Does quadrupole stability imply LLSVP fixity?

Abstract

arising from C. P. Conrad, B. Steinberger & T. H. Torsvik Nature 498, 479–482 (2013)10.1038/nature12203

The African and Pacific large low-shear-velocity provinces (LLSVPs) at present dominate the structure of the Earth’s lowermost mantle, but there is considerable debate as to whether these structures have remained fixed throughout geologic time or whether they have shifted in response to the changing configurations of mantle downwellings associated with zones of surface tectonic plate convergence. In a recent Letter, Conrad et al.¹ performed a multipole expansion of the Earth’s plate motions from 250 million years (Myr) ago to the present and used the relatively stationary positions of quadrupole divergence to argue that the two LLSVPs have remained stationary at least for the past 250 Myr and further speculated that the two LLSVPs formed “stable anchors” in the more distant geologic past. Here we show that the quadrupole divergence of plate motions is not a representative diagnostic for overall plate divergence patterns, owing to cancellation effects in the multipole expansion. Hence, the conclusion by Conrad et al.¹ that the presence of stationary quadrupole divergence implies fixity of the LLSVPs is not well founded. There is a Reply to this Brief Communication Arising by Conrad, C. P., Steinberger, B. & Torsvik, T. H. Nature 503, doi:10.1038/nature12793 (2013).