Viscous Mantle Research Articles

Convective Heat Transfer in the Strongly Viscous Mantle in Different Aspect Ratio Cells

Mantle convection, a fundamental mechanism controlling the dynamics of the Earth’s surface and interior, shows different behaviors caused by different factors such as viscosity variation, viscous dissipation, internal heating, and so on. In this paper, the effects of temperature-dependent viscosity, temperature and pressure-dependent viscosity and viscous dissipation on mantle convection are investigated in elongated and narrow cells. The Rayleigh-B´enard convection model is solved numerically with the full form of the Arrhenius viscosity function at a high Rayleigh number for viscosity contrasts up to 1030. The root mean square velocity and Nusselt number are computed and tabulated. The thermal characteristics and flow dynamics inside the convection cell are presented by temperature profiles and stream function contours. These simulated results indicate that increasing viscosity contrasts with the incorporation of viscous dissipation weakens the convection vigour and heat transfer in the mantle. The selected narrow cell remains stable for a very high viscosity contrast at different viscous pressure number μ, whereas the selected elongated cell with temperature-dependent viscosity and strong viscous dissipation becomes unstable and single-cell pattern breaks down at high viscosity variation. J. Bangladesh Math. Soc. 44.2 (2024) 077–096

New constraints to subduction zone arc and backarc mantle temperatures: a test of the corner flow model

The heat source for the hot volcanic arc of subduction zones and the uniformly warm backarc is not well understood, particularly where backarc extension is absent. In the widely studied corner flow model, the traction of the underthrusting plate drags down the overlying viscous mantle, and the induced shallower seaward flow brings in heat from the landward and deeper asthenosphere. However, earlier comparisons with observations, especially backarc heat flow, gave poor agreement. There are several new constraints to the temperatures and structures above the underthrusting plate and in the backarc, especially those in western North America, for comparison with model predictions. Seismic receiver functions map horizontal boundaries, including the lithosphere-asthenosphere boundary (LAB). Seismic shear wave tomography gives mantle velocity-depth that provides an approximate proxy for temperature. Attenuation of seismic wave speed indicates high temperature or partial melting. The presence of recent backarc volcanics and their geochemistry give the depth and temperature of the LAB. From these constraints in western North America, the LAB reflects ponding of partial melt, is consistently at a depth of 65±3 km and temperature of 1350±25°C, and overlies nearly constant temperature (adiabatic) asthenosphere across the backarc from Mexico to Alaska. These depth and temperature estimates are greatly different from corner flow model predictions. An alternative hypothesis more consistent with these constraints is vigorous small-scale convection in the backarc asthenosphere. Whether or how this convection coexists with the large-scale flow patterns inferred from the interpretation of seismic anisotropy analyses, and to what degree it prevails in the arc-forearc region, deserve further research.

Vertical Crustal Deformation Due To Viscoelastic Earthquake Cycles at Subduction Zones: Implications for Nankai and Cascadia

AbstractDespite significant progress in studying subduction earthquake cycles, the vertical deformation is still not well understood. Here, we use a generic viscoelastic earthquake‐cycle model that has recently been validated by horizontal observations to explore the dynamics of vertical earthquake‐cycle deformation. Conditioned on two dimensionless parameters (i.e., the ratio of earthquake recurrence interval T to mantle Maxwell relaxation time tM (T/tM) and the ratio of downdip seismogenic depth D to elastic upper‐plate thickness Hc (D/Hc)), the modeled viscoelastic deformation exhibits significant spatiotemporal deviations from the simple time‐independent elastic solution. Caution thus should be exercised in interpreting fault kinematics with vertical observations if ignoring Earth's viscoelasticity. By systematically exploring these two parameters, we further investigate three metrics that characterize the predicted vertical deformation: the coastal pivot line (CPL), the uplift zone (UZ) landward above the downdip seismogenic extent, and the secondary subsidence zone (SSZ) in the back‐arc region. We find that these metrics can all be time‐dependent, subject to D/Hc and T/tM. The CPL location and the UZ width are mainly controlled by D/Hc and T/tM, respectively. The presence of the SSZ is prevalent during the interseismic phase due to viscous mantle flow driven by ongoing plate convergence. Contemporary vertical deformation in Nankai and Cascadia is largely consistent with the model predictions and features differences mainly related to contrasting D/Hc values in the two margins. These findings suggest that vertical crustal deformation bears fruitful information about subduction‐zone dynamics and is potentially useful for inversions of key subduction‐zone parameters, deserving properly designed monitoring.

