HR: 17:45h
AN: T32F-08    [PDF]
TI: On-Going Regional Metamorphism Under the Japanese Islands
AU: * Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Omori, S
EM: omori@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AB: Combining the phase diagrams of MORB + water, pelitic-psammitic rocks, and of peridotite + water, with thermal structure of the descending slabs of the Pacific in NE Japan and Philippine Sea (PHS) in SW Japan, and with velocity structure of sub-arc mantle, we can draw the metamorphic facies distribution of regional metamorphism, water circulation under the Japanese islands and small-scale mantle convection in the wedge mantle. The most uncertain parameter is a thermal structure calculated by numerical simulation, because of difficulty of evaluation of frictional heating, heat transportation by dehydration and mantle convection in the hanging wall. To overcome this problem, the direct seismic determination of the depth of blueschist-eclogite transformation (50-60km depth in the Pacific slab), seismogenic zone ($150\deg$C to $350\deg$C), depth limit of tremor seismicity (Obara, 2002, dehydration limit at 40km depth, ca.$500\deg$C) slab melting in SW Japan (750-$800\deg$C at 80km depth) were used as fixed temperatures. Moreover, the thickness of Pacific MORB crust (6km) and overlying sedimentary mass (3km), Moho depth and shape of wedge mantle in both SW and NE Japan were used through the recent seismic investigations. Under the NE Japan arc, the MORB crust on the descending Pacific slab is regionally metamorphosed at 50-60km depth ca. $550\deg$C from blueschist to amphibole eclogite to release 1.5 wt% water by the discontinuous reaction. The complex dehydration reaction of dense-hydrous silicates including antigorite occur at depths ca.200-180km from which water-rich fluids penetrate upwards into dry eclogite where it may be partially melted to form felsic magma. It may react to generate pyroxenite in the mantle wedge and become a rising wet column to cause arc magmatism as documented by the high-resolution P-wave tomographic images (Zhao et al., 1992). On the other hand, the low-T (ca.$550\deg$C) wet rising column at 50-60 km depth right above the Pacific slab hydrates the hanging wall. The two rising wet columns merge at Moho below the arc-trench gap to form a dowelling flow. The small-scale convection cell in the wedge mantle at shallow depth $<$200km may preserve the 3 km thick felsic regional metamorphic belt at depth range from 70-80km to 50km on the downgoing Pacific slab. On the other hand, the active volcanoes are rare in SW Japan and dip of subducting PHS plate is small, a thin wedge mantle is hydrated extensively in the oceanward half, but not in the continental side. The possible presence of thin (ca.2km) felsic materials on the PHS slab may exhume as a plastic flow against the subducting PHS slab (4cm/y) due to the small-scale corner flow in the wedge mantle.
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting