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総合地質 5巻 1号 2021年10月20日発行 [full-issue PDF link]
General Geology, Vol. 5 No. 1, October 20, 2021


[アイデア]

日本海の拡大と中新世瀬戸内火山帯の高Mg 安山岩火成活動とを関連づける造構モデル [PDF link]

君波和雄
総合地質, 5巻, 1–12 頁
A plausible tectonic model linking the opening of the Sea of Japan with high-Mg andesite magmatism in the Miocene Setouchi volcanic belt, SW Japan [PDF link]
Kazuo Kiminami
General Geology, Vol. 5, Page 1–12.

要旨

中新世の瀬戸内火山帯は,高Mg 安山岩(HMA)で特徴づけられる.従来の研究結果は,HMA の形成環境として,次の束縛条件を要求している:1) 前弧域での火成活動,2)急激な温度上昇,3)通常よりも多くのH2O.HMA は,現在のマントル・ウエッジ(以下,ウエッジ)の海溝側末端の上に位置する.日本海の拡大終了後 2−3 my 経過してHMA が噴出した.日本海下で上昇したアセノスフェアは,コーナー流に乗って海溝側に移動し,ウエッジを高温化した.ウエッジの海溝側末端には,一般に加水したマントルが存在する.ウエッジの末端も加熱され,蛇紋岩中の緑泥石やアンチゴライトが分解して多くのH2O が放出されたことにより,カンラン岩の含水融解開始温度が低下して部分溶融した.沈み込んだ堆積物も溶融し,溶融したマントルと混合して,HMAマグマが形成された.紀伊半島西部でHMA が中央構造線の南側まで分布しているのは,フィリピン海スラブのシンフォームが関係していると推定される.

Abstract

The Miocene Setouchi volcanic belt is characterized by the occurrences of High-Mg andesites distributing on the north side of the Median Tectonic Line, excluding the western part of the Kii Peninsula. Previous works demand the following key constraints for the genesis of high-Mg andesite magmas: 1) magmatism beneath the forearc region, 2) abruptly increasing thermal condition, and 3) unusually high concentration of H2O in the mantle. To find the plausible geotectonic model satisfying these constraints has been required. Most high-Mg andesites are situated above the tip of the present-day mantle wedge. The high-Mg andesites were erupted 2−3 my after the end of opening of the Sea of Japan. Opening of a backarc basin is expected to lead to elevated geothermal gradient in a mantle wedge. The opening of the Sea of Japan resulted in upwelling of hotter asthenospheric mantle. It was entrained by slab-induced corner flow and carried toward the tip of the mantle wedge called cold forearc nose. Cold forearc nose is generally made up of hydrated mantle (serpentinite). The opening of the Sea of Japan is presumed to lead to a temperature increase of the cold forearc nose, resulting in breakdown of hydrous minerals in the serpentinite such as chlorite and antigorite. This induced partial melting of the forearc mantle at the vapor-saturated solidus. Sediment-derived melt also contributed to formation of the high-Mg andesite magmas. Distribution of high-Mg andesite in the western part of the Kii Peninsula exceptionally expands southward into the Cenozoic Shimanto Belt. The Philippine Sea slab beneath the western part of the Kii Peninsula is folded into a NS-trending synform, suggesting the mantle in this region extends southward.


[論説]

北海道石狩湾沿岸の分部越観測井の火山灰分析と埋没地形の検討 [PDF link]

嵯峨山 積・藤原与志樹・井島行夫・岡村 聡
総合地質, 5巻, 13–18 頁
Analysis of volcanic ash in the Bunbegoe observation well and investigation of buried landforms at the Ishikari Bay area, Hokkaido, Japan [PDF link]
Tsumoru Sagayama, Yoshiki Fujiwara, Yukio Izima and Satoshi Okamura
General Geology, Vol. 5, Page 13–18.

要旨

火山灰対比のために,石狩湾新港地域の分部越観測井の深度 38.0 m 付近に狭在する軽石混じりの細砂について火山灰分析を行った.火山ガラスの屈折率レンジは1.495~1.501 で最頻値は1.496 で,洞爺火山灰や支笏軽石流堆積物の値とは一致せず,恵庭a 降下軽石層とほぼ同じである.火山灰直下の埋没地形 Bt2 を形成する礫混じり粗粒砂の礫は夕張川~石狩川から供給されたと推定され,当時の石狩川の流路は南西から北東に移動しながら埋没地形 Bt2~Bt4 を順次形成していった.

