Mantle dynamics, geoid, inertia and TPW since 120 Myr

We investigate the effect of internal masses redistributions on the position of the Earth rotational pole for the last 120 Myr. We use a geodynamic model based on plate reconstructions that estimates the location and rate of subducted slabs under the assumption that they sink vertically into the mantle (Ricard et al., 1993b). Our model also takes into account the effect of large-scale upwellings (domes) derived from an analysis of seismic tomography. Their location is assumed to remain stable with time. We then compute the geoid associated with the time-dependent mantle density heterogeneities. In order to reconcile the computed and observed geoids, we investigate the influence of the depth down to which the subducted Pacific plates beneath the Americas present a significant density contrast with respect to the surrounding mantle on the present-day geoid, and propose a plate model in which we obtain a variance reduction greater than 0.9 for the degree 2. We show that the vertical oscillation of domes within mantle only modulates the amplitude of the associated geoid. The temporal variation of the mantle density heterogeneities is consequently essentially due to changes in the subduction history. The temporal evolution of the Principal Inertia Axis (PIA) of the Earth derived from our model (the rotational axis is aligned to the maximum PIA) is then investigated, and finally compared to estimations of TPW. Both the maximum and intermediate PIAs have moved in a plane perpendicular to Africa, along a circle corresponding to the low of geoid induced by subduction around the Pacific. The minimum PIA seems to be relatively stable since 120 Myr, and close to the maximum degree 2 geoid high under Africa. This can account for observed TPW or African APW in the last 200 Myr.