HR: 09:50h
AN: U31B-08    [Abstracts]
TI: A Short Review of True Polar Wander
AU: * Courtillot, V
EM: courtil@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 place Jussieu, paris, 75252 France
AB: Polar wander on Earth has been suggested since the 19th century. With the discovery that continental drift accounted for much of the apparent polar wander of continents, one could ask whether there was a remaining ("true") fraction in polar wander not accounted for by plate tectonics, which would be a characteristic of "Earth as a whole". TPW results from conservation of angular momentum in a rotating, deformable body. Seen from the surface of the Earth, TPW appears as the wave-like propagation of the Earth's bulge, whose rate of motion is controlled by mantle viscosity. The bulge adjusts in about 104 years, the characteristic time for glacio-isostatic rebound. Paleomagnetic poles (used to derive APWPs) and oceanic data (used to derive plate kinematic models) can be blended to produce a "synthetic " APWP for all plates. Motion of hotspots with respect to plates can then be integrated to derive an estimate of TPW: this displays in succession a standstill at 160-130 Ma, a quasi-circular track from 130 to 70 Ma (rate 30 km/m.y.), a standstill at 50-10 Ma and faster motion up to the present (rate 100 km/m.y.). Suggested episodes of superfast TPW seem to be artefacts. Field geometry is unlikely to severely alter TPW estimates. A legitimate concern is that the analysis is not truly global (it fails to encompass the Pacific plate). And there are ongoing debates on the fixity of hotspots with respect to each other. We find little evidence for significant inter-hotspot motion (larger than 5 km/m.y.) either within the Pacific or Indo-Atlantic hemispheres. Other authors do find some motion (e.g. between Hawaii and Louisville). We suggest that (primary) hotspots form two slowly deforming subsets in the two geodynamically distinct hemispheres. The two subsets would have been in slow motion for the last 45 m.y., but in faster motion prior to that. Other authors (Gordon) conclude that there is no significant motion in the past 125 m.y. It has been suggested that a quasi-discontinuous 90ø rotation is possible in which the rotation axis will align with a new axis of maximum non-hydrostatic moment of inertia (inertial interchange event IITPW). Oscillatory polar wander is thought by some to be documented in the Precambrian, which is attributed to TPW about a long-lived inertial axis inherited from the super-continent of Rodinia (Evans). Modeling of TPW has made significant progress. Seismic tomography is used to infer 3D maps of density heterogeneities that drive flow in the viscous mantle. The maximum speed of polar wander driven by mantle convection is about 100km/m.y. A significant viscosity increase in the lower mantle is required to bring TPW rates closer to observed values. Recent modeling (Steinberger) involves limited, predictable inter-hotpot motion. One can still derive a TPW curve in the "mean mantle" reference frame that takes hotspot motions into account. This captures most of the features of the observed TPW. Possible links between 1) TPW episodes, or major changes between TPW episodes, or IITPW events, 2) reorganizations in the geometry of subduction zones (e.g. avalanches), or plume and superplume generation, and 3) major biotic changes (e.g. mass extinctions) will likely keep analyses of polar wander very much alive in the coming decade, despite the somewhat bothering feeling that true polar wander still remains an elusive geophysical phenomenon.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 1714 Geomagnetism and paleomagnetism
DE: 1239 Rotational variations
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1560 Time variations--secular and long term
SC: Union [U]
MN: 2004 AGU Fall Meeting