Sanbagawa subduction: what went in, how deep, and how hot did it get?


Overprint of secondary Du folding in the Sambagawa metamorphic belt, SW Japan: implications for strain ellipsoids and Paleogene tectonics of the east-Eurasian margin


Recognition of the early Sambagawa metamorphism and a schematic cross-section of the Late-Cretaceous Sambagawa subduction zone

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．


Phase equilibria modelling of blueschist and eclogite from the Sanbagawa metamorphic belt of southwest Japan reveals along-strike consistency in tectonothermal architecture


A new approach to develop the Raman carbonaceous material geothermometer for low-grade metamorphism using peak width

日本国内の接触変成帯，広域変成帯から変成温度既知の19試料を集めてラマン分光分析を行い，炭質物のラマンスペクトルにおけるdisorderピークの半値幅が有効な温度の指標となることを示した．これにより165∼400˚Cの低温域でも適用可能なラマン炭質物温度計が新たに構築された．


Paleo-mantle wedge preserved in the Sambagawa high-pressure metamorphic belt and the thickness of forearc continental crust

四国中央部における詳細な地質調査に基づき，三波川変成帯の低変成度領域(緑泥石帯)には超塩基性岩が一切存在しないことを明らかにした．これにより，三波川帯高変成度部に散在する超苦鉄質岩類がすべて沈み込み帯上盤側のマントルウェッジ起源であることが明確になった．また，超塩基性岩の出現線の位置から，後期白亜紀のユーラシア東縁における大陸地殻の厚さを30∼35kmと見積もった．


Slow subduction and buoyant exhumation of the Sanbagawa eclogite

四国三波川帯の粗粒エクロジャイト岩体である西五良津岩体にシュードセクション解析を用い，詳細な変成温度・圧力履歴を導いた．また得られた履歴に対して熱モデル計算によるフィッティングを行い，西五良津岩体が沈み込み帯浅部で高速沈み込み，また深部ではスラブからはがれて引きずられることによる低速沈み込みを行っており，これによって粗粒エクロジャイトが生じたことを論じた．


E-W extension at 19 Ma in the Kung Co area, S. Tibet: Evidence for contemporaneous EW and NS extension in the Himalayan orogen

南チベットKun Co地域におき，主に構造地質学的解析から約19Maに地殻上部で東西伸長が起こっていたことを示した．また同時期に地殻中部で起こっていた南北伸長(学術論文19)と併せて当時のチベット地殻における3次元的な流動構造図を提案した．野外調査を担当．


長野県飯田市，卯月山苦鉄質複合岩体の岩石学的性質—領家帯における苦鉄質火成作用の成因解明への予察的検討—

領家帯における苦鉄質マグマ活動の成因解明への予察的検討として，長野県飯田市，卯月山苦鉄質複合岩体の岩石学的性質を明らかにした．岩石の組織，鉱物組合せ及び鉱物化学組成の検討の結果，この岩体を構成する苦鉄質深成岩類は，かんらん石とスピネル，単斜輝石，斜方輝石，斜長石そして角閃石の順に含水苦鉄質・非アルカリ質の親マグマからの結晶作用により形成されたキュムレイトであることが明らかとなった．

Bulletin of the Geological Survey of Japan


Middle to late Miocene extremely rapid exhumation and thermal reequilibration in the Kung Co rift, southern Tibet

南チベットにおいて約12Maに急速な東西伸長変形が起こり，深部岩石を上昇させたことを，ジルコンを用いたU-Th/He年代測定法によって示した．野外調査を担当．


Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks

既存のラマン炭質物温度計の適用範囲を接触変成帯に拡張した．また測定の際に問題となる試料の不均質，及び測定方位依存について考察し，測定方位を気にせず測定数を稼いだ方が信頼性の高い温度見積もりが得られることを示した．一部試料の採取，測定戦略，及び論文全体の執筆を担当．


Boundary between the Sambagawa and Chichibu belts in the Odai area, Mie Prefecture, eastern Kii Peninsula, Japan

