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NW Canada and Alaska are the continuation of the North American Cordillera through Mexico, western USA and western Canada. I show that they have similar thermal regimes and thermal control of tectonics and seismicity. I first summarize the multiple constraints to crust and upper mantle temperatures and then discuss some consequences. There are bimodal crust and upper mantle temperatures characteristic of most subduction zones: cool forearc, uniformly hot backarc (Yukon Composite Terrane to Southern Brooks Range, and Mackenzie Mountains), and stable cratonic backstops (Arctic Alaska Terrane and Canadian Shield). The main constraints are as follows: (a) Heat flow measurements, (b) Temperature‐dependent upper mantle velocities and seismic attenuation, (c) Temperature‐dependent topographic elevations; thermal isostasy, (d) Depth and temperature of the seismic lithosphere‐asthenosphere boundary (LAB), (e) Origin temperature and depth of craton kimberlite xenoliths, (f) Geochemically inferred source temperature and depth of recent volcanic rocks. (g) Depth to the magnetic Curie temperature, (h) Depth extent of seismicity. The backarc lithosphere is thin, LAB at 50–85 km and ∼1,350°C ± 25°C. Moho temperatures at 35 km are 850°C ± 100°C compared to cool cratonic areas of 400°C–500°C. The consequences include the following: (a) Thin and weak backarc lithosphere that accommodates pervasive tectonic deformation indicated by wide‐spread seismicity and GPS‐defined motions, in contrast to the stable cratonic regions; (b) Weak backarc lower crust that flattens the Moho and allows detachment and thrusting of the upper crust over the cold strong Arctic Alaska Terrane and Canadian Shield. This article provides a model for how to estimate deep temperatures from multiple constraints. Plain Language Summary Alaska and NW Canada are the continuation of the North American Cordillera mountain belt through Mexico, western USA and western Canada. I show that they have similar high temperatures in the earth's crust and upper mantle, and that these deep temperatures are the principal control of earthquake occurrence and ongoing mountain belt deformation processes. Hot crustal areas are weak and deform easily, whereas cold strong areas like the Canadian Shield deform very little and have fewer earthquakes. I first outline the constraints to deep earth temperatures in Alaska and NW Canada and then the earthquake and mountain building consequences. The key association in this region and globally is that there are uniformly high crustal temperatures in the weak “backarcs,” the high mountain belts that extend 100 s of kilometers landward of current and recent subduction zones (where ocean crust is being thrust beneath continents generating volcanoes, and great earthquakes). I summarize eight methods that provide estimates of deep temperatures. They give a consistent picture, with deep temperatures in the backarc mountain belts of central Alaska, the Yukon and western Northwest Territories that are double those in the cold strong cratons (shields) of the northern part of arctic Alaska and the Canadian Shield in NW Territories. Earthquakes and mountain building processes follow this deep temperature difference, with most occurring where crustal temperatures are high and the crust weak. Key Points The regional distribution of seismicity and tectonic deformation in Alaska/NW Canada is controlled by crust and upper mantle temperatures Eight principal constraints are presented that give consistent deep temperature estimates; bimodal factor of two, backarc versus craton 300–800 km wide Cordillera backarc is uniformly hot and tectonically active in contrast to the cold stable Arctic Alaska and Canadian Shield

A mantle thermal plume may be tilted, deflected, or even split-up by mantle lateral flows (mantle wind) during its ascent, which in turn changes the spatial distribution of various geological-magmatic responses, such as magmatic activity in the overriding plate and hotspot tracks on the surface, affecting the reliability of the constraints on absolute plate motion history. Previous research on tilted mantle plumes has focused mainly on the lower/whole mantle regions. Whether mantle plumes formed in whole/layered mantle convection suffer lateral tilt in the upper mantle, and how this affects the magmatic activity along the surface hotspot track as well as the plume-related tectonic processes, are important scientific issues in mantle thermal-plume dynamics and plate tectonics theory. This study introduces a thermal Stokes-fluid-dynamics numerical model (in ASPECT software) and pyrolite parameters constrained by mineral physics data, and quantitatively analyzes the tilted/deflected morphology of upper-mantle plumes and the concomitant surface-hotspot location-evolution characteristics under the combined effects of overriding-plate-motion driven flow (Couette) and upper mantle counter-flow (Poiseuille). We find that this composite upper-mantle wind can lead to (1) Plume head-and-upper-conduit horizontal motion in the opposite direction of the overriding plate motion and also with respect to the conduit roots, such that the magmatic spacing is increased; (2) Near-periodic split-up and ascent of a laterally-moving plume conduit, whose split-up/ascent period depends mainly on the thermo-chemical buoyancy of the plume itself; and (3) Under specific conditions of thermo-chemical buoyancy of a main “parent” plume interacting with upper mantle winds, two secondary “child” plumes hundreds of kilometers apart can sprout and ascend sequentially/sub-simultaneously through the upper mantle in a very short period of time (2–4 Myr). The resulting oscillating/jumping behavior of hotspot locations along the overriding plate motion direction can be used to explain the observations on some of Earth’s igneous provinces and hotspot tracks (for example, the Kerguelen hotspot) and related-tectonics, that: (i) younger hotspot-magmatic-tectonic regions can superimpose-to and situate-amidst older ones (surface-hotspot-motion or plume-deflection distances greater than overriding-plate-motion distances, with magmatism separated closely in space but largely in time), and (ii) plume-related magmatism can be widely separated in space but closely in time or age (near-simultaneous ascent of two distant “child” plumes from the same “parent” mantle-plume conduit). Our study suggests that the complex dynamic environment within the upper mantle should be considered when constraining absolute plate motions by the moving-hotspot-reference-frame, especially when these hotspots are located near mid-ocean ridges and/or subduction zones.

Enigmatic discontinuities in the upper mantle beneath Africa, such as the mid‐lithosphere and X‐discontinuities, have prompted various theories regarding their causes. However, most studies rely on S‐to‐P receiver functions, raising concerns about resolution. In this study, we enhance the depth‐resolution of mantle discontinuities using high‐resolution P‐to‐S receiver functions that are free from crustal reverberations. Our analysis reveals the depths and the polarities of both single and multiple discontinuities. The most prevalent observation is an unpaired (i.e., no high velocity discontinuity beneath or above) low‐velocity discontinuity at depths between 50 and 100 km, which shows minimal secular variation. Additionally, paired discontinuities are detected either within or below the heterosphere. Comparisons with thermal, chemical and geophysical models that predict rapid seismic velocity gradients due to partial‐melting (induced by volatiles or sub‐solidus grain‐boundary mechanisms) suggest that the common sharp velocity drop is most likely caused by a sub‐solidus mechanism, while the paired discontinuities, including a newly resolved high‐velocity discontinuity, may indicate the base of a melt layer. This mantle stratification might be linked to magmatism or remnants of past continental assembly.

We use receiver functions calculated for data collected by the INDEPTH‐IV seismic array to image the three‐dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70 km in northern Tibet. In addition, we observe a 20 km Moho offset beneath the northern margin of the Kunlun Mountains, a 10 km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong‐Nujiang Suture at 80 km depth which dips ∼10° to the north, reaching ∼120 km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20 km depression of the 660‐km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10 km depression of the 410‐km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small‐scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau. Key Points We obtained a 2D Moho depth map view of the NE Tibet plateau We observed a preserved Lhasa block lithosphere slab in the upper mantle The deflection of the 660 km and 410 km interface supports a subducting model

The fluctuations based on the “phase space electron hole” in the Venusian upper mantle boundary due to the Parker Solar Probe (PSP) and Pioneer Venus Orbiter (PVO) observations are detected. This study examines the characteristics of ion-acoustic solitary waves and double-layer structures. The influence of various plasma parameters on the wave properties is investigated to gain a better understanding of the formation and propagation of these non-linear structures. The Schamel-KdV equation is derived using a hydrodynamic description, accounting for a small percentage of non-isothermal trapped electrons. A parametric investigation was then conducted, utilizing spacecraft observations of the plasma configuration at Venus, including observed data of densities, temperatures, and velocities. Solitary waves and double-layer structures can exist in two different modes: slow and fast. In order to identify which structure is most closely matched with PVO observation for Doppler-shifted frequencies of 5.4 kHz and an electric field of 15 mV/m, we examined these structures in the context of available data. It is found that our results of the ion-acoustic wave in fast mode are in agreement with the mission data from PSP and PVO.

