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The migration of mid-ocean ridges is driven by asymmetric plate motions on either ridge flank transmitted from far-field subduction forces. Within this model, the geometry and location of mid-ocean ridges are independent of lower-mantle dynamics. However, this fails to recognise the attraction between mid-ocean ridges and mantle plumes. Using numerical models of mantle convection, we show that plumes with high buoyancy flux (> 6000 kg/s) can capture mid-ocean ridges within a 1000 km radius and anchor them in place. If the plume buoyancy flux wanes below 1000 kg/s the ridge may be released, potentially resulting in rapid migration rates that trigger a major plate reorganisation. Plume-ridge interactions are commonly preserved as conjugate large igneous provinces (LIPs), which form on each flank of a mid-ocean ridge as new crust is created. The decoupling of ridges from plumes are demarcated by a switch from conjugate LIPs, formed by a plume beneath a spreading ridge, to trails of intraplate hotspot volcanoes signifying the plume and ridge have separated. We demonstrate that the waning buoyancy flux of the Kerguelen plume, inferred from the geochemistry of eruption products, resulted in its decoupling with the SE Indian Ridge spurring rapid northward migration of the Australian plate. Our modelling predicts that following plume-ridge decoupling, the waning plume can tilt 15° within the upper mantle towards the migrating ridge, providing an explanation for diffuse volcanism and low eruption volumes along the Kerguelen Archipelago. Our results have significant implications for other plume-ridge interactions globally such as the Iceland, Tristan, and Easter plumes, and the generation of intraplate hotspot volcanoes proximal to mid-ocean ridges.

Despite quotas, increased advocacy and movements like Me Too and Time’s Up, sexism in science and the wider world is as prevalent as ever. So why could this be? Is it the expectation that as women and minorities are more common in the workplace, the fight for equality and inclusivity has been “won” or the greater recognition of sexist behaviors and microaggressions? Or is it the rise of “incels” and the encouragement of “masculinity influencers” who subscribe to a brand of extreme misogyny?Now in its seventh year, the Did This Really Happen?! project provides a safe space for the submission of anonymised testimonies documenting real lived experiences of everyday sexism in scientific environments, including sexist biases and a range of micro and macro aggressions. These are, in turn, converted into comic strips by the DTRH team, as a way to visualize the stories and start a conversation.Since the project’s inception in 2016, we have received over 150 testimonies which have been turned into about 50 comics, with many more in various stages of preparation. Testimonial topics have ranged from treating women as objects to questioning female competencies and confining males to stereotypical roles (Bocher et al., 2020). In this presentation, we will assess if and how the topics of the stories received within our project have evolved since 2016, and we will attempt to reply to the following question, through the lenses of our project: how has the state of sexism evolved in academia since the start of Did this really happen?!. Using a quantitative analysis of all the stories we have received so far, we will prove that (sadly) our project is as relevant today as when it was started.

It is well-established that the mantle exerts a strong control on the geodynamic processes and their expressions observed at the surface. One of the key components in mantle-surface interaction is rheology, yet, the details of this rheological coupling remain not well understood. This is particularly true for large to global scales, which are difficult to assess in the field or in the lab. For instance, rheological inheritance is thought to influence the onset location of plate boundaries and their subsequent evolution. But the originating physical processes and timescales over which such rheological inheritance applies are still debated. Moreover, differences in the rheological coupling between mantle and surface are expected to change first-order surface observables, such as topography.Here, we use numerical geodynamic models of whole-mantle convection to test the sensitivity of surface tectonics to different rheological assumptions, for instance in terms of deformation mechanisms at play and rheological inheritance. We show that the self-consistent generation of plate tectonics from mantle convection is altered by the use of a composite rheology (with co-existing diffusion and dislocation creep), by the consideration of mantle grain-size evolution, as well as by simple parameterisations of rheological memory such as strain-weakening. Using a set of quantitative diagnostics, we also demonstrate how such rheological complexities affect surface topography.

The past decade has seen the rise of fully kinematic palaeogeographic models that explicitly define the evolution of both plate boundaries and tectonic plates. Included in these models are (possible) interpretations of spreading systems in extinct ocean basins. Typically, the primary constraint on controlling these synthetic mid-ocean ridges is ensuring that at known (i.e. preserved in the geological record) subduction zones there is convergence, and that at modelled mid-ocean ridges there is divergence. The most common way this is expressed in models is through a quasi-stable triple junction. While obviously subject to large inherent uncertainties, the advantage of modelling such ocean basins is that they can provide an internally consistent model of (tectonic) ocean evolution, tied to the underlaying palaeomagnetic and palaeotectonic framework. Here we explore this inherent uncertainty in such synthetic ocean basins, by introducing the concept of ‘structural uncertainty’ within a full-plate model. We describe structural uncertainty as the answer to the question, “how much oceanic-oceanic subduction (i.e. not preserved in the geological record) is required to balance the modelled synthetic spreading ridges?” While an initial inclination that models tending to ‘zero’ might be best, we entertain the possibility that there is a range of ‘lost’ subduction. To assess this hypothesis, we also interrogate whole-mantle convection models that produce self-consistent plate tectonics to determine the proportion of subduction around or adjacent to continents (representative of what might be preserved in the geological record), and subduction occurring within ocean basins (representative of what might be lost to the geological record).

