INCREASE OF SEISMIC VELOCITIES IN UPPER OCEANIC-CRUST AND HYDROTHERMAL CIRCULATION IN THE JUAN-DE-FUCA PLATE

Two multi-channel seismic reflection profiles have been analyzed for interval velocities and thicknesses of seismic layer 2A in crust 0-4.5 Ma created at the Endeavour segment of the Juan de Fuca plate. Stacking velocities were interpreted using the Dix assumption for interval velocity and thickness. Stacking velocities were picked from constant velocity stacks as well as normally moved-out gathers at an interval of .2 to 2 km. Interval velocities are 3000-3500 m/s close to the ridge axis and begin to increase at 0.6 Ma when the crust is covered with a few hundred meters of Pleistocene sediments. Velocities are over 5000 m/s by 1.2 Ma, 20 km from the nearest basement outcrop. This increase is associated with an increase in basement temperature from 2-25-degrees-C to 30-50-degrees-C as heat flow values approach the predicted conductive cooling curve and the basement hydrothermal regime changes from fully open to mostly closed. Mineralisation in the abundant porosity of the upper crust is probably triggered by these thermal changes and results in increased seismic velocity. This is the first study to correlate an increase in seismic velocity with the hydrothermal regime in upper oceanic crust and demonstrates that such changes can occur in a remarkably short time.