The Mohorovicic discontinuity as a phase transition

Abstract

The temperature and pressure regime at the Mohorovicic discontinuity is developed for four specific geographic areas in the Central Stable Region of the United States by detailed geophysical crustal modeling in these areas. Currently available geophysical and geological data for each area are combined with the steady-state thermodynamic equilibrium assumptions to determine a temperature-pressure relationship for the Moho. The parameters of the models are constrained by the seismic refraction data, the surface heat flow and the observed gravity in each area. A modified Monte Carlo solution technique is developed and processed on an electronic computer to calculate the resultant temperature-pressure regime and to determine the precision or confidence limits of the solutions. The depths to the Moho for the modeled areas range from 33 kilometers to 52 kilometers allowing a wide rangeof temperatures and pressures to be developed. A linear relationship between temperature and pressure is derived for the Moho in this region by calculating the least squares line for the four data points. The correlation of this relationship with experimental investigations of phase transition properties of various mineral assemblages allows the interpretation that it is highly probable for the Moho to exist as a first order isochemical phase transformation for at least this one limited region of the earth. The pressure-temperature line for the central United States Moho, with dP/dT (approximate equal sign) +26 bars/°C, not only parallels the experimental phase transition zones for gabbroic-basaltic rocks, but it is also seen as the direct and immediate linear extension of these zones into the lower pressure and temperature regions..