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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.
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.
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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.
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.
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Stratigraphy of the Pleistocene SCH boring core drilled in the Sapporo fan, Hokkaido, Japan [PDF link]
Tsumoru Sagayama, Akira Sato, Yukio Izima and Satoshi Okamura
General Geology, Vol. 5, Page 19–23.
Abstract
The SCH boring core, drilled in Kita 1 Nishi 2 of the Sapporo, is 54 m long and mainly the Sapporo fan 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 fan deposits is MIS 5e, and peat bed underlies the fan deposits is the Middle Pleistocene.
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Serpentine precipitation and low-temperature serpentinization around the Kamuikotan tectonic belt, Hokkaido [PDF link]
Takayuki Katoh, Yukihiro Mizuochi, Atusi Ninomiya, Ikuo Okamoto, Tatsuya Yajima, Koki Saito and Masao Sorai
General Geology, Vol. 5, Page 25-48.
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- type; pH 11-12).
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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.
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 AMS 14C-dating and pollen analysis for the thinlayered mud facies of the upper part and obtained the measurement age of 20.6ka 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.
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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.
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.
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