HR: 1340h
AN: T43B-1328    [Abstracts]
TI: Re-arrangements of Global Plate Motion: Role of True Polar Wander (TPW)
AU: * Bostrom, R C
EM: rbostrom@washington.edu
AF: Earth & Space Sciences, University of Washington, Box 35-1310, Seattle, WA 98195-1310 United States
AB: Plate-motion models constructed by R.G. Gordon and D.M. Jurdy (1986) and at Harvard by R.J. O'Connell et al. (1991), further developed by O. Cadek and V. Ricard (1992), show that in sum Cenozoic plate motion has been concentrated around the degree 1 harmonic spectral component, representing net lithosphere rotation (NLR) about the contemporary Pole. Participants allowed for uncertainties in using hotspots as benchmarks. In the absence of an alternative to tidal action to account for perennial convection asymmetry, otherwise enigmatic (D. Bercovici 2003), plate motion under the observed degree 1 system is here compared with the regime to be expected under geocentric plus external gravity. Mantle convection takes place under a minute westward tilt in globally-averaged {\it g}, contributed by a tidal component having the value -$2.16\deg$ identified by G.J.R. MacDonald (1964). Previously impossible, R.D. Ray et al. (2001) recently have combined satellite tracking data and Topex/Poseidon altimetry to separate the solid-earth fraction of the associated dissipation, 110 +/- 25 GW, from the obscuring marine signal. Hitherto, in model construction it has been necessary to assume almost perfect elasticity. Surprisingly large in terms of earlier estimates of the dissipation factor 1/Q, the value obtained conforms notably with supposition as per Cadek and Ricard, that the upper mantle remains constantly at the point of failure representing advance, in precisely this mode, of the internally driven convection basically responsible for global tectonics. The regime is self-reinforcing and prone to be stable. How might it be interrupted? Some 25 years ago P.A. Rona and E.S. Richardson (1978) identified the global plate reorganization which took place in the Lower Cenozoic. Essentially, reorganization was directional in nature, characterized by supplantation of basic geotectonic features such as seafloor spreading and subduction having a N-S orientation, by similar features oriented E-W. It may now be significant that with increased understanding of the paleomagnetic record (M. Prevot et al. 2000; J. Besse and V. Courtillot 2002), shift of Earth's pole of rotation (TPW) has become a well demonstrated occurrence, although agreement is far from universal as to its modus operandi. Prior to -130 Ma interpretation still is subject to much uncertainty, the major subsequent event having been the end of the -130 to -60 Ma period of relatively fast polar wander. Standstill may have set in as from about -50 Ma. An earlier period of standstill, currently estimated to extend from -200Ma to about -150 Ma, was co-extensive with the Jurassic, being replaced by the highly TPW-active Cretaceous. Agreement has not been reached as to the rapidity of a $20\deg$ ultra-fast episode or `jump' culminating at around -115 Ma, at the time of most active break-up of Gondwana into its constituent continents and formation of such ranges as the Rockies and European Alps. If the Cadek/Ricard summary of plate motion or one similar is sound, TPW entails equivalent change in the direction 'west'. The liberty is now taken to suggest that rather than being subsidiary to unexplained reorganizations of global tectonics, TPW is causative and primary. Thermodynamics requires that under TPW, junctures must occur at which Earth's heat efflux is effected most efficiently via convection compliant with a greatly different Pole location, compelling the occurrence of a fast episode or `jump' of the ilk reported by Prevot et al. Within the grossly inhomogeneous, convective Earth, the operation of the conjugate factors NLR and TPW seems basic; of which, the latter has constituted the primary geotectonic determinant.
DE: 9820 Techniques applicable in three or more fields
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1214 Geopotential theory and determination
DE: 1249 Tides--Earth
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting