Mantle Anchor Structure: An argument for bottom up tectonics

Close examination of the long wavelength shear velocity signal in the lowermost mantle in the wavenumber domain ties several geophysical observations together and leads to fundamental inferences. When mantle shear velocity model S362ANI at a depth of 2800 km is expanded in spherical harmonics up to degree 18, more than one half of the seismic model's total power is contained in a single spherical harmonic coefficient: the “recumbent” Y20 spherical harmonic; a Y20 with its axis of symmetry rotated to the equatorial plane. This degree 2 signal, which continues with decreasing amplitude for more than 1000 km above the core–mantle boundary (CMB), is characterized by two antipodal regions of low velocities, separated by a circum-polar torus of higher than average velocities. If the slow regions are associated with net excess mass, then any axis of rotation located in the plane of the circum-polar torus will be close to the maximum moment of inertia axis; this includes, of course, the current axis of rotation. We suggest that the recumbent Y20 is a very stable feature: once established, it is difficult to erase, and only relatively small departures from this equilibrium configuration are possible. This anomaly correlates strongly with the degree 2 terms of the residual geoid expansion, distribution of the hot spots above the slow regions, high attenuation in the transition zone, and position of subduction zones above the fast band during the last 200 Ma. Also, the preferred paths of the virtual geomagnetic pole and true polar wander locations for the last 200 Ma lie within the fast band. Since the non-hydrostatic perturbation of the moment of inertia tensor depends only on degree-2 anomalies in the density distribution and deformation of discontinuities, it is natural to infer that rotational dynamics of the Earth have influenced the distribution of heterogeneities in the Earth's deep interior. We propose that the large-scale heterogeneity at the base of the mantle, which we name Mantle Anchor Structure (MAS) may have formed early in the history of the convecting mantle, remained locked in place with respect to the Earth's rotation axis ever since, and is currently imposing the planform of flow in the mantle and of plate tectonics at the surface.