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3‐D iso‐surface representations of wave speed anomalies with magnitude greater than +1% (green bodies) and less than −0.8% (red bodies), as well as the fast axes (yellow arrows) at a depth of 500 km in model US32. Topography is superimposed on top of the 3‐D iso‐surface bodies. Cyan arrows are used to highlight slab features while orange arrows are used to denote mantle flow fields.
Source publication
Several hypotheses have been proposed to explain intriguing circular shear wave splitting patterns in the Pacific Northwest, invoking either 2‐D entrained flows or 3‐D return flows. Here, we present some hitherto unidentified, depth‐dependent anisotropic signatures to reconcile different conceptual models. At depths shallower than 200 km, the fast...
Citations
... However, this feature is not well recovered by recent Rayleigh-wave azimuthal anisotropy tomography at similar periods (Zhang et al., 2021). A Vs model obtained by full wave inversion also shows trench-normal FVDs at depths <200 km beneath Cascadia (Zhu et al., 2020). By assuming horizontal hexagonal symmetry, P-wave anisotropic tomography of the Cascadia subduction zone revealed clear trench-parallel FVDs in the subducting JdF slab under the forearc (Zhao and Hua, 2021). ...
... Compared to previous tomographic models from the JdF ridge to the Cascadia subduction zone (e.g., Bell et al., 2016;Gao, 2018;Hawley and Allen, 2019;Janiszewski et al., 2019;Bodmer et al., 2020a;Zhu et al., 2020;Zhang et al., 2021), our present 3-D Vs model provides a new and clearer image of the JdF lithosphere-asthenosphere system in the upper mantle down to ~200 km depth (Figs. 8, 9 and 12), especially for its azimuthal anisotropic structure before and after the subduction. A notable new feature revealed by this study is distinct azimuthal anisotropies between the JdF plate beneath the Pacific Ocean and the subducting JdF slab beneath the Cascadia land area. ...
To clarify the internal structure of the oceanic lithosphere-asthenosphere system, here we determine a new 3-D model of azimuthal anisotropic shear-wave velocity (Vs) down to ~200 km depth from the Juan de Fuca (JdF) mid-ocean ridge to the Cascadia subduction zone, by inverting newly measured teleseismic fundamental mode Rayleigh-wave phase and amplitude data at periods of 25–100 s. The JdF lithosphere is clearly imaged as a highvelocity anomaly from the ridge to the subduction zone. Distinct azimuthal anisotropies are revealed in the JdF lithosphere before and after its subduction. Obvious trench-normal fast-velocity directions (FVDs) exist in the JdF
plate beneath the Pacific Ocean, whereas the subducting JdF slab beneath the Cascadia margin generally exhibits trench-parallel FVDs. We propose that the subduction-induced structure and mineral alignment in the JdF slab transform the fossil fabrics in the JdF plate formed at the mid-ocean ridge and altered during the plate motion and cooling.
... Although anisotropic tomography has better depth resolution, the obtained images are less consistent with each other. For example, some models of azimuthal anisotropy tomography show trench-parallel FVDs (Gosselin et al., 2020;Huang & Zhao, 2013;Wagner et al., 2013) in the Cascadia margin, whereas other anisotropic models show FVDs sub-normal to the trench (Yuan & Romanowicz, 2010;Zhu et al., 2020). In addition, seismic reflection profiles (Han et al., 2016) show that faults are produced in the upper part of the Juan de Fuca plate due to the upward plate bending in the outer-rise area, but the deep portion of the subducted slab is not imaged. ...
... Tilting-axis anisotropy has a more general assumption in which the HSA can be freely orientated in 3-D space. Here we compare our slow and fast HSA models with previous azimuthal and radial anisotropy models (See Text S5 and Figures S37-S40 in Supporting Information S1 for details) ( & Romanowicz, 2010; Zhou et al., 2022;Zhu et al., 2020). These models are generally similar in the forearc and arc areas, suggesting that the tilting-axis anisotropy can reconcile the contradictory assumptions of azimuthal and radial anisotropies. ...
... Therefore, the interpretation of the trench-normal FVDs is challenging as they could reflect both entrained and toroidal flows and are hard to discriminate (Long, 2016). Recent tomographic studies for azimuthal anisotropy, with a better depth resolution, have revealed potential mantle flow patterns (Zhao & Hua, 2021;Zhu et al., 2020), but they still cannot exclude the case that the trench-normal FVDs may reflect toroidal flow (Zandt & Humphreys, 2008). Hence, this paradox (Long, 2016) has not been addressed adequately. ...
