In order to investigate the impact of off-axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust (layer 2A), we performed an extensive geophysical survey on the eastern flank of the East Pacific Rise at 14 ¿S. Seismic refraction and heat flow data were obtained along a 720-km-long and 25 to 40-km wide corridor, covering thinly sedimented seafloor created since 8.5 Ma. The seismic data yield a seismic velocity of ~2.9 km/s at the top of 0.5-m.y.-old basement rocks. Within about 8 m.y. the velocity increases gradually to a value of mature oceanic crust (~4.3 km/s). Heat flow data, derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements, suggest that an open hydrothermal circulation system persists for at least 6--7 m.y. In crust older than 7 Ma, regional heat flow is close to values predicted by plate cooling models, suggesting that hydrothermal circulation is going to cease. Considering published dating of alteration minerals, it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10--15 m.y. This idea may explain why seismic velocities in the Pacific ocean have not changed significantly in igneous crust older than 8--10 Ma. In regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different; velocities increase rapidly and reach values of mature oceanic crust within 1--2 m.y. We therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust <Stephen and Harding, 1983; Rohr, 1994>. ¿ 1999 American Geophysical Union