Formation‐scale hydraulic and mechanical properties of oceanic crust inferred from pore pressure response to periodic seafloor loading

Abstract

Observations of fluid pressure variations in young igneous oceanic crust have been made in five sealed and instrumented Ocean Drilling Program boreholes on the flanks of the Mid‐Atlantic and Juan de Fuca Ridges. The holes penetrate locally well sedimented, and hence hydrologically well‐sealed crust, and are situated 1 to 85 km from areas where water can flow freely through the seafloor at extensive basement exposures. Amplitudes and phases of formation pressure variations have been determined relative to tidal and noritidal pressure variations measured simultaneously at the seafloor for periods ranging from 4.8 hours to 14 days. Formation pressure variations are reduced to amplitudes between 98% and 28% relative to those at the seafloor and shifted in phase by up to 20°. Simple theory for porous media response to periodic loading predicts that the scale of diffusive signal propagation from locations of basement outcrop through buried parts of the igneous crust should be proportional to basement permeability and the inverse square root of the period of the variation. This behavior is consistent with the observations, and the characteristic half wavelength of the diffusive signal defined by the data from the sites near basement exposures is 14 km at diurnal periods. If signals propagate in a simple one‐dimensional manner, this requires a formation‐scale permeability of 1.7×10−10 m2. No constraints are provided on the thickness of material characterized by this permeability, but the high‐permeability pathway must be laterally continuous. At two sites near basement exposures the bulk modulus of the rock matrix estimated from the elastic component of the pore pressure response is 3 GPa. Where the igneous crust is regionally well sealed by sediment, the elastic response yields a bulk modulus of 16 GPa. The increase in bulk modulus with age and distance from basement outcrop is consistent with an observed increase in crustal alteration, an increase in seismic velocity, and a decrease in permeability. Observed lateral gradients of pressure, coupled with the estimated permeability, suggest that the amplitude of semidiurnal tidal volumetric flux (Darcy velocity) exceeds 10−6 m s−1; semidiurnal fluid particle excursions would reach 0.25 m if the full volume of water contained in 10% porosity of the rock matrix were involved. If flow is channelized along discrete pathways, tidally modulated fluid flow velocities and particle excursions would be locally greater. The amplitude of tidal velocity is similar to that estimated for buoyancy‐driven hydrothermal convection, but the direction is generally different. Thus tidal flow may enhance water‐rock interactions significantly. Energy dissipated in this manner would approach 0.3 μW m−3.