Despite progress in tomographic imaging of Earth’s interior, a number of critical questions
regarding the large-scale structure and dynamics of the mantle remain outstanding. One of those questions is the impact of phase-boundary undulations on global imaging of mantle heterogeneity and on geodynamic (i.e. convection-related) observables. To address this issue, we developed a joint seismic-geodynamic-mineral physical tomographic inversion procedure that incorporates lateral variations in the depths of the 410- and 660-km discontinuities. This inversion includes S-wave traveltimes, SS precursors that are sensitive to transition-zone topography, geodynamic observables/data (free-air gravity, dynamic surface topography, horizontal divergence of tectonic plates and excess core-mantle boundary ellipticity) and mineral physical constraints on thermal heterogeneity. Compared to joint tomography models that do not include data sensitivity to phase-boundary undulations in the transition zone, the inclusion of 410- and 660-km topography strongly influences the inference of volumetric anomalies in a depth interval that encompasses the transition zone and mid-mantle. It is notable that joint tomography inversions, which include constraints on transition-zone discontinuity topography by seismic and geodynamic data, yield more pronounced density anomalies associated with
subduction zones and hotspots. We also find that the inclusion of 410- and 660-km topography may improve the fit to the geodynamic observables, depending on the weights applied to seismic and geodynamic data in the inversions. As a consequence, we find that the amplitude of non-thermal density anomalies required to explain the geodynamic data decreases in most of the mantle. These findings underline the sensitivity of the joint inversions to the inclusion of transition-zone complexity (e.g. phase-boundary topography) and the implications for the inferred non-thermal density anomalies in these depth regions. Finally, we underline that our inferences of 410- and 660-km topography avoid a commonly employed approximation that represents the contribution of volumetric heterogeneity to SS-wave precursor data. Our results suggest that this previously employed correction, based on a priori estimates of uppermantle heterogeneity, might be a significant source of error in estimating the 410- and 660-km topography.
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The effects of discontinuity topography in the mantle transition zone on global geodynamic observables and mantle heterogeneity
Despite progress in tomographic imaging of Earth's interior, a number of critical questions regarding the large-scale structure and dynamics of the mantle remain outstanding. One of those questions is the impact of phase-boundary undulations on global imaging of mantle heterogeneity and on geodynamic (i.e. convection-related) observables. To address this issue, we developed a joint seismic-geodynamic-mineral physical tomographic inversion procedure that incorporates lateral variations in the depths of the 410- and 660-km discontinuities. This inversion includes S-wave traveltimes, SS precursors that are sensitive to transition-zone topography, geodynamic observables/data (free-air gravity, dynamic surface topography, horizontal divergence of tectonic plates and excess core-mantle boundary ellipticity) and mineral physical constraints on thermal heterogeneity. Compared to joint tomography models that do not include data sensitivity to phase-boundary undulations in the transition zone, the inclusion of 410- and 660-km topography strongly influences the inference of volumetric anomalies in a depth interval that encompasses the transition zone and mid-mantle. It is notable that joint tomography inversions, which include constraints on transition-zone discontinuity topography by seismic and geodynamic data, yield more pronounced density anomalies associated with subduction zones and hotspots. We also find that the inclusion of 410- and 660-km topography may improve the fit to the geodynamic observables, depending on the weights applied to seismic and geodynamic data in the inversions. As a consequence, we find that the amplitude of non-thermal density anomalies required to explain the geodynamic data decreases in most of the mantle. These findings underline the sensitivity of the joint inversions to the inclusion of transition-zone complexity (e.g. phase-boundary topography) and the implications for the inferred non-thermal density anomalies in these depth regions. Finally, we underline that our inferences of 410- and 660-km topography avoid a commonly employed approximation that represents the contribution of volumetric heterogeneity to SS-wave precursor data. Our results suggest that this previously employed correction, based on a priori estimates of upper-mantle heterogeneity, might be a significant source of error in estimating the 410- and 660-km topography.
more » « less- Award ID(s):
- 1903108
- NSF-PAR ID:
- 10364448
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 230
- Issue:
- 1
- ISSN:
- 0956-540X
- Format(s):
- Medium: X Size: p. 623-642
- Size(s):
- p. 623-642
- Sponsoring Org:
- National Science Foundation
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Abstract Using recently collected high‐resolution seismic data along a dense linear transect across Ohio, West Virginia, and Virginia (called Mid‐Atlantic Geophysical Integrative Collaboration (MAGIC) profile), we analyze P‐to‐S receiver functions to investigate the undulations of the mantle transition zone (MTZ) discontinuities (410‐ and 660‐km) beneath the central Appalachian region. Our results incorporating the effects of local crustal and mantle structures suggest shallowing of both the 410‐ and the 660‐km discontinuities from the northwest (inland) to the southeast (coast) along MAGIC profile. Hydro‐thermal upwelling beneath the eastern U.S. coastal plain due to a hydrated MTZ and hot upwelling return flow associated with the descending lower mantle Farallon slab is consistent with our observations of MTZ structure considering 3D velocity heterogeneity. The inferred hydrous hot upwelling rising into the upper mantle may trigger dehydration melting atop the 410‐km discontinuity, which may help to explain the presence of a low velocity upper mantle anomaly beneath the region today.
