included in Table S1. The 1-D transversely isotropic and azimuthally anisotropic models and their uncertainties from this study can be found in the supporting information. This work was supported by NSF grants OCE-0928270 and OCE-1538229 (J. B. Gaherty), EAR-1361487 (G. Hirth), and OCE-0938663 (D. Lizarralde, J. A. Collins, and R. L. Evans), and an NSF Graduate Research Fellowship DGE-16-44869 to J. B. Russell. The authors thank the editor as well as reviewers Donald Forsyth, Hitoshi Kawakatsu, and Thorsten Becker for their constructive comments, which significantly improved this manuscript. J. B. Russell thanks Natalie J. Accardo for kindly sharing codes and expertise that contributed greatly to the analysis.
Publisher Copyright:
©2018. American Geophysical Union. All Rights Reserved.
PY - 2019/1
Y1 - 2019/1
N2 - Lithospheric seismic anisotropy illuminates mid-ocean ridge dynamics and the thermal evolution of oceanic plates. We utilize short-period (5–7.5 s) ambient-noise surface waves and 15- to 150-s Rayleigh waves measured across the NoMelt ocean-bottom array to invert for the complete radial and azimuthal anisotropy in the upper ∼35 km of ∼70-Ma Pacific lithospheric mantle, and azimuthal anisotropy through the underlying asthenosphere. Strong azimuthal variations in Rayleigh- and Love-wave velocity are observed, including the first clearly measured Love-wave 2θ and 4θ variations. Inversion of averaged dispersion requires radial anisotropy in the shallow mantle (2-3%) and the lower crust (4-5%), with horizontal velocities (V SH ) faster than vertical velocities (V SV ). Azimuthal anisotropy is strong in the mantle, with 4.5–6% 2θ variation in V SV with fast propagation parallel to the fossil-spreading direction (FSD), and 2–2.5% 4θ variation in V SH with a fast direction 45° from FSD. The relative behavior of 2θ, 4θ, and radial anisotropy in the mantle are consistent with ophiolite petrofabrics, linking outcrop and surface-wave length scales. V SV remains fast parallel to FSD to ∼80 km depth where the direction changes, suggesting spreading-dominated deformation at the ridge. The transition at ∼80 km perhaps marks the dehydration boundary and base of the lithosphere. Azimuthal anisotropy strength increases from the Moho to ∼30 km depth, consistent with flow models of passive upwelling at the ridge. Strong azimuthal anisotropy suggests extremely coherent olivine fabric. Weaker radial anisotropy implies slightly nonhorizontal fabric or the presence of alternative (so-called E-type) peridotite fabric. Presence of radial anisotropy in the crust suggests subhorizontal layering and/or shearing during crustal accretion.
AB - Lithospheric seismic anisotropy illuminates mid-ocean ridge dynamics and the thermal evolution of oceanic plates. We utilize short-period (5–7.5 s) ambient-noise surface waves and 15- to 150-s Rayleigh waves measured across the NoMelt ocean-bottom array to invert for the complete radial and azimuthal anisotropy in the upper ∼35 km of ∼70-Ma Pacific lithospheric mantle, and azimuthal anisotropy through the underlying asthenosphere. Strong azimuthal variations in Rayleigh- and Love-wave velocity are observed, including the first clearly measured Love-wave 2θ and 4θ variations. Inversion of averaged dispersion requires radial anisotropy in the shallow mantle (2-3%) and the lower crust (4-5%), with horizontal velocities (V SH ) faster than vertical velocities (V SV ). Azimuthal anisotropy is strong in the mantle, with 4.5–6% 2θ variation in V SV with fast propagation parallel to the fossil-spreading direction (FSD), and 2–2.5% 4θ variation in V SH with a fast direction 45° from FSD. The relative behavior of 2θ, 4θ, and radial anisotropy in the mantle are consistent with ophiolite petrofabrics, linking outcrop and surface-wave length scales. V SV remains fast parallel to FSD to ∼80 km depth where the direction changes, suggesting spreading-dominated deformation at the ridge. The transition at ∼80 km perhaps marks the dehydration boundary and base of the lithosphere. Azimuthal anisotropy strength increases from the Moho to ∼30 km depth, consistent with flow models of passive upwelling at the ridge. Strong azimuthal anisotropy suggests extremely coherent olivine fabric. Weaker radial anisotropy implies slightly nonhorizontal fabric or the presence of alternative (so-called E-type) peridotite fabric. Presence of radial anisotropy in the crust suggests subhorizontal layering and/or shearing during crustal accretion.
KW - Love-wave anisotropy
KW - ambient-noise tomography
KW - oceanic lithosphere
KW - seismic anisotropy
KW - surface waves
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U2 - 10.1029/2018JB016598
DO - 10.1029/2018JB016598
M3 - Article
AN - SCOPUS:85060254441
SN - 2169-9313
VL - 124
SP - 631
EP - 657
JO - Journal of Geophysical Research: Solid Earth
JF - Journal of Geophysical Research: Solid Earth
IS - 1
ER -