We present a new, 3-D model of seismic velocity and anisotropy in the Pacific upper mantle, PAC13E. We invert a data set of single-station surface-wave phase-anomaly measurements sensitive only to Pacific structure for the full set of 13 anisotropic parameters that describe surface-wave anisotropy. Realistic scaling relationships for surface-wave azimuthal anisotropy are calculated from petrological information about the oceanic upper mantle and are used to help constrain the model. The strong age dependence in the oceanic velocities associated with plate cooling is also used as a priori information to constrain the model. We find strong radial anisotropy with vSH > vSV in the upper mantle; the signal peaks at depths of 100–160 km. We observe an age dependence in the depth of peak anisotropy and the thickness of the anisotropic layer, which both increase with seafloor age, but see little age dependence in the depth to the top of the radially anisotropic layer. We also find strong azimuthal anisotropy, which typically peaks in the asthenosphere. The azimuthal anisotropy at asthenospheric depths aligns better with absolute-plate-motion directions while the anisotropy within the lithosphere aligns better with palaeospreading directions. The relative strengths of radial and azimuthal anisotropy are consistent with A-type olivine fabric. Our findings are generally consistent with an explanation in which corner flow at the ridge leads to the development and freezing-in of anisotropy in the lithosphere, and shear between the lithosphere and underlying asthenosphere leads to anisotropy beneath the plate. We also observe large regions within the Pacific basin where the orientation of anisotropy and the absolute-plate-motion direction differ; this disagreement suggests the presence of shear in the asthenosphere that is not aligned with absolute-plate-motion directions. Azimuthal-anisotropy orientation rotates with depth; the depth of the maximum vertical gradient in the fast-axis orientation tends to be age dependent and agrees well with a thermally controlled lithosphere–asthenosphere boundary. We observe that azimuthal-anisotropy strength at shallow depths depends on half-spreading rate, with higher spreading rates associated with stronger anisotropy. Our model implies that corner flow is more efficient at aligning olivine to form lattice-preferred orientation anisotropy fabrics in the asthenosphere when the spreading rate at the ridge is higher.
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.
more » « less- NSF-PAR ID:
- 10359825
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 4
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
SUMMARY -
more » « less
Abstract We present a novel geodynamic approach that can potentially tighten existing constraints on mantle rheology. This new approach, which we call probabilistic geodynamic modeling, is applied here to the rheology of the upper mantle. We combine the numerical modeling of plate‐driven corner flow and the seismic observation of radial anisotropy, aiming to reduce substantial uncertainties associated with experimentally derived flow laws, but our results also highlight the complex competition among different deformation mechanisms under mantle conditions. Despite the remaining rheological uncertainty, our study suggests that significant background shear flow is required near the lithosphere‐asthenosphere boundary to explain the strong radial anisotropy observed at 100–200 km depth underneath the Pacific plate, and the plausible nature of this background flow is characterized using our new probabilistic approach. Our analysis also provides a new insight into the asthenospheric water content and the grain size distribution in the upper mantle, but these results are also subject to nontrivial nonuniqueness. The merit of our probabilistic approach lies in its ability to assess the extent of such nonuniqueness, and we demonstrate this by quantifying the robustness of some of our results.