One of the most discussed issues of whole-mantle geodynamic models is the need of an 'ad hoc' yield stress which is lower than any strength measurement of natural samples in the brittle or plastic regimes. It is commonly believed that grain size evolution, in particular grains size reduction due to dynamic recrystallization, may decrease the strength of the lithosphere and therefore aid the onset and persistence of the mobile-lid regime. In this work, we carry out an investigation of 2D whole-mantle annulus models with varying yield stress. We compare cases with different grain growth and grain reduction parameters to cases with constant grain size to make inferences on the feasibility of a plate-like convective regime as a function of the yield strength of the lithosphere.Our results show that viscosity profiles of models with dynamic grain-size evolution are inherently different to those with constant grain size, and that those profiles vary little when changing grain-size evolution parameters. In this context, the lower mantle shows greater variations in viscosity than the upper mantle: with viscosity contrasts between upper and lower mantle and plume widths comparable to those of the Earth, in particular in models with enhanced grain growth. Furthermore, our models show that, while enhancing grain size reduction reduces episodicity and increases mobility up to some point, increasing grain growth favors mobile-lid convection even more. This is at odds with previous conceptions of the grain-size-evolution-induced mobile-lid regime, where grain groth should promote healing of the lithosphere and therefore inhibit subduction. We hypothesize that increased stiffness of the bottom of the lithosphere, together with a more viscous lower mantle, are the main reasons for the grain-grouth-favored mobile-lid regime.

The last few years have been marked by a number of motivating novel ideas and methodological advancements in paleomagnetic analysis (e.g. trans-hierarchical uncertainty propagation), observational and theoretical geodynamics, and paleogeographical modeling (e.g. optimisation and Bayesian approaches). Many of these developments offer new insights on, and/or approaches to estimating, the past motions of tectonic plates—but so far these developments have largely unfolded in isolation of one another. In November 2021 an international group of 15 young scientists with highly complementary backgrounds (spanning the aforementioned fields) gathered to explore and discuss these exciting new developments and to brainstorm strategies that may enable their integration. We anticipate that the integration of these diverse new ideas and methods will open new frontiers in plate tectonic research, and notably lead to much better-constrained paleogeographic models. In this presentation, we will share some of the insights and strategies that emerged from the workshop, including the advantages of conducting paleomagnetic analysis at the site-level, the application of emerging paleomagnetic Euler pole analysis frameworks, and the use of insights extracted from Earth-like geodynamic models (which self-generate plate tectonic behavior) to further constrain the results of these paleomagnetic methods. We also present some preliminary results of early experiments putting these strategies into practice on a paleomagnetic dataset from North America.

Deep mantle plumes are buoyant upwellings rising from the Earth’s core-mantle boundary to its surface, and describing most hotspot chains. Mechanisms to explain dual chains of hotspot volcanoes for the Hawaiian-Emperor and Yellowstone chains fail to explain the geochemical similarity and large distances between contemporaneous volcanoes of the Tasmantid and Lord Howe chains in the SW Pacific. Using numerical models of mantle convection, we demonstrate how slab-plume interaction can lead to sustained plume branching over a period of >40 million years to produce parallel volcanic chains that track plate motion. We propose a three-part model: first, slabs stagnate in the upper mantle, explaining fast upper mantle P-wave velocity anomalies; second, deflection of a plume conduit by a stagnating slab splits it into two branches 650-900 km apart, aligning to the orientation of the trench axis; third, plume branches heat the stagnating slab causing partial melting and release of volatiles which percolate to the surface forming two contemporaneous volcanic chains with slab-influenced EM1 signatures. Our results highlight the critical role of long-lived subduction on the evolution and behaviour of intraplate volcanism.

Mantle plumes provide valuable information about whole-mantle convection: they originate at the core-mantle boundary, cross Earth's mantle and interact with the lithosphere. For instance, it has been proposed that the mobility/stability of plumes depends on plume intrinsic properties, on how slabs interact with the basal boundary layer, on mantle flow, or on their proximity to mid-ocean ridges. Here, we use 3D-spherical models of mantle convection generating self-consistent plate-like behaviour to investigate the mechanisms linking tectonics and mantle convection to plume dynamics. Our models produce fully-dynamic mantle plumes that rise vertically with deflection 5 cm yr−1 over 25%, which suggests that fixed hotspot reference frames can be defined from carefully selected hotspot tracks.

Like any other workplace, academia is not free from everyday gender stereotypes and sexist behaviours. They participate in the feeling of insecurity and devaluation of women and gender non-conforming individuals, which ultimately contributes to a persisting gender imbalance in this environment. Building on this observation, the Did this really happen?! project, born in 2018, aims at reporting real occurrences of everyday sexism experienced within the scientific community. Using comic strips, we raise awareness about such behaviours and their pernicious consequences, which are often difficult to notice. Through our website www.didthisreallyhappen.net, we have now collected more than 100 contributions from researchers all over the world, describing sexist biases that they have faced or witnessed in academia. We have already turned 25 of them into anonymous comics that we publish without any comments on the website. In hindsight, we have identified six repeating categories of sexist behaviours: 1) those that aim at maintaining women in stereotypical feminine roles, 2) those that aim at maintaining men in stereotypical masculine roles, 3) those that question the scientific skills of female researchers, 4) those where women have the position of an outsider, especially in informal networking contexts, 5) those that objectify women, and 6) those which express neosexist views. Here, we present our project in more details, propose a detailed analysis of these sexist situations, and we are happy to discuss further ways to engage with the scientific community on this topic.

3D spherical dynamic models show how the force balance between tectonic plates and mantle evolves over supercontinent cycles., Does Earth’s mantle drive plates, or do plates drive mantle flow? This long-standing question may be ill posed, however, as both the lithosphere and mantle belong to a single self-organizing system. Alternatively, this question is better recast as follows: Does the dynamic balance between plates and mantle change over long-term tectonic reorganizations, and at what spatial wavelengths are those processes operating? A hurdle in answering this question is in designing dynamic models of mantle convection with realistic tectonic behavior evolving over supercontinent cycles. By devising these models, we find that slabs pull plates at rapid rates and tear continents apart, with keels of continents only slowing down their drift when they are not attached to a subducting plate. Our models show that the tectonic tessellation varies at a higher degree than mantle flow, which partly unlocks the conceptualization of plate tectonics and mantle convection as a unique, self-consistent system.

International audience; The head of a mantle plume can weaken the oceanic lithosphere through the combined action of its buoyancy forces and excess temperature, which eventually induces intra-oceanic subductions. However, the dynamics of plume-induced subductions, their lifetime and their ability to propagate at the scale of an entire ocean remain unknown. Here, we combine a quantitative synthesis of the ophiolitic record of the Neotethys subduction in the Late Cretaceous with 3D spherical modeling of mantle convection self-generating plate-like tectonics to propose that the pre-Deccan plume initiated the Southern Neotethys Subduction. We suggest that this subduction retreated at 5-10 cm yr−1 until it reached the continental margins of Arabia and India. The sequential segmentation of this subduction during its retreat and the diachronous deactivation of the resulting segments drove the Late Cretaceous-Eocene series of plate reorganization events recorded in the Indian Ocean's fabric.

Mantle plume fixity has long been a cornerstone assumption to reconstruct past tectonic plate motions. However, precise geochronological and paleomagnetic data along Pacific continuous hotspot tracks have revealed substantial drift of the Hawaiian plume. The question remains for evidence of drift for other mantle plumes. Here, we use plume-derived basalts from the Mid-Atlantic ridge to confirm that the upper-mantle thermal anomaly associated with the Azores plume is asymmetric, spreading over ~2,000 km southwards and ~600 km northwards. Using for the first time a 3D-spherical mantle convection where plumes, ridges and plates interact in a fully dynamic way, we suggest that the extent, shape and asymmetry of this anomaly is a consequence of the Azores plume moving northwards by 1–2 cm/yr during the past 85 Ma, independently from other Atlantic plumes. Our findings suggest redefining the Azores hotspot track and open the way for identifying how plumes drift within the mantle., Tectonic plate motions are often reconstructed based on the assumption that mantle plumes are fixed within the mantle. Here, the authors provide geochemical and geodynamic evidence to suggest that the asymmetry of the Azores thermal anomaly can be explained by northward motion of the Azores plume.

International audience; Mantle convection shapes Earth's surface by generating dynamic topography. Observational constraints and regional convection models suggest that surface topography could be sensitive to mantle flow for wavelengths as short as 1,000 and 250 km, respectively. At these spatial scales, surface processes including sedimentation and relative sea-level change occur on million-year timescales. However, time-dependent global mantle flow models do not predict small-scale dynamic topography yet. Here we present 2-D spherical annulus numerical models of mantle convection with large radial and lateral viscosity contrasts. We first identify the range of Rayleigh number, internal heat production rate and yield stress for which models generate plate-like behavior, surface heat flow, surface velocities, and topography distribution comparable to Earth's. These models produce both whole-mantle convection and small-scale convection in the upper mantle, which results in small-scale (\textless500 km) to large-scale (\textgreater10(4) km) dynamic topography, with a spectral power for intermediate scales (500 to 10(4) km) comparable to estimates of present-day residual topography. Timescales of convection and the associated dynamic topography vary from five to several hundreds of millions of years. For a Rayleigh number of 10(7), we investigate how lithosphere yield stress variations (10-50 MPa) and the presence of deep thermochemical heterogeneities favor small-scale (200-500 km) and intermediate-scale (500-10(4) km) dynamic topography by controlling the formation of small-scale convection and the number and distribution of subduction zones, respectively. The interplay between mantle convection and lithosphere dynamics generates a complex spatial and temporal pattern of dynamic topography consistent with constraints for Earth.