On unusual conditions for the exhumation of subducted oceanic crustal rocks: How to make rocks hotter than models

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.

Gravitational Instability in the Earth's Viscoelastic Crust

This paper studies instability of a heavy inclusion in the Earth's upper layers by the linear theory method for small perturbations. The existence of such inclusions with increased density is associated with chemical inhomogeneity or phase transitions. The viscoelasticity of the geomaterial is described by the Maxwell rheological model. Two layouts of the inclusion with increased density are considered. The heavy inclusion in the cold upper elastic layer of the crust does not change its location under small perturbations, i.e., it is stable according to the linear theory. The heavy inclusion which is located in the hot viscous crustal layer underlying the upper cold layer, is unstable (slowly sinking into the underlying viscous mantle layers).

The temporal viscoelastic model of flexural isostasy for estimating the elastic thickness of the lithosphere

SUMMARY The (effective) elastic thickness of the lithosphere defines the strength of the lithosphere with respect to a load on it. Since the lithosphere is buoyant on a viscous mantle, its behaviour with respect to a load is not fully elastic, but rather viscoelastic. Fennoscandia is a well-known area in the world where the lithosphere has not yet reached its isostatic equilibrium due to the ongoing uplift after the last glacial period at the end of the Pleistocene. To accommodate for this changing property of the lithosphere in time, we present the flexural model of isostasy that accommodates temporal variations of the lithospheric flexure. We then define a theoretical model for computing the elastic thickness of the lithosphere based on combining the flexural and gravimetric models of isostasy. We demonstrate that differences between the elastic and viscoelastic models are not that significant in Fennoscandia. This finding is explained by a relatively young age of the glacial load when compared to the Maxwell relaxation time. The approximation of an elastic shell is then permissible in order to determine the lithospheric structure and its properties. In this way, the elastic thickness can be estimated based on combining gravimetric and flexural models of isostasy. This approach takes into consideration the topographic and ocean-floor (bathymetric) relief as well as the lithospheric structural composition and the post-glacial rebound. In addition, rheological properties of the lithosphere are taken into consideration by means of involving the Young modulus and the Poisson ratio in the model, both parameters determined from seismic velocities. The results reveal that despite changes in the Moho geometry attributed to the glacial isostatic adjustment in Fennoscandia are typically less than 1 km, the corresponding changes in the lithospheric elastic thickness could reach or even exceed ±50 km. The sensitivity analysis confirms that even small changes in input parameters could significantly modify the result (i.e. the elastic thickness estimates). The reason is that the elastic thickness estimation is an inverse problem. Consequently, small changes in input parameters can lead to large changes in the elastic thickness estimates. These findings indicate that a robust estimation of the elastic thickness by our method is possible if comprehensive information about structural and rheological properties of the lithosphere as input parameters are known with a relatively high accuracy. Otherwise, even small uncertainties in these parameters could result in large errors in the elastic thickness estimates.

GPS Imaging of Vertical Bedrock Displacements: Quantification of Two‐Dimensional Vertical Crustal Deformation in China

AbstractThe Global Positioning System (GPS) derived bedrock displacements respond to multiple geophysical effects, ranging from surface elastic loads to tectonic sources or viscoelastic uplifts stemming from Earth’s viscous mantle. In this study, the GPS‐inferred vertical crustal velocities are rigorously estimated in mainland China. We integrate the GPS vertical velocity field with Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GFO) data, adopting an empirical Spatial Structure Function (SSF), to image tectonic deformation in mainland China with respect to the International Terrestrial Reference Frame (ITRF) 2014. We present four profiles across China, which indicate that our new robust results are superior to kriging. Furthermore, we use the GRACE/GFO products to account for elastic deformation due to surface mass changes to isolate tectonic deformation signals at GPS sites within mainland China from 2002 to 2019. By integrating GPS and GRACE/GFO measurements, our results reveal the long‐term spatial patterns of vertical tectonic motion in different blocks in mainland China. We conclude that significant steep velocity gradients occur at tectonic block boundaries that are attributable to locking and elastic strain accumulation on active block boundary faults.

