Thesis Summary
Tectonic processes during Maizuru back-arc basin closure: Geochemical,geochronological, and structural approaches(ᆅ⌫໬Ꮫ࣭ᆅ㉁ᖺ௦Ꮫ࣭ᵓ㐀ᆅ㉁Ꮫⓗᡭἲ࡟ᇶ࡙ࡃ⯙㭯⫼ᘼᾏ┅㛢㙐ࣉࣟࢭࢫࡢゎ᫂ )                  LARISSA NGOMBI MAVOUNGOU
The aim of this research is to have a precise understanding of the evolution of backarc basins and more specifically to unravel the tectonic processes that occur during
their closure. The paleo Maizuru back-arc basin (BAB) which opened in Early Permianand closed possibly during the Permo-Triassic (P-T) boundary at the East Asian
continental margin, is the target to elucidate those back-arc basins closure-related
processes.

Petrological,

petrographic,mineralogical,geochemical,andgeochronological studies have been carried out on the sedimentary rocks of this basin
in order to achieve the above-mentioned goals. The present research work is organized
into eight chapters. Chapter 1 introduces the research with brief information on back arc basin studies in general with a special emphasis on the Maizuru BAB. It states
the problem and the main objectives of this research. Chapter 2 presents thegeological background of the Maizuru BAB whose rocks are exposed in the presentday Maizuru Terrane. The Maizuru Terrane is subdivided into three zones, thegranitoid-dominated Northern zone, the mafic and ultramafic ophiolitic basementrock-dominated Southern zone, and the sedimentary rock-bearing Central zone.
Chapter 3 focuses on the different methods and techniques used in this study.
Petrological and petrographic studies were conducted through field investigations,
optical and scanning electron microscopy (SEM). The mineral chemical compositions
were determined by electron microprobe analysis (EPMA), and the geochemical
characteristics were defined using X-ray fluorescence (XRF) and inductively coupledplasma mass spectrometry (ICP-MS). Laser ablation inductively coupled plasma mass
spectrometry (LA-ICP-MS) was used for geochronology. Chapter 4 describes the mainrock types in the study area (Mimasaka-Doi, Okayama) and their field relations. The
massive mudstone-dominated Maizuru Group, the breccia-dominated Tonoshiki
Formation, and the Yakuno Ophiolitic group of rocks are the three main units foundin the study area. Chapters 5 and 6 focus on the main outcomes of this research.
Chapter 5 presents the mineralogical, geochemical characteristics, and the zircon UPb geochronological data of the breccia and its surrounding units. The results showthat Tonoshiki breccia is a product of submarine slope failure and bothtypes of the breccia contain mudstone clasts that have similar mineralogical andgeochemical features as the mudstones of the Maizuru Group. Chapter 6 deals withthe characteristics of Tonoshiki breccia and shows that it is affected by several
calcite-filled fractures and sediment-filled fractures, which are evidence of hydraulicfracturing. Chapters 7 and 8 discuss the main results of this research and address
the problems stated in the introduction. The major findings and conclusionsof thisresearch revealed that Tonoshiki Formation is the topmost unit of the Maizuru
Group and the rock record during the closure of the Maizuru back-arc basin in Late
Permian time. The closure of the basin occurred during a period of intense tectonic
activity as a result of the subduction of the basin basement under the East Asian
continent and was characterized by two episodes of debris-flow events at ca. 259 Maand ca. 251 Ma, which led to the deposition of extensively fractured breccia. The
study of closure-related tectonic processes for back-arc basin has its global
importance as similar basins were formed and eventually closed in similar tectonic
setting. Therefore, such understanding of the tectonic development of the Maizuru
area in southwest Japan leads to the understanding of the East Asian continental
margin during Permo-Triassic boundary, and also help to decipher the closing stage
mechanisms and structural style of back-arc basins in the world. Very few studieson paleo back-arc basins have been published and this leads to the very limited
amount of information regarding those closure-related mechanisms. The current
study therefore provides additional and important information or ideas about thelate development stage of back-arc basins. The results obtained from the present
study suggest that coarse-grained sediments such as breccia and processes such as
slope failure, sediment provenance switch from opposite sides of the basin, intensive
fracturing, and veining could constitute typical sedimentary and structural rockrecord of back-arc basin closure.

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