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SUMMARY We report finite-frequency imaging of the global 410- and 660-km discontinuities using boundary sensitivity kernels for traveltime measurements made on SS precursors. The application of finite-frequency sensitivity kernels overcomes resolution limits in previous studies associated with large Fresnel zones of SS precursors and their interferences with other seismic phases. In this study, we calculate the finite-frequency sensitivities of SS waves and their precursors based on a single-scattering (Born) approximation in the framework of travelling-wave mode summation. The global discontinuity surface is parametrized using a set of triangular gridpoints with a lateral spacing of about 4°, and we solve the linear finite-frequency inverse problem (2-D tomography) based on singular value decomposition (SVD). The new global models start to show a number of features that were absent (or weak) in ray-theoretical back-projection models at spherical harmonic degree l > 6. The thickness of the mantle transition zone correlates well with wave speed perturbations at a global scale, suggesting dominantly thermal origins for the lateral variations in the mantle transition zone. However, an anticorrelation between the topography of the 410-km discontinuity and wave speed variations is not observed at a global scale. Overall, the mantle transition zone is about 2–3 km thicker beneath the continents than in oceanic regions. The new models of the 410- and 660-km discontinuities show better agreement with the finite-frequency study by Lawrence & Shearer than other global models obtained using SS precursors. However, significant discrepancies between the two models exist in the Pacific Ocean and major subduction zones at spherical harmonic degree >6. This indicates the importance of accounting for wave interactions in the calculations of sensitivity kernels as well as the use of finite-frequency sensitivities in data quality control.more » « less
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Abstract The mantle transition zone (MTZ) plays an important role in modulating material transport between the upper mantle and the lower mantle. Constraining this transport is essential for understanding geochemical reservoirs, hydration cycles, and the evolution of the Earth. Slabs and hotspots are assumed to be the dominant locations of transport. However, the degree of material transport in other areas is debated. We apply
P ‐to‐S receiver functions to an amphibious data set from Cascadia to image the MTZ discontinuities beneath mid‐ocean ridges, a hotspot, and a subduction zone. We find a MTZ thinned by 10 ± 6 km beneath the ridges and by 8 ± 4 km beneath the base of the slab, closely resembling the 660 discontinuity topography. Depressions on the 410 discontinuity are smaller, 5 ± 2 km on average, focused in the north and the south and accompanied by supra‐410 discontinuity melt phases. The depressions occur away from locations of uplifted 660 discontinuity, but near slow seismic velocity anomalies imaged in the upper mantle. This suggests lower mantle upwellings occur beneath ridges and beneath the base of slabs but stall in the transition zone, with upper mantle convection determining upward material transport from the transition zone. Therefore, upper mantle dynamics play a larger role in determining transfer than typically assumed. -
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Abstract The mantle transition zone (MTZ) of Earth is demarcated by solid‐to‐solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660‐km depths. Mineral physics experiments predict that wadsleyite can have strong single‐crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, significant seismic anisotropy is possible in the upper MTZ where lattice‐preferred orientation of wadsleyite is produced by mantle flow. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We stack the data to explore the azimuthal dependence of travel‐times and amplitudes of SS precursors and constrain the azimuthal anisotropy in the MTZ. Beneath the central Pacific, we find evidence for ~4% anisotropy with a SE fast direction in the upper mantle and no significant anisotropy in the MTZ. In subduction zones, we observe ~4% anisotropy with a trench‐parallel fast direction in the upper mantle and ~3% anisotropy with a trench‐perpendicular fast direction in the MTZ. The transition of fast directions indicates that the lattice‐preferred orientation of wadsleyite induced by MTZ flow is organized separately from the flow in the upper mantle. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle but negligible anisotropy (<1%) in the MTZ. Finally, we correct for the upper mantle and MTZ anisotropy structures to obtain a new MTZ topography model. The anisotropy correction produces
± 3 km difference and therefore has minor overall effects on global MTZ topography.