The geometry and deformation of the Indian continental mantle lithosphere (ICML) beneath the India-Eurasia collision zone are critical to understanding the accommodation of continent-continent convergence. In this paper, the distribution of residual gravity anomalies in the upper mantle of southern Tibet is estimated using the gravity data and seismic velocity models, and the heterogeneous density distribution of the upper-mantle is then recovered through three-dimensional gravity inversion. The results reveal a low-density anomaly (~300 km W-E and ~100 km N-S) in the upper mantle under the eastern Himalaya, while there is no obvious density anomaly under the western Himalaya. The western boundary of the low-density anomaly coincides with the Yadong-Gulu Rift (YGR) on the surface (89°-90°E), and its southern boundary is located at ~28°N, approximately 130 km southward from the Indus-Yarlung suture, probably representing the mantle suture at depth. This observation indicates that, in contrast to the western ICML which is probably underthrusting at a shallow angle, the eastern ICML be likely subducting steeply, accompanying asthenosphere upwelling. Such a laterally varying geometry suggests that a major tearing of the ICML may have taken place from the intersection of the mantle suture and the YGR in the upper mantle. The tearing and the steep subduction of the ICML might be associated with the magmatic and mineralization events in the eastern Himalaya-Gangdese and the formation of the YGR.

Transient creep in olivine aggregates has been studied by stress‐relaxation experiments at pressures of 1.7–3.6 GPa and at temperatures of ≤1020 K in a DIA apparatus. Time‐dependent deformation of olivine at small strains (<0.07) was monitored with an ∼1 s of time resolution using a combination of a high‐flux synchrotron X‐ray and a cadmium telluride imaging detector. The observed deformation was found to follow the Burgers creep function with the transient relaxation time ranging from 50 (±20) to 1,880 (±750) s. We show that the Burgers creep for olivine cannot account for the low viscosities in early post‐seismic deformation reported by geodetic observations (<7 × 1017 Pa·s). In contrast, the time‐dependent increase in viscosity observed in late post‐seismic deformation (1018−1020 Pa·s) is explained by the Burgers rheology, suggesting that the combination of the Burgers model and another model is needed for the interpretation of post‐seismic deformation. Plain Language Summary Geodetic observations reveal that the deformation of the crust and upper mantle after a great earthquake continues for decades. Viscosities of the upper mantle estimated from early post‐seismic deformation are often significantly low (1017−1018 Pa·s) and continuously increase to a typical value (∼1020 Pa·s). This characteristic of post‐seismic mantle flow cannot be explained by partial melting (nor water weakening) of upper mantle rocks. Here we performed small‐strain deformation experiments on natural olivine, which is the major mineral in the upper mantle, via a state‐of‐the‐art technology large‐volume deformation apparatus combined with high‐flux synchrotron X‐ray observations. We have successfully shown that the reported time‐dependent crustal deformation, which continues for decades after a great earthquake, is explained by the transient creep of olivine. Key Points In situ deformation experiments on olivine were performed using a high‐flux synchrotron X‐ray and a cadmium telluride imaging detector Transient creep of olivine aggregates follows the Burgers creep function at upper mantle pressures and temperatures Time‐dependent rheology of the shallow mantle observed in the late post‐seismic deformation is explained by the Burgers model

We present new 3‐D tomographic models of VP, VS and VP/VSratio anomalies in the upper mantle beneath EC and adjacent areas. This data was collected and interpreted with the goal of clarifying geodynamic processes that caused spatially variable event histories throughout Eastern China (EC) during Mesozoic to Cenozoic time. The tomographic images were constructed by inverting body wave travel‐times recorded at ∼1300 stations within the upgraded China National Seismic Network, and 9 temporary arrays. Resolution tests for different depths and the featured velocity anomalies verify that the tomographic images capture the velocity heterogeneities in the upper mantle to depths of 700 km. The salient features of VP, VS and VP/VSratio anomalies can be clearly identified. These include strong multiscale heterogeneities occupying the upper mantle beneath EC and differences in the spatial scale of anomalies found beneath northern and southern areas of EC. These features demonstrate a degree of spatial variability in the geodynamic evolution of EC. We propose two mechanisms to explain these patterns. First, the western front of the subducted slab may have imparted greater horizontal compressional stress in areas where it impinged further eastward into EC. These areas would experience stronger convection and an altered stress regime in the upper mantle, creating significant thermal anomalies beneath the South China Block (SCB) relative to the eastern North China Craton (NCC). Second, differing thermal states and viscosities for the eastern NCC and the Cathaysia Block (CaB) resulted in differing responses to regional deformation. The Archean hinterland of the eastern NCC specifically has a colder thermal state and higher viscosity, and therefore exhibits only small‐scale heterogeneities due to the effect of shear localization. The Neoproterozoic CaB has a relatively warm thermal state with lower viscosity, and thus deformed more continuously. Key Points First tomographic models of Vp, Vs, Vp/Vs in upper mantle beneath east China Strong multi‐scale heterogeneities occupy the upper mantle in eastern China Strong contrast in spatial scale of anomalies from north to south in east China

