初期三波川変成作用の認識，及び後期白亜紀三波川沈み込み帯の描像

Recent petrological studies on the Sambagawa metamorphic belt in Shikoku have recognized that the coarse-grained eclogite-bearing lithologies (so-called ‘tectonic blocks’ in earlier studies) in the Besshi area exclusively preserve evidence for the ‘early’ Sambagawa metamorphism, which can be related to onset of the Sambagawa subduction system during Early Cretaceous (c.116Ma). Geological mapping and associated multidisciplinary studies on the regional (spatially widespread) Sambagawa metamorphism (both the eclogite-facies and main metamorphisms) have revealed the tectonic framework of the Late-Cretaceous Sambagawa subduction zone as follows: (i) a spreading ridge was approaching close to the trench; (ii) the subducting slab was coupled with the convective mantle at depth of >65 km; (iii) thickness of the hanging-wall continental crust was 30-35 km; and (iv) the forearc mantle wedge (30-65 km depth) was largely serpentinized. These features allow us to draw a semi-quantitative cross-section of the Sambagawa subduction zone at around 89-85Ma, implying that boundary conditions for thermo-mechanical modeling aiming to simulate exhumation of high-P/T metamorphic rocks are now well constrained. It has also become clear that ultramafic blocks dispersed in the higher-grade part of the Sambagawa belt were derived from the mantle wedge, i.e. the corresponding part of the belt has been re-evaluated as a ‘fossil subduction boundary’ of a relatively warm subduction zone. Field-based petrological studies in the Sambagawa belt can, therefore, have potential to provide invaluable information on material behaviors at the slab-mantle wedge interface including domains of episodic tremor and slip (ETS) in present-day warm subduction zones.

近年の四国三波川帯に関する研究から，テクトニックブロックと認識されて来た粗粒の含エクロジャイト岩体群に前期白亜紀（約116Ma）の初期三波川変成作用が記録されていることが判明した．一方，後期白亜紀（約89～85Ma）の主たる三波川変成作用については，海嶺の海溝への接近というテクトニックな状況，蛇紋岩化したマントルウェッジの存在，またスラブとマントル対流の結合深度（約65km）や上盤側大陸地殻の厚さ（30～35km）が認識され，当時の沈み込み帯模式断面図がかなり具体的に描けるようになった．同時に，散在するブロック状超苦鉄質岩類が上盤側マントルウェッジ起源であることも判明し，三波川帯は「深部沈み込み境界の化石」として再認識された．深部低周波微動やスロースリップなど，現世沈み込み帯で観測される注目すべき地質現象の解釈に当たり，物質科学的な情報を提供し得るフィールドとして新たな期待がかかる．