On the relative influence of heat and water transport on planetary dynamics

International audienceThe dynamics of a planet and its evolution are controlled to a large extent by its viscosity. In this study, we demonstrate that the dependence of mantle viscosity on temperature and water concentration introduces strong dynamic feedbacks. We derive a dimensionless parameter to quantitatively evaluate the relative strength of those feedbacks, and show that water and heat transport are equally important in controlling present-day dynamics for the Earth. A simple parameterized evolution model illustrates the strong feedbacks and behavior of the system and agrees well with our analytic results. The analysis identifies characteristic times for changes of viscosity, temperature, and water concentration and demonstrates, for time scales greater than a few hundred million years, that the system should either be degassing while warming or regassing while cooling. This yields a characteristic evolution in which, after an initial period of rapid adjustment, the mantle warms while degassing, and subsequently cools rapidly while regassing. As the planet continues to cool, the entire surface ocean may eventually return to the mantle. Our results suggest that a simple relationship may exist between the rate of change of water concentration and the rate of change of temperature in the mantle. This connection is extended by deriving an explicit equation for the Urey ratio that depends on both heat and water transport

Flat-slab subduction, topography, and mantle dynamics in southwestern Mexico

Topography above subduction zones arises from the isostatic contribution of crustal and lithospheric buoyancy, as well as the dynamic contribution from slab-driven mantle flow. We evaluate those effects in southwestern Mexico, where a segment of the Cocos slab subducts horizontally. The eastern part of the volcanic arc - the Trans-Mexican Volcanic Belt - stands at an average elevation of 2.3 km, nearly 1.3 km above the fore-arc. Lateral changes in bulk crustal density are relatively small, and seismic imaging shows that there is little variation in crustal thickness between these two regions. Thus, the elevation difference between the arc and the fore-arc should arise from differences in mantle properties. We present finite element models of flat-slab subduction that provide a simultaneous match to topography, plate velocities, and stress state in the overriding plate. We find that the dynamic effects are primarily controlled by the amount of coupling at the subduction interface and in the mantle wedge, the lack of slab anchoring into the lower mantle, and the absence of continental mantle lithosphere. With a mantle wedge and a subduction interface that are, respectively, 2 and 4 orders of magnitude weaker than the asthenosphere, the flat slab exerts a downward pull that can explain most of the elevation difference between the fore-arc and the arc. We infer that lateral viscosity variations play a significant role in shaping dynamic topography in complex tectonic settings and that sublithospheric dynamics can influence the topography at wavelengths that are significantly shorter than previously recognize

L'élargissement des Andes: une interaction entre dynamique de subduction et croissance de prismes d'accrétion crustaux

International audienceShortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere. We show that the locus of shortening in the western margin of South America has largely been controlled by the geometry of the slab. Numerical models confirm that horizontal subduction favors compression far from the trench, above the asthenospheric wedge and steeply dipping segment of the subducting slab. As a result, a second crustal wedge grows in the hinterland of the continent, and widens the Andes. In the Bolivian orocline, this wedge corresponds to the Eastern Cordillera, whose growth was triggered by a major episode of horizontal subduction. When the slab returned to a steeper dip angle, shortening and uplift pursued, facilitated by the structural and thermo-chemical alteration of the continental lithosphere. We review the successive episodes of horizontal subduction that have occurred beneath South America at different latitudes and show that they explain the diachronic widening of the Andes. We infer that the present-day segmented physiography of the Andes results from the latitudinally variable, transient interplay between slab dynamics and upper plate tectonics over the Cenozoic. We emphasize that slab flattening, or absence thereof, is a major driving mechanism that sets the width of the Andes, at any latitude.Les modèles mécaniques de déformation de la lithosphère montrent que le raccourcissement d'une plaque continentale est généralement accommodé par des prismes d'accrétion crustaux qui croissent au dessus de méga-chevauchements recoupant le manteau lithosphérique. Nous montrons que la localisation du raccourcissement sur la bordure ouest du continent sud-américain est fortement contrôlée par la géométrie de la plaque plongeante. En effet, nous présentons des modèles numériques qui confirment que les subductions horizontales entrainent une migration de la zone de raccourcissement loin de la fosse de subduction. Il en résulte l'apparition d'un second prisme d'accrétion crustal qui se développe à l'intérieur du continent, et dont la croissance entraine l'élargissement de la chaîne andine. Dans l'orocline bolivien, ce second prisme correspond à la Cordillère orientale. Sa croissance est due à un épisode de subduction horizontale survenu au cours du Paléogène. Lorsque le pendage de la plaque plongeante redevient incliné, le raccourcissement et le soulèvement de la chaîne se poursuivent, facilités par les changements thermiques et rhéologiques que subit alors la plaque continentale. Nous passons en revue les épisodes de subduction horizontale survenus sous les Andes au cours du Tertiaire, à différentes latitudes, et montrons qu'ils expliquent l'élargissement diachrone de la chaîne. Nous concluons que l'aspect contrasté actuel de la morphologie de la Cordillère des Andes est le résultat des interactions entre la dynamique de la subduction et la tectonique continentale au cours du Tertiaire. L'apparition ou l'absence de segments de subductions horizontales est un mécanisme essentiel qui explique la largeur actuelle de la chaîne, à toutes les latitudes

