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Abstract Understanding the temporal variability of plate tectonics is key to unraveling how mantle convection transports heat, and one critical factor for the formation and evolution of plate boundaries is rheological “memory,” that is, the persistence of weak zones. Here, we analyze the impact of such damage memory in global, oceanic‐lithosphere‐only models of visco‐plastic mantle convection. Self‐consistently‐formed weak zones are found to be reactivated in distinct ways, and convection preferentially selects such damaged zones for new plate boundaries. Reactivation of damage zones increases the frequency of plate reorganizations, and hence reduces the dominant periods of surface heat loss. The inheritance of distributed lithospheric damage thus dominates global surface dynamics over any local boundary stabilizing effects of weakening. In nature, progressive generation of weak zones may thus counteract and perhaps overcome any effects of reduced convective vigor throughout planetary cooling, with implications for the frequency of orogeny and convective transport throughout Wilson cycles. 

Abstract The style of convective force transmission to plates and strain‐localization within and underneath plate boundaries remain debated. To address some of the related issues, we analyze a range of deformation indicators in southern California from the surface to the asthenosphere. Present‐day surface strain rates can be inferred from geodesy. At seismogenic crustal depths, stress can be inferred from focal mechanisms and splitting of shear waves from local earthquakes via crack‐dependent seismic velocities. At greater depths, constraints on rock fabrics are obtained from receiver function anisotropy,P_nandPtomography, surface wave tomography, and splitting ofSKSand other teleseismic core phases. We construct a synthesis of deformation‐related observations focusing on quantitative comparisons of deformation style. We find consistency with roughly N‐S compression and E‐W extension near the surface and in the asthenospheric mantle. However, all lithospheric anisotropy indicators show deviations from this pattern.P_nfast axes and dipping foliations from receiver functions are fault‐parallel with no localization to fault traces and match post‐Farallon block rotations in the Western Transverse Ranges. Local shear wave splitting orientations deviate from the stress orientations inferred from focal mechanisms in significant portions of the area. We interpret these observations as an indication that lithospheric fabric, developed during Farallon subduction and subsequent extension, has not been completely reset by present‐day transform motion and may influence the current deformation behavior. This provides a new perspective on the timescales of deformation memory and lithosphere‐asthenosphere interactions. 

We estimate seismic azimuthal anisotropy for the Juan de Fuca ‐ Gorda plates from inversion of a new 10–80 s period Rayleigh wave dataset, resulting in a two‐layer model to 80 km depth. In the lithosphere, most anisotropy patterns reflect the kinematics of plate formation, as approximated from seafloor‐age‐based paleo‐spreading, except for regions close to propagator wakes and near plate boundaries. In the asthenosphere, the fast propagation orientations align with convective shear as inferred from the NUVEL1A plate motion model, which is indicative of a ∼3 Myr average, rather than with the more recent, ∼0.8 Myr, motions inferred from MORVEL. Regional anisotropy of this young plate system thus records convection like older plates such as the Pacific. On smaller scales, anisotropy imaging provides insights into dynamics of plate generation and can further elucidate plate reorganizations and changes in boundary loading. 

o what extent mechanical anisotropy is required to explain the dynamics of the lithosphere is an important yet unresolved question. If anisotropy affects stress and deformation, and hence processes such as fault loading, how can we quantify its role from observations? Here, we derive analytical solutions and build a theoretical framework to explore how a shear zone with linear anisotropic viscosity can lead to deviatoric stress heterogeneity, strain-rate enhancement, as well as non-coaxial principal stress and strain rate. We develop an open-source finite-element software based on FEniCS for more complicated scenarios in both 2-D and 3-D. Mechanics of shear zones with transversely isotropic and orthorhombic anisotropy subjected to misoriented shortening and simple shearing are explored. A simple regional example for potential non-coaxiality for the Leech River Schist above the Cascadia subduction zone is presented. Our findings and these tools may help to better understand, detect and evaluate mechanical anisotropy in natural settings, with potential implications including the transfer of lithospheric stress and deformation through fault loading. 

