The Osbourn Trough is a 900 km long, east-west trending gravity low, visible in satellite altimetry data, which intersects the Tonga Trench at 25 ¿30'S. A recent survey collected gravity, magnetic, echo sounder, and swath bathymetry data on three adjacent, north-south trending ship tracks centered on the trough. The linear gravity low is 20--30 mGal less than the regional value and is accompanied by a flat-lying, 200--500 m deep sediment-filled valley. Swath bathymetry images reveal several parallel, east-west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers, including a prominent inside corner high and possible pseudo-fault trace. The presence of magnetic anomalies (possibly anomalies 33 and 32) suggests that the seafloor here was formed after the end of the Cretaceous Normal Superchron (anomaly 34). These data support the conclusion that this trough is a spreading center, which stopped spreading in Late Cretaceous time. The existence of this feature has important implications for tectonic reconstructions in this region. The Osbourn Trough could be part of the fossil spreading center between the Pacific Plate and a fragment of the Phoenix Plate, the Bellingshausen Plate. This would require the seafloor between the Osbourn Trough and the Chatham Rise to the south to be a remnant fragment of the Bellingshausen Plate. This remnant may have joined to the Pacific Plate when the Hikurangi Plateau entered the Gondwana subduction zone at the Chatham Rise possibly causing the cessation of spreading on the Osbourn Trough. ¿ 2000 American Geophysical Union