True polar wander affects the Earth dynamic topography and favours a highly viscous lower mantle

Abstract

True Polar Wander (TPW), the global motion of the Earth's mantle relative to the axis of rotation, is considered for its impact on the degree 2 surface dynamic topography induced by mass anomalies in a viscously stratified mantle. These sources, responsible for a geoid and topographic signal at the Earth's surface, mimic the effects of subductions and other thermal instabilities on long‐term Earth's rotation. A rotating, viscous planet has the ability to displace the axis of rotation with respect to internal mass anomalies in order to maintain, on the average, the excess of non‐hydrostatic geoid along the equator, affecting at the same time the degree 2 pattern of the dynamic topography which supports the geoid. By means of a time series of synthetic mass anomalies, randomly distributed in order to avoid any bias due to the clustering of the sources on time scales of 107 yr, we show that TPW introduces a selection rule which favours in the dynamic topography the order m=0 for an isoviscous mantle and the order m=2 for lower mantle viscosities at least a factor 30 higher than in the upper mantle. The dominance of the order m=2 in the dynamic topography of the real Earth, induced by the mass anomalies inferred from seismic tomography, agrees with the requirements of a rotating planet deformed by internal sources, characterized by a substantial viscosity increase in the lower mantle. This agreement seems to indicate that the intimate physical reason of the high order 2 content in the topography stands on the interplay between the dynamic response of the mantle to internal sources and Earth rotation.