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TitreStagnant lids and mantle overturns: Implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics
AuteurBédard, J H
SourceGeoscience Frontiers 2017., https://doi.org/10.1016/j.gsf.2017.01.005 (Accès ouvert)
Année2017
Séries alt.Secteur des sciences de la Terre, Contribution externe 20160233
ÉditeurElsevier BV
Documentpublication en série
Lang.anglais
DOIhttps://doi.org/10.1016/j.gsf.2017.01.005
Mediapapier; en ligne; numérique
Formatspdf
Sujetscroûte continentale; croûte océanique; marges continentales; marges plaques; subduction; zones de subduction; panneau orienté vers le bas; Archéen; convection; manteau terrestre; lithosphère; tectonique de plaques
Illustrationscross-sections, structural; geochemical plots; schematic diagrams
ProgrammeInitiative géoscientifique ciblée (IGC-3), 2005-2010
Diffusé2017 02 09
Résumé(disponible en anglais seulement)
The lower plate is the active agent in modern convergent margins characterized by active subduction, as negatively buoyant oceanic lithosphere sinks under its own weight into the mantle. This is a plate-driving force because tensional stresses generated by slab-pull can be transmitted through a stiff sub-oceanic lithospheric mantle. Geological and geochemical data do not support the existence of modern-style ridges and arcs in the Archaean, instead favouring a stagnant-lid scenario. Induced convection beneath stagnant-lid crust would erode the sub-oceanic lithospheric mantle and keep the oceanic lithosphere hot, weak and buoyant, making it effectively unsubductable. Intervals of stagnant-lid behaviour may correspond to periods of layered mantle convection where efficient convective cooling was restricted to the upper mantle, perturbing Earth¿s heat generation/loss balance, and eventually triggering a mantle overturn. Mixing and rehomogenization during mantle overturns would retard development of the isotopically depleted MORB (mid-ocean ridge basalts) mantle reservoir, but allow incremental extraction of buoyant felsic continental crust. Upwelling zones related to Hadean and Archaean mantle overturns were probably larger and longer-lived than post-Archaean mantle plumes, and early cratons probably formed above OUZOs (Overturn Upwelling Zones), which delivered large volumes of basalt and komatiite for protracted periods, allowing basal crustal cannibalism, restite delamination, and coupled development of continental crust and sub-continental lithospheric mantle. Archaean basalts lack the subduction-recycled enriched/depleted mantle zoo components found in plumes, and were mostly derived from this upwelling, relatively undepleted mantle. Pre-existing cratons located above later OUZOs would be strongly reworked; whereas OUZO-distal cratons would drift in response to mantle currents. The leading edges of drifting continents would be convergent margins where other proto-continents and unsubductable stagnant-lid oceanic lithosphere were imbricated and subcreted. As Earth cooled and the background oceanic lithosphere became colder and stiffer, there would be an increasing probability that older oceanic crustal segments would founder in an organized way when subjected to compression, leading to a gradual evolution of convergent margins into modern-style active subduction margins after 2.5 Ga. The start of true subduction led to unidirectional mantle differentiation towards depleted MORB mantle, driven principally by the sequestration of subducted slabs at the core-mantle boundary. Plate tectonics today is constituted of: (1) a continental drift system that started in the Archaean, driven by deep mantle currents pressing against the Archaean-age, sub-continental lithospheric mantle keels that underlie Archaean cratons; (2) a subduction-driven system that started near the end of the Archaean.
Résumé(Résumé en langage clair et simple, non publié)
La géologie d'avant 2.5 Ga ne manifeste pas d'évidence claire de tectonique des plaques et la Terre est interprétée comme ayant évoluée comme planète à carapace stagnante à cette époque. Les carapaces stagnantes n'évacuent pas la chaleur efficacement, et auraient déclenchés des basculements mantelliques périodiques à intervalle de 300-500 Ma qui auraient ré-homogénéisés le manteau. Ceci peut expliquer l'absence d'appauvrissements élémentaire et isotopique manifestés par la plupart des basaltes Archéens. Les zones de remonté mantelliques seraient des sites préférés de la genèse des continents.
GEOSCAN ID299350