323 research outputs found
The dynamics of plate boundaries over a convecting mantle
International audienceTrench motion and upper plate deformation ultimately respond to mantle flow. Herein I build upon the mantle flow model results of Conrad and Behn (2010) and compute the drag forces underneath all plates, and show that they control the dynamics of plates and plate boundaries. The small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics. Less intuitive is the fact that the interplay between the drag forces from the upper and lower plates, that amounts to 5 to 8.5 1012Nm 1 (per unit trench length), dictates both trench migration rates and upper plate deformation. At odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones (slab age, interplate friction, water content etc), I find that the intrinsic properties of subduction zones in fact only modulate this behavior. More specifically, the mean value of the integrated trenchward mantle drag force from the lower and upper plates (from 2 to 6.5 1012Nm 1) controls upper plate deformation. Conversely, it is the difference between the lower and upper plates mantle drag forces (from 3 to 10 1012Nm 1) that controls trench migration rates. In addition, I find that a minimum trenchward force of 2.5 1012Nm 1 must be supplied by mantle drag before trenches can actually advance, and before upper plates undergo compression. This force results from the default tendency of slabs to rollback when solely excited by their own buoyancy, and is thus the effective tensional force that slab pull exerts on the plate interface
Propagation of tectonic waves
Mountain building depends on the disequilibrium between boundary stresses, either at the base of the deforming lithosphere or its lateral boundaries, and buoyancy stresses arising form lateral density variations within the lithosphere itself. On the basis of the thin viscous sheet approximation, we propose a model which accounts for both crustal and lithospheric thicknesses variations. The deformation is controlled by the sum of the moments of density anomalies (i.e. density anomalies times depth) of compositional and thermal origins. The transport of the compositional moment is obtained from the continuity equation while the transport of the thermal moment is obtained from the heat equation. The resulting set of equations controls the coupled behavior of the crust and lithosphere. It shows that various type of solutions can exist: unstable, stable and propagating. When propagation occurs, the crustal and the lithospheric thickness variations are out of phase. The tectonic waves propagate with velocities around 5 mm yr that increase with the crustal thickness and decrease with the lithospheric viscosity. We discuss these solutions and argue that continents may in large part be in a domain of propagating tectonic waves
On the location of hotspots in the framework of mantle convection
International audiencePutative mechanisms that have been proposed to explain intraplate "hotspot" volcanism extensively depart from the early plume theory, and many do not involve deep mantle flow. Here, we look for a relationship between hotspot volcanism and mantle flow using flow models excited by density anomalies inferred from seismic tomography. We show that previously identified major hotspots are preferentially located, to a high degree of statistical significance, above regions of positive divergence of horizontal shear tractions beneath the lithosphere. This observation renders it difficult to discard some contribution of mantle flow as a control on hotspot volcanism and instead suggests that mantle plumes are drawn toward, and conveyed by, mantle upwellings (either active or passive), which are revealed by the positive stress divergence. This allows us to exclude a variety of external or shallow mechanisms for the major hotspots. Because we also find that many secondary hotspots do fall at random locations with respect to mantle flow, we emphasize that alternative processes are also required to trigger the less productive volcanism
Philippe Labrecque, Comprendre le conservatisme en quatorze entretiens
Le prĂ©sent ouvrage propose quatorze entretiens de dix Ă vingt pages chacun prĂ©cĂ©dĂ©s dâune brĂšve introduction pour « comprendre le conservatisme », face au constat censĂ©ment dressĂ© par lâauteur dâune mĂ©connaissance de ce dernier, dans la mesure oĂč il est « de nos jours souvent mal compris, caricaturĂ© et raillé » (p. 10). Ce faisant, il vise Ă contribuer Ă remettre les grandes questions philosophiques et politiques au cĆur de nos sociĂ©tĂ©s et lâaider Ă mieux saisir « lâĂ©tat rĂ©el de la modernitĂ© ..
Jean-Luc Marion, Ce que nous voyons et ce qui apparaĂźt
Lâouvrage est constituĂ© par la transcription dâune sĂ©ance du 24 juin 2003 du « CollĂšge iconique » autour dâune confĂ©rence de Jean-Luc Marion. Le texte de la confĂ©rence proprement dite (pp. 29-60) est encadrĂ© par une « Ouverture » de François Soulages (pp. 5-27) puis par la transcription du dialogue qui sâest instaurĂ© avec les participants de la sĂ©ance (pp. 61-86), avec des interventions de François Soulages, Itzhak Golberg, Jean Montenot, Jacob Rogozinski, Alain Flageul, Michel Costantini, Se..
Patrick Charaudeau, dir., La LaĂŻcitĂ© dans lâarĂšne mĂ©diatique. Cartographie dâune controverse sociale
Ces derniers mois, la laĂŻcitĂ© a Ă©tĂ© Ă lâhonneur, mais pour des raisons dont on se serait passĂ©, et câest tout un ensemble dâouvrages de statut et dâhorizons disciplinaires fort divers qui ont Ă©tĂ© rĂ©cemment publiĂ©s. Celui-ci est le fruit du travail de lâĂ©quipe de recherche â disparue depuis â du Centre dâanalyse du discours (universitĂ© Paris 13) avec le Laboratoire de communication politique â devenu une Ă©quipe de lâInstitut de recherche interdisciplinaire en sciences sociales (universitĂ© Pari..
Patrick Charaudeau, Petit traitĂ© de politique Ă lâusage du citoyen
Patrick Charaudeau est un spĂ©cialiste de lâanalyse du discours. Avec cet ouvrage, il Ă©largit son champ dâintervention (lâanalyse du discours, puis plus spĂ©cifiquement du discours politique et de ses effets de pouvoir) pour le situer dans le cadre de la philosophie politique classique. Ce « petit traité » se prĂ©sente comme un travail de synthĂšse, mettant en jeu et situant les uns par rapport aux autres un certain nombre de concepts (accentuĂ©s par leur mise en italique, du moins dans la premiĂšr..
De la dynamique de la lithosphĂšre
Le mĂ©moire qui suit est articulĂ© sous la forme dâune compilation de mes rechercheseffectuĂ©es au cours dâune dizaine dâannĂ©es. Essentiellement construit dans un ordrechronologique -modulo quelques adaptations thĂ©matiques-, il illustre l'Ă©volution dema philosophie des Sciences de la Terre, sous la forme de lâĂ©largissement spatialautour de lâobjet dâĂ©tude quâa emprunttĂ© mon cheminement scientifique dans letemps. Jâai commencĂ© par Ă©tudier des structures unitaires sur le front des chaĂźnesde montagnes pour ensuite considĂ©rer les chaĂ©ines de montagnes comme des bourreletsqui rident la surface de la Terre et enfin dĂ©velopper une rĂ©flexion `a lâĂ©chellede la gĂ©odynamique globale
Pascal Robert, La Bande dessinĂ©e, une intelligence subversiveVilleurbane, Presses de lâenssib, coll. Papiers, 2018, 311 pages
AprĂšs plusieurs ouvrages envisageant certains aspects de la bande dessinĂ©e, lâauteur donne avec cet ouvrage son « systĂšme de la bande dessinĂ©e » (pour reprendre le titre dâun ouvrage de Thierry Groensteen, Paris, Presses universitaires de France, 1999). La spĂ©cificitĂ© de cette analyse, et notamment son irrĂ©ductibilitĂ©, tient Ă la dimension essentiellement subversive de la « technologie intellectuelle narrative » (p. 279) et ses trois formes de « crĂ©ativité » (p. 16) ou dâintelligences â « cog..
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