紀伊半島東部，三重県多気郡大台地域における三波川帯-秩父帯境界

The Sambagawa and Chichibu pelitic rocks are in direct contact in Kohtaki outcrop, which has been found out in field studies carried out in the Odai area, eastern Kii Peninsula. In the Kohtaki outcrop, it is suggested that difference in degree of metamorphic recrystallization between the two lithologies can be recognized by naked eyes, i.e. the Sambagawa pelitic phyllite is more highly recrystallized than the Chichibu pelitic mixed rock. In addition to this observation, overall difference in their constituent protolith was used to determine the boundary between the Sambagawa and Chichibu belts. The result suggests that the pelitic mixed rock in the Chichibu belt overlies pelitic phyllite in the Sambagawa belt with a moderately south dipping tectonic boundary, in contradiction with a previous study. It is also discussed that the juxtaposition of the two belts already completed by the end of the Sambagawa main deformation stage, Ds, which is associated with regional-scale E-W stretching. The Kohtaki outcrop likely preserves original structural relationships between the Sambagawa and Chichibu belts.

三波川帯と秩父帯を構成する泥質岩の間にみかけ結晶化度の差違があることを，両者が直接する露頭の発見によって示した．またこの観察事実を加味して地質図上での三波川帯と秩父帯の境界を新たに提案した．


Regional-Scale Excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau

東チベットの複変成地域(約200Maと約60Ma)におき，系統的なK-Ar及びAr-Ar年代測定から，2回目の変成作用の温度が約650度以下の地域では年代がリセットされないことを示した．同時に遷移帯が異常に古い年代を示すことから，リセット時に放出された過剰アルゴンを捕獲した地域であることを示唆した．野外調査，測定試料の選定，及び論文の概要作成を担当．


Plate movements, ductile deformation and geochronology of the Sanbagawa belt, SW Japan: tectonic significance of 89-88 Ma Lu-Hf eclogite ages

Lu-Hf法によって瀬場及び高越エクロジャイトのざくろ石形成年代(約89Ma)を導出した．同時にこのエクロジャイト相変成が海嶺沈み込みを伴うプレートテクトニクスの大改変(約85Ma)の直前期にあたることを示し，学術論文13の予言を裏付けた．野外調査と測定試料の選定を担当．


High-P metamorphism, pattern of induced flow in the mantle wedge, and the link with plutonism in paired metamorphic belts

三波川エクロジャイトの形成温度・圧力条件をコンパイルし，後期白亜紀のユーラシア東縁沈み込み帯におけるスラブ-マントル結合深度を約65kmと見積もった．同時に，65km以浅のウェッジマントルが蛇紋岩化していたこと，及び海嶺沈み込みによって加熱された蛇紋岩類が脱水したことで下部地殻の部分溶融が起こり，領家花崗岩類の起源マグマが生じたことを論じた．データコンパイルと論文の大部分の執筆を担当．


Contact metamorphism in the Malashan dome, North Himalayan gneiss domes, southern Tibet: an example of shallow extensional tectonics in the Tethys Himalaya

南チベット変成岩ドームの核部をなす花崗岩類の貫入深度(変成温度・圧力条件)を制約し，一部の花崗岩の貫入年代を示した．野外調査，及び全岩化学組成分析の一部を担当．


The Malashan gneiss dome in south Tibet: comparative study with the Kangmar dome with special reference to kinematics of deformation and origin of associated granites

南チベット変成岩ドームの核部をなす花崗岩類がヒマラヤ造山運動時(約50Ma以降)に貫入したこと，及び周囲の変成岩類における変形が広域的な南北伸長変形と同一視できることを構造地質学的解析，及び全岩化学組成分析に基づいて論じた．野外調査，構造解析，全岩化学組成分析の一部，及び論文全体の執筆を担当．


Origin of eclogitic metagabbro mass in the Sambagawa belt: geological and geochemical constraints

野外での産状と全岩化学組成分析に基づき，エクロジャイトを含む三波川帯の変斑れい岩(瀬場変斑れい岩)が海山に由来すること，すなわち沈み込み帯の下盤側起源であることを示唆した．野外調査，全岩化学組成分析，及び論文全体の執筆を担当．


