Rotational bulge and TPW in a reference frame described by hotspots since 200Ma: influence of an elastic crust.

We aim to investigate the influence of a viscous or elastic layer in the upper part of the lithosphere on the Earth's dynamics. We start with a simple mantle dynamic model integrating subducted lithosphere, domes located at the bottom of the mantle and upwelling plumes for the last 300 Ma. Our calculations are performed in a new hot spot reference frame model constructed with various surface indicators (geology, thermal data from boreholes and a compilation of global surface volcanism), a reassessment of hot spots classification and paleomagnetic data. Using a MonteCarlo approach, we first research the two best models of mantle viscosity which explain the present day geoid, gravity and gradients of gravity induced by these mantle density heterogeneities, for two cases: with and without an elastic crust. The temporal evolution of mantle mass anomalies in these two models displays a large degree 2 order 2 component over the last 200Ma which may be explained by the quasi permanence of a girdle of subduction around the Pacific ocean (or its ancestor) and by the presence of two quasi antipodal domes found to remain close to the paleomagnetic equator. We then calculate the time-dependent Principal Inertia Axis (PIA) and the associated True Polar Wander (TPW, the shifting of the Earth's rotation axis with respect to the mantle). The integration of the elastic crust generates a remnant rotational bulge perpendicular to the initial axis of rotation which tends to stabilize the polar motion. We discuss this stabilizing effect dependent on the amplitudes of the inertia tensor perturbations due to the remnant bulge and the ones due to the mantle density heterogeneities.