A New Assessment of the Relationships Between Crack Morphology, Porosity, and Seismic Velocity in Young Upper Oceanic Crust

Water carried into subduction zones such as Cascadia, in pore spaces and in hydrous alteration products, affects the hydrogeology of the subduction complex, as well as volcanic activity and seismicity. Though the water content of the crust entering subduction zones cannot be measured directly, it can be estimated from the seismic structure. This study is a new assessment of the relationships between porosity, crack morphology, and seismic velocity, in the upper oceanic crust - the lava pile, transition zone and dike section, or seismic Layers 2A, 2B and 2C. In a theoretical model velocity is a quadratic function of porosity Φ, and the coefficients are functions of the fractional area of asperity contact Af across the cracks - a measure of crack morphology; v = v(Af, Φ). Fits to data sets from the lava and dike sections in Holes 504B and 1256D yield Af = 0.096×0.008 and 0.045×0.003), with rms errors in the estimated fractional porosity of 0.017 and 0.003, respectively. The mean porosities in the lavas and the dikes at these sites are 0.070 and 0.012. Velocities that are characteristic of the lavas are less than 5.8 km/sec, while velocities in the dike section range from 5.8 to 6.7 km/sec. The best-fitting models apply to the upper crust in mature crust sampled in Holes 504B and 1256D, but not to Layer 2A in young crust, i.e., on the flanks of the Juan de Fuca Ridge. However, lower values of Af account for the high porosities and low velocities observed in Layer 2A. An unexpected finding is that, after 0.2 Ma, the porosities of both Layer 2A and the deeper part of the lava pile are in the range 8×2% and do not decrease with age. The observed increase of Layer 2A velocities is caused by changing crack morphology - increasing values of Af probably caused by progressive alteration - not by decreasing porosity; at 8% porosity, an increase of Af from ~0.003 to 0.1 causes velocities to increase from ~3 to > 5 km/sec. Af is thus the primary variable affecting seismic velocities in the uppermost oceanic crust. Conversely velocity is a measure of Af, not of porosity. Thus the porosities of the dike and lava sections, including Layer 2A, of the crust are near 8 and 1.2%, and volume of water carried in subduction zone depends only on the thickness of these units. The fractional area of asperity contact is also related to the permeability of the uppermost crust on the East flank of the Endeavour Ridge, and elsewhere.