1. Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK
2. Institute of Geophysics, ETH Zurich, 8093 Zurich, Switzerland
3. Present address: School of Mathematical Sciences, Monash University, Clayton, Victoria 3800, Australia.

Correspondence to: Saskia Goes¹ Correspondence and requests for materials should be addressed to S.G. (Email: s.goes@imperial.ac.uk).

It is well accepted that subduction of the cold lithosphere is a crucial component of the Earth's plate tectonic style of mantle convection. But whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate, which has substantial implications for the chemical and thermal evolution of the mantle^{1, 2}. Here we identify lower-mantle slab penetration events by comparing Cenozoic plate motions at the Earth's main subduction zones³ with motions predicted by fully dynamic models of the upper-mantle phase of subduction, driven solely by downgoing plate density⁴. Whereas subduction of older, intrinsically denser, lithosphere occurs at rates consistent with the model, younger lithosphere (of ages less than about 60 Myr) often subducts up to two times faster, while trench motions are very low. We conclude that the most likely explanation is that older lithosphere, subducting under significant trench retreat, tends to lie down flat above the transition to the high-viscosity lower mantle, whereas younger lithosphere, which is less able to drive trench retreat and deforms more readily, buckles and thickens. Slab thickening enhances buoyancy (volume times density) and thereby Stokes sinking velocity, thus facilitating fast lower-mantle penetration. Such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle^{5, 6}. Thus we identify a direct expression of time-dependent flow between the upper and lower mantle.

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