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TitleStagnant lids and mantle overturns: Implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics
AuthorBédard, J H
SourceGeoscience Frontiers 2017., Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20160233
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
Subjectscontinental crust; oceanic crust; continental margins; plate margins; subduction; subduction zones; downgoing slab; Archean; convection; mantle; lithosphere; plate tectonics
Illustrationscross-sections, structural; geochemical plots; schematic diagrams
ProgramTargeted Geoscience Initiative (TGI-3), 2005-2010
Released2017 02 09
AbstractThe 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.
Summary(Plain Language Summary, not published)
Hadean-Archaean (pre- 2.5 Ga) geology lacks clear evidence for plate tectonics, and the Earth is interpreted to have evolved as an unstable stagnant lid planet at that time. Stagnant lid systems are inefficient at evacuating planetary heat, and would trigger periodic mantle overturns at 300-500 Ma intervals that would rehomogenize the mantle. This would explain the lack of isotopic and trace element depletion shown by most Archaean basalts. Overturn upwelling zones would be preferred sites of continent genesis.

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