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Published October 15, 2015 | Accepted Version
Book Section - Chapter Open

Top driven asymmetric mantle convection


The role of decoupling in the low-velocity zone is crucial for understanding plate tectonics and mantle convection. Mantle convection models fail to integrate plate kinematics and thermodynamics of the mantle. In a first gross estimate, we computed at >300 km^3/yr the volume of the plates lost along subduction zones. Mass balance predicts that slabs are compensated by broad passive upwellings beneath oceans and continents, passively emerging at oceanic ridges and backarc basins. These may correspond to the broad low-wavespeed regions found in the upper mantle by tomography. However, west-directed slabs enter the mantle more than three times faster (~232 km^3/yr) than in the opposite east- or northeast-directed subduction zones (~74 km^3/yr). This difference is consistent with the westward drift of the outer shell relative to the underlying mantle, which accounts for the steep dip of west-directed slabs, the asymmetry between flanks of oceanic ridges, and the directions of ridge migration. The larger recycling volumes along west-directed subduction zones imply asymmetric cooling of the underlyingmantle and that there is an "easterly" directed component of the upwelling replacement mantle. In this model, mantle convection is tuned by polarized decoupling of the advecting and shearing upper boundary layer. Return mantleflow can result from passive volume balance rather than only by thermal buoyancy-driven upwelling.

Additional Information

© 2015 Geological Society of America. Accepted 2 February 2015; First published on June 5, 2015. Discussions with Enrico Bonatti, Eugenio Carminati, Françoise Chalot-Prat, Marco Cuffaro, Giuliano Panza and Federica Riguzzi were very much appreciated. Three anonymous referees constructively reviewed the manuscript. Thanks to Gillian Foulger for several helpful suggestions. The research was supported by the Sapienza University and Prin-Miur grants.

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