Alternative TitleRadiolarian-based oceanic plate stratigraphy of the melanges and subduction-accretion processes in the western sector of the Yarlung-Tsangpo suture zone, southern Tibet
チベット南部ヤールンツァンポー縫合帯西域におけるメランジュの放散虫にもとづく海洋プレート層序の復元と沈み込み-付加過程
Note (General)The southernmost Yarlung–Tsangpo suture zone (YTSZ) in the Tibet plateau is widely accepted as the youngest suture zone that separates the Indian continent to the south and the Lhasa terrane to the north. Early studies argued that the entire Tethyan oceanic crust was subducted along the southern margin of the Lhasa terrane through one subduction system. An alternate model proposes the existing of multiple subductions. The oceanic plate stratigraphy (OPS) and the subduction-accretion processes, which are required to reconcile these two models, are far poorly understood, especially in the western YTSZ.\
Our research area is located in the western segment of the YTSZ, where the suture zone is divided by the Zhada–Zhongba terrane into the northern and the southern belts. The southern belt of the YTSZ contains ophiolitic melanges (Tanwa melange), volcanic-sedimentary melanges (Erba melange, Maquanhe melange, Tanong melange) and clastic dominated melange (Yisang melange). Detailed radiolarian analyses are conducted on melanges near Zhongba County. Macroscopic-, me soscopic- and microscopic observations are combined to elucidate the relationships of age, lithology and structure. New data are applied to reconstruct the oceanic plate stratigraphy and the subduction-accretion process.The Tanwa melange, which is located south to the Zhada–Zhongba terrane, is characterized by a huge sequence of upper mantle peridotites and mafic rocks thrust north to a siliceous melange sequence. The Erba melange is characterized by highly sheared volcanoclastic or siliceous mudstone matrix including blocks of chert, claystone, basalt. Broken formations of tens-meters are observable in this melange. In the Tanwa melange and the Erba melange, radiolarian assemblages from the highly sheared siliceous mudstone matrix of this melange are comparable with the Arisaturnalis carinatus (KR3) zone, the Turbocapsula costata Zone and the Spoletoensis Zone, Aptian to Albian in age. Radiolarian assemblages from the chert blocks are comparable with the Trillus elkhomensis (JR2) Zone, the Loopus primitivus (JR8) zone, the Pseudodictyomitra carpatica (KR1) zone, and the Cecrops septemporatus (KR2) zone, Pliensbachian–Toarcian and Tithonian–Barremian in age.\
The Maquanhe melange is located south of the Tanwa melange or the Erba melange. They are connected by faults. It consists of siliceous mudstone, chert, basalt, and limestone. This melange does not display the typical blocks-in-matrix fabric. Strata are grossly continuous. Radiolarian assemblages from the chert of the Maquanhe melange are comparable with the Spoletoensis Zone, late Albian in age. The lowermost record of the OPS is thin Lower Jurassic bedded red chert with some layers of limestone. In the Late Jurassic, thin bedded green cherts interbedded with shales were deposited. Trace fossils were well developed. Above them, red cherts and claystones were accumulated from Late Jurassic to Early Cretaceous. During the Aptian to Albian, siliceous mudstones were accumulated in the northern part (the Tanwa melange and the Erba melange), while chert interbedded with limestone were dominant in the southern part (the Maquanhe melange).
Siliceous mudstones, which are Aptian to Albian in age, are always highly sheared. Isoclinally folds and pinch-and-swelled structure are well developed in the Tithonian to Barremian chert blocks. Well developed block-in-matrix fabric in the Late Jurassic to Early Cretaceous strata and rare preservation of strata older than Tithonian indicate a decollement fault formed in a siliceous claystone in the Late Jurassic near Tithonian. Type 2 brecciation developed in Tithonian to Barremian chert blocks and the Aptian to Albian siliceous mudstones indicates continuous tectonic shearing and fluid overpressure in the sediments above the decollement fault. Strata with different lithification and ratio of competent and incompetent materials displayed different deformations styles. No deformation in the radiolarian skeletons indicates that the subduction-accretion processes were in a shallow depth.Because of the lack of terrrigenous trubidites in the Tanwa melange, the Erba melange, and the Maquanhe melange, the age of the youngest siliceous mudstones is the maximum constraint on the timing of accretion. The melanges become younger southward, which is consistent with the oceanic plate subducting northward under the Lhasa terrane during the Cretaceous. In the Albian, the presence of limestone interbedded with chert in the Maquanhe melange indicates sedimentation above or near the carbonate compensation depth (CCD). Compared with the coeval strata in the Tanwa melange and the Erba melange, the shallower water depth and the dominance of siliceous mudstone in the Maquanhe melange reflect the close proximity to the northernmost edge of the Indian continent.\
Within the Tanwa melange, type 1 breccia is interpreted as proximal debris flow from the old accretionary complex. No clast of terrestrial material indicates the sources are only pelagic or hemipelagic. This breccia can be interpreted by the existing of an intra-oceanic subduction within the Neo-Tethys.\
The lack of terrigenous turbidites in the melanges may caused by increasing distance from the continent, topographic barrier of intraoceanic island arcs and Zhada–Zhongba terrane, and the huge accomodation of the Xigaze forearc basin.
学位の種類: 博士（理学）. 報告番号: 甲第4326号. 学位記番号: 新大院博（理）甲第425号. 学位授与年月日: 平成29年3月23日
新大院博（理）甲第425号
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2017-07-03T04:10:06+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション