Alexandre Janin

Les réorganisations des plaques tectoniques, de l'observation sur la faille transformante d'Owen à la modélisation globale: À la surface de la Terre les plaques tectoniques sont en mouvement les unes par rapport aux autres et se déforment à leur frontière. Ce lent mouvement des plaques, de l’ordre d’une dizaine de centimètres par an, est à l’origine de la majeure partie de l’énergie radiée par les séismes. Le déplacement de deux plaques voisines peut être stable sur plusieurs dizaines de millions et brusquement changer. Lors de ces changements cinématiques, les frontières des plaques s’ajustent aux nouveaux mouvements. La cartographie du fond marin, mais surtout l’analyse des anomalies magnétiques portées par les roches de la lithosphère océanique mettent en lumière ces réorganisations des plaques et de leurs frontières. Cette thèse propose d’étudier les réorganisations du mouvement des plaques via deux approches complémentaires et à deux échelles différentes : l’analyse détaillée d’une frontière de plaque active observée sous l’angle de la géophysique marine et la modélisation globale où la dynamique du manteau et de la lithosphère peut être étudiée sur plusieurs centaines de millions d’années d’évolution. J’introduis tout d’abord les concepts clés liés au sujet de cette thèse et présente une synthèse générale sur les réorganisations du système d’accrétion et des failles transformantes (FT) océaniques. Ensuite, je m’intéresse plus particulièrement à la FT océanique d’Owen formant avec la ride médio-océanique de Carlsberg la frontière entre les plaques indienne et somalienne. Le traitement de nouvelles données de bathymétrie multi-faisceaux et de sismique réflexion allié à un travail de stratigraphie sismique me permet de révéler les différents régimes tectoniques de cette frontière de plaque sur les derniers 8.6 millions d’années (Ma). Je constate qu’un changement cinématique ayant eu lieu il y a 2.4 à 1.5 Ma est à l’origine de la transpression affectant l’ensemble de la FT d’Owen et responsable de la formation d’une ride médiane. Ce changement abrupt de mouvement de plaques vient mettre fin à une période antérieure de relative quiescence tectonique. L’analyse de la géométrie des dépôts sédimentaires les plus superficiels ainsi que l’étude de la sismicité régionale montrent que la transpression le long de la FT d’Owen est toujours active. Ces résultats me permettent de proposer un modèle d’évolution de cette frontière depuis sa formation, contemporaine de l’ouverture du Golfe d’Aden vers 20 Ma, jusqu’à l’actuel. Je discute la singularité de la vallée transformante d’Owen liée notamment à son état thermique et son important remplissage sédimentaire porté par l’Indus. Par la suite, je change d’échelle et après avoir présenté comment la modélisation globale du manteau permet d’étudier la tectonique des plaques, je présente une nouvelle méthode de détection et de suivi automatique des plaques émergeant à la surface de ces modèles. Cette méthode de segmentation est basée sur une analyse topologique de la vitesse de surface couplée à un diagnostic cinématique. J’utilise cette méthode pour analyser le déplacement des plaques à la surface d’un modèle numérique de manteau sur plus de 260 Ma. Je montre que la cinématique des plaques ainsi modélisée oscille comme sur Terre entre des périodes de stabilité et des évènements abrupts de réorganisation. La dynamique des zones de subduction joue un rôle essentiel dans les changements cinématiques des plaques et peut conduire à un changement des mouvements de plaques à l’échelle globale. La méthode de détection et suivi des plaques développée dans cette thèse me permet d’étudier la durée d’existence des plaques tectoniques. Je montre que la distribution de durée de vie des plaques terrestres peut être modélisée avec un modèle global de manteau et suit une distribution fractale. Je constate que plus une plaque tectonique vieillie, plus elle devient apte à vieillir davantage. La modélisation globale me permet d'investiguer quantitativement les paramètres agissant sur cette distribution.

One efficient driver for atmospheric CO2 removal over 10-100 Ma timescales is silicate-rich rock weathering, which is notably favored in the context of arc magmatism (Gernon et al., 2021). The modeling of the past evolution of atmospheric CO2 therefore requires to finely reconstruct the evolution of past subduction zones, which is challenging due to the permanent recycling of oceanic lithosphere. One difficulty notably resides in the reconstruction of intra-oceanic arcs, which leave almost no direct imprints in the geological record, although they could significantly contribute to the atmospheric CO2 removal through silicate weathering, especially in the tropics (Gaillardet et al., 2011).We propose to test how the variability of intra-oceanic arcs can affect the amount of CO2 removed from the atmosphere through supercontinent cycles. To do so, we use 3D numerical models of whole-mantle convection self-generating Earth-like plate tectonics in order to quantify the temporal evolution of the number and length of intra-oceanic arcs, in a fully-dynamic context, independent of any plate reconstructions. We use the automatic plate tessellation algorithm MAPT3 based on the open-source library Topology ToolKit (Janin et al., 2025) to detect intra-oceanic subduction zones. We show that the total length of intra-oceanic arcs varies significantly depending on the continental configuration in the models. We then test the sensitivity of atmospheric CO2 absorption level through silicate weathering to mantle convective parameters, to the latitudinal distribution of the intra-oceanic arcs, their width and fraction above sea-level, and the potential effect of True Polar Wander. We show that in a fully-dynamic model, it is possible to reach the amount of extra-weathering required to possibly explain the atmospheric CO2 and temperature drops observed, especially during periods of continental aggregation. Nevertheless, the amount of intra-oceanic subduction zones in the geodynamic models varies over longer timescales than in the plate reconstruction, and cannot explain alone, rapid cooling events, such as during the Hirnantian (Marcilly et al., 2022).

Earth’s interior plays a fundamental role in the long-term evolution of the surface, climate, and biosphere. However, Earth's mantle evolution remains largely ambiguous, as imaging techniques are limited to present-day observations and geochemical or geological constraints apply to a non-global scale. Plate tectonic reconstructions coupled with convection models could provide constraints on the evolution of mantle structure. In this study, we employ both fully self-consistent and kinematically constrained mantle convection models[1]. The mantle rheology is temperature-, pressure-, phase-, and stress-dependent, with the latter represented through pseudo-plasticity. The novelty of this work lies in employing a composite rheology with “realistic” rheological parameters[2] in a fully three-dimensional geometry. By using both fully self-consistent models and plate-driven models, we aim to address the discrepancies in terms of long-term convective and tectonic behaviour that arise when forcing plate velocities onto the surface. The latter is done by imposing time-dependent surface velocity boundary conditions provided by a plate tectonic reconstruction[3].To evaluate the extent to which the models reproduce plate-like tectonics, we explore several independent constraints. In particular, we compute slab sinking rates and compare them to estimates inferred from seismic tomography[4]. Slab sinking rates in self-consistent models provide insight into the mantle’s rheology. For example, sinking rates that are lower than those based on tomographic and geological data may indicate an overly viscous mantle. Our results show that the slab sinking rate is generally higher in models with imposed plate velocities compared to fully self-consistent models. Furthermore, a tessellation algorithm[5] will be applied to the surface of the models to detect plates in the self-consistent models with plate-like behaviour. Based on these results, a plate-size frequency distribution can be calculated and compared to present-day Earth[6]. Results show that low yield stresses generate too many small plates, and too few large plates [Fig. 1]. In order to generate Earth-like plate tectonics, yield stress needs to be sufficiently high to reproduce the plate-size frequency distribution of present-day Earth, but also sufficiently low to facilitate a long-term mobile lid regime.[1] Tackley, P. J. (2008). Phys. Earth Planet. Inter. 171, 1–4.[2] Tackley, P. J., Ammann, M., Brodholt, J. P., Dobson, D. P., & Valencia, D. (2013). Icarus 225, 50–61.[3] Merdith, A. S., Williams, S. E., Collins, A. S., et al. (2021). Earth-Sci. Rev. 214, 103477.[4] Van der Meer, D. G., van Hinsbergen, D. J. J., & Spakman, W. (2018). Tectonophysics 723, 309–448.[5] Janin, A., Coltice, N., Chamot-Rooke, N., & Tierny, J. (2025). Nat. Geosci. 18, 1041–1047.[6] Bird, P. (2003). Geochem. Geophys. Geosyst. 4, 2001GC000252.

Oceanic transform faults (OTFs) have long been viewed exclusively as vertical, strike-slip structures offsetting mid-ocean ridges, yet their deep geometry and structural complexity remain poorly constrained. Thus, key questions persist, including whether OTFs are single-stranded and continuous, whether they maintain vertical dip angles, if they accommodate mixed-mode slip, and what factors control their geometry. Our study addresses these questions through a global statistical analysis of teleseismic earthquake focal mechanisms from 150 OTFs across diverse tectonic settings. We introduce 'stack maps', a novel method that quantifies fault dip and rake, providing a graphical representation of average focal mechanisms. Our findings reveal that while OTFs tend to conform to the classical vertical, strike-slip model, nearly half exhibit deviations, either in dip or motion, challenging the simplified view of these plate boundaries. We identify four distinct OTF categories: (1) those adhering to the standard model, (2) non-vertical faults with transtensive/transpressive components, (3) non-vertical faults accommodating strike-slip motion, and (4) vertical faults with a vertical component of motion. Tectonic regime shifts emerge as a primary driver of structural changes, with non-vertical geometries persisting even after the regime reverts to pure strike-slip motion. This structural memory suggests that fault geometry, once established, remains stable over geological timescales of several tens of Myr. By reconciling previously 'unusual' focal mechanisms with fault structure and dynamics, this work demonstrates that global seismic catalogues, when analysed statistically, offer robust insights into OTF geometry and tectonic regimes.

SUMMARY: Oceanic transform faults (OTFs) have long been viewed exclusively as vertical, strike-slip structures offsetting mid-ocean ridges, yet their deep geometry and structural complexity remain poorly constrained. Thus, key questions persist, including whether OTFs are single-stranded and continuous, whether they maintain vertical dip angles, if they accommodate mixed-mode slip, and what factors control their geometry. This study addresses these questions through a global statistical analysis of teleseismic earthquake focal mechanisms from 150 OTFs across diverse tectonic settings. We introduce stack maps, a novel method that quantifies fault dip and rake, providing a graphical representation of average focal mechanisms. Our findings reveal that while OTFs tend to conform to the standard vertical, strike-slip model, nearly half exhibit deviations, either in dip or motion, challenging the classical view of these plate boundaries. We identify four distinct OTF categories: (1) those adhering to the standard model, (2) non-vertical faults with transtensive/transpressive components, (3) non-vertical faults accommodating strike-slip motion and (4) vertical faults with a vertical component of motion. Tectonic regime shifts emerge as a primary driver of structural changes, with non-vertical geometries persisting even after the regime reverts to pure strike-slip motion. This structural memory suggests that fault geometry, once established, remains stable over geological timescales of several tens of Myr. By reconciling previously ‘unusual’ focal mechanisms with fault structure and dynamics, this work demonstrates that global seismic catalogues, when analysed statistically, offer robust insights into OTF geometry and tectonic regimes.

Earth’s lithosphere is fragmented into tectonic plates that move relative to each other and the mantle, following a dynamic equilibrium that evolves over time. Sporadically, abrupt kinematic changes occur that typically lead to regional-scale plate reconfigurations, and hypothetically to global-scale events, affecting the motion of all major plates. Identifying and understanding such global plate reorganizations has proven challenging because of the geometric nature of plate tectonic theory and the scarcity of preserved kinematic changes. Here we combine Earth-like global mantle convection with topological data analysis to show that global plate reorganization events can emerge from mantle–lithosphere interactions. Unlike regional changes, during these rare global events, the entire plate network shifts to a new dynamic equilibrium by the formation and destruction of plate boundaries at global scale, affecting the dynamics of the entire mantle. Such events represent transitions between two stable states in the coupled mantle–plates system. Our findings suggest that Earth may have experienced a similar global tectonic reorganization during the mid-Eocene around 47 million years ago. Our new approach provides a quantitative framework to unveil mantle–lithosphere interactions from global geodynamic models.

