The role of plume-lithosphere interaction in Hawaii-Emperor chain formation

Paleolatitudes of volcanic rocks reveal that prominent changes in volcanic trend of the Hawaii-Emperor hotspot chain represent meridional migration of the magma source. However, models assuming latitudinal plume migration fail to explain the observed age distribution, rock composition, and erratic paleolatitude changes of the oldest Emperor seamounts. Here we use data-assimilation models to better reproduce the Hawaii-Emperor hotspot track by systematically considering plate reconstruction, plume-lithosphere interaction, and simplified melt generation and migration. Our results show that plate drag and plume-ridge interaction are both important in explaining the observed seamount ages. These shallow dynamic processes could account for 50% of the observed paleolatitude's secular reduction and erratic variations over time, where the necessary southward migration of the Hawaiian plume root is significantly less than previously thought. We conclude that plume-lithosphere interaction represents a common mechanism in affecting hotspot track, and has important implications in understanding mantle dynamics and plate reference frames.

Western US intraplate deformation controlled by the complex lithospheric structure

The western United States is one of Earth's most tectonically active regions, characterized by extensive crustal deformation through intraplate earthquakes and geodetic motion. Such intracontinental deformation is usually ascribed to plate boundary forces, lithospheric body forces, and/or viscous drag from mantle flow. However, their relative importance in driving crustal deformation remains controversial due to inconsistent assumptions on crustal and mantle structures in prior estimations. Here, we utilize a fully dynamic three-dimensional modeling framework with data assimilation to simultaneously compute lithospheric and convective mantle dynamics within the western United States. This approach allows for quantitative estimations of crustal deformation while accounting for the realistic three-dimensional lithospheric structure. Our results show the critical role of the complex lithospheric structure in governing intraplate deformation. Particularly, the interaction between the asthenospheric flow and lithospheric thickness step along the eastern boundary of the Basin and Range represents a key driving mechanism for localized crustal deformation and seismicity.

Paleogene India-Eurasia collision constrained by observed plate rotation

The Cenozoic India-Eurasia collision has had profound impacts on shaping the Tibetan plateau, but its early history remains controversial due to uneven availability of constraints. Recent plate reconstructions reveal two prominent counterclockwise rotation (azimuthal change) rate peaks of the Indian plate at 52-44 and 33-20 Ma, respectively, which could bear key information about this collision history. Using fully dynamic three-dimensional numerical modeling, we show that the first rotation rate peak reflected the initial diachronous collision from the western-central to eastern Indian front, and the second peak reflected the full collision leading to strong coupling between India and Eurasia. Further comparison with observation suggests that the initial and complete India-Eurasia collision likely occurred at 55 ± 5 and 40 ± 5 Ma, respectively, an inference consistent with key geological observations. We suggest that this collision history is instructive for studying the tectonic history of the Tibetan plateau and its surrounding areas.

A South Pole-Aitken impact origin of the lunar compositional asymmetry

The formation of the largest and most ancient lunar impact basin, South Pole-Aitken (SPA), was a defining event in the Moon's evolution. Using numerical simulations, we show that widespread mantle heating from the SPA impact can catalyze the formation of the long-lived nearside-farside lunar asymmetry in incompatible elements and surface volcanic deposits, which has remained unexplained since its discovery in the Apollo era. The impact-induced heat drives hemisphere-scale mantle convection, which would sequester Th- and Ti-rich lunar magma ocean cumulates in the nearside hemisphere within a few hundred million years if they remain immediately beneath the lunar crust at the time of the SPA impact. A warm initial upper mantle facilitates generation of a pronounced compositional asymmetry consistent with the observed lunar asymmetry.

The evolution of basal mantle structure in response to supercontinent aggregation and dispersal

Seismic studies have revealed two Large Low-Shear Velocity Provinces (LLSVPs) in the lowermost mantle. Whether these structures remain stable over time or evolve through supercontinent cycles is debated. Here we analyze a recently published mantle flow model constrained by a synthetic plate motion model extending back to one billion years ago, to investigate how the mantle evolves in response to changing plate configurations. Our model predicts that sinking slabs segment the basal thermochemical structure below an assembling supercontinent, and that this structure eventually becomes unified due to slab push from circum-supercontinental subduction. In contrast, the basal thermochemical structure below the superocean is generally coherent due to the persistence of a superocean in our imposed plate reconstruction. The two antipodal basal thermochemical structures exchange material several times when part of one of the structures is carved out and merged with the other one, similarly to “exotic” tectonic terranes. Plumes mostly rise from thick basal thermochemical structures and in some instances migrate from the edges towards the interior of basal thermochemical structures due to slab push. Our results suggest that the topography of basal structures and distribution of plumes change over time due to the changing subduction network over supercontinent cycles.