-
The structure of the lithosphere-asthenosphere boundary (LAB) beneath oceanic plates is key to understanding how plates interact with the underlying mantle. Prior contradictory geophysical observations have been used to argue for a thin, melt-rich boundary that decouples the plate from the rest of the mantle, or for a much broader anisotropic and thermally controlled boundary that indicates significant coupling with the rest of the mantle. The predictions of models based on these interpretations can be tested most easily in a subduction zone setting where the steady increase in pressure at the base of the subducting plate’s LAB will have differing effects on melt and anisotropy. Melt remains stable within the mantle to ~150-250 km (for carbonate melt) or to ~330 km (for silicate melt), while anisotropy induced by different processes should have no significant change until ~250 km to ~440 km depth. We calculate P-to-S receiver functions (PRFs) using varying frequency bands at broadband seismic stations with >4 years of data from the Servicio Geológico Colombiano’s Red Sismológica Nacional de Colombia to investigate the characteristics of the LAB of the subducting Nazca oceanic plate from the coast to the Andean foreland (corresponding to slab LAB depths of ~50 km to >400 km). The use of PRFs permits identification and analysis of anisotropy across the boundary while calculation at a range of frequency bands permits tuning of the PRFs to differing spatial scales to determine the size and abruptness of the boundary. We find that the P-to-S converted phase of the subducted Nazca plate’s LAB is detectable 4-5 seconds after the converted phase of the plate’s Moho to at least ~150 km depth. Assuming the slab has an average Vp/Vs of 1.75 to 1.78 and Vp of 8.2 km/s (+2.5% dVp), this corresponds to a plate thickness of ~50 km, matching the expected thickness given the Nazca plate’s age in the region (~10-20 Myrs). We find that the Nazca plate’s LAB is most consistently detectable in the <0.24 Hz band and largely undetectable in the <2.4 Hz band, indicating the LAB is gradational and between 10 and 30 km in thickness. Amplitude variations and complexities in the LAB converted phases further indicate that the boundary marks a change in anisotropy most consistent with the LAB representing a sheared zone between the plate and underlying mantle.more » « less
-
In recent years there have been several attempts to make the link between mineral properties and seismic anisotropy in the D’’ region but have yet to reach consensus with regards to the dynamics in lower mantle minerals that could give rise to the observed seismic anisotropy. Here, we aim to provide further constraints on the observed long wavelength shear velocity patterns seen in seismic tomography studies. We introduce a forward model of deformation in a subducting slab as it impacts the core mantle boundary (D’’ layer) and proceeds to upwelling at the edge of a simulated LLSVP. By implementing the most recent results from atomistic modeling and high-pressure deformation experiments coupled with a 3-dimensional geodynamic model, we compare the microstructural evolution of an aggregate with a pyrolytic composition to the macroscopically observed seismic anisotropy of the lowermost mantle. We account for topotaxial relations in the forward and reverse phase transitions of MgSiO3-perovskite (Pv) to post-perovskite (pPv) within the slab as well as explore the effects introduced by partial melting near the CMB. Comparisons in the two leading candidate deformation mechanisms in the post-perovskite phase, (001) and (010), are compared. In this study we find that the reverse transition (pPv to Pv) occurs at a depth which is ~ 150 km deeper than that of the forward transition due to increasing temperature near the CMB providing a varying topography of the D’’ discontinuity. Our model also produces good fits with the isotropic velocities of PREM for the bulk lower mantle. When coupled with temperature and pressure dependent forward and reverse phase transitions, a pPv system with dominant (001) slip provides good correlation with the currently observed VSH fast horizontal (~ 1 – 6%) in D’’ and with VSV consistently fast in upwelling areas. Azimuthal variations along the streamline are also investigated showing a symmetry lower than that of the assumed VTI in D’’ introduced by ‘rolling’ effects near the slab’s edge. The addition of 1% partial melting at the CMB is shown to increase S and P wave anisotropy beneath the slab at the base of upwelling with up to ~2.5 & 4.0% P and S wave reductions respectively compared to the global reference.more » « less
-
more » « less
Abstract Little has been seismically imaged through the lithosphere and mantle at rifted margins across the continent‐ocean transition. A 2014–2015 community seismic experiment deployed broadband seismic instruments across the shoreline of the eastern North American rifted margin. Previous shear‐wave splitting along the margin shows several perplexing patterns of anisotropy, and by proxy, mantle flow. Neither margin parallel offshore fast azimuths nor null splitting on the continental coast obviously accord with absolute plate motion, paleo‐spreading, or rift‐induced anisotropy. Splitting measurements, however, offer no depth constraints on anisotropy. Additionally, mantle structure has not yet been imaged in detail across the continent‐ocean transition. We used teleseismic
S ,SKS ,SKKS , andPKS splitting and differential travel times recorded on ocean‐bottom seismometers, regional seismic networks, and EarthScope Transportable Array stations to conduct joint isotropic/anisotropic tomography across the margin. The velocity model reveals a transition from fast, thick, continental keel to low velocity, thinned lithosphere eastward. Imaged short wavelength velocity anomalies can be largely explained by edge‐driven convection or shear‐driven upwelling. We also find that layered anisotropy is prevalent across the margin. The anisotropic fast polarization is parallel to the margin within the asthenosphere. This suggests margin parallel flow beneath the plate. The lower oceanic lithosphere preserves paleo‐spreading‐parallel anisotropy, while the continental lithosphere has complex anisotropy reflecting several Wilson cycles. These results demonstrate the complex and active nature of a margin which is traditionally considered tectonically inactive.
