HR: 10:20h
AN: T12A-01    [Abstracts]
TI: The Manihiki Plateau, Hikurangi Plateau, Wishbone Scarp, and Osbourn Trough: A Review and Analysis
AU: Henig, A
EM: glowstargi@aol.com
AF: Scripps Institution of Oceanography, University of California, San Diego 9500 Gilman Drive, mail code 0208, La Jolla, CA 92093,
AU: * Luyendyk, B P
EM: luyendyk@geol.ucsb.edu
AF: Dept. Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106,
AB: The extinct Osbourn Trough spreading system in the southwestern Pacific played a key role in separating the once joined Hikurangi and Manihiki Plateaus in Cretaceous time. Recent studies by Downey et al. [2007] and Taylor [20006] have provided new data and concepts on that history. Studies by Larson et al. [2002] describe Cretaceous histories adjacent and east of the Osbourn system and Eagles et al. [2004] describe a history for the southwest Pacific just after the Osbourn system became extinct. The lack of identifiable magnetic anomalies allows tectonic events during separation of the plateaus and spreading on the Osbourn Trough to occur between about 124.6 and 84 Ma (Chron 34). Satellite gravity maps of the region identify what are interpreted as Cretaceous fracture zone trends. Swath bathymetry data reveal at least four provinces of abyssal hill trends. What is known are the following: Minimum age of the Manihiki High Plateau at 123.4 Ma from DSDP Site 317, the age of seafloor, 115 Ma, from a dredge sample from the southern segment of the West Wishbone Scarp 300 km northeast of the Hikurangi Plateau, the east-west trend and extent of the extinct Osbourn spreading center at latitude 26° S, the trends (NNE-SSW to NE-SW) and extent of portions of the West Wishbone Scarp east of the Hikurangi Plateau, the N-S trend of the East Wishbone Scarp that appears to truncate the West Wishbone Scarp, the NNE- SSW trend and extent of the prominent Manihiki (Eastern) Scarp at the eastern boundary of the Manihiki High Plateau, the NW-SE trend and extent of the Rapuhia Scarp on the northwestern Hikurangi Plateau, and trends in abyssal hills; E-W near the Osbourn Trough and WNW-ESE closer to the plateaus. Differences in abyssal hill morphology suggest one or more changes in spreading rates. The Osbourn Trough began rifting apart a Large Igneous Province formed at 123.5 +/- 1.5 Ma, into the separate Ontong-Java, Manihiki, and Hikurangi Plateaus by 121 Ma. After an initial 6 Myr interval of slow spreading at 4.7 cm/yr (full rate -fsr), the rate at 115 Ma increased to ~18.3 cm/yr fsr at an azimuth of 197.4° (holding Manihiki Plateau fixed) and continued for 10 Myr to 105 Ma. At 105 Ma, the Hikurangi Plateau entered a subduction zone at the north edge of the present Chatham Rise. It is assumed that this caused a change in spreading direction to an azimuth of 183.5° as observed from abyssal hill strike data. Fast spreading is assumed to have continued in this new direction for another 3.5 Myr until 101.5 Ma when a change in abyssal hill morphology suggests a large decrease in rate. From this time until the extinction of the Osbourn Trough at 97-93 Ma, slow spreading proceeded at a rate between 2.6-4.9 cm/yr fsr. Spreading rate calculations are based on assuming the changes in spreading direction seen in abyssal hill trends reflects a change in the tectonic regime of the Chatham subduction zone at 105 Ma. This history accounts for the common origin of the Manihiki Eastern Scarp and West Wishbone Ridge and adjoins the Manihiki High Plateau with deeper portions of the east Hikurangi Plateau. The East Wishbone Ridge is a fracture zone that postdates the end of Osbourn Trough spreading and is responsible for the southward removal of missing Osbourn system fracture zones and ridge segments east of it.
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8157 Plate motions: past (3040)
DE: 9355 Pacific Ocean
DE: 9609 Mesozoic
SC: Tectonophysics [T]
MN: 2007 Fall Meeting