The Cascadia margin is an unusual subduction zone characterized by the downdip movement of young and thin oceanic plates, where mantle flow and intraslab deformation are still unclear. Here we present new anisotropic tomography of the Cascadia subduction zone, in which the hexagonal symmetry axis of anisotropy is tilting rather than horizontal or vertical as assumed in previous studies of seismic anisotropy. Subduction-induced entrained and toroidal flows under the Cascadia margin are discriminated well by the spatial relationship between the tilting-axis anisotropy and slab geometry. The obliquely entrained flow is trapped in a narrow zone (<100 km wide) above and below the subducting slab and reaches ~200 km depth, which is surrounded by large-scale sub-horizontal toroidal flow. The intraslab anisotropy is trench-normal above 80 km depth but changes to trench-parallel at 100-400 km depths, which may reflect fossil anisotropy overprinted by deep deformation beneath the arc, or joint effect of serpentinization and hydrous faulting.
... Several significant anisotropic features have been revealed in the western United States, such as edge-parallel flows along the western and southern edges of the NA craton Yang et al., 2017), fast velocity directions (FVDs) parallel with the APM direction of NA in the continental interior Yang et al., 2014), and a circular pattern of anisotropy fast directions centered in south-central Nevada (e. g., West et al., 2009;Eakin et al., 2010). In addition, toroidal and poloidal flows developed around both the tip and edges of the descending slab are also demonstrated by laboratory experiments and geodynamic simulations (Buttles and Olson, 1998;Humphreys, 2009;Faccenda and Capitanio, 2013;Faccenda, 2014;Zhu et al., 2020aZhu et al., , 2020b. However, limited by different sensitivities of different data types, the origin and mechanism of observed continental anisotropies are still under debate, sequentially contentious between the hypotheses of one-or two-or three-layer anisotropy models (e.g., Yuan and Romanowicz, 2010a;Yuan and Levin, 2014;Yang et al., 2014Yang et al., , 2017Refayee et al., 2014). ...
To investigate lateral and depth variations of seismic anisotropy beneath the central-western United States, we determined a detailed 3-D model of P-wave anisotropic tomography by inverting a large number of arrival-time data of local and teleseismic events. Our results reveal significant azimuthal anisotropies in the crust and lithosphere, which are associated with ancient orogenic collisional and magmatic activities. As depth increases, the fast-velocity direction (FVD) pattern becomes gradually trended and small features fade away. There is a boundary in the FVD distribution, which separates the tectonically active region in the west from the stable cratonic region in the east. Frozen-in anisotropy with a NW-SE FVD is preserved in the thick Wyoming cratonic lithosphere that exhibits as a high-velocity (high-V) anomaly to a depth of ~250 km. In the asthenosphere beneath the western thin lithosphere, FVDs are generally parallel with the absolute motion direction of the North American plate due to shearing between the plate and the asthenosphere. In the deeper areas, the subducted and fragmented slab exhibiting as high-V anomalies leads to slab-related mantle flows. These results indicate that seismic anisotropies exist in both the lithosphere and asthenosphere with different geodynamic mechanisms and it is feasible to link the P-wave azimuthal anisotropy to lithospheric deformations, fossil anisotropy in the lithosphere, and flows in the asthenosphere.
... Toward the northeast, the SWS measurements rather align with the spatial distribution of low-velocities beneath the YSRP. Zhu et al. (2020) have distinguished the decisive flow geometries beneath Pacific Northwest in detail. Their findings agree with a fast slab rollback assumption from Zandt and Humphreys (2008), causing toroidal flow. ...
... Their findings agree with a fast slab rollback assumption from Zandt and Humphreys (2008), causing toroidal flow. These flows (below 300 km depths in Zhu et al., 2020) encompass the Gorda slab on one side, while crossing a slab gap further north. In addition, horizontal pressure gradients near the slab gap potentially enable return flows. ...
... Beside upper mantle anisotropy, deeper regions might further contribute to observed Δt values (e.g., Savage, 1999). Indications for transition zone anisotropy (>400 km ) beneath our area of interest, for example, were revealed just recently by Zhu et al. (2020) and Zhang et al. (2021). ...
Shear-wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, as caused by the occurrence of olivine, to constrain the dominant directions of upper mantle deformation. While SK(K)S splitting measurements at individual seismic stations are often averaged before interpretation, it is useful to consider additional information, for example, based on the variation of splitting parameters with azimuth due to the non-vertical incidence of core-phases. These constraints in theory enable a differentiation between various types of olivine and may allow us to infer otherwise poorly known upper mantle parameters such as stress, temperature, and water content. In this study, we predict the azimuthal variation of splitting parameters for A-, C-, and E-type olivine fabrics and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain in the Pacific Northwest US. This helps to constrain the amount of water in the upper mantle in the back-arc of the Cascadia subduction zone, known for its consistent E-W oriented seismic anisotropy, and particularly large splitting delay times. Our investigation renders the C-type olivine mechanism improbable for this location; A- and E-type fabrics match the observations, although differentiating between them is difficult. However, the agreement of the amplitude of backazimuthal variation of the fast orientation, plus the potential to explain large splitting delay times, suggest the occurrence of E-type olivine, and thus the likely presence of a moderately hydrated upper mantle beneath Cascadia's back-arc.
