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Although an average westward rotation of the Earth's lithosphere is indicated by global analyses of surface features tied to the deep mantle (e.g., hot spot tracks), the rate of lithospheric drift is uncertain despite its importance to global geodynamics. We use a global viscous flow model to predict asthenospheric anisotropy computed from linear combinations of mantle flow fields driven by relative plate motions, mantle density heterogeneity, and westward lithosphere rotation. By comparing predictions of lattice preferred orientation to asthenospheric anisotropy in oceanic regions inferred from SKS splitting observations and surface wave tomography, we constrain absolute upper mantle viscosity (to 0.5–1.0 × 1021 Pa s, consistent with other constraints) simultaneously with net rotation rate and the decrease in the viscosity of the asthenosphere relative to that of the upper mantle. For an asthenosphere 10 times less viscous than the upper mantle, we find that global net rotation must be <0.26°/Myr (<60% of net rotation in the HS3 (Pacific hot spot) reference frame); larger viscosity drops amplify asthenospheric shear associated with net rotation and thus require slower net rotation to fit observed anisotropy. The magnitude of westward net rotation is consistent with lithospheric drift relative to Indo‐Atlantic hot spots but is slower than drift in the Pacific hot spot frame (HS3 ≈ 0.44°/Myr). The latter may instead express net rotation relative to the deep mantle beneath the Pacific plate, which is moving rapidly eastward in our models.

Individual sinking slabs present markedly different geometries between 410 and 660 km depths, from vertical slabs penetrating the lower mantle to slabs stagnating above the lower mantle. The proposed factors determining these contrasted geometries include mantle viscosity and the magnitude and evolution of trench retreat. Here, we assess the success of paleo‐geographically driven global mantle flow models in matching slabs in tomographic models between 400 km and 1,000 km depth. We quantify the spatial match between predicted present‐day mantle temperature anomalies and vote maps of tomographic models. We investigate the sensitivity of the spatial match to input parameters of the mantle flow model: imposed tectonic reconstruction, model start age, and viscosity contrast between the upper and lower mantle. We evaluate the visual match between model slabs and tomographic vote maps for three circum‐Pacific regions with contrasted slab dip angles between 400 km and 1,000 km depth. Predicted model slabs better match slabs inferred from tomography when there is an increase in viscosity at 660 km depth. The temporal evolution of the models and the global match at present day suggest that the subduction history could be refined in the global tectonic reconstructions that we considered. For example, we suggest that the subduction to the east of Japan should be offset by approximately 100 km to the west at ∼80 Ma to match the anchoring of a continuous slab into the lower mantle suggested by tomography. Plain Language Summary Oceanic lithosphere is recycled as sinking slabs into Earth's mantle, and the analysis of global earthquake data compiled in tomographic models has revealed that the dip angle of slabs varies between regions. In this manuscript, we analyze the geometry of mantle slabs predicted by forward models of past global mantle flow that follow imposed surface tectonic motions. We present some of the first quantifications of the spatial match between the present‐day mantle temperature predicted by reconstructions of past global mantle flow and that imaged by global tomographic models. Few studies have quantified this match and clearly exposed it spatially. We quantify the match between the upper mantle slabs predicted by these models and those inferred from a series of tomographic models. We show that the models successfully reproduce the steeply dipping Mariana slab and stagnating Western Pacific slab under Japan and the intermediate geometry of the Farallon and Nazca slabs under South America. We find that trench retreat is the main driver of the geometry of these slabs. Our results suggest that the geometry of slabs can be used to refine global tectonic reconstructions and to calibrate the viscosity contrast between the upper and lower mantle in global mantle flow models. Key Points We quantitatively compare slab locations in mantle flow and tomographic models Trench retreat and mantle viscosity influence the location of sinking slabs Our comparison indicates where global tectonic reconstructions could be improved

It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (> 1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high ³He/⁴He (up to 8.9RA) in low-temperature (≤50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source ³He/⁴He > 10.3RA (and potentially up to 26RA, similar to Galápagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1RA). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high ³He/⁴He values in central Panama are likely derived from the infiltration of a Galápagos plume–like mantle through a slab window that opened ∼8 Mya. Two potential transport mechanisms can explain the connection between the Galápagos plume and the slab window: 1) sublithospheric transport of Galápagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galápagos plume material blown by a “mantle wind” toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galápagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits.

