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Present True Polar Wander in the Frame of the Geotectonic Reference System

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Present True Polar Wander in the Frame of the Geotectonic Reference System 1661-8726/08/030629-8 Swiss J. Geosci. 101 (2008) 629–636 DOI 10.1007/s00015-008-1284-y Birkhäuser Verlag, Basel, 2008 Present true polar wander in the frame of the Geotectonic Reference System NAZARIO PAVONI Key words: Geotectonic Reference System, polar wander, geodynamics, geotectonic bipolarity ABSTRACT The Mesozoic-Cenozoic evolution of the Earth’s lithosphere reveals a funda- mA which are responsible for the two geoid highs results in the long-standing mental hemispherical symmetry inherent in global tectonics. The symmetry stability of location of the geotectonic axis PA. The Earth’s rotation axis is is documented by the concurrent regular growth of the Pacific and African oriented perpendicularly to the PA axis. Owing to the extraordinary stabil- plates over the past 180 million years, and by the antipodal position of the two ity of the geotectonic axis, any change of orientation of the rotation axis, e.g. plates. The plates are centered on the equator, where center P of the Pacific induced by post-glacial rebound, occurs in such a way that the axis remains plate is located at 170° W/0° N, and center A of the African plate at 10° E/0° N. in the equatorial plane of the geotectonic system, i.e. in the 80° W/100° E P and A define a system of spherical coordinates, the Geotectonic Reference meridional plane. Observed present polar wander is directed towards 80° W System GRS, which shows a distinct relation to Cenozoic global tectonics. P (79.2 ± 0.2° W). This indicates that the present true polar wander is directed and A mark the poles of the geotectonic axis PA. exactly along the GRS-equator, identical with the 80° W/100° E great circle of The degree-two pattern of the residual geoid displays two major highs, the the Earth. It is concluded that the phenomenon of polar wander is basically Pacific high and the African high. Poles P and A are located in the centers of related to the symmetry of the geotectonic system defined by the Pacific pole the respective geoid highs. The stable configuration of excess masses mP and P, at 170° W/0° N, and the African pole A, at 10° E/0° N. Introduction wander. Wegener (1912, 1915), a geophysicist, was well aware of the fundamental insights into the structure and dynamics It is generally agreed that the present drift of the Earth’s rota- of the Earth provided by the geodetic observations of Earth tion pole is primarily due to late-Pleistocene melting of con- rotation changes. In all of his papers on continental drift he tinental ice-sheets, such as the ice-sheets of Laurentia, Fen- emphasized the fundamental role that isostasy plays in geody- noscandia, and related redistribution of mass within the Earth namics, and thus favoured a plastic deformable Earth. Wegener system (e.g. Vermeersen et al. 1997; Mitrovica & Milne 1998; stated: “polar motion must be understood as a consequence of Johnston & Lambeck 1999; Mitrovica et al. 2006). In addition to mass displacements within the Earth” (Wegener 1912, p. 194). external loads, internal loads due to endogene processes, such From the pattern of observed motion of the North pole in the as thermal convection, will undoubtedly also contribute to po- period 1900–1910, he found a weak indication for a systematic lar drift. Little is known about the spatial and temporal dis- displacement of the solid Earth’s axis of figure towards the At- tribution of uncompensated loads in the Earth’s interior. This lantic ocean (Wegener 1912, p. 309). may explain why traditional predictions of glaciation-induced In a fundamental paper Gold (1955) discussed the absence polar wander neglect endogene contributions to the motion of of long-term rigidity of the solid Earth. Apart from isostasy and the spin axis. In the present paper the contribution of endogene other observations, the absence of long-term rigidity is clearly processes to the phenomenon of polar wander will be discussed documented by the “free” or “Eulerian” nutation of the Earth, and emphasized. To that end a brief historical review will be the Chandler wobble. The wobble corresponds to a periodi- presented. cal movement of the Earth around its axis of rotation, with a The systematic observation of polar motion in the frame period of 435 days, whereby the direction of the rotation axis of the International Latitude Service, ILS, beginning in 1900, remains nearly fixed in space. Origin, period and damping of brought about a growing interest into the phenomenon of polar the wobble are related to the physical conditions on and within Sonnenbergstrasse 11, CH-8134 Adliswil, Switzerland. E-mail: [email protected] Present true polar wander in the frame of the GRS 629 the Earth. Gold (1955) pointed to the observed high damping (Pavoni 1985b) with axis of symmetry located in the equatorial of the wobble. plane represents a very stable configuration (Busse 1975; Pers. In a plastic deformable Earth, the existence of gravity Comm.). The geotectonic axis corresponds to the axis of sym- anomalies over large regions must be understood as being metry, connecting centers P and A. related to geotectonic processes which control the origin and New space geodetic techniques, such as very long baseline redistribution of lateral heterogeneities in the mantle. Since interferometry (VLBI), satellite and lunar laser ranging (SLR these geotectonic processes are long-lived, the related non-hy- and LLR), and utilization of the Global Positioning System drostatic loads and gravity anomalies must be also long-lived. (GPS) technology, which became available in the 1970s and The distribution and extent of the excess masses shows up in 1980s allowed an accurate determination of the Earth’s gravi- the undulations of the geoid. Since there is plastic flow, the ro- tational field. The observed long-wavelength geoid (Lerch et tation axis of the Earth will follow closely any imposed changes al. 1979), referred to the equilibrium hydrostatic figure of the of the axis of maximum non-hydrostatic moment of inertia. The Earth (Nakiboglu 1982), shows little resemblance to the distri- stability of the spin axis is therefore dependent upon the stabil- bution of continental and oceanic lithosphere. The same holds ity of the geoidal shape (Gold 1955). An extensive summary of true for the residual geoid, which is obtained when the effect of previous studies dealing with rotation of the Earth is presented subducted slabs in the upper mantle is removed from the ob- in the book of Munk & MacDonald (1960). served geoid heigths (Chase 1979; Crough & Jurdy 1980; Hager Goldreich & Toomre (1969) extended the ideas of Gold 1984). As stated by these authors, the residual geoid heigths (1955) and also concluded that the gradual redistribution of have the form of two broad, elliptical positive areas surrounded density inhomogneities produced by convective processes by lows. With regard to the geotectonic bipolartiy model men- within the Earth can cause large motions of the entire mantle tioned above, the residual geoid displays the same Pacific/Afri- relative to the space fixed axis of rotation (true polar wander, can hemispheric symmetry as expressed in the model, with the TPW). Since these works, satellite-derived data regarding the broad geoid highs corresponding to the cylindrical upwellings, Earth’s gravitational field and the geoid have become available the belt of geoid lows trending along 100° E/80° W correspond- (e.g. Kaula 1966). According to Goldreich & Toomre (1969) the ing to the downwelling flow of the proposed mantle convection inertial moment of the Earth may be separated into a hydro- (Pavoni 1983, Figs. 10, 11). static part due to the equilibrium bulge under rotation and a Concurrently with the survey of the Earth’s gravitational non-hydrostatic part due the topical distribution of mass anom- field, the world-wide installation of modern seismograph net- alies. The authors show that after subtraction of the hydrostatic works in the 1960’s, and new technologies of geophysical data part, the remaining non-hydrostatic part of the Earth’s inertia aquisition and interpretation allowed a precise determination figure is distinctly triaxial. of the global pattern of seismicity, marking the advent of the By the end of the 1960’s, it had become clear that the Earth theory of plate tectonics (e.g. McKenzie 1969). The global seis- displays a high internal mobility due to plastic flow. TPW can mic data allowed the establishment of a preliminary reference occur on a varity of time scales, on the order of thousands to Earth model PREM (Dziewonski & Anderson 1981), and a de- millions of years. On a shorter time scale, it is generally assumed scription of the lateral heterogeneities of seismic velocity and that present TPW results from Pleistocene deglaciation. On density in the Earth’s mantle (Dziewonski 1984; Woodhouse & longer time scales polar wander is caused by gradually chang- Dziewonski 1984). The large-scale pattern of lateral heteroge- ing density heterogeneities generated by long-lived geotectonic neities of seimic velocity in the lower mantle shows the same processes, such as thermal convection in the mantle. hemispherical symmertry as expressed in the residual geoid. In this context attention is drawn to the geotectonic bi- World-wide geophysical und space-geodetic research done polarity model, which is of relevance to the discussion below. in the 1970’s and 1980’s thus confirmed and completed the Pa- The model is an attempt to describe the large-scale pattern of cific/African hemispheric symmetry observed in global tecton- mantle convection as derived from the Mesozoic-Cenozoic ics. In particular, it confirmed the large-scale pattern of mantle- evolution of the Earth’s lithosphere. The bipolarity model was wide (or thermally coupled) convection in the Earth’s mantle, introduced in the 1960’s, before the advent of plate tectonics.
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