... However, the amplitude of the sensitivity kernels reduces with depth, leaving uncertainty about anisotropy at >400 km depth. Recent full-waveform inversion (FWI) for anisotropic velocities using regional earthquakes suggests that subduction-driven flow beneath the PNW creates anisotropy at transition zone depths (Zhu et al., 2020). The fast orientations from FWI tomography generally agree with SWS, but the depth-integrated magnitude of anisotropy is much smaller than that obtained by SWS. ...
Plain Language Summary
Earth’s mantle convects like a fluid over geological time and it organizes mineral fabrics resulting in directional dependence of seismic velocities, that is seismic anisotropy. There is abundant evidence for flow‐induced seismic anisotropy at depths above about 400 km, but it is less clear if anisotropy is developed in the mantle transition zone at about 400–700 km deep. Here, we use seismic waves generated from the bottom and top of the transition zone to constrain anisotropy within the layer. Localized evidence of strong anisotropy is found beneath the Pacific Northwest near locations where prior imaging studies show gaps between subducted oceanic plate fragments. We propose that focused flow through constrictions like slab gaps may cause seismic anisotropy in the mantle transition zone.
Forest productivity projections remain highly uncertain, notably because underpinning physiological controls are delicate to disentangle. Transient perturbation of global climate by large volcanic eruptions provides a unique opportunity to retrospectively isolate underlying processes. Here, we use a multi‐proxy dataset of tree‐ring records distributed over the Northern Hemisphere to investigate the effect of eruptions on tree growth and photosynthesis and evaluate CMIP6 models. Tree‐ring isotope records denoted a widespread 2–4 years increase of photosynthesis following eruptions, likely as a result of diffuse light fertilization. We found evidence that enhanced photosynthesis transiently drove ring width, but the latter further exhibited a decadal anomaly that evidenced independent growth and photosynthesis responses. CMIP6 simulations reproduced overall tree growth decline but did not capture observed photosynthesis anomaly, its decoupling from tree growth or the climate sensitivities of either processes, highlighting key disconnects that deserve further attention to improve forest productivity projections under climate change.
The lithospheric structure of the contiguous US and surrounding regions offers clues into the tectonic history, including interactions between subducting slabs and cratons. In this paper, we present a new radially anisotropic shear wave speed model of the upper mantle (70–410 km) of the contiguous US and surrounding regions, constrained by seismic full‐waveform inversion. The new model (named CUSRA2021) utilizes frequency‐dependent travel time measurements, from 160 earthquake events recorded by 5,280 stations. The data coverage in eastern US is improved by incorporating more intraplate earthquakes. The final model exhibits clear and detailed shear wave speed anomalies correlating well with tectonic units such as North America Craton (high‐Vs), Cascadia subduction zones (high‐Vs), Columbia Plateau (low‐Vs), Basin and Range (low‐Vs), etc. In particular, the detailed structure of the North America Craton beneath Illinois basin is revealed. The depth of high‐Vs anomaly beneath the North America Craton correlates well with S‐to‐P receiver function and SH reflection results. Besides, the radial anisotropy in the Craton lithosphere shows a layering structure, which may relate to the process of lithospheric accretion and the origin of mid‐lithosphere discontinuities.
To clarify the upper mantle structure and dynamics of the Kamchatka subduction zone, we conduct Rayleigh‐wave phase‐velocity azimuthal anisotropy tomography beneath Kamchatka using amplitude and phase data of teleseismic fundamental mode Rayleigh‐waves at periods of 25–150 s. With the obtained anisotropic phase‐velocity model, a 3‐D azimuthal anisotropic shear‐wave velocity (Vs) model is determined down to ∼300 km depth. Beneath Kamchatka, the subducting Pacific slab is clearly imaged as a dipping high‐Vs zone on the south of the Shiveluch volcano at ∼57°N latitude, whereas this high‐Vs zone is absent on the north of the volcano, suggesting that a slab edge exists in the region. Visible low‐Vs anomalies are revealed around the slab edge. The subducting Pacific slab beneath Kamchatka generally exhibits a fast‐velocity direction (FVD) of ∼NE‐SW in the area south of ∼54°N latitude, whereas the dominant FVD in the slab rotates to ∼N‐S or ∼NNW‐SSE in the area of ∼54°–57°N latitude. The FVDs in the subducting Pacific slab are generally parallel to the Kamchatka trench, which may result from the shape‐preferred orientation of stratified oceanic lithosphere. At shallow depths (<∼150 km), the low‐Vs mantle wedge generally exhibits ∼ NW‐SE FVDs, whereas ∼ E‐W FVDs appear on the north of the slab edge, which form semi‐toroidal FVDs around the slab edge. The semi‐toroidal FVDs may reflect a combination of corner flow in the mantle wedge and toroidal mantle flow around the slab edge beneath Kamchatka, which may deform and heat or even melt the slab near its edge.