Subduction zones are fundamental features of Earth's mantle convection and plate tectonics, but mantle flow and pressure around slabs are poorly understood because of the lack of direct observational constraints on subsurface flow. To characterize the linkages between slabs and mantle flow, we integrate high‐resolution representations of Earth's lithosphere and slabs into a suite of global mantle convection models to produce physically plausible present‐day flow fields for Earth's mantle. We find that subduction zones containing wide, thick, and long slabs dominate regional mantle flow in the neighboring regions and this flow conforms to patterns predicted by simpler regional subduction models. These subduction zones, such as Kuril‐Japan‐Izu‐Bonin‐Mariana, feature prismatic poloidal flow coupled to the downgoing slab that rotates toward toroidal slab‐parallel flow near the slab edge. However, other subduction zones, such as Sumatra, deviate from this pattern because of the competing influence of other slabs or longer‐wavelength mantle flow, showing that upper mantle flow can link separate subduction zones and how flow at subduction zones is influenced by broader scale mantle flow. We find that the non‐linear dislocation creep reduces the coupling between slab motion and asthenospheric flow and increases the occurrence of non‐ideal flow, in line with inferences derived from seismological constraints on mantle anisotropy. Key Points We embed detailed representations of Earth's subduction zones within global convection models to predict mantle flow at subduction zones Some subduction zones conform to ideal models, with upper mantle flow coupled to downgoing slabs, while others deviate from this pattern Dislocation creep partially decouples upper mantle flow from slab motion, increasing the fraction of non‐ideal flow

Upper mantle anisotropy has been mapped beneath continents at high spatial resolution. Beneath the oceans, however, shear wave splitting constraints on upper mantle anisotropy are sparse, due to the paucity of seismic receivers. A technique that does not require the availability of seismic stations close to the region under study is differential PS‐SKS splitting. Here, we use global wavefield simulations to investigate circumstances under which PS‐SKS splitting can be applied, and then use this technique to measure upper mantle anisotropy beneath the Pacific Ocean basin. Our results demonstrate that upper mantle anisotropy in our study region mostly reflects shearing due to the Pacific plate. North of Fiji, we observe a rotation of fast polarization directions, away from the direction of absolute plate motion of the Pacific plate. This may reflect far‐field mantle flow effects associated with the subduction of the Australian plate beneath the Pacific. Plain Language Summary Earthquakes cause seismic waves whose speeds sometimes depend on their polarization and propagation direction. This material property, called seismic anisotropy, can be used to infer the direction of flow in Earth's upper mantle. Seismic anisotropy is straightforward to measure directly beneath a seismic station, but harder to study if station coverage is sparse. We use a technique that allows us to infer upper mantle seismic anisotropy beneath the Pacific Ocean in places without nearby seismic stations. Our measurements show that while seismic anisotropy varies laterally beneath the Pacific Ocean, in most cases it can be explained by the movement of the Pacific tectonic plate, leading to horizontal shearing of the underlying mantle. North of Fiji, we can observe the effects that the subduction of the Australian beneath the Pacific tectonic plate has on upper mantle flow. Key Points We test the robustness of differential PS‐SKS shear‐wave splitting measurements to characterize anisotropy near the PS bounce point We use this technique infer seismic anisotropy beneath the Pacific Ocean A majority of our measurements can be explained by plate motion induced shearing beneath the Pacific plate

The crystallographic preferred orientation (CPO) of polycrystalline olivine affects both the viscous and seismic anisotropy of Earth's upper mantle with wide geodynamical implications. In this methods paper, we present a continuous field formulation of the popular directors method for modeling the strain‐induced evolution of olivine CPOs, assuming the activation of a single preferred crystal slip system. The formulation reduces the problem of CPO evolution to a linear matrix problem that can easily be integrated alongside large‐scale geodynamical flow models, and conveniently minimizes the degrees of freedom necessary to represent CPO fields. We validate the CPO model against existing deformation experiments and naturally deformed samples, as well as the popular discrete grain model D‐Rex. A numerical model of viscoplastic thermal convection is built to illustrate how flow and CPO evolution may be two‐way coupled, suggesting that CPO‐induced viscous anisotropy does not necessarily strongly affect convection time scales, boundary (lid) stresses, and seismic anisotropy, compared to isotropic viscoplastic rheologies. As a consequence, geodynamical modeling that relies on an isotropic rheology (one‐way coupling) might suffice for predicting seismic anisotropy under some circumstances. Finally, we discuss limitations and shortcomings of our method, such as representing D‐ and E‐type fabrics or modeling flows with mixed fabric types, and potential improvements such as accounting for the effect of dynamic recrystallization. Plain Language Summary The orientation of olivine crystals in Earth's upper mantle can affect how seismic waves and mantle viscosity depend on the direction of propagation and stress, respectively, with implications for interpreting seismic data, the interactions between plate motions and the sublithospheric mantle, and the circulation around subducting plates, among other things. In this methods paper, we re‐write the popular directors method as a continuous field problem, allowing for efficient and seamless prediction of how grain orientations co‐evolve with large‐scale mantle flow. We build an idealized model of thermal convection to illustrate the effect of letting flow and grain orientations co‐evolve compared to decoupled models where grain orientations do not affect flow. Our results suggest that, for certain cases, decoupled models might suffice for understanding for example, large‐scale seismic data. Although our methodology has several shortcomings, such as the neglect of recrystallization, it presents a new path forward for understanding how grain orientations can affect flow, and vice–versa, generally thought to be important for understanding seismic signatures and mantle–plate dynamics. Key Points The directors method is posed as a continuous field problem for large‐scale modeling of crystallographic preferred orientation (CPO)‐induced viscous and seismic anisotropies A model of thermal convection demonstrates that seismic anisotropy and boundary stresses are affected when flow and CPO are two‐way coupled Challenges are discussed for representing D‐ and E‐type fabrics, dynamic recrystallization, and non‐uniform fabric types

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.

Seismic anisotropy provides essential constraints on mantle dynamics and continental evolution. One particular question concerns the depth distribution and coherence of azimuthal anisotropy, which is key for understanding force transmission between the lithosphere and asthenosphere. Here, we reevaluate the degree of coherence between the predicted shear wave splitting derived from tomographic models of azimuthal anisotropy and that from actual observations of splitting. Significant differences between the two types of models have been reported, and such discrepancies may be due to differences in averaging properties or due to approximations used in previous comparisons. We find that elaborate, full waveform methods to estimate splitting from tomography yield generally similar results to the more common, simplified approaches. This validates previous comparisons and structural inversions. However, full waveform methods may be required for regional studies, and they allow exploiting the back‐azimuthal variations in splitting that are expected for depth‐variable anisotropy. Applying our analysis to a global set ofSKSsplitting measurements and two recent surface wave models of upper‐mantle azimuthal anisotropy, we show that the measures of anisotropy inferred from the two types of data are in substantial agreement. Provided that the splitting data is spatially averaged (so as to bring it to the scale of long‐wavelength tomographic models and reduce spatial aliasing), observed and tomography‐predicted delay times are significantly correlated, and global angular misfits between predicted and actual splits are relatively low. Regional anisotropy complexity notwithstanding, our findings imply that splitting and tomography yield a consistent signal that can be used for geodynamic interpretation. Key Points Simplified approaches of predicting splitting from tomography work The fit between actual and predicted splitting is very good, globally Azimuthal anisotropy is consistently mapped and related to mantle flow

