Półkule aktywności płaszcza

Abstract

MANTLE ACTIVITY HEMISPHERES Summary The major geological process, reflecting differentiation at the mantle-core boundary and leading to thermogravitational convection, takes place in zones of accretion of oceanic lithosphere. Origin of new oceanic litosphere is the final result of this process. Such conclusion may be drawn on the basis of the Holmes - Dietz - Hess idea and views of R.W. van Bemmelen (5, 6), J.V. Artiushkov (2, 3), O.G. Sorochtin (14, 15), V.P. Miasnikov and others (9) as well as results of the latest studies of American geophysicists (e.g. Don L. Anderson, R.W. Clayton, and A. Dziewonski, 8), using seismic waves and the most advanced technology to reconstruct spatial distribution of temperature and density in the whole mantle. The available data show asymmetry in distribution of accretion zones in the scale of the globe. It appears possible to differentiate two hemispheres (Fig. 1): one, on which almost the whole system of accretion zones is situated, and the other, almost without such zones. Assuming the leading role of the phenomenon of differentiational activity of the lower mantle the author called the former as the hemisphere of mantle activity, and the latter - the quiet mantle hemisphere. The process of differentiation at large depths and thermogravitational convection presumably also takes place in the quiet mantle hemisphere but its vertical extent is smaller and influence on lithosphere less significant because of lower temperatures. Therefore, this does not speak against the above presented asymmetry of the mantle. In discussing the question of hot spots it is assumed that they are of astenospheric origin. Their mobility may be due to horizontal currents in that layer, probably induced by some agents including thermogravitational convection diapirs (rising from the upper mantle to astenosphere but not further). The subdivision of the globe with reference to distribution of hot spots, presented by R.W. Clayton and R. Corner of Caltech (8), is shown to be fairly consistent with the above presented subdivision into hemispheres differing in activity of the mantle. The historical aspects of the subdivision are briefly discussed. The differences in mantle activity can be traced back to about 100 m. y. ago, i.e. the end of a period transitional from that of the Paleozoic activity, characterized by concentration of activity of the mantle in the present day quiet mantle hemisphere. Horizontal mobility of active accretion zones, interpreted as of special importance by S.C. Solomon and others (12, 13) and several other authors of continental drift reconstructions, appears so limited in scale that it fails to explain the above discussed asymmetry. A special attention should be paid to clear interrelationship of relief of the geoid and the mantle activity hemispheres (Fig. 1). Two major branches may be differentiated in the global system of accretion zones. One of them is called here as meridional and used to establish meridian 0° for both hemispheres, and the other - as equatorial with reference to its setting in equatorial zones of the two hemispheres of mantle activity (Fig. 1). Both branches are characterized by markedly sinusoidal outline (Figs 2A, B). Attention should be also paid to homologous orientation of transform faults in relation to both meridians 0° and 180° and equator separating the hemispheres (Figs 2A, B). The knowledge of factors responsible for the differences in activity of the mantle in the two hemispheres, and the explanation of the recorded correlations and characteristic features of the system of accretion zones appear highly important from the point of view of theory of mantle differentiation and convection as well as that of mobility of lithosphere.