Drawing everyday sexism in academia: observations and analysis of a community-based initiative

International audienceAbstract. Sexist behaviour in the workplace contributes to create a hostile environment, hindering the chance of women and gender non-conforming individuals to pursue an academic career, but also reinforcing gender stereotypes that are harmful to their progress and recognition. The Did this really happen?! project aims at publishing real-life, everyday sexism in the form of comic strips. Its major goal is to raise awareness about unconscious biases that transpire in everyday interactions in academia and increase the visibility of sexist situations that arise within the scientific community, especially to those who might not notice it. Through the website didthisreallyhappen.net, we collect testimonies about everyday sexism occurring in the professional academic environment (universities, research institutes, scientific conferences…). We translate these stories into comics and publish them anonymously without any judgement or comments on the website. By now, we have collected over 100 testimonies. From this collection, we identified six recurrent patterns: (1) behaviours that aim at maintaining women in stereotypical feminine roles, (2) behaviours that aim at maintaining men in stereotypical masculine roles, (3) the questioning of the scientific skills of female researchers, (4) situations where women have the position of an outsider, especially in informal networking contexts, (5) the objectification of women, and (6) the expression of neosexist views. We first present a detailed analysis of these categories, then we report on the different ways we interact and engage with the Earth science community, the scientific community at large and the public in this project

Drawing everyday sexism in academia: Observations and analysis of a community-based initiative

Sexist behaviour in the workplace contributes to create a hostile environment, hindering the chance of women and gender non-conforming individuals to pursue an academic career, but also reinforcing gender stereotypes that are harmful to their progress and recognition. The Did this really happen?! project aims at publishing real-life, everyday sexism in the form of comic strips. Its major goal is to raise awareness about unconscious biases that transpire in everyday interactions in academia and increase the visibility of sexist situations that arise within the scientific community, especially to those who might not notice it. Through the website didthisreallyhappen.net, we collect testimonies about everyday sexism occurring in the professional academic environment (universities, research institutes, scientific conferences…). We translate these stories into comics and publish them anonymously without any judgement or comments on the website. By now, we have collected over 100 testimonies. From this collection, we identified six recurrent patterns: (1) behaviours that aim at maintaining women in stereotypical feminine roles, (2) behaviours that aim at maintaining men in stereotypical masculine roles, (3) the questioning of the scientific skills of female researchers, (4) situations where women have the position of an outsider, especially in informal networking contexts, (5) the objectification of women, and (6) the expression of neosexist views. We first present a detailed analysis of these categories, then we report on the different ways we interact and engage with the Earth science community, the scientific community at large and the public in this project.ISSN:1680-7340ISSN:1680-735

Dynamic uplift and subsidence in the Central Andes associated with flat-slab subduction

International audienc