Seismic anisotropy in the upper mantle reveals geodynamic processes and the tectonic evolution of the Earth. The two most powerful methods, surface wave tomography, and shear‐wave splitting observations, cannot investigate the deep local anisotropy with good vertical and lateral resolution, resulting in poor constraints on plate deformation processes of the complex plate boundary beneath the Southern California region. Here, we show that the amplitude ratio of translational displacement and rotation makes it possible to retrieve the local anisotropy in the upper mantle. Azimuthal anisotropy in the asthenosphere is well determined and resolved in lateral and vertical directions. The fast axis retrieved from amplitude observations indicates the local rapid changes in plate deformation and complex pattern of mantle flow, which is compatible with the distributions of horizontal mantle flow illuminated by geodetic measurements, providing new insights on geodynamic processes of the Southern California region. Plain Language Summary Rotational motion is the angle of ground rotation observed during Earth's deformation, and the ratio of amplitude to translational motion is sensitive to local structure. In the past few decades, study on the mantle structure inside the Earth has mainly relied on the time difference of seismic waves to calculate azimuth‐dependent velocity changes, namely azimuthal anisotropy. Due to the correlation between the direction of maximum velocity propagation and the direction of mantle flow and plate deformation, the study of mantle anisotropy can provide evidence for the evolution of the Earth. However, studying anisotropy based on seismic wave travel time is often affected by heterogeneity, especially in extremely complex structures such as Southern California. The splitting of shear waves can effectively constrain the anisotropy of the mantle in the lateral direction, but its depth resolution is poor. Additional rotational amplitude observations with local sensitivity and depth resolution can provide better constraints on the study of mantle anisotropy, providing new evidence for the direction of mantle flow and plate motion. Key Points Local seismic anisotropy revealed for the first time from rotational amplitude observations Depth dependence of anisotropy in the local upper mantle of Southern California region is well resolved in lateral and vertical directions The asthenospheric fast axis matches absolute plate motions, providing new insights on geodynamic processes in Southern California

Seismic tomography of Earth's mantle images abundant slab remnants, often located in close proximity to active subduction systems. The impact of such remnants on the dynamics of subduction remains underexplored. Here, we use simulations of multi‐material free subduction in a 3‐D spherical shell geometry to examine the interaction between visco‐plastic slabs and remnants that are positioned above, within and below the mantle transition zone. Depending on their size, negatively buoyant remnants can set up mantle flow of similar strength and length scales as that due to active subduction. As such, we find that remnants located within a few hundred km from a slab tip can locally enhance sinking by up to a factor 2. Remnant location influences trench motion: the trench advances toward a remnant positioned in the mantle wedge region, whereas remnants in the sub‐slab region enhance trench retreat. These motions aid in rotating the subducting slab and remnant toward each other, reducing the distance between them, and further enhancing the positive interaction of their mantle flow fields. In this process, the trench develops along‐strike variations in shape that are dependent on the remnant's location. Slab‐remnant interactions may explain the poor correlation between subducting plate velocities and subducting plate age found in recent plate tectonic reconstructions. Our results imply that slab‐remnant interactions affect the evolution of subducting slabs and trench geometry. Remnant‐induced downwelling may also anchor and sustain subduction systems, facilitate subduction initiation, and contribute to plate reorganization events. Plain Language Summary Subduction, the process where cold oceanic lithosphere descends into the mantle, is a time‐dependent process: old subduction zones cease while new subduction zones initiate, in cycles of tectonic plate motions. The cessation of subduction is accompanied by break‐off of the subducting slab from the surface plate, forming a slab remnant. The remnant continues sinking into the mantle and, in doing so, generates a flow field that may influence adjacent subduction systems. In this study, we present numerical simulations of subduction in a 3‐D spherical shell domain, and examine how subduction systems interact with a range of slab remnants. Our models show that sinking remnants can significantly enhance the sinking velocity of slabs within a few 100–1,000 km of the remnants, and can influence the spatial and temporal evolution of trench shape. Our results suggest that the existence of slab remnants may help to anchor and sustain subduction systems, and lead to an environment more favorable for the initiation of new subduction zones. Since such events are closely linked to reorganizations in global plate motions, we suggest that the location of pre‐existing remnants influences tectonic plate movements and, potentially, super continent cycles. Key Points Subducted slab remnants can enhance the sinking velocities of actively subducting plates by up to a factor 2 Slab remnants strongly influence trench motions and the evolution of trench shape at subduction zones located within a few 100–1,000 s of km The flow fields interact such that the slab tip and remnant approach, thus strengthening mantle flow that can anchor subduction location