Diapiric ascent of magmas through power law crust and mantle
Abstract
There has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust. We argue here that this is mainly because the country rocks have previously been assumed to be Newtonian, and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten. The ascent velocity of diapirs is calculated through an equation with the form of the HadamardRybczynski equation for the rise of spheres through Newtonian ambient fluids. This wellknown equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid. These correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of Newtonian drops through power law fluids. The equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks. The velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth. The depth of solidification of the diapir is systematically studied as a function of the geothermal gradient, buoyancy of the body, solidus temperature of the magma, and rheological parameters of the wall rock. The results show that when the wall rock behaves as a power law fluid, the diapir's ascent rate increases, without a similar increase in the rate of heat loss. In this way, diapirs rising at 10 to 10^{2} m/yr can ascend into the middle or upper crust before solidification. Strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates. The thermal energy of the diapir is used to soften the country rock only at late stages of ascent. The transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures.
 Publication:

Journal of Geophysical Research
 Pub Date:
 May 1994
 DOI:
 10.1029/93JB03461
 Bibcode:
 1994JGR....99.9543W
 Keywords:

 Earth Crust;
 Earth Mantle;
 Geodynamics;
 Geophysics;
 Granite;
 Magma;
 Mass Transfer;
 Melting;
 Rheology;
 Solidification;
 Viscous Flow;
 Volcanoes;
 Volcanology;
 Cooling;
 Drops (Liquids);
 Mathematical Models;
 Polynomials;
 Strain Rate;
 Subduction (Geology);
 Temperature Effects;
 Thermal Energy;
 Velocity;
 Viscosity;
 Geophysics;
 Tectonophysics: Physics of magma and magma bodies;
 Volcanology: Magma migration;
 Physical Properties of Rocks: Fracture and flow;
 Tectonophysics: Structural geology (crustal structure and mechanics)