A seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the Juan de Fuca Ridge at 45¿N, at the northern Cleft segment and at the overlapping rift zone between the Cleft and Vance segments. These data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low-velocity layer, which we interpret as the extrusive volcanic layer. The experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small (20 km by 35 km) area. These travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust, presumed to be caused by ridge-parallel fracturing. Compressional velocities are 3.35 km/s in the ridge strike direction and 2.25 km/s across strike. Travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low-velocity layer. Extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m. The zone of the thinnest low-velocity layer is within the northern Cleft segment axial valley, in a region of significant hydrothermal activity. Layer thickness variability is greater near the Cleft-Vance overlapping rift zone, where changes of 300 m occur over as little as several kilometers laterally. These low-velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system, where the low side of each fault block accumulates more extrusive volcanics.