Abstract

In order to elucidate the volcanic ash correlation, we carried out volcanic ash analysis of pumiceous fine sand bed intercalated in 38 m in depth of the Bunbegoe observation well, located in the Ishikari Bay area, in the north of Sapporo. The result of the analysis shows possibility of that the fine sand bed is correlative with the Eniwa-a pumice fall deposits, ca 17 ka erupted. It is estimated that the gravels on Bt2, buried landform, deposited just underlies the fine sand bed were transported by the Ishikari River, and buried landforms, Bt2~Bt4, were formed in order by the river, shifted gradually from southwest to northeast.


[論説]

札幌扇状地で掘削された更新統SCH ボーリングコアの層序 [PDF link]

嵯峨山 積・佐藤 明・井島行夫・岡村 聡
総合地質, 5巻, 19–23 頁
Stratigraphy of the Pleistocene SCH boring core drilled in the Sapporo fun, Hokkaido, Japan [PDF link]
Tsumoru Sagayama, Yukio Izima, Satoshi Okamura and Yoshitaka Sakata
General Geology, Vol. 5, Page 19–23.

要旨

札幌市中央区北1条西2丁目のSCH ボーリングにより,主に札幌扇状地堆積物からなる長さ54 m のコアが採取された.深度53.45 mの泥炭質シルトの珪藻分析結果は,低塩分の汽水域という堆積環境を示す.SCH ボーリング,N8·1 ボーリング,北海道大学観測井(HU)および地質研究所観測井(GSH)の地層対比から,札幌扇状地堆積物の基底年代はMIS 5e で,同扇状地堆積物直下の泥炭層は中部更新統と推定した.

Abstract

The SCH boring core, drilled in Kita 1 Nishi 2 of the Sapporo, is 54 m long and mainly the Sapporo fun deposits. Result of diatom analysis of peaty silt, deposited in 53.45 m depth of the core, indicates weak brackish in sedimentary environment. Correlation with four borings, SCH, N8·1, HU and GSH, shows geologic age of the basement of the Sapporo fun deposits is MIS 5e, and peat bed underlies the fun deposits is the Middle Pleistocene.


[論説]

神居古潭帯周辺における蛇紋石の沈澱現象と低温蛇紋岩化作用 [PDF link]

加藤孝幸・水落幸広・二ノ宮 淳・岡本征雄・矢島達哉・斉藤晃生・徂徠正夫
総合地質, 5巻, 25–48 頁
Serpentine precipitation and low-temperature serpentinization around the Kamuikotan tectonic belt, Hokkaido [PDF link]
Takayuki Katoh, Yukihiro Mizuochi, Atusi Ninomiya, Ikuo Okamot, Tatsuya Yajima, Koki Saito and Masao Sorai
General Geology, Vol. 5, Page 25−48.

要旨

神居古潭帯の蛇紋岩体周辺の湧水から,現在沈澱しつつある蛇紋石が発見された. この湧水はpH 9-10 のMg2+-HCO3タイプであり,地下の蛇紋岩化作用はすでに停止している.また,複数の蛇紋岩体の周辺から,かつて地表付近で蛇紋石の沈澱が起こったと考えられるさまざまの産状が確認された.これらの蛇紋石は蛇紋岩化作用の産物ではなく,地下水・湧水から沈澱したと考えられるので,低温沈澱性蛇紋石と呼ぶ.このような常温・常圧ないしこれに近い条件で生成した低温沈澱性蛇紋石類や同ブルーサイトは蛇紋岩自体の重要な構成要素となっている.これに対し, 蛇紋岩化作用が現在起こっている岩体は約40 ℃と考えられる蛇紋岩化作用の下限温度を地下で越えている岩体,すなわち比較的低緯度に位置するか,岩体が厚いか, 地温勾配が高い地域の岩体で,かつかんらん石や輝石を残し,適切な深度に地下水が存在する岩体に限られ,pH 11-12 のCa2+-OHタイプの温泉を生成する.

Abstract

A product of precipitation of serpentine under the present-day low-temperature conditions, has been discovered from the cold springs (Mg2+-HCO3 type; pH = 9-10) within the landslides of the Akaiwa and other serpentinite masses, which belong to around the Kamuikotan tectonic belt of Hokkaido, Japan. In the same area, such supergene serpentine occurs in various modes, within or adjacent to several serpentinite masses, either as (i) a constituent of the matrix of the consolidated debris flow deposits derived from serpentinite masses, or (ii) a filling material within the fractures of serpentinites, and the fractures formed in the mudstone adjacent to the serpentinites. In the absence of signs of replacement of the primary minerals, such as olivine and pyroxenes, the observed serpentine is not a product of serpentinization. Instead, it is the result of precipitation from groundwater or spring water. These types of serpentine and brucite, derived from precipitation at near-atmospheric conditions, are estimated to be 10-15% of total serpentine in the studied areas. Unlike the process of formation of low-temperature precipitated serpentine, described above, serpentinization as a mineral transformation process may occur under near-atmospheric conditions only if the following requirements are met within a host represented by an ultramafic rock mass: (i) serpentinization is yet incomplete; (ii) existence of deep-seated groundwater as a promoter of serpentinization; and (iii) presence of relatively high temperature, exceeding the lower limit (ca. 40 °C) required for serpentinization. Therefore, present-day serpentinization is limited to only a few ultramafic masses, which meet one or several of the following criteria: (a) location in low latitude regions; (b) relatively large thickness of the rock mass; (c) relatively high local geothermal gradient; (d) presence of olivine and pyroxenes as relict minerals; and (e)presence of groundwater at appropriate depth as a source of the hot spring (Ca2+-OH; pH 11-12).