Mantle Evolution of Asia Inferred from Pb Isotopic Signatures of Sources for Late Phanerozoic Volcanic Rocks

We present a systematic study of Pb isotope ages obtained from sources of the late Phanerozoic volcanic rocks from unstable Asia and also volcanic rocks and kimberlites from stable regions of the Siberian and Indian paleocontinents. In the mantle sources, we have recorded events of the Early, Middle, and Late epochs of the Earth’s evolution. Evidence on the Early epoch are preserved in sources of the protolithosphere and viscous lower protomantle likely generated from the Hadean magma ocean about 4.51 and 4.44 Ga and in sources of the viscous upper mantle that acquired low µ and elevated µ (LOMU and ELMU) signatures in the early Archean (4.0–3.7 Ga). The Middle and Late epochs are denoted by sources of the viscous upper mantle that was generated, respectively, in the late Archean-Paleoproterozoic (2.9–2.6 Ga and 2.0–1.8 Ga) and in the Neoproterozoic-late Phanerozoic (0.7–0.6 Ga and < 0.25 Ga). Our results show the specific role of the mantle beneath unstable Asia in terms of globally varied µ signatures and the same mantle epochs in sources of the late Phanerozoic volcanic rocks and kimberlites from stable regions of the Siberian and Indian paleocontinents, but with high μ (HIMU) signatures that are distributed worldwide and explained by sulfide sequestration of Pb from the mantle to the core. We refer the LOMU-ELMU mantle sources to the Asian high-velocity lower mantle domain and propose that the HIMU generating processes were focused mainly in the South Pacific and African low-velocity lower mantle domains in the Middle Mantle Epoch of the Earth’s evolution due to influence of the unbalanced solid core.

Buoyant rise of anorthosite from a layered basic complex triggered by Rayleigh-Taylor instability: Insights from a numerical modeling study

AbstractA major unsolved problem of the Proterozoic is the genesis and tectonic evolution of the massif type anorthosites. The idea of large-scale floating of plagioclase crystals in a basaltic magma chamber eventually generating massif type anorthosite diapirs from the floatation cumulates is not supported by observations of the major layered basic complexes of Proterozoic to Eocene age. In this paper, we test and propose a new genetic process of anorthosite diapirism through Rayleigh-Taylor instability. We have carried out a numerical modeling study of parallel, horizontal, multiple layers of norite and anorthosite, in a model layered basic complex, behaving like Newtonian or non-Newtonian power law fluids in a jelly sandwich model of the continental lithosphere. We have shown that in this pressure-temperature-rheology configuration the model lithosphere generates Rayleigh-Taylor instability, which triggers diapirism of the anorthosite. In our model, the anorthosite diapirs buoyantly rise through stages of simple, symmetrical upwelling and pronounced bulbous growth to a full-blown mushroom-like form. This is the growth path of diapirs in nearly all analog and numerical previous studies on diapirism. Our anorthosite diapirs fully conform to this path. Furthermore, we demonstrate that the progressive diapirism brings in striking internal changes within the diapir itself. In the process, the lowermost anorthosite layer rises displacing the upper norite and anorthosite layers as progressively stretched and isolated segments driven to the margin of the rising diapir—a feature commonly seen in natural anorthosite massifs. We propose that a large plume-generated basaltic magma chamber may be ponded at the viscous lower crust or ductile-plastic upper mantle or further down in the weaker mantle of the jelly sandwich type continental lithosphere. The magma may cool and crystallize very slowly and resolve into a thick-layered basic complex with anorthosite layers. Rheologically behaving like Newtonian or non-Newtonian power law fluids, the layers of the basic complex with built-in density inversions would generate RT (Rayleigh-Taylor) instability. The RT instability would trigger a buoyant rise of the unstable anorthosite from the layered complex. The upward driven anorthosite, accumulated as anorthosite plutons, would gradually ascend across the lower and middle crust as anorthosite diapirs.

