Free water and seismic reflectivity in the lower continental crust

Abstract

Many deep seismic reflection data sets show bright reflectivity filling much of the lower continental crust. Where these reflections are well developed, they have apparent reflection coefficients of at least 0.1. It has been proposed that this reflectivity could be explained by the existence of free water in the lower crust with stratified porosity or pore pressure, and that this free water might also explain observations of high electrical conductivity. If this explanation is correct then it follows that seismic observations of apparent heterogeneity in the lower crust reveal only heterogeneities in fluid distribution, and are not necessarily related to changes in rock type. That hypothesis is tested here. Water in the lower crust has a density and velocity comparable with liquid water at the Earth's surface. Numerical modelling of the seismic properties of rocks containing free water is used to delineate the range of porosity and pore geometry required to produce bright reflections. Porosities of several per cent or thin sheet-like pores with high crack density are required. Rocks in textural equilibrium do not contain thin pores and require porosities in excess of 5% to explain the reflection coefficients. The lower crust will compact, water will be expelled, and sharp boundaries in porosity and pore pressure will diffuse in geologically short times unless the water is distributed in isolated equi-dimensional pores. It is concluded that free water cannot explain the seismic reflections seen from the lower crust in stable areas, and that there are good geophysical and petrological reasons for supposing that the lower crust in the stable continental interior and on stable continental shelves typically contains little or no free water. The widespread observations of seismic reflectivity in the lower continental crust do indeed therefore imply significant heterogeneities in rock type.