Fluid composition and fluxes in subduction zones are primarily governed by the nature and degree of hydrothermal alteration of the subducting oceanic lithosphere. However, spatial and temporal heterogeneities inherited from mid‐ocean ridge and oceanic transform fault (OTF) systems introduce significant uncertainties in constraining these fluid variations. Here, we focus on the effect of subducting Fe‐Ti‐rich gabbroic rocks (oxide gabbros), which are commonly found in (ultra)slow‐spread oceanic crust along OTF walls, in detachment faults forming at the inside corners of ridge‐transform intersections (RTIs) and within subducted oceanic metamorphic units. We carried out a petrological and geochemical characterization of oxide gabbros from the Vema OTF which segments the mid‐Atlantic Ridge to document and discuss their abundance, composition, formation and transformation processes at RTIs. Results illustrate spatially variable magmatic and hydrothermal processes at RTIs, resulting in variable Fe‐Ti‐(P)‐(H₂O)‐V enrichment (ilmenite + titanomagnetite ± apatite ± amphibole ± olivine) of primary gabbroic rocks. Thermodynamic modeling reveals significant variability in the stability of hydrated phases across different gabbroic compositions, indicating that, in subduction zones, Fe‐Ti‐enriched lithologies release fluids at shallower depths. Oxide gabbros, like the ones studied, represent a significant but often overlooked source of H₂O, halogens and large ion lithophile elements to the mantle wedge. In addition, subducted P‐rich oxide gabbros may serve as a deep (>700 km) source of fluorine in the asthenosphere. Our results demonstrate that subduction of a compositionally heterogeneous slab containing significant amounts of oxide gabbros generates a broad dehydration domain with implications for seismicity, water transport along the subduction interface and fluid‐mediated tectonic slicing.

Little is known about the geometry of oceanic transform faults. Although their surface trace and curvature along kinematic small circles has been known since the advent of plate tectonics, their structure at depth remains poorly constrained. The classical assumption is that oceanic transform faults are vertical and delimited at depth by the 600°C isotherm. It is only recently that the deployment of local OBS arrays on major oceanic transform faults have allowed us to investigate their geometry at depth and the link with their seismicity. Seismic moment tensors of teleseismic events also contain first order information about the geometry of oceanic transform faults, giving us access to the dip of the fault plane that has ruptured. Abercrombie and Ekström (2001) investigated focal mechanisms along the Chain transform fault (TF) in the equatorial Atlantic Ocean, which indicated a consistent northward dip of the fault along its entire 300-km length.In this study, we take advantage of the increasing data from global seismic catalogs and conduct a statistical exploration of the dip variations of strike-slip focal mechanisms along more than 80 oceanic transform faults. Most of them are either vertical to subvertical, depending on the local variability of the data, or show no preferential dip towards a given side of the fault. Although the optimal dip for a strike-slip fault in a classical Andersonian stress state is vertical, we show here that the case of the Chain TF is not isolated and several other oceanic transform faults show a similar deviation to the vertical, including Owen TF in the Indian Ocean, Vema TF in the Atlantic Ocean, and Tharp TF along the Pacific-Antarctic ridge. The measured deviations to the vertical show a maximum at ~20° (dip 70°). We discuss our observations within the tectonic setting and history of each of these plate boundaries, and speculate on the implications of this maximum dip for the mechanical properties and/or stress conditions in oceanic lithosphere.

The rigid surface of the Earth is divided into a jigsaw puzzle of about 50 tectonic plates separated by boundaries. Nowadays, three-dimensional spherical mantle modelling manages to produce self-consistently a stiff surface fragmented into several rigid caps that exhibit a plate-like behaviour. It thus becomes possible to analyse the dynamics of these models through the prism of plate tectonics theory and compare it to plate reconstruction models for the Earth. Such an analysis requires a robust method to automatically detect plates and their boundaries from continuous geophysical fields. The method should further recognize diffuse plate boundaries, as observed on Earth and reproduced in mantle convection models. We propose here a method to automatically detect and track plates through time, based on a trans-disciplinary approach combining a geodynamical and kinematic analysis with applied mathematics and computer sciences. This analysis is performed using the free and open-source software Paraview and the open-source software platform TTK (Topology ToolKit) designed for an efficient topological analysis of scalar fields. We apply our method to a three-dimensional spherical mantle convection model generating Earth-like plate tectonics at its surface. Our results show that, as for the Earth, the motion of modelled plates is stable over million-years-long periods separated by abrupt reorganizations occurring in less than 5 Myrs. The full plate-motion analysis over 262 Myrs in the model allows us to discuss the spatial extent of kinematic changes and shows that a plate reorganization can have regional to global effects on the plate network.