We present a three-dimensional (3-D) crustal velocity model for part of the western Woodlark Basin in the SW Pacific, where rifting of a continental arc transitions to seafloor spreading. Velocities to ~14 km depth are constrained by regularized inversion of first arrival traveltimes recorded at 14 ocean bottom instruments from a network of airgun profiles within an 80 km¿80 km area. The 3-D velocity model images the axis and flanks of the rift basin adjacent to the westernmost spreading segment. Velocity contours that dip north at ~10¿ parallel seismic reflectors from within a thick ophiolitic basement. High velocities (7.5 km/s) at the base of our model and preliminary analysis of earthquake P-wave arrival times are consistent with an average crustal thickness of 15--20 km. Anomalously shallow upper mantle (8 km/s) velocities occur 6 km south of the spreading center at a depth of 3 km below the seafloor, and in the western part of the model, ~30 km east of the D'Entrecasteaux Islands, at a depth of 6 km below the seafloor. ¿ 2001 American Geophysical Union