The adjoint equations for thermochemical compressible mantle convection: derivation and verification by twin experiments

The adjoint method is an efficient way to obtain gradient information in a mantle convection model relative to past flow structure, allowing one to retrodict mantle flow from observations of the present-day mantle state. While adjoint equations for isochemical mantle flow have been derived for both incompressible and compressible flows, here we extend the method to thermochemical mantle flow models, and present thermochemical adjoint equations in the elastic-liquid approximation. We verify the method with twin experiments, and retrodict the flow history of a thermochemical reference model (reference twin) assuming for the final state, either a consistent thermochemical interpretation, using the thermochemical adjoint equations, or an inconsistent purely thermal interpretation, using the isochemical adjoint equations. The consistent simulation correctly retrodicts the flow evolution of the reference twin. The inconsistent case, instead, restores a false flow history whereby internal buoyancy forces and convectively maintained topography are overestimated. Because the cost function is reduced in either case, our results suggest that the adjoint method can be used to link assumptions on the role of chemical mantle heterogeneity to geologic inferences of dynamic topography, thus providing additional means to test hypotheses on mantle composition and dynamics.

A rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow

Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth's mantle. Seismic imaging reveals that these plumes can be of deep origin--probably rooted on thermochemical structures in the lower mantle. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.

Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept

The Earth's biggest magmatic events are believed to originate from massive melting when hot mantle plumes rising from the lowermost mantle reach the base of the lithosphere. Classical models predict large plume heads that cause kilometre-scale surface uplift, and narrow (100 km radius) plume tails that remain in the mantle after the plume head spreads below the lithosphere. However, in many cases, such uplifts and narrow plume tails are not observed. Here using numerical models, we show that the issue can be resolved if major mantle plumes contain up to 15–20% of recycled oceanic crust in a form of dense eclogite, which drastically decreases their buoyancy and makes it depth dependent. We demonstrate that, despite their low buoyancy, large enough thermochemical plumes can rise through the whole mantle causing only negligible surface uplift. Their tails are bulky (>200 km radius) and remain in the upper mantle for 100 millions of years.

The classic mantle plume concept explains large igneous provinces and hotspot magmatism, but often contradicts observed surface uplift and plume morphology. Here, the authors present a plume model that better supports observations by considering low-buoyancy plumes containing up to 15% of recycled oceanic crust.

Visualization of Flow Behavior in Earth Mantle Convection

A fundamental characteristic of fluid flow is that it causes mixing: introduce a dye into a flow, and it will disperse. Mixing can be used as a method to visualize and characterize flow. Because mixing is a process that occurs over time, it is a 4D problem that presents a challenge for computation, visualization, and analysis. Motivated by a mixing problem in geophysics, we introduce a combination of methods to analyze, transform, and finally visualize mixing in simulations of convection in a self-gravitating 3D spherical shell representing convection in the Earth's mantle. Geophysicists use tools such as the finite element model CitcomS to simulate convection, and introduce massless, passive tracers to model mixing. The output of geophysical flow simulation is hard to analyze for domain experts because of overall data size and complexity. In addition, information overload and occlusion are problems when visualizing a whole-earth model. To address the large size of the data, we rearrange the simulation data using intelligent indexing for fast file access and efficient caching. To address information overload and interpret mixing, we compute tracer concentration statistics, which are used to characterize mixing in mantle convection models. Our visualization uses a specially tailored version of Direct Volume Rendering. The most important adjustment is the use of constant opacity. Because of this special area of application, i. e. the rendering of a spherical shell, many computations for volume rendering can be optimized. These optimizations are essential to a smooth animation of the time-dependent simulation data. Our results show how our system can be used to quickly assess the simulation output and test hypotheses regarding Earth's mantle convection. The integrated processing pipeline helps geoscientists to focus on their main task of analyzing mantle homogenization.

Thermochemical structures beneath Africa and the Pacific Ocean

Large low-velocity seismic anomalies have been detected in the Earth's lower mantle beneath Africa and the Pacific Ocean that are not easily explained by temperature variations alone. The African anomaly has been interpreted to be a northwest-southeast-trending structure with a sharp-edged linear, ridge-like morphology. The Pacific anomaly, on the other hand, appears to be more rounded in shape. Mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes. It has not been shown, however, that such models can lead to thermochemical structures that satisfy the geometrical constraints, as inferred from seismological observations. Here we present numerical models of thermochemical convection in a three-dimensional spherical geometry using plate velocities inferred for the past 119 million years. We show that Earth's subduction history can lead to thermochemical structures similar in shape to the observed large, lower-mantle velocity anomalies. We find that subduction history tends to focus dense material into a ridge-like pile beneath Africa and a relatively more-rounded pile under the Pacific Ocean, consistent with seismic observations.