HR: 08:40h
AN: T31A-03    [PDF]
TI: Sedimentation rate curves as a key to understand the evolution of arc and backarc basin -Arc type and Basin type-
AU: * Fujioka, K
EM: fujioka@godi.co,jp
AF: Global Ocean Development Inc, 1-13-8, Kamioooka-Nishi, Kounan-ku, Yokohama, 233-0002 Japan
AU: Matsuoka, H
EM: matusoka@cc.kochi-u.ac.jp
AF: Kochi University, 2-5-1, Akebono-cho, Kochi, 780-8520 Japan
AB: The deepsea cores recovered from about 50 drilling sites in the Philippine Sea, equally distributed in marginal basins, remnant arcs, present arcs and other tectonic settings, during the DSDP/IPOD/ODP offer significant geotectonic information. Sediment accumulation rates, lithologic changes and frequency of tephras were examined in the light of the recent advanced nannofossil biostratigraphy of the sediment on these cores. Sediment accumulation rate curves of these drill sites were classified into two major types, A and B types, respectively. A type has rapid accumulation rates just above the arc basement and then shows a decreasing pattern. In contrast, B type has rather constant accumulation rate throughout the cores. Sediments from the rapid sediment accumulation rates imply volcanogenic debris flow and volcaniclastic turbidite sequences derived which were derived from arcs that represent an activity of magmatic arc consisting of tholeiitic and calc-alkalic volcanic rocks. On the contrary, sediments from the low sediment accumulation rates imply mostly biogenic materials instead of volcaniclastic materials. This may mean the termination of intense arc volcanism. Frequency of volcanic ash layers deduced from these cores has maxima just after the rapid sediment accumulation stage of A type curves. As for the remnant arcs such as the Kyushu-Palau and the Daito Ridge, tephra maxima exist at both late Eocene to early Oligocene time and the present arc such as the Izu-Bonin Arc, there are two major maxima at Eocene-Oligocene and Pliocene-Pleistocene time, respectively. Explosive volcanism may take place when oceanic arc develops as shallow as the pressure compensation level (PCL). If this is the case, we may draw the volcanic history of oceanic island arc. At the incipient stage of arc evolution the style of volcanism is quiet resultant formation of pillow lavas and hyaloclastite. On the contrary, the intense volcanism takes place with formation of marine tephra layers at the explosive stage, and at the subaerial stage, large amount of tephras are emitted from the summit of the volcanoes. No volcanisms were happened instead continuous rapid subsidence of the arc at the remnant stage. These stages of the evolution of oceanic island arc are quite similar to those of the volcanic islands such as the Hawaiian volcanoes.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 5475 Tectonics (8149)
DE: 5480 Volcanism (8450)
DE: 8404 Ash deposits
DE: 9355 Pacific Ocean
SC: Tectonophysics [T]
MN: 2003 Fall Meeting