History of the Cretaceous Osbourn spreading center

Abstract

The Osbourn Trough is a fossil spreading center that rifted apart the Manihiki and Hikurangi Plateaus during Cretaceous time. Previous models of the Osbourn spreading center are based on data collected near the trough axis, and therefore only constrain the history of the Osbourn spreading center during the last few Ma of spreading. Our data set includes multibeam data collected northward to the Manihiki Plateau, allowing us to examine seafloor morphology created during the entire active period of the Osbourn spreading center, as well as several additional multibeam data sets that provide the opportunity to examine the relationship between the Osbourn paleospreading center and the Cretaceous Pacific‐Phoenix ridge. The axial gravity of the trough is similar to the gravity found at other extinct slow‐intermediate spreading rate ridges. Magnetic field measurements indicate that spreading at the trough ceased during Chron C34. Abyssal‐hill trends indicate that spreading during the early history of the Osbourn spreading center occurred at 15°–20°. The east‐west component of this spreading explains the modern east‐west offset of the Manihiki and Hikurangi Plateaus. Spreading rotated to 2°–5° prior to extinction. Abyssal‐hill RMS amplitudes show that a decrease in spreading rate, from >7 cm/yr to 2–6 cm/yr full‐spreading rate, occurred ∼2–6 Ma prior to ridge extinction. Our data analysis is unable to determine the exact spreading rate of the Osbourn spreading center prior to the slowing event. The temporal constraints provided by our data show that the Osbourn spreading center ceased spreading prior to 87 Ma or 93 Ma, depending on whether the Manihiki and Hikurangi Plateaus rifted at 115 Ma or 121 Ma. Our model resolves the conflict between regional models of Osbourn spreading with models based on trough characteristics by showing that spreading at the Osbourn spreading center was decoupled from Pacific‐Phoenix spreading.