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 about 306 km3/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 wave speed regions found in the upper mantle by tomography. However, W-directed slabs enter the mantle more than 3 times faster (about 232 km3/yr) than the opposite E- or NE-directed subduction zones (about 74 km3/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 W-directed slabs, the asymmetry between flanks of oceanic ridges and the directions of ridge migration. The larger recycling volumes along W-directed subduction zones imply asymmetry cooling of the underlying mantle 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 mantle flow can result from passive volume balance rather than thermal buoyancy driven upwelling.