Tectonically asymmetric Earth: From net rotation to polarized westward drift of the lithosphere

• Plate margins show asymmetries consistent with a “westward” drift of the lithosphere.
• Plate motions occur along an undulate flow which is about 30° wrt the equator.
• A net rotation >1°/Ma toward the west is required in order to fit the asymmetries.
• An asymmetric mantle convection is expected in which the mantle moves.

The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources (below or within the asthenosphere, which decouples the lithosphere from the deep mantle) predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation (NNR) reference frame, and low (<0.2°–0.4°/Ma) net rotation rates (deep hotspots source) predict an average net rotation in which some plates move eastward relative to the mantle (e.g., Nazca). With fast (>1°/Ma) net rotation (shallow hotspots source), all plates, albeit at different velocity, move westerly along a curved trajectory, with a tectonic equator tilted about 30° relative to the geographic equator. This is consistent with the observed global tectonic asymmetries.

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