Western U.S. seismic anisotropy revealing complex mantle dynamics

Zhou, Quan; Hu, Jiashun; Liu, Liang; Chaparro, T.; Stegman, D. R.; Faccenda, Manuele

doi:10.1016/j.epsl.2018.08.015

Cited by 40 publications

(44 citation statements)

References 60 publications

(111 reference statements)

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“…Our observation of the circular pattern of azimuthal anisotropy (Fig. 3 ), though at a smaller scale, is comparable to the observations in the western United States, which is ascribed to mantle flow that is locally modified by variation in the lithospheric thickness, the presence of slab segments and a lithospheric drip 38 – 40 . Similar observations in other regions also suggest that the interaction between different components involved in a subduction/collision system can have significant influence on the flow field 41 – 44 .…”

Section: Discussionsupporting

confidence: 86%

Mantle-flow diversion beneath the Iranian plateau induced by Zagros’ lithospheric keel

Kaviani

Mahmoodabadi

Rümpker

et al. 2021

Sci Rep

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Previous investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role.

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Section: Discussionsupporting

confidence: 86%

Mantle-flow diversion beneath the Iranian plateau induced by Zagros’ lithospheric keel

Kaviani

Mahmoodabadi

Rümpker

et al. 2021

Sci Rep

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“…Studies that utilize station‐averaged values of splitting as constraints for geodynamic calculations often find the need to consider both contributions, especially in North America following the deployment of the USArray (e.g., Wang & Becker, ; Zhou et al, ). However, station averages that reduce the complexity of shear wave splitting signal to two numbers are an imperfect proxy.…”

Section: Discussionmentioning

confidence: 99%

Localized Anisotropic Domains Beneath Eastern North America

Levin

Elkington

et al. 2019

Geochem Geophys Geosyst

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Seismic anisotropy beneath eastern North America likely reflects both the remnant lithospheric fabrics and the present‐day deformation of the asthenosphere. We report new observations of splitting in core‐refracted shear phases observed over 3–5 years at 33 sites in New Jersey, New York, and states in the New England region and also include data from eight previously studied locations. Our data set emphasizes back azimuthal coverage necessary to capture the directional variation of splitting parameters expected from vertically varying anisotropy. We report single‐phase splitting parameters as well as station‐averaged values based on splitting intensity technique that incorporates all observed records regardless of whether they showed evidence of splitting or not. Trends of averaged fast shear wave polarizations appear coherent and are approximately aligned with absolute plate motion direction. The general disparity between the fast axes and the trend of surface tectonic features suggests a dominant asthenosphere contribution for the observed seismic anisotropy. Averaged delay values systematically increase from ~0.5 s in New Jersey to ~1.4 s in Maine. Splitting parameters vary at all sites, and neighboring stations often show similar patterns of directional variation. We developed criteria to group stations based on their splitting patterns and identified four domains with distinct anisotropic properties. Splitting patterns of three domains suggest a layered anisotropic structure that is geographically variable, outlining distinct regions in the continental mantle, for example, the Proterozoic lithosphere of the Adirondack Mountains. A domain coincident with the North Appalachian Anomaly displays virtually no splitting, implying that the lithospheric fabric was locally erased.

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“…Rather, this well-constrained deep anisotropy results from flow around the southern end of the subducting plate (Figure 8), part of global flow pattern partly driven by motion of the Pacific plate, which aligns approximately with the fast direction we see. Dynamical modeling (Zhou et al, 2018) predicts that return flow around the slab edge coupled with large-scale entrainment by the Pacific plate should produce strong WNW-ESE fabrics at >100-km depth beneath Gorda. The exact directions are somewhat contingent on the boundary conditions of the models, but generally hold.…”

Section: Eilon and Forsythmentioning

confidence: 99%

Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System

Eilon

Forsyth

2020

JGR Solid Earth

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We use surface wave measurements to reveal anisotropy as a function of depth within the Juan de Fuca and Gorda plate system. Using a two‐plane wave method, we measure phase velocity and azimuthal anisotropy of fundamental mode Rayleigh waves, solving for anisotropic shear velocity. These surface wave measurements are jointly inverted with constraints from SKS splitting studies using a Markov chain approach. We show that the two data sets are consistent and present inversions that offer new constraints on the vertical distribution of strain beneath the plates and the processes at spreading centers. Anisotropy of the Juan de Fuca plate interior is strongest (~2.4%) in the low‐velocity zone between ~40‐ to 90‐km depth, with ENE direction driven by relative shear between plate motion and mantle return flow from the Cascadia subduction zone. In disagreement with Pn measurements, weak (~1.1%) lithospheric anisotropy in Juan de Fuca is highly oblique to the expected ridge‐perpendicular direction, perhaps connoting complex intralithospheric fabrics associated with melt or off‐axis downwelling. In the Gorda microplate, strong shallow anisotropy (~1.9%) is consistent with Pn inversions and aligned with spreading and may be enhanced by edge‐driven internal strain. Weak anisotropy with ambiguous orientation in the low‐velocity zone can be explained by Gorda's youth and modest motion relative to the Pacific. Deeper (≥90 km) fabric appears controlled by regional flow fields modulated by the Farallon slab edge: anisotropy is strong (~1.8%) beneath Gorda, but absent beneath the Juan de Fuca, which is in the strain shadow of the slab.

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Western U.S. seismic anisotropy revealing complex mantle dynamics

Cited by 40 publications

References 60 publications

Mantle-flow diversion beneath the Iranian plateau induced by Zagros’ lithospheric keel

Mantle-flow diversion beneath the Iranian plateau induced by Zagros’ lithospheric keel

Localized Anisotropic Domains Beneath Eastern North America

Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System

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