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 i nterval velocities and thicknesses of seismic layer 2A in crust 0-4.5 Ma created at the Endeavour segment of the Juan de Fuca plate. Stackin g velocities were interpreted using the Dix assumption for interval ve locity and thickness. Stacking velocities were picked from constant ve locity 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 a xis and begin to increase at 0.6 Ma when the crust is covered with a f ew hundred meters of Pleistocene sediments. Velocities are over 5000 m /s by 1.2 Ma, 20 km from the nearest basement outcrop. This increase i s associated with an increase in basement temperature from 2-25-degree s-C to 30-50-degrees-C as heat flow values approach the predicted cond uctive 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 corr elate 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.