North-south extension in the Tibetan crust triggered by granite emplacement

南チベット変成岩ドームの形成が核部花崗岩の貫入による接触変成に起因すること，及び，接触変成の熱的影響が南北伸長変形の引き金となったことを示した．またこの変形がchannel flowモデルで剛体的とされた上部地殻に及んでいたことから，モデルに修正(花崗岩マグマ上昇の考慮)が必要なことが示唆された．野外調査，年代測定や全岩化学組成分析の一部，及び論文全体の執筆を担当．


Thermal model for approach of a spreading ridge to subduction zones and its implications for high-P/high-T metamorphism: Importance of subduction versus ridge approach ratio

海嶺の海溝への接近を組み込んだ沈み込み帯熱モデルを構築し，様々な変数を用いた計算結果から，沈み込み速度に対する海嶺の接近速度比が重要な変数であることを示した．同時に，高圧型変成岩の沈み込みP-T pathが変成岩形成時のテクトニックセッティングを指標する重要な情報であることを論じた．モデルの原案と論文の主要部分の執筆を担当．


Chloritoid-bearing basic schists from the Sanbagawa metamorphic belt, central Shikoku : their petrologic significance and tectonic implications

Chloritoid- and taramite-bearing basic schists occur on the streambed at Ikadadu in the garnet zone of the Sanbagawa metamorphic belt, central Shikoku. Their major constituent phases are chloritoid, taramite, chlorite, paragonite, phengite, epidote, ilmenite, titanite, albite and quartz. Chloritoid occurs only as inclusions in garnet. The appearance of chloritoid in the basic schists is strongly dependent on their restricted FeO-rich and high Al2O3/(CaO + Na2O + K2O) bulk-rock compositions. The metamorphic pressure (P) and temperature (T) conditions, 0.9 GPa and 505-515°C, are estimated by using equilibrium relationships among chloritoid, taramite, chlorite, paragonite, garnet and quartz. These P-T conditions are distinctly lower than those estimated for a chloritoid-bearing assemblage at the eclogite facies stage of pelitic schists in the Seba eclogite unit (1.7 GPa/525-555°C), 0.5-1 km north of the Ikadadu outcrop. This implies that there is a significant difference in the peak metamorphic pressure of the chloritoid-forming stages between the Ikadadu and Seba chloritoid-bearing schists, indicating the existence of a tectonic discontinuity between the two localities. Chloritoid and amphibole-bearing assemblages are stable at various metamorphic P-T conditions within suitable bulk rock compositions and their phase equilibria are helpful in discussing P-T conditions for various metamorphic rocks.

三波川帯において，ガーネットがクロリトイドを包有することは必ずしもエクロジャイト相変成の指標とならないこと，及びエクロジャイトユニットとそれ以外のユニットの間に有意な変成圧力のギャップがあることを明確にした．野外調査と全岩化学組成分析を担当．

Journal of Mineralogical and Petrological Sciences


Chloritoid and barroisite-bearing pelitic schists from the eclogite unit in the Besshi district, Sanbagawa metamorphic belt

三波川帯，瀬場地域では塩基性の岩石のみならず，近接する泥質岩類もエクロジャイト相に達していたことをクロリトイドを含むガーネットの包有物解析から示した．野外調査と全岩化学組成分析を担当．


Research history of the Seba region in the Sambagawa metamorphic belt, central Shikoku, with special focus on its field-geological aspect.

四国三波川帯・瀬場地域の研究史:高度変成地域の野外調査にまつわる諸事情

In the Seba area of the Sambagawa belt it was previously proposed that eclogitic basic schists found in the area adjacent to a metagabbro body formed by solid-state contact metamorphism as a result of tectonic emplacement of the metagabbro. This explanation has been reexamined through later studies, which include discovery of a number of eclogitic basic schists (or their constituting mineral, omphacite) distant from the metagabbro and recognition of relationship between multiple deformation stages and growth of omphacite. The results indicate formation of all the eclogitic basic schists in the Seba area can be attributed to a single regional eclogite-facies metamorphism. This research history gives the following implications to field studies for high-grade metamorphic regions: 1) recognition of key metamorphic minerals and of its map distribution should be done with great care; and 2) multiple deformation stages and their associated structures should be classified in relationship to growth timing of metamorphic minerals.

三波川帯，瀬場地域のエクロジャイトに関する近年の学説の変遷を紹介し，変成岩研究における包括的アプローチの重要性を論じた．

Bulletin of the Geological Survey of Japan


Derivation of Subduction-Stage Pressure-Temperature Path of Sambagawa Eclogite and its Tectonic Significance : Towards the Goal of a Comprehensive Petrology

三波川エクロジャイトの沈み込みP-T経路の導出とそのテクトニックな意味付け : 包括的岩石学への布石として

In general, derivation of subduction-stage P-T (pressure-temperature) paths for high-P metamorphic rocks is difficult, because in most cases they are affected by subsequent metamorphism, during which the pre-peak P mineral assemblages are significantly modified. However, petrological studies during the last 10 years in the Sambagawa belt have recognized unusually well-constrained sections of the subduction-stage P-T path. In particular, derivation of the subduction-stage P-T path of the Kotsu glaucophane eclogite was backed-up by insights obtained from a combination of petrological and structural analyses. Application of a new thermal model shows that the derived subduction-stage P-T paths are most likely to have been generated in a setting where an active spreading ridge is close to being subducted. This tectonic interpretation suggests that exhumation of high-P/T metamorphic rocks in oceanic subduction zones is triggered by regional heating events such as ridge subduction. A review of recent studies on the Sambagawa belt indicates the significance of promoting a comprehensive petrology, which encompasses the full set of constraints provided by disciplines such as petrology, structural geology, geochronology and thermal modeling.

主に学術論文(Aoya et al., 2003; Geology)の解説．相平衡岩石学，構造地質学，モデル計算，年代学などを取り入れた包括的岩石学の重要性を論じた．


Subduction stage pressuer-temperature path of eclogite from the Sambagawa belt : Prophetic record for oceanic-ridge subduction

Petrologic studies of the Kotsu eclogite from the Sambagawa high-pressure (P), low-temperature (T) metamorphic belt show that the eclogite has recorded a very steep subduction-stage P-T path reaching 20 kbar and 600 8C; the dP/dT is .7.1 kbar/100 8C. Compilation of previously derived subduction-stage P-T paths from the Sambagawa belt and the result of this study show that these P-T paths are not on a straight line from the origin, but define a curve with dP/dT increasing with pressure. A new model incorporating the progressive approach of an oceanic ridge to a subduction zone shows that the Sambagawa P-T paths are best explained by a setting in which a ridge is close to being subducted at a slow rate relative to the plate-subduction rate. Exhumation of high-P, low-T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges.

三波川帯，高越エクロジャイトの沈み込みP-T pathが非常に険しいdP/dT勾配を持つことを岩石学的に示し，熱モデル計算によるフィッティングから，エクロジャイト形成時期が海嶺沈み込みの直前期であることを論じた．岩石学的解析とモデル原案を担当．


Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet

New zircon U-Pb and mica 40Ar/39Ar dating combined with structural studies in the Longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of Tibet is Mesozoic. However, at lower structural levels, apatite U-Pb and monazite electron microprobe dating reveals a previously unknown domain of Cenozoic (ca. 65 Ma) Barrovian-type metamorphism and deformation. This discovery shows that the crust in the eastern margin of Tibet was already a substantial thickness around the time of the India-Asia collision. Associated deformation has a N-S-oriented stretching lineation, implying that deformation was not driven by topographic gradients in the Tibetan Plateau. The observed moderate amounts of distributed postmetamorphic E-W shortening can probably explain the present thickness of the continental crust in the area. These results do not support models of crustal thickening caused by solid-state lateral flow of midcrustal metamorphic rocks.

東チベットで約65Maの中圧型変成作用を認識，これらがヒマラヤ造山運動について提案されているchannel flow モデルの部分溶融帯とみなせないことを論じた．SHRIMP年代測定や熱史の構築に関わった．


Role of nappe boundaries in subduction-related regional deformation: spatial variation of meso- and microstructures in the Seba eclogite unit, the Sambagawa belt, SW Japan

The Seba eclogite unit of the Sambagawa high-P/T metamorphic belt has a kilometre-scale synformal structure and its base is the boundary between an overlying eclogite nappe and an underlying non-eclogitic nappe. The formation of the nappe boundary is related to a phase of strong penetrative ductile deformation (DA). Spatial variations of meso- and microstructures of schistose eclogite in the Seba unit reveal that there is (i) a decreasing intensity of DA, and (ii) a time-dependent widening of the DA shear zone, away from the nappe boundary. These features indicate that the nappe boundary behaved as a core zone during the DA phase and, furthermore, that development of the regional deformation, DA, is strongly controlled by the presence of the nappe boundary. A new model is proposed where each phase of regional deformation in subduction-type metamorphic belts is related to ancient activity of a particular nappe boundary. In this model a single phase of deformation may be related to exhumation in one nappe and subduction in an adjacent one.

海洋沈み込み型の変成帯におき，ナップ境界が変形の伝搬の核となること，及び新規ナップの付加が変形時階の更新を引き起こすことを主にガーネットの微細構造観察に基づいて論じた．野外調査，構造解析，大部分の執筆を担当．


Petrological constraints on the formation conditions and retrograde P-T path of the Kotsu eclogite unit, central Shikoku

三波川帯の高越エクロジャイトの上昇が瀬場エクロジャイトとは異なり，低温の状態を保ちながら起こったことを岩石学的に示した．微細構造観察，変形史の解析を担当．


Structural position of the Seba eclogite unit in the Sambagawa Belt : Supporting evidence for an eclogite nappe

三波川帯，別子地域における巨視的な褶曲構造を認識し，地質図上でばらばらに見えるエクロジャイト岩体群が同一の構造位置を占めるナップとみなせることを示した．


Thermal consequences of a subduction boundary jump: a numerical model for generating subduction-related clockwise pressure-temperature paths

沈み込み込んだスラブが厚さ数kmのナップとして付加した後にナップ内部で温度上昇が起こることをモデル計算によって示し，三波川変成岩の時計回りP-T pathに解釈を与えた．モデルの原案と適用部分を担当．


P-T-D path of eclogite from the Sambagawa Belt deduced from combination of petrological and microstructural analyses

構造地質学と岩石学を融合し，三波川帯，瀬場エクロジャイトの経験した圧力・温度・変形履歴を導出した．


A re-evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe

徳島県高越地域で三波川帯初の藍閃石エクロジャイト発見，記載し，三波川エクロジャイトはテクトニックブロック群ではなく，単一ユニット(エクロジャイトナップ)とみなせることを主張した．鉱物化学分析を担当．


Structural and microstructural constraints on the mechanism of eclogite formation in the Sambagawa belt, SW Japan

三波川帯の瀬場エクロジャイト質片岩が全て三波川広域変成作用によって生じたものであることを構造解析，微細構造解析によって示した．野外調査，構造解析，本文執筆を担当．


Thermal calculation for high-pressure contact metamorphism : Application to eclogite formation in the Sebadani area, the Sambagawa belt, SW Japan

三波川帯，瀬場エクロジャイト質片岩の成因が，変はんれい岩体の貫入に伴う固体接触変成作用によるものではないことを熱モデル計算によって示した．


伸長剪断帯の認識とその造山帯における重要性

伸長剪断帯の認識とその造山帯における重要性

造山帯において，剪断歪みの変位に垂直な方向の伸長・短縮が組み合わさった変形様式(transpression, transtension)が重要である可能性を指摘した．英文の和訳を担当．


Prograde eclogites from Sambagawa basic schists in the Sebadani area, central Shikoku, Japan

四国中央部三波川帯瀬場谷地域の塩基性片岩中に産する累進的エクロジャイト

愛媛県新居浜市別子山の三波川帯，瀬場地域におき，それまで未発見だったエクロジャイト片岩の複数地点における産出を報告した．野外調査と執筆の大部分を担当．

Memoirs of the Faculty of Science and Engineering Shimane University. Series A

島根大学総合理工学部紀要. シリーズA