The Woodlark rift system is one of the few places where active ocean basin formation can be studied. Within this rift system, continental extension rates are some of the fastest on the planet, and extension progresses eastwards to full seafloor spreading. We use results from a recent passive seismic experiment to address the role of magmatism prior to the onset of seafloor spreading. We invert local earthquake P and S traveltimes for 3-D structure around and ahead of the active spreading tip. From the local earthquake tomography we observe three main structures. Seismic velocities in the crust show a sharp contrast between regions that resemble continental crust ahead of the spreading tip and oceanic crust to the east. In the continental portion, P velocities increase from 5.6 km s(-1) to 6.9 km s(-1) between 10 and 25 km depth, indicating a bulk felsic to intermediate composition, similar to other continental regions. Beneath the seismic Moho (23-28 km depth, as defined by receiver functions) a 10-15-km-thick gradient zone exist locally with velocities from 7.0 to 7.9 km s(-1), which may reflect an underplated mafic, granulite facies lowermost crust, or perhaps magmatic intrusion into the lithospheric mantle. In contrast, farther east and closer to the active spreading tip, velocities rapidly increase from 6.5 to 7.2 km s(-1) between 8 and 18 km depth. These fast crustal velocities appear in a narrow zone roughly 60 km wide and indicate a mafic crust, similar to oceanic crust. Our velocity results suggest that the transition from diffuse continental rifting to localized seafloor spreading likely occurs across a narrow zone. Magmatism prior to the onset of seafloor spreading may not play a significant role in altering crust until the onset of seafloor spreading, except through underplating at the base of the crust.
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