ㄽᩥᑂᰝࡢせ᪨(Summary of Dissertation Review)༤ኈࡢᑓᨷศ㔝ࡢྡ⛠(Degree)Ꮫ఩ᤵ୚ࡢせ௳༤ ኈ㸦 ⌮Ꮫ 㸧ẶྡᏛ఩つ๎➨ 4 ᮲➨ձ࣭2 㡯ヱᙜ(Author)

LARISSA NGOMBI MAVOUNGOUㄽᩥ㢟┠ (Title)
7HFWRQLFSURFHVVHVGXULQJ0DL]XUXEDFNDUFEDVLQFORVXUH*HRFKHPLFDOJHRFKURQRORJLFDODQGVWUXFWXUDODSSURDFKHV㸦ᆅ⌫໬Ꮫ࣭ᆅ㉁ᖺ௦Ꮫ࣭ᵓ㐀ᆅ㉁Ꮫⓗᡭἲ࡟ᇶ࡙ࡃ⯙㭯⫼ᘼᾏ┅㛢㙐ࣉࣟࢭࢫࡢゎ᫂㸧ㄽᩥᑂᰝᢸᙜ⪅ (Dissertation Committee)୺  ᰝ (Committee chair)  ෸ᩍᤵ  DAS KAUSHIKᑂᰝጤဨ (Committee member)  ᩍ ᤵ  Ᏻᮾ ῟୍         ᑂᰝጤဨ (Committee member)  ᩍ ᤵ  ∦ᒣ 㑳ኵ         ᑂᰝጤဨ (Committee member)  ᩍ ᤵ  㡲⏣ ┤ᶞᑂᰝጤဨ (Committee member)  ᩍ ᤵ  ᰘ⏣ ▱அᑂᰝጤဨ (Committee member)  ᩍ ᤵ  ஭ୖ ᚭᑂᰝጤဨ (Committee member)  ᩍ ᤵ  ⸈⏣ ࡦ࠿ࡿᑂᰝጤဨ (Committee member)  ◊✲ဨ᪩ᆏ ᗣ㝯㸦ᗈᓥ኱Ꮫ㸧   ࠝㄽᩥᑂᰝࡢせ᪨ (Summary of Dissertation Review)ࠞ
 The Maizuru back-arc basin was a paleo back-arc basin that opened during Early
Permian time at the East Asian continental margin with the deposition of a black andmassive mudstone-dominated sedimentary cover (the Maizuru Group) and closed
possibly during the Permo-Triassic (P-T) boundary with so far unknown tectonic process.
The rocks of this paleo back-arc basin are exposed on land, in the Maizuru Terrane,
which is subdivided into Northern, Central, and Southern zones. The Northern zone ismostly made up of granitoids, the Southern zone is mainly composed of mafic andultramafic ophiolitic rocks representing the basement of the Maizuru back-arc basin,
and the Central zone, which contains the sediments of the back-arc basin, is dominatedby Permian sedimentary rocks with some Triassic rocks. Except for the above-mentioned
Maizuru Group, sandstones, and another major lithological unit, the breccia-dominatedTonoshiki Formation, are exposed in the present study area of Mimasaka-Doi,
Okayama. The Tonoshiki breccia was initially thought to have been deposited directly on
top of the ophiolitic basement rocks, but its stratigraphic position is yet to be clearly
understood. Moreover, these rocks have the potential to unravel the exact basin
closure-related tectonic processes and that of the East Asian continental margin, which
is one of the main aims of this study.
To achieve this goal, the determination of the basin closure-related sedimentological andstructural rock record is of utmost importance and for this purpose, petrological,

petrographic, mineralogical, geochemical, geochronological, and structural studies havebeen carried out in the key localities of the present study area.
The petrographic observations revealed that Tonoshiki breccia rocks are made up ofdifferently-sized extremely angular clasts, which are typical characteristics ofdebris-flow deposits, and can be subdivided into two major types. The first type (Type I),a major type in this area mainly consists of mafic to intermediate rock clast fragments,
and the second type (Type II) is mostly made up of felsic rock clast materials. Both types
contain mudstone clasts that also look black and massive and have similar
mineralogical and geochemical characteristics as the mudstone of the Maizuru Group.
This implies that the mudstone clasts in the Tonoshiki breccia were derived from theunderlying Maizuru Group bringing the Tonoshiki Formation stratigraphically on top ofthe Maizuru Group and representing the rock record during the closure of the basin.
The U-Pb detrital zircon geochronology revealed two pulses of debris flow separated by amore tranquil phase characterized by the deposition of mudstones and sandstones. The
first debris flow event occurred at ca. 259 Ma with the deposition of Type I with sources
mainly from the Southern Zone and the second debris flow event took place at ca. 251
Ma with the formation of Type II breccia sourced mainly from the Northern zone of theMaizuru Terrane. This switch of provenance suggests that the closure of the Maizuru
back-arc basin at the P-T boundary was characterized by intense tectonic activity, whichis evident from different fracture patterns in Tonoshiki breccia. Structural studies
revealed the presence of calcite and sediment-filled fractures in Tonoshiki breccia, aproduct of hydraulic fracturing, which possibly triggered the slope failure and theformation of the breccia. The conspicuous lack of similar fractures in the overlying
Triassic rocks of the Fukumoto Formation suggests that the fracturing events werelimited to the P-T boundary.
Ms. Larissa Mavungou Ngombi generated all the high-precision data in this study andshowed that the closure of the Maizuru back-arc basin was related to the subduction of
the basement crust of the basin under the East-Asian continent during the P-Tboundary and accompanied by the deposition of fractured coarse-grained materials
through pulses of debris flow. This in-depth understanding of the tectonic process andprecise timing of the closure of the back-arc basin are new and original contribution to
the geological evolution of Japanese archipelago and that of the East Asia. Moreover,such in-depth understanding of the back-arc basin closure has its global importance assimilar rock record should be targeted to study other closed paleo-backarc basins andalso many of the present-day back-arc basins which are in the closing stage.
Finally, it is agreed from the above review result that the author of the present thesis
has enough credentials to be awarded the doctoral degree (Science).

බ⾲ㄽᩥ Published Article
 [1] Ngombi, M. L., Das, K., Kawaguchi, K., Hayasaka, Y., and Shibata, T., 2022. Back-arc basin
closure at the East Asian margin during Permo-Triassic boundary: Evidence from geochemistry
and U-Pb zircon data of sedimentary breccia from Maizuru Terrane, Southwest Japan.
Geosystems and Geoenvironment, 1(3) (2022) 100080.