HR: 0830h
AN: V31C-0945    [PDF]
TI: Evolution of mantle-crust thermal structures in the late Cenozoic NE Honshu arc, Japan
AU: * Yoshida, T
EM: tyoshida@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Aramaki-aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Kimura, J
AF: Department of Geoscience, Shimane University, 1060, Nishikawatsu-machi, Matsue, 690-8504 Japan
AU: Sato, H
AF: Earthquake Research Institute, The University of Tokyo, 1-1-1, Yayoi, Bunkyou-ku, Tokyo, 113-0032 Japan
AU: Iwasaki, T
AF: Earthquake Research Institute, The University of Tokyo, 1-1-1, Yayoi, Bunkyou-ku, Tokyo, 113-0032 Japan
AU: Nakajima, J
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Aramaki-aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Yamada, R
AF: Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Aramaki-aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: NE Honshu is an island arc associated with the back-arc Japan Sea basin, and three prominent stages of the volcanic activity in the late Cenozoic have been recognized. These are continental margin volcanism (66-21 Ma), back-arc basin volcanism (21-13 Ma), and island-arc volcanism (13-0 Ma). In the continental margin stage, the volcanic front was located deeper in the back-arc side compared to the present. The volcanic front expanded toward the trench side, and volcanic arc was at its widest between 25 and 21 Ma. After the back-arc opening event ended, the volcanic front began to migrate to the rear-arc, and settled at its present position. Geological and petrological approaches combined with seismological images enable examination of deep structures beneath island arcs. Across-arc profiles of source mantle temperatures and heterogeneity of mantle compositions inferred from petrological data are compared with the seismic velocity structures. Volcanic products in each stage have unique patterns of across-arc chemical variations that should be closely related with the thermal structure of the wedge mantle. The spatial heterogeneity in the basalt source regions of the NE Honshu arc mantle wedge may be formed by injection of depleted asthenospheric mantle into enriched continental lithosphere during the back-arc basin opening stage. This was related to three rifting episodes in the arc, firstly at 21 Ma in the back arc Yamato Basin, secondly at the rear-arc edge of the present island arc at 16 Ma, and finally at 14 Ma in the imediately inside of the Backbone Range of NE Honshu. In the back-arc basin volcanic stage, the inferred head structure of the 1320?O`C mantle isotherm was very flat. This temperature increase in the back-arc mantle region may be related to upwelling of asthenosphere, resulting in eruption of tholeiite from shallower mantle depth. Slow subsidence of the rear-arc edge area began at 14.5Ma, perhaps due to cooling of the injected hot mantle and sub-arc lithosphere. In the island arc volcanic stage, alkali contents of erupted lavas increase toward the rear-arc. This suggests that the 1320 ?O`C isotherm was inclined toward the rear-arc, and hence magma segregation depths gradually increased westward. This inclination would result from the cooling of back-arc mantle after back-arc basin opening. The present day mantle wedge has a low velocity zone in the middle of the mantle wedge, and this zone is inclined toward the rear-arc. The coincidence of these features provides evidence for the thermal structure of the arc mantle wedge. The geothermal gradient in the fore-arc and the Backbone Range of the NE Honshu arc appears to have fallen through the late Miocene to the Quaternary (Ban et al., 1992; Yamaji, 1994). The westward retreat of the volcanic front (Ohguchi et al., 1989) is also in accord with the regional cooling which has occurred in the NE Honshu arc since the late Miocene.
DE: 3640 Igneous petrology
DE: 7218 Lithosphere and upper mantle
DE: 8109 Continental tectonics--extensional (0905)
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting