HR: 1340h
AN: T13A-1128    [Abstracts]
TI: Seamounts, knolls and petit spots on the NW Pacific Plate represent intra-plate volcanism from the Cretaceous to present
AU: * Hirano, N
EM: nhirano@eqchem.s.u-tokyo.ac.jp
AF: Laboratory for Earthquake Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
AU: Nakanishi, M
EM: nakanisi@earth.s.chiba-u.ac.jp
AF: Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba, 263-8522, Japan
AU: Koppers, A A
EM: akoppers@coas.oregonstate.edu
AF: College of Oceanic & Atmospheric Sciences, Oregon State University, 104 COAS Admin Bldg, Corvallis, OR 97331-5503, United States
AB: Most of seamounts of the western Pacific formed before 70 Ma in the so-called West Pacific Seamount Province (WPSP) which is characterized by relatively short seamount chains maybe indicating a significant short-lived hotspot system (Koppers et al., 2003). As for the NW Pacific Plate offshore of Northeast Japan, the Joban and Japanese Seamount Trail are also composed of middle Cretaceous seamounts, which are erupted on the northern margin of WPSP. The 120 to 100 Ma seamounts in the Joban seamount chain do not show a middle Cretaceous hotspot track, whereas the Japanese seamount chain shows a well-established ENE to WSW trend in this age range. On the other hand, the unnamed knolls, which are well-circular and flat-topped in shape, are scattered on the NW Pacific Plate and are not aligned to any volcanic chains. These were correspond to eruptive ages of 70 Ma based on Ar-Ar ages of a second volcanic event in the NW Pacific. As the last stage, we should note that Hirano et al. (2006) reported the 0-1, 2, 4.2, 6.0 and 8.6 Ma volcanoes, called petit spots, in the Japan Trench on the outer-rise system. The petit spot volcanoes imply episodic eruptions of magma over a distance of 600 km of plate motion on the flexural Pacific Plate before its subduction but with low volumes of magma production. The volume of volcanic edifice of the petit spot volcanoes certainly is several orders of magnitude less than the Cretaceous seamounts and knolls. Therefore, we can interpret that the petit spot volcanoes are not related to any mantle plumes and hotspots. Evidence includes the geochemical data and the tectonic alignment of the volcanoes which show that the petit spot lavas escaped along fractures in the lithosphere and were sourced from small pockets of asthenospheric melts. The bathymetric map and sidescan sonar imagery of the ocean-ward slope in the Tonga Trench also shows a possible presence of young volcanoes. Such small volcanoes, therefore, may be ubiquitous on the ocean floor, where incipient melts in the asthenosphere can be squeezed out by tectonic forces associated with plate flexure of outer-rise.
UR: http://www.geo.titech.ac.jp/lab/takahashi/staff/nhirano/
DE: 3037 Oceanic hotspots and intraplate volcanism
DE: 3075 Submarine tectonics and volcanism
DE: 8138 Lithospheric flexure
DE: 9604 Cenozoic
DE: 9609 Mesozoic
SC: Tectonophysics [T]
MN: 2007 Fall Meeting