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

Observations of fluid pressure variations in young igneous oceanic crust ha ve been made in five sealed and instrumented Ocean Drilling Program borehol es on the flanks of the Mid-Atlantic and Juan de Fuca Ridges. The holes pen etrate 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 for mation pressure variations have been determined relative to tidal and nonti dal pressure variations measured simultaneously at the seafloor for periods ranging from 4.8 hours to 14 days. Formation pressure variations are reduc ed to amplitudes between 98% and 28% relative to those at the seafloor and shifted in phase by up to 20 degrees. Simple theory for porous media respon se to periodic loading predicts that the scale of diffusive signal propagat ion from locations of basement outcrop through buried parts of the igneous crust should be proportional to basement permeability and the inverse squar e root of the period of the variation. This behavior is consistent with the observations, and the characteristic half wavelength of the diffusive sign al defined by the data from the sites near basement exposures is 14 km at d iurnal periods. If signals propagate in a simple one-dimensional manner, th is requires a formation-scale permeability of 1.7 x 10(-10) m(2). No constr aints are provided on the thickness of material characterized by this perme ability, but the high-permeability pathway must be laterally continuous. At two sites near basement exposures the bulk modulus of the rock matrix esti mated from the elastic component of the pore pressure response is 3 GPa. Wh ere the igneous crust is regionally well sealed by sediment, the elastic re sponse yields a bulk modulus of 16 GPa. The increase in bulk modulus with a ge and distance from basement outcrop is consistent with an observed increa se in crustal alteration, an increase in seismic velocity, and a decrease i n permeability. Observed lateral gradients of pressure, coupled with the es timated permeability, suggest that the amplitude of semidiurnal tidal volum etric flux (Darcy velocity) exceeds 10(-6) m s(-1); semidiurnal fluid parti cle 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 ex cursions would be locally greater. The amplitude of tidal velocity is simil ar to that estimated fur buoyancy-driven hydrothermal convection, but the d irection is generally different. Thus tidal flow may enhance wafer-rock int eractions significantly. Energy dissipated in this manner would approach 0. 3 mu W m(-3).