Convection in a non-Newtonian mantle, continental drift, and mountain building

Abstract

A condition for thermal convection has been derived for a mantle of non-Newtonian viscosity, with the properties of a crystalline material in the hot creep range. The critical magnitude of the yield stress (creep limit) at which convection can take place with plausible values of the dimensions of the hot column (ca. 1000 km) and of the temperature difference (ca. 100 degC), practically coincides with the stress drop estimated from the energy release in deep focus earthquakes. Thus, convection in the mantle appears probable. It is likely to take for the form of the rise of relatively narrow hot dikes. If the oceanic ridges are ascribed to convective dikes, their characteristic profile, sharp curvatures, and the direction of the intersecting Murray-Menard wrench faults can be understood. Non-Newtonian dike convection involves a mechanism which drives the ascending current towards the central position between two continents. Bernal’s view that the oceanic heat flow may not be caused by an excess of radioactivity but by the rise of convection currents in the oceanic mantle can be based on the assumption that the present hot convective dikes originated under primeval continents before their disruption. The cohesive, gravitational, and frictional components of the strength of the continental and oceanic lithosphere are quantitatively compared, and reasons are given for the relative incompressibility of continents and for the relative frequency of strike-slip earthquakes. The coefficient of quasi-viscosity of sedimentary rocks, as manifested in the rise of salt domes, is estimated; it has the Fennoscandian magnitude. The soft layer (asthenosphere) in the upper mantle is attributed to low melting and volatile constituents forming glassy grain boundary envelopes. Absence of isostatic adjustments, as in India or in the Moon, may be due to loss of water (in the case of India, perhaps by the outpouring of the Deccan trap). The velocity of the westward drift of the Americas is estimated both from the gravity spreading pressure under the Mid-Atlantic Ridge, and from the energy release in circumpacific earthquakes. Both estimates give the order of magnitude of 1 cm/y. The plasticization and fluxing of the crust on which the orogenic process is based may be the consequence of the precipitation of water from serpentine flowing continentward from the oceanic ridge where the horizontal flow is directed downward at the marginal continental shear plane. The accumulation of water and other low melting and volatile components under the shear plane (above the 500 °G isotherm) reduces the density until the density inversion assumed by Daly occurs. The density inversion reverses the gravitational stabilization of the crust against horizontal buckling and permits orogenic folding by the compressive stress exerted by the rising hot dikes. Geosynclinal subsidence may be a consequence of the outpouring of lava from under geosynclinal areas. With recent order-of-magnitude estimates of the annual outpouring of lava and of the magnitude of the active geosynclinal areas, a rate of subsidence of about 0-1 mm/y is obtained.