HR: 0830h
AN: T31D-0861    [PDF]
TI: Arrangement of Convection in the Earth by Lunar Gravity,II: Geotectonics Under a Minute Wsstward Tilt, With TPW
AU: * Bostrom, R C
EM: rbostrom@washington.edu
AF: ESS-Planetology Box 351310, Univ./Washington, Seattle, WA 98195 United States
AB: G. Darwin's lunar retarding torque is magnitude orders too small to cause lateral motion in a viscous passive Earth [1]. Nevertheless plate-motion data suggesting an apparent net lithosphere rotation seem to accumulate, confirming that given convection under gravity, this can scarcely be immune to an asymmetrical field component. Investigative obstacles have lain in establishing an ITRF tying surface benchmarks to Earth's interior, and a dynamics quantitatively capable of shaping the convection. By delimiting the lunar orbital expansion (irrespective of whether due to marine or body-tide dissipation, or yield under convection itself), LLR [2] delimits the secular, whole-Earth, day-averaged field under which mantle convection takes place. Thus a derived value 600 seconds of the luni-tidal interval indicates that masses not reaching equilibrium add to the secular field a component tilted by arcsin[(600secs)/(25hrs24 min)] = 0.38 degrees (relative to symmetrical standard-{\bf g},- the latter pertinent only to an isolated Earth). The derived value delimits also the dissipation, and accords with the increase in l.o.d. and Earth/Moon astronomic history. Conversely, were {\bf g$_{tot}$} not minutely west-tilted, a couple would not exist, hence Earth-Moon distance not increase. Assumption that the convection develops under a symmetrical tensor field {\bf g} in strict accordance with NNR, neglecting the tilt inherent in observed tidal components, is thermodynamically untenable. Convection at all scales must be to some extent asymmetrical. How to assess the effect in a heterogeneous Earth of a system so minute, but operative throughout geological time? Plate motion and ocean development combined with paleomagnetically established TPW [3,4,5,6] display the following:- During Mesozoic times until -110Ma the pole was located at 'quasi-still-stand' in extreme NE Siberia,present coordinates; the regime of convection then operative resulted in N Atlantic birth, under NW-SE extension. Associated with a regime change at about -85Ma, causing or in consequencec of mass displacement in IndOcean development [7], the pole then drifted towards its present location. Coincident with the change in tilt azimuth there took place the Cenozoic global plate reorganization early discovered by NOAA [8], entailing "reorientation of relative plate motions with large N-S components into large E-W components"; constituting birth of the S Atlantic, development of the Atlantic Ocean as a whole, still ongoing, and E-W motion of the large Pacific plate. In sum, the record back through Mesozoic times suggests that the minute tilt inherent in the tidal component of {\bf g$_{tot}$} is not without consequence: that given autonymous mantle convection and equally inevitable TPW [9], a flow bias is likely since early times, in polarity surface-westward {\it referred to the contemporary pole of rotation}. Theory and data suggest that as base for modeling convection and geotectonics it would be rewarding to relax confinement to NNR, admitting an element of net-lithosphere-rotation whilst still conserving {\it system} angular momentum. [1] Bostrom, R.C., 1973, {\it Phil. Trans. Roy. Soc. A274: 397-407}. [2] Dickey, J.O. et al., 1994, {\it Science 265:482-490}. [3] Van der Voo, R., 1993. {\it Paleomagnetism. Cambridge U.P., 411 pp.} [4] Prevot, M.P., et al., 2000, {\it EPSL 179:517-528}. [5] Sager, W.W., \& A. Koppers, 2000, {\it Science 287: 455-459}.[6] Besse,J. \& V. Courtillot, 2002, {\it JGR 107(B11) EPM 6-1}. [7] Bostrom, R.C., 1990, {\it Tectonophysics 182: 393-402}. [8] Rona, P.A. \& E.S. Richardson, 1978, {\it EPSL 40: 1-11}. [9] Goldreich, P.,\& A. Toomre, 1969, {\it JGR 74:2555-2567}.
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 5450 Orbital and rotational dynamics
DE: 8122 Dynamics, gravity and tectonics
DE: 8155 Plate motions--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting