日高変成帯の形成テクトニクス  [in Japanese] Tectonics of the Hidaka metamorphic belt, Hokkaido, northern Japan  [in Japanese]

変形・変成・火成活動史の統合的解析に基づき, 古第三紀〜中新世の日高変成帯の造構史は次のように説明される。D_1期には, 地殻底部への苦鉄質マグマの大量底付けにより, 伸張性造構場が生じるとともに, D_0期に構造的に厚化した地質体に多量の熱が供給され, グラニュライト相に達する増進変成作用が起こり, 大量のSタイプ花崗岩マグマが生じた。続くD_2期には, 日高変成帯は右ずれトランスカレントにより, アナテクシスの著しい層準と少ない層準との境界面をデコルマンとし上盤が南へ移動するセンスをもって, Sタイプ花崗岩マグマの分離・貫入を伴いながら最下部地殻から分離し上昇を開始した。D_3期の日高変成帯は右ずれトランスプレッションによって上昇し, 東に急傾斜した状態となった。D_0-D_3期に基本構造が形成された日高変成帯は, その後, 南北性の左横ずれ剪断変形や東南東-西南西圧縮変形など, いくつかの時期の脆性変形による部分的な構造的改変を受けた。

The tectonic evolution of the Hidaka metamorphic belt is divided into 3 stages based on the nature and sequence of deformation, metamorphism and igneous activity during early Paleocene to Miocene time; I: formation of magmatic arc (D_0 and D_1), II: exhumation of the crust (D_2 and D_3), and III: post-kinematic brittle deformation. Pre-metamorphic and -igneous stage (D_0) involved tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex. D_1 is the prograde metamorphic stage, accompanying surfacial extension and vertical uplifting of metamorphic rocks caused by mafic magmatic underplating. The D_1-mafic magma is a possible heat source for the high-temperature metamorphism up to granulite facies, and for a large amount of S-type granitic magmas formed by crustal anatexis in the lowest to lower crust. Subsequent to the D_1 stage, top-to-the south subhorizontal displacement of detached lower to upper crustal rocks occurred under an N-S compression, associated with the formation of duplex structure, high-temperature mylonitization, and S-type granitic intrusion (D_2). The S-type magma intruded into a basal detachment (decollement) and upper zones along floor, ramp, and roof thrusts of the duplex. The decollement was formed along the transitional zone from amphibolite to granulite facies, where partial melting incipiently took place and was intensified in the deeper part. The depth of the intra-crustal decollement was possibly defined by relative amounts of partial melt fraction and fluid flux. The D_2-event was resulted from transcurrent tectonics in a flat-lying structural state. Subsequently, D_3-dextral transpressional movement accompanied by retrograde mylonitization resulted in eastward tilting (stee pening) on N-S axis and exhumation of the belt. The granitic intrusion enhanced the crustal deformation and exhumation in the D_2- and D_3 stages. The exhumed crustal rocks were in part modified by brittle deformations of later stages, especially by sinistral shearing to form a symmetric kink folds, and then by conjugate faulting associated with the formation of pseudotachylites. The conjugate fault set was formed under an ENE-WSW compression during the latest Miocene, cutting the general trend of the belt nearly at right angles.