[論説]

最終氷期後期の有珠成層火山の形成で生じた長流川の堰き止め湖 [PDF link]

岡 孝雄・星野フサ・関根達夫・米道 博・近藤 務・若松幹男
総合地質, 5巻, 49–70 頁
The lake formed by the damming of Osaru river related to the formation of Usu stratovolcano in the late stage of the Last glacial period [PDF link]
Takao Oka, Fusa Hoshino, Tatsuo Sekine, Hirosi Yonemichi, Tsutomu Kondou and Mikio Wakamatsu
General Geology, Vol. 5, Page 49−70.

要旨

北海道胆振地方西部の長流川下流域の壮瞥市街(滝之町)付近において,同川の南東岸側には現河床からの比高25~45m の中位段丘面が存在している.中位段丘堆積物は有珠成層火山の活動により長流川が堰き止められたことが関与した堆積物(層厚20m 前後)であり,下部,上部,最上部に三分できる.下部は礫層(河川の河道および氾濫原堆積物)より成り,堰き止め発生以前のものである.一方,上部は湖沼を示す薄板状泥相と水中ファン~デルタを示す粗~極粗粒砂相より構成される.最上部は火山灰質ローム層と腐植土層より成り風成層である.この一連の層序は,河谷状態から,突然の堰き止め・湖水の出現を経て,その収束に至るプロセスを示している.上部の上半部(厚さ5~6m の薄板状泥層)の基底より2m 部分について,AMS14C 年代を測定した結果,20.6ka 頃の年代が得られ,花粉分析結果からは亜寒帯針葉樹林要素を主体とし寒冷な気候が示唆された.これらのことから,堰き止めは20ka 頃以降のMIS 2 の最終氷期最寒冷期に生じており,有珠成層火山の形成も20ka 頃であることが明らかとなった.

Abstract

There is the middle terrace whose surface is 25 to 45 meters in height from riverbed, southwest of Soubetsu town area in the Osaru valley, western Iburi in Hokkaido. The deposits of middle terrace, 20± meters in thickness, consists of lower, upper and uppermost parts. The lower part constitutes gravel layer which indicates river channel and flood plain. The upper part consists of coarse sandy facies and thin-layered mud facies. The former part indicates submarine fan or delta and the latter part indicates lake. The uppermost part consists of volcanic ashy loam bed and humus soil bed. The succession as shown above express the process from start of lake to end of it, after the stage of river valley. The authors conducted AMS14C-dating and pollen analysis for the thin-layered mud facies of the upper part and obtained the measurement age of 20.6 ka and the results of cold climate shown by sub arctic coniferous forest mainly. Those results reveal that the damming of Osaru river related to the formation of Usu stratovolcano in the late stage of the Last glacial Period.


[論説]

古倶知安湖の検証-その2-:北海道倶知安町の上部更新統真狩別層の14C年代値,火山灰,花粉および珪藻の分析 [PDF link]

嵯峨山 積・星野フサ・井島行夫・近藤玲介・関根達夫・小田桐 亮・宮入陽介・横山祐典
総合地質, 5巻, 71−80 頁
Investigation of the Paleo-Lake Kutchan (part 2): 14C dating, volcanic ash, pollen and diatom analyses of the late Pleistocene Makkaribetsu Formation in the Kutchan, Hokkaido, Japan [PDF link]
Tsumoru Sagayama, Fusa Hoshino, Yukio Izima, Reisuke Kondo, Tatsuo Sekine, Ryo Odagiri, Yosuke Miyairi and Yusuke Yokoyama
General Geology, Vol. 5, Page 71−80.

