Morphology and origin of the Osbourn Trough

The Osbourn Trough is a 900 km long, east-west trending gravity low, visibl e in satellite altimetry data, which intersects the Tonga Trench at 25 degr ees 30'S. A recent survey collected gravity, magnetic, echo sounder, and sw ath bathymetry data on three adjacent, north-south trending ship tracks cen tered on the trough. The linear gravity low is 20-30 mGal less than the reg ional value and is accompanied by a flat-lying, 200-500 m deep sediment-fil led valley. Swath bathymetry images reveal several parallel, east-west tren ding linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers, inc luding 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 Superc hron (anomaly 34). These data support the conclusion that this trough is a spreading center, which stopped spreading in Late Cretaceous time. The exis tence of this feature has important implications for tectonic reconstructio ns 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 T rough and the Chatham Rise to the south to be a remnant fragment of the Bel lingshausen Plate. This remnant may have joined to the Pacific Plate when t he Hikurangi Plateau entered the Gondwana subduction zone at the Chatham Ri se possibly causing the cessation of spreading on the Osbourn Trough.