Phase relations of magnesioferrite (MgFe2O4) have been studied between 8 and 18 GPa and 1000-1600 °C using multi-anvil experiments. At 8-10 GPa and 900-1200 °C, MgFe2O4 breaks down to Fe2O3+MgO. At higher temperatures, a new phase appears along with Fe2O3 Although this new phase is unquenchable, EPMA and TEM data point to a composition with Mg5Fe2O8 or Mg4Fe2O7 stoichiometry. Depending on pressure and temperature, other stoichiometries also appear to be stable together with Fe2O3 In terms of pressure, the stability field of the unquenchable phases + hematite widens with increasing temperature to 3 ± 1 GPa at ∼1400 °C, and then narrows to ∼1 GPa at 1600 °C. The recoverable assemblage of Mg2Fe2O5+Fe2O3 becomes stable between 11-13 GPa. The Mg2Fe2O5+Fe2O3 assemblage is stable up to at least 18 GPa at 1300 °C without any evidence of a hp-MgFe2O4 phase. In addition, hematite plays an important role in the phase relations of MgFe2O4 by being present over a wide range in pressure and temperature together with a Mg-rich Fe-oxide. Interestingly, hematite incorporates variable amounts of Mg whereby its concentration appears to be a function of temperature. This experimental study has implications for interpreting inclusions in natural diamonds where magnesioferrite occurs by placing a maximum pressure stability on the formation of this phase. Through these inclusions, it also provides constraints on diamond formation and their subsequent evolution prior to eruption. For example, the occasional observation of nano-sized magnesioferrite within (Mg,Fe)O inclusions must have either formed from a high-pressure precursor phase with a different stoichiometry at transition zone or upper lower mantle conditions, or it exsolved directly from the host (Mg,Fe)O under upper mantle conditions (i.e., <9-10 GPa). Since several studies report various non-silicate inclusions with simple oxide compositions, including magnesioferrite, magnetite, or ferropericlase, such inclusions provide evidence for variable redox conditions at the time of entrapment.

Recently, the continually increasing availability of seismic data has allowed high‐resolution imaging of lithospheric structure beneath the African cratons. In this study, S‐wave seismic tomography is combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure is employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S‐wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons. Plain Language Summary Cratons are the ancient cores of continents that, with few exceptions, are underlain by a cold, strong lithospheric root with a thickness of about 250 km. The physical properties of lithospheric roots, principally temperature and composition, shed light on the origin and evolution of the most ancient portions of the Earth's lithosphere, the Precambrian cratons. We use an iterative method to process S‐wave seismic tomography and satellite gravity data to calculate the thermal and compositional state of the lithosphere. Our results reveal great diversity in the thickness and physical properties of the African lithosphere. The West African, northern Congo, and Zimbabwe cratons are underlain by relatively cold, thick and chemically depleted lithosphere. In contrast, the Uganda, Tanzania, southern Congo, and Kaapvaal cratons are warmer, thinner and have a less depleted (or non‐depleted) composition, indicating either refertilization (metasomatism) or formation in a non‐depleted state. These results document the formation of the Africa continent during the past 3.7 Ga from a diverse collection of cratons, each with a unique evolutionary history. Key Points A new Moho map was constructed from available seismic data to improve thermo‐compositional modeling of the African cratonic lithosphere Lithosphere of the West African, central to northern Congo, and Zimbabwe cratons is cold, up to 250 km thick, and chemically depleted Hot, thin (<200 km) and mostly undepleted lithosphere of Uganda, Tanzania, southern Congo, and Kaapvaal cratons indicates refertilization