要旨

古倶知安湖の存在を検証するために,2020 年に真狩別層より採取した地質試料について14C 年代測定と火山灰,花粉および珪藻の分析を行った.14C 年代値は48,050-46,750 cal BP,45,750-45,000 cal BP および44,500-43,750 cal BP が得られた.火山灰分析では,成分は火山ガラスがほぼ80% を占め,支笏軽石流堆積物の再堆積物と推定した.花粉分析ではA~E の花粉帯を区分し,約3,280 年間はやや冷涼で乾性な気候が安定的に推移したと推定した.珪藻分析から,浮遊性淡水生種の多産により流れの静かな湖沼と,付着性淡水生種の多産により比較的流れがあった堆積環境を想像した.今回測定した14C 年代値は羊蹄山の崩壊(約3.8 万年前)よりも古いことから,同崩壊は古倶知安湖出現の原因とは考えられず,他の地質現象を推定する必要がある.

Abstract

To investigate the Paleo-Lake Kutchan, which was situated in the Kutchan area during the late Pleistocene, analyses of geologic samples were carried out using radiocarbon dating, volcanic ash, pollen and diatom analyses. The samples originate from the late Pleistocene sediments that outcrop in the Kutchan area. Radiocarbon dating indicates 48,050-46,750 cal BP in the G-2 geologic outcrop, 45,750-45,000 cal BP in the G-1 and 44,500-43,750 cal BP in the G-3. Volcanic ash analysis shows that the sample from the G-4 geologic outcrop is a redeposit of the Shikotsu Pumice Flow deposits. The results of pollen analysis in the G-2 and G-3 geologic outcrops reveal 5 pollen zones, A to E, and estimate that the area experienced lower temperatures and drier air when compared with the present day. Diatom analysis indicates considerable planktonic freshwater species yielded from the G-2 geologic outcrop and epiphytic freshwater species yielded primarily from the G-3 and G-5 outcrops. The authors believe that the lake was born before ca 48 ka, the oldest dating in this paper, and the debris avalanche of the Yotei in ca 38 ka did not cause the lake’s genesis. To fully understand this late Pleistocene lake’s beginning, additional data regarding other geologic phenomena is required.


[自由投稿]

樽前山1874年火口を見たイザベラ・バード [PDF link]
宮坂省吾
総合地質, 5巻, 81–88 頁
Isabella Bird, who saw the 1874 crater of Mt. Tarumae [PDF link]
Seigo Miyasaka
General Geology, Vol. 5, Page 81–88.


[報告・資料]

室蘭・絵鞆半島に1,000 万年前の海底火山の活動の跡を見る [PDF link]

松田義章・山岸宏光
総合地質, 5巻, 89−98 頁
Trace of submarine volcanic activity of ca.10 million years ago observed in the Etomo Peninsula, Murorann [PDF link]
Yoshiaki Matsuda and Hiromitsu Yamagishi
General Geology, Vol. 5, Page 89−98.


[論文紹介]

総合地質, 5巻, 99−106 頁 [PDF link]
General Geology, Vol. 5, Page 99−106. [PDF link]

Artemieva, I.M., Thybo, H. and Shulgin, A., 2016. Geophysical constraints on geodynamic processes at convergent margins: A global perspective. Gondwana Res., 33, 4–23.

Vauchez, A., Tommasi, A. and Mainprice, 2012. Faults (shear zones) in the Earth's mantle. Tectonophys., 558–559, 1–27.

Sun, M., Chen, H., Milan, L.A., Wilde, S.A., Jourdan, F. and Xu, Y., 2018. Continental arc and back-arc migration in Eastern NE China: New constraints on Cretaceous paleo-Pacific subduction and rollback. Tectonics, 37, 3893–3915.

Shen, L., Yu, J.-H., O’Reilly, S.Y., Griffin, W,L. and Wang, Q., 2016. Widespread Paleoproterozoic basement in the eastern Cathaysia Block: Evidence from metasedimentary rocks of the Pingtan–Dongshan metamorphic belt, in southeastern China. Precambrian Res., 285, 91−108.

Ma, Q and Xu, Y.-G., 2021. Magmatic perspective on subduction of paleo-Pacific plate and initiation of big mantle wedge in East Asia. Earth-Science Reviews, 213, doi.org/10.1016/j.earscirev.2020.103473.

Xu, C., Zhang, L., Shi, H., Brix, M.R., Huhma, H., Chen, L., Zhang, M. and Zhou, Z., 2017. Tracing an Early Jurassic magmatic arc from South to East China Seas. Tectonics, 36, 466−492.

Lee, S., Oh, C. and Jung, S., 2021. Jurassic igneous activity in the Yuseong area on the southern margin of the Gyeonggi Massif, Korean Peninsula, and its implications for the tectonic evolution of Northeast Asia during the Jurassic. Minerals, 11, https://doi.org/10.3390/min11050466.


[追悼 山岸宏光会員]

総合地質, 5巻, 107−116 頁 [PDF link]
Memorial to Hiromitsu Yamagishi
General Geology, Vol. 5, Page 107−116. [PDF link]


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