3-D crustal velocity structure at the rift tip in the western Woodlark Basin

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 a rc transitions to seafloor spreading. Velocities to similar to 14 km depth are constrained by regularized inversion of first arrival traveltimes; reco rded at 14 ocean bottom instruments from a network of airgun profiles withi n an 80 km x 80 km area. The 3-D velocity model images the axis and flanks of the rift basin adjacent to the westernmost spreading segment. Velocity c ontours that dip north at similar to 10 degrees 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 ti mes are consistent with an average crustal thickness of 15-20 km. Anomalous ly shallow upper mantle (8 km/s) velocities occur 6 km south of the spreadi ng center at a depth of 3 km below the seafloor, and in the western part of the model, similar to 30 km east of the D'Entrecasteaux Islands, at a dept h of 6 km below the seafloor.