The asthenosphere plays a fundamental role in present-day plate tectonics as its low viscosity controls how convection in the mantle below it is expressed at the Earth’s surface above. The origin of the asthenosphere, including the role of partial melting in reducing its viscosity and facilitating deformation, remains unclear. Here we analysed receiver-function data from globally distributed seismic stations to image the lower reaches of the asthenospheric low-seismic-velocity zone. We present globally widespread evidence for a positive seismic-velocity gradient at depths of ~150 km, which represents the base of a particularly low-velocity zone within the asthenosphere. This boundary is most commonly detected in regions with elevated upper-mantle temperatures and is best modelled as the base of a partially molten layer. The presence of the boundary showed no correlation with radial seismic anisotropy, which represents accumulated mantle strain, indicating that the inferred partial melt has no substantial effect on the large-scale viscosity of the asthenosphere. These results imply the presence of a globally extensive, partially molten zone embedded within the asthenosphere, but that low asthenospheric viscosity is controlled primarily by gradual pressure and temperature variations with depth. 

SUMMARY EarthScope's USArray seismic component provided unprecedented coverage of the contiguous United States and has therefore spurred significant advances in tomographic imaging and geodynamic modelling. Here, we present a new global, radially anisotropic shear wave velocity tomography model to investigate upper mantle structure and North American Plate dynamics, with a focus on the contiguous United States. The model uses a data-adaptive mesh and traveltimes of both surface waves and body waves to constrain structure in the crust and mantle in order to arrive at a more consistent representation of the subsurface compared to what is provided by existing models. The resulting model is broadly consistent with previous global models at the largest scales, but there are substantial differences under the contiguous United States where we can achieve higher resolution. On these regional scales, the new model contains short wavelength anomalies consistent with regional models derived from USArray data alone. We use the model to explore the geometry of the subducting Farallon Slab, the presence of upper mantle high velocity anomalies, low velocity zones in the central and eastern United States and evaluate models of dynamic topography in the Cordillera. Our models indicate a single, shallowly dipping, discontinuous slab associated with the Farallon Plate, but there are remaining imaging challenges. Inferring dynamic topography from the new model captures both the long-wavelength anomalies common in global models and the short-wavelength anomalies apparent in regional models. Our model thus bridges the gap between high-resolution regional models within the proper uppermost mantle context provided by global models, which is crucial for understanding many of the fundamental questions in continental dynamics. 

Abstract Azimuthal variations in receiver function conversions can image lithospheric structural contrasts and anisotropic fabrics that together compose tectonic grain. We apply this method to data from EarthScope Transportable Array in Alaska and additional stations across the northern Cordillera. The best-resolved quantities are the strike and depth of dipping fabric contrasts or interfaces. We find a strong geographic gradient in such anomalies, with large amplitudes extending inboard from the present-day subduction margin, the Aleutian arc, and an influence of flat-slab subduction of the Yakutat microplate north of the Denali fault. An east–west band across interior Alaska shows low-amplitude crustal anomalies. Anomaly amplitudes correlate with structural intensity (density of aligned geological elements), but are the highest in areas of strong Cenozoic deformation, raising the question of an influence of current stress state. Imaged subsurface strikes show alignment with surface structures. We see concentric strikes around arc volcanoes implying dipping magmatic structures and fabric into the middle crust. Regions with present-day weaker deformation show lower anomaly amplitudes but structurally aligned strikes, suggesting pre-Cenozoic fabrics may have been overprinted or otherwise modified. We observe general coherence of the signal across the brittle-plastic transition. Imaged crustal fabrics are aligned with major faults and shear zones, whereas intrafault blocks show imaged strikes both parallel to and at high angles to major block-bounding faults. High-angle strikes are subparallel to neotectonic deformation, seismicity, fault lineaments, and prominent metallogenic belts, possibly due to overprinting and/or co-evolution with fault-parallel fabrics. We suggest that the underlying tectonic grain in the northern Cordillera is broadly distributed rather than strongly localized. Receiver functions thus reveal key information about the nature and continuity of tectonic fabrics at depth and can provide unique insights into the deformation history and distribution of regional strain in complex orogenic belts.