Geodetic Evidence of Time‐Dependent Viscoelastic Interseismic Deformation Driven by Megathrust Locking in the Southwest Japan Subduction Zone

AbstractThe time‐variable features of interseismic deformation in subduction zones are poorly understood and commonly ignored. Here, by incorporating century‐long leveling data and contemporary global navigation satellite systems (GNSS) velocities, we investigate the temporal evolution of interseismic deformation in southwest Japan using two‐dimensional viscoelastic earthquake‐cycle models. We find that a steady‐state continental mantle viscosity of 1019 Pa·s is required to explain these two geodetic data sets. Our preferred model predicts significant variations in the surface velocity field throughout the entire interseismic period due to viscous mantle flow driven by ongoing megathrust locking, indicating that the multiyear GNSS‐derived velocity field represents a snapshot of time‐varying interseismic deformation. The locking depth and locking time exert significant influences on the horizontal and vertical deformation patterns. Our findings highlight the importance of the interseismically relaxing mantle and the necessity of considering such an effect in determining the current locking state along the Nankai subduction zone.

Megathrust Locking and Viscous Mantle Flow Induce Continental Shortening in Central Andes

Crustal faults at subduction zones show evidence of activity over geological time, but at the scale of earthquake cycle the mechanical behavior of these faults is not fully understood. Here we construct 2-D viscoelastic models constrained by both horizontal and vertical GPS-derived interseismic velocities to investigate the contribution and interrelation between subduction zone locking, viscous mantle flow, and upper plate faulting on surface deformation in Central Andes. Main pattern of horizontal velocities can be explained by a combination of locking degree and viscous flow, whereas vertical signal is found essential for estimating the locking depth. A sharp deformation gradient near the major back-arc fault suggests an active interseismic shorting across this structure. We further conduct mechanical viscoelastic models with a frictional back-arc fault to analyze its displacement and activation conditions. Our results suggest that the back-arc fault is creeping at ~ 3 mm/year and its motion is mainly driven by the interseismic viscous mantle flow, which spreads plate tectonic stresses broadly across the continent. Moreover, the frictional strength of the back-arc fault must be remarkably weak and its mechanics re-distributes the interseismic deformation and shortens the continental plate in Central Andes. Geological estimates suggest that the long-term shortening rate at the back-arc fault is ~ 10 mm/year, suggesting that this structure can accumulate ~ 7 mm/year of slip deficit, confirming the seismic potential of this structure.

On Some Properties of the Glacial Isostatic Adjustment Fingerprints

Along with density and mass variations of the oceans driven by global warming, Glacial Isostatic Adjustment (GIA) in response to the last deglaciation still contributes significantly to present-day sea-level change. Indeed, in order to reveal the impacts of climate change, long term observations at tide gauges and recent absolute altimetry data need to be decontaminated from the effects of GIA. This is now accomplished by means of global models constrained by the observed evolution of the paleo-shorelines since the Last Glacial Maximum, which account for the complex interactions between the solid Earth, the cryosphere and the oceans. In the recent literature, past and present-day effects of GIA have been often expressed in terms of fingerprints describing the spatial variations of several geodetic quantities like crustal deformation, the harmonic components of the Earth’s gravity field, relative and absolute sea level. However, since it is driven by the delayed readjustment occurring within the viscous mantle, GIA shall taint the pattern of sea-level variability also during the forthcoming centuries. The shapes of the GIA fingerprints reflect inextricable deformational, gravitational, and rotational interactions occurring within the Earth system. Using up-to-date numerical modeling tools, our purpose is to revisit and to explore some of the physical and geometrical features of the fingerprints, their symmetries and intercorrelations, also illustrating how they stem from the fundamental equation that governs GIA, i.e., the Sea Level Equation.

Southwest Pacific Absolute Plate Kinematic Reconstruction Reveals Major Cenozoic Tonga-Kermadec Slab Dragging.

Tectonic plates subducting at trenches having strikes oblique to the absolute subducting plate motion undergo trench‐parallel slab motion through the mantle, recently defined as a form of “slab dragging.” We investigate here long‐term slab‐dragging components of the Tonga‐Kermadec subduction system driven by absolute Pacific plate motion. To this end we develop a kinematic restoration of Tonga‐Kermadec Trench motion placed in a mantle reference frame and compare it to tomographically imaged slabs in the mantle. Estimating Tonga‐Kermadec subduction initiation is challenging because another (New Caledonia) subduction zone existed during the Paleogene between the Australia and Pacific plates. We test partitioning of plate convergence across the Paleogene New Caledonia and Tonga‐Kermadec subduction zones against resulting mantle structure and show that most, if not all, Tonga‐Kermadec subduction occurred after ca. 30 Ma. Since then, Tonga‐Kermadec subduction has accommodated 1,700 to 3,500 km of subduction along the southern and northern ends of the trench, respectively. When placed in a mantle reference frame, the predominantly westward directed subduction evolved while the Tonga‐Kermadec Trench underwent ~1,200 km of northward absolute motion. We infer that the entire Tonga‐Kermadec slab was laterally transported through the mantle over 1,200 km. Such slab dragging by the Pacific plate may explain observed deep‐slab deformation and may also have significant effects on surface tectonics, both resulting from the resistance to slab dragging by the viscous mantle.

Heat transport and coupling modes in Rayleigh–Bénard convection occurring between two layers with largely different viscosities

A high-resolution numerical simulation model in two-dimensional cylindrical geometry was used to discuss the heat transport and coupling modes in two-layer Rayleigh-Bénard convection with a high Rayleigh number (up to the order of 109), an infinite Prandtl number, and large viscosity contrasts (up to 10−3) between an outer, highly viscous layer (HVL) and an inner, low-viscosity layer (LVL). In addition to mechanical and thermal interaction across the HVL-LVL interface, which has been investigated by Yoshida and Hamano [“Numerical studies on the dynamics of two-layer Rayleigh-Bénard convection with an infinite Prandtl number and large viscosity contrasts,” Phys. Fluids 28(11), 116601 (2016)], the spatiotemporal analysis in this study provides new insights into (1) heat transport over the entire system between the bottom of the LVL and the top of the HVL, in particular that associated with thermal plumes, and (2) the convection regime and coupling mode of the two layers, including the transition mechanism between the mechanical coupling mode at relatively low viscosity contrasts and the thermal coupling mode at higher viscosity contrasts. Although flow in the LVL is highly time-dependent, it shares the spatially opposite/same flow pattern synchronized to the nearly stationary upwelling and downwelling plumes in the HVL, corresponding to the mechanical/thermal coupling mode. In the transitional regime between the mechanical and thermal coupling modes, the LVL exhibits periodical switching between the two phases (i.e., the mechanical and thermal coupling phases) with a stagnant period. A detailed inspection revealed that the switching was initiated by the instability in the uppermost boundary layer of the LVL. These results suggest that convection in the highly viscous mantle of the Earth controls that of the extremely low-viscosity outer core in a top-down manner under the thermal coupling mode, which may support a scenario of top-down hemispherical dynamics proposed by the recent geochemical study.

Along-strike variation in subducting plate velocity induced by along-strike variation in overriding plate structure: Insights from 3D numerical models

Subduction dynamics can be understood as the result of the balance between driving and resisting forces. Previous work has traditionally regarded gravitational slab pull and viscous mantle drag as the main driving and resistive forces for plate motion respectively. However, this paradigm fails to explain many of the observations in subduction zones. For example, subducting plate velocity varies significantly along-strike in many subduction zones and this variation is not correlated to the age of subducting lithosphere. Here we present three-dimensional and time-dependent numerical models of subduction. We show that along-strike variations of the overriding plate thermal structure can lead to along-strike variations in subducting plate velocity. In turn, velocity variations lead to significant migration of the Euler pole over time. Our results show that the subducting plate is slower beneath the colder portion of the overriding plate due to two related mechanisms. First, the mantle wedge beneath the colder portion of the overriding plate is more viscous, which increases mantle drag. Second, where the mantle wedge is more viscous, hydrodynamic suction increases, leading to a lower slab dip. Both factors contribute to decreasing subducting plate velocity in the region; therefore, if the overriding plate is not uniform, the resulting velocity varies significantly along-strike, which causes the Euler pole to migrate closer to the subducting plate. We present a new mechanism to explain observations of subducting plate velocity in the Cocos and Nazca plates. These results shed new light on the balance of forces that control subduction dynamics and prove that future studies should take into consideration the three-dimensional structure of the overriding plate.

