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1 Journal of Geography 113 (5) On-going Regional Metamorphism beneath the Japanese Islands Shigenori MARUYAMA *, Soichi OMORI * and Yasuyuki IWASE * * Abstract Recent advances in seismic studies have revealed that earthquakes have a close link with chemical processes, i.e. metamorphic dehydration. From this point of view, we provide a new scheme for observation of on-going regional metamorphism in a subduction zone. Combining the phase diagrams of MORB + water and peridotite + water with the thermal structure of the descending Pacific oceanic plate in NE Japan and in the Philippine Sea (PHS) plate in SW Japan, we can draw the distribution of metamorphic facies of regional metamorphism. However, the most uncertain parameter is thermal structure, even though it has been calculated numerically, because of the difficulty of evaluating frictional heating, heat transportation by dehydrated fluids and mantle convection in the hanging wall. To overcome this problem, we have carried out different approach from seismic observations in estimating the thermal structure of a subduction zone, by applying the dehydration-induced earthquake hypothesis. This hypothesis involves the assumptions as follows : 1) any dehydration in the subducted slab induces earthquakes, 2) peridotite of the subducting plate is more or less hydrated, as well as the oceanic crust, and 3) the dehydration reactions proceed in near equilibrium condition. Direct seismic determination of the depths of the blueschist or epidote-amphibolite facies to eclogite transformation, decomposition of serpentine (antigorite), and the stability limit of clinochlore enable us to establish fixed points for the slab temperature. The seismogenic zone (150 Ž to 350 Ž), the depth limit of non-volcanic tremor seismicity, and the slab melting in SW Japan ( Ž) were also used to fix temperatures at given depths. Three profiles in NE- Japan and two profiles in SW-Japan were examined, and their P-T paths along the Wadati- Benioff zone were estimated to be anti-clockwise in all cases. The P-T paths are consistent with those of metamorphic facies series from well-studied on-land regional metamorphic belts. The P-T path of the subducting slab in NE-Japan is colder than that of the eastern-shikoku section in SW-Japan, except for the Kii-peninsula section which has an almost similar P-T path to that in NE-Japan. Comparison between the on-going metamorphism beneath the Japanese islands and the Department of Earth and Planetary Sciences, Tokyo Institute of Technology National Defense Academy

2 on-land regional metamorphic belts in the Sanbagawa and Kokchetav shows that the P-T conditions of these two metamorphic belts are located between that of NE-Japan and of the eastern-shikoku profile. A numerical model for wedge-mantle convection shows that the direction of the small corner flow of the wedge mantle causes a back current along the subducting slab. The area of the corner flow is wider in a shallow subduction zone corresponding to that in eastern Shikoku, and is narrower in a steeper subduction zone represented by NE-Japan. Since the exhumed metamorphic belts have intermediate P-T conditions between those in NE Japan and eastern Shikoku, We suggest that a change in the mode of wedge-corner flow from steep to shallow subduction plays some role in the exhumation of a metamorphic belt. Such a change in Cretaceous time from 120 Ma to 80 Ma may have promoted the exhumation of the Sanbagawa belt. Key words : subduction zone, intraslab eathquake, regional metamorphism, dehydraioninduced earthquake hypothesis, northeastern Japan, southwestern Japan

4 Fig. 1 Schematic diagram of the method of this study.

5 Fig. 2 a) Petrogenetic grid for the MORB+H20 system (modified after Okamoto and Maruyama, 1999). PA : Pumpellyite-Actinolite facies, B: blueschist facies, GS : greenschist facies, EA: epidote-amphibolite facies, AM : amphibolite facies, EpEc : epidote-eclogite facies, HGR : high-p granulite facies, GR: granulite facies. Numbers in the fields correspond approximate water content in the rock. b) Petrogenetic grid for the peridotite+ H2O system (modified after Omori et al., 2004). Atg : antigorite, Tc : talc, Chl : clinochlore, A : phase A, Mg-Sur :, Mgsurssasite

6 Fig. 3 Epicenters (black dots), quatarnary volcanoes (circle) and active volcanoes (triangles) in the NE-Japan. Epicenter map was taken from Igarashi et al. (2001). Lines A, B, C correspond to the location of the cross-sections in Fig. 4.

7 Fig. 4 Hypocenter distributions in NE- Japan. The location of each section is shown in Fig. 3.

8 Fig. 5 On-going metamorphic reactions and the thermal fixed-points in NE-Japan. Abbreviations are similar to Fig. 2.

9 Fig. 6 Distribution of the non-volcanic tremors (after Obara, 2002) and depth contours for the Wadati- Beniof plane in SW-Japan. Circles represent epicenters of the non-volcanic tremors. The depth contours were drawn for each 5 km. D-

10 Fig. 7 Hypocenter distributions in SW-Japan. a) D-D' section (modified after Kodaira et al., 2002), Gray area is the oceanic crust of the Philippine Sea plate. Dotted curve is an approximate bottom of the plate. b) E-E' section (modified after Smith et al., 2004). Gray area is the oceanic crust of the Philippine Sea plate. Dotted curve is an approximate bottom of the plate. Shaded area in the mantle wedge is a possible source region of the non-volcanic tremor.

11 Fig. 8 On-going metamorphic reactions and the thermal fixed-points in SW- Japan. Abbreviations are similar to Fig. 2. a) D-D'-D" section. b) E-E' section.

12 Fig. 9 Corner flow in a model mantle-wedge by the result of 2-dimensional numerical simulation (Iwase and Honda, 1993). a) Stream line field after 2.4 My. b) Thermal structure. The area of 500 km ~ 150 km were considered, and a 30 km-thick continent is assumed. Left end is trench axis, and subduction angle is 45 deg.

13 Fig. 10 Schematic diagrams showing corner flow in mantle-wedge. a) NE-Japan and b) SW-Japan.

14 Fig. 11 Subduction P-T paths of on-going metamorphism at the uppermost oceanic crust, regional metamorphic rocks, and numerical simulations for the uppermost subducted crust. NEJ : NE-Japan, SWJ (D) : profile D in SW-Japan, SWJ (E) : profile E in SW- Japan, SMB : Iratsu-eclogite in Sambagawa belt (Ota et al., 2004), KCH : Kokchetav Ultrahigh-pressure metamorphic belt (Maruyama et al., 2002; Masago, 2003; Omori and Masago, 2004 in this issue), H93 : numerical estimate for NE-Japan by Hacker et al., 2003, P99 : numerical estimate for SW-Japan by Peacock and Wang (1999).

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地質調査研究報告/Bulletin of the Geological Survey of Japan

地質調査研究報告/Bulletin of the Geological Survey of Japan Kazuhiro Miyazaki (2003) Tentative standardized legends of metamorphic rocks for 1/200,000 scale seamless geological map of Japan. Bull. Geol. Surv. Japan, vol. 54(9/10), p.295-302, 2 figs. Abstract: This