GNGTS 2014 - Atti del 33° Convegno Nazionale

In a Coriolis-effect-free Earth’s mantle, the westward drift of tidal origin of the lithosphere has been adopted by main stream (Bostrom, 1971; Doglioni et al. , 2011; among others) as explanation of the East-West asymmetry of the Wadati-Benioff zones, with additional assumptions. But the same argument of high viscosity can be used to reject the westward drift because the negligible value of tidal force in comparison to viscous friction (Jordan, 1974; Ranalli, 2000; Caputo and Caputo, 2012). In the first years of plate tectonics the hypothesis was proposed that the plates could be decoupled from underlying mantle at level of asthenosphere, but Jordan (1974) proved that the depths of oceanic and continental lithospheres are very different and that the roots of continents can be detected up to 350 km. The consequent undulations of the ideal surface that defines the roots of oceans and continents do not allow for a tidal westward drift on it. Plate tectonicists have resolved the problem by hypothesizing a thin low viscosity layer at the depth of about 400 km, immediately upon the transition zone but still not observed by seismology (Caputo and Caputo, 2012). Besides the lacking of evidence in favor of this thin layer, evidence exist of regional upwelling of the 400 km discontinuity (Scalera et al. , 1981; Piromallo and Morelli, 2003). The evidence of strong undulations of transition zone produces consequent problems in the hypothesized thin low viscosity layer, which could be uplifted by the upwelling transition zone or cutted and interrupted by it – a difficulty for the horizontal motion of the plates in the explanation of the E-W asymmetry without Coriolis effect. Although the mutual importance of all the forces acting on the mantle materials must first be assessed, it is important to note that the observable facts seem to indicate a non-negligible action Fig. 1 – a) The plate tectonics representation of the plate motion does not allows Coriolis effects. Subduction zones are well defined at the leading edges of the plate motions. – b) If inertial Coriolis effects would be present the plate paths would be circular, with a problematic definition of the subductive margins. This pattern is not possible on an expanding Earth because of the extremely limited horizontal motions. Fig. 2 – a) Convective motions in the Earth’s mantle divided in upper-mantle and lower-mantle cells. In this representation Coriolis inertial effect is not taken into account because – following the main stream conception – only a laminar flow of few cm/yr are believed to occur. The Ekman ratio E k ≈ 109 does not allows significant Coriolis effects. – b) The mantle motions of a rotating Earth could be deformed by the Coriolis effect in their upward and downward flows if impulsive motions occur. During earthquakes the mantle materials can slip with velocities in the range V = 1.0 - 10.0 m/s. In this case inertial forces cannot be neglected in comparison to viscous friction. In the expanding Earth framework the surfacewards impulsive motions are very likely to occur. 176 GNGTS 2014 S essione 1.2