Three-Dimensional Thermal Model of the Costa Rica-Nicaragua Subduction Zone

The thermal structure of a subduction zone controls many key processes, including subducting plate metamorphism and dehydration, the megathrust earthquake seismogenic zone and volcanic arc magmatism. Here, we present the first three-dimensional (3D), steady-state kinematic-dynamic thermal model for the Costa Rica-Nicaragua subduction zone. The model consists of the subducting Cocos plate, the overriding Caribbean Plate, and a viscous mantle wedge in which flow is driven by interactions with the downgoing slab. The Cocos plate geometry includes along-strike variations in slab dip, which induce along-strike flow in the mantle wedge. Along-strike flow occurs primarily below Costa Rica, with a maximum magnitude of 4 cm/year (~40 % of the convergence rate) for a mantle with a dislocation creep rheology; an isoviscous mantle has lower velocities. Along-margin flow causes temperatures variations of up to 80 °C in the subducting slab and mantle wedge at the volcanic arc and backarc. The 3D effects do not strongly alter the shallow (<35 km) thermal structure of the subduction zone. The models predict that the megathrust seismogenic zone width decreases from ~100 km below Costa Rica to just a few kilometers below Nicaragua; the narrow width in the north is due to hydrothermal cooling of the oceanic plate. These results are in good agreement with previous 2D models and with the rupture area of recent earthquakes. In the models, along-strike mantle flow is induced only by variations in slab dip, with flow directed toward the south where the dip angle is smallest. In contrast, geochemical and seismic observations suggest a northward flow of 6–19 cm/year. We do not observe this in our models, suggesting that northward flow may be driven by additional factors, such as slab rollback or proximity to a slab edge (slab window). Such high velocities may significantly affect the thermal structure, especially at the southern end of the subduction zone. In this area, 3D models that include slab rollback and a slab edge are needed to investigate the mantle structure and dynamics.

Water circulation and global mantle dynamics: Insight from numerical modeling

AbstractWe investigate water circulation and its dynamical effects on global‐scale mantle dynamics in numerical thermochemical mantle convection simulations. Both dehydration‐hydration processes and dehydration melting are included. We also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals. Heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed, due to effective slab dehydration at shallow depths. Water still affects significantly the global dynamics by weakening the near‐surface oceanic crust and lithosphere, enhancing the activity of surface plate motion compared to dry mantle case. As a result, including hydrous minerals, the more viscous mantle is expected with several orders of magnitude compared to the dry mantle. The average water content in the whole mantle is regulated by the dehydration‐hydration process. The large‐scale thermochemical anomalies, as is observed in the deep mantle, is found when a large density contrast between basaltic material and ambient mantle is assumed (4–5%), comparable to mineral physics measurements. Through this study, the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection.

On the temporal evolution of long‐wavelength mantle structure of the Earth since the early Paleozoic

AbstractThe seismic structure of the Earth's lower mantle is characterized by a dominantly degree‐2 pattern with the African and Pacific large low shear velocity provinces (i.e., LLSVP) that are separated by circum‐Pacific seismically fast anomalies. It is important to understand the origin of such a degree‐2 mantle structure and its temporal evolution. In this study, we investigated the effects of plate motion history and mantle viscosity on the temporal evolution of the lower mantle structure since the early Paleozoic by formulating 3‐D spherical shell models of thermochemical convection. For convection models with realistic mantle viscosity and no initial structure, it takes about ∼50 Myr to develop dominantly degree‐2 lower mantle structure using the published plate motion models for the last either 120 Ma or 250 Ma. However, it takes longer time to develop the mantle structure for more viscous mantle. While the circum‐Pangea subduction in plate motion history models promotes the formation of degree‐2 mantle structure, the published pre‐Pangea plate motions before 330 Ma produce relatively cold lower mantle in the African hemisphere and significant degree‐1 structure in the early Pangea (∼300 Ma) or later times, even if the lower mantle has an initially degree‐2 structure and a viscosity as high as 1023 Pas. This suggests that the African LLSVP may not be stationary since the early Paleozoic. With the published plate motion models and lower mantle viscosity of 1022 Pas, our mantle convection models suggest that the present‐day degree‐2 mantle structure may have largely been formed by ∼200 Ma.

Tectonic speed limits from plate kinematic reconstructions

The motion of plates and continents on the planet's surface are a manifestation of long-term mantle convection and plate tectonics. Present-day plate velocities provide a snapshot of this ongoing process, and have been used to infer controlling factors on the speeds of plates and continents. However, present-day velocities do not capture plate behaviour over geologically representative periods of time. To address this shortcoming, we use a plate tectonic reconstruction approach to extract time-dependent plate velocities and geometries from which root mean square (RMS) velocities are computed, resulting in a median RMS plate speed of ∼4 cm/yr over 200 Myr. Linking tectonothermal ages of continental lithosphere to the RMS plate velocity analysis, we find that the increasing portions of plate area composed of continental and/or cratonic lithosphere significantly reduces plate speeds. Plates with any cratonic portion have a median RMS velocity of ∼5.8 cm/yr, while plates with more than 25% of cratonic area have a median RMS speed of ∼2.8 cm/yr. The fastest plates (∼8.5 cm/yr RMS speed) have little continental fraction and tend to be bounded by subduction zones, while the slowest plates (∼2.6–2.8 cm/yr RMS speed) have large continental fractions and usually have little to no subducting part of plate perimeter. More generally, oceanic plates tend to move 2–3 times faster than continental plates, consistent with predictions of numerical models of mantle convection. The slower motion of continental plates is compatible with deep keels impinging on asthenospheric flow and increasing shear traction, thus anchoring the plate in the more viscous mantle transition zone. We also find that short-lived (up to ∼10 Myr) rapid accelerations of Africa (∼100 and 65 Ma), North America (∼100 and 55 Ma) and India (∼130,80 and 65 Ma) appear to be correlated with plume head arrivals as recorded by large igneous province (LIPs) emplacement. By evaluating factors influencing plate speeds over the Mesozoic and Cenozoic, our temporal analysis reveals simple principles that can guide the construction and evaluation of absolute plate motion models for times before the Cretaceous in the absence of hotspot tracks and seafloor spreading histories. Based on the post-Pangea plate motions, one principle that can be applied to pre-Pangea times is that plates with less than ∼50% continental area can reach RMS velocities of ∼20 cm/yr, while plates with more than 50% continental fraction do not exceed RMS velocities of ∼10 cm/yr. Similarly, plates with large portions of continental or cratonic area with RMS velocities exceeding ∼15 cm/yr for more than ∼10 Myr should be considered as potential artefacts requiring further justification of plate driving forces in such scenarios.

Glacial isostatic adjustment in the static gravity field of Fennoscandia

AbstractIn the central part of Fennoscandia, the crust is currently rising, because of the delayed response of the viscous mantle to melting of the Late Pleistocene ice sheet. This process, called Glacial Isostatic Adjustment (GIA), causes a negative anomaly in the present‐day static gravity field as isostatic equilibrium has not been reached yet. Several studies have tried to use this anomaly as a constraint on models of GIA, but the uncertainty in crustal and upper mantle structures has not been fully taken into account. Therefore, our aim is to revisit this using improved crustal models and compensation techniques. We find that in contrast with other studies, the effect of crustal anomalies on the gravity field cannot be effectively removed, because of uncertainties in the crustal and upper mantle density models. Our second aim is to estimate the effects on geophysical models, which assume isostatic equilibrium, after correcting the observed gravity field with numerical models for GIA. We show that correcting for GIA in geophysical modelling can give changes of several kilometer in the thickness of structural layers of modeled lithosphere, which is a small but significant correction. Correcting the gravity field for GIA prior to assuming isostatic equilibrium and inferring density anomalies might be relevant in other areas with ongoing postglacial rebound such as North America and the polar regions.