Evidence for a 20 degrees tilting of the Earth's rotation axis 110 millionyears ago

True polar wander (TPW), the shift of the Earth's rotation axis with respec t to the entire globe, is most probably due to mass redistribution in the E arth's mantle as a result of convection. Using a new rigorously selected pa leomagnetic database gathering only directions obtained from magmatic rocks , we find that TPW has been clearly intermittent over the last 200 Myr with two long periods of strict standstill from the present to 80 Ma and from a pproximately 150 to 200 Ma. A single period of shifting is observed, betwee n 80 and about 150 Ma. This period culminates around 110 Ma in an 20 degree s abrupt tilting during which an angular speed exceeding 5 degrees/Myr (0.5 m/yr) may have been reached. Assuming that the time-averaged geomagnetic f ield is axial, our results indicate that the changes in the position of the rotation axis, and therefore in the inertia tensor of the Earth, are inter mittent. We suggest that a major reorganization of the mass distribution in the Earth's mantle occurred in the Lower Cretaceous. This event, concomita nt with plume hyperactivity at the Earth's surface and probable drastic cha nges at the core-mantle boundary attested by the inhibition of geomagnetic reversals, suggests unmixing of upper and lower mantle by avalanching of up per mantle material down to the core-mantle boundary. The astonishingly str ict stability of the time-averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convec tion which does not modify the large-scale distribution of mass within the mantle. Given the intermittence of mantle avalanching, we suggest that thes e long periods of stability correspond to the temporary reestablishment of a basically two-layered convection system within the mantle. (C) 2000 Elsev ier Science B.V. All rights reserved.