Northern Fossa Magna Research Articles

Upper Cenozoic sedimentary basin in the northern Fossa Magna —its development history and generating mechanism—

Upper Cenozoic sedimentary basin in the northern Fossa Magna —its development history and generating mechanism—

Geology of the lower to middle Miocene Uchimura Formation in the northern Fossa Magna region ―Interfingering relationship between normal and volcanic sedimentary members―

Geology of the lower to middle Miocene Uchimura Formation in the northern Fossa Magna region ―Interfingering relationship between normal and volcanic sedimentary members―

Geochemical evidence for “cryptic amphibole fractionation” and lower crust melting for the generation of island arc tholeiitic rocks from northern Fossa Magna, central Japan

Amphibole plays an important role in arc-related magma differentiation. This relates to the fact that it contains a variety of elements and volatiles, and because it is one of the main minerals comprising the lower crust. Pliocene-Pleistocene Yoneyama volcanic rocks (northern Fossa Magna, central Japan) show a high-K sub-alkalic composition, and pargasite bearing ultramafic–mafic rocks and megacrysts are frequently contained within them. Volcanic activities are divided into five stages. Based on petrography and petrochemistry, “cryptic amphibole fractionation” was found to contribute to the magma differentiation process for the 1st stage rocks even though amphibole is rarely included as a phenocryst. Partial melting of amphibole bearing lower crust, on the other hand, was controlled the generation of the 3rd stage basaltic magma, with the lower crust being one of the important sources supplying H2O to basaltic magma. On the basis of mineral chemistry and isotopes, amphibole bearing ultramafic–mafic cumulates and megacrysts are considered to have crystallized from these basaltic magmas at a lower crustal depth. We conclude that these cumulates are the direct evidence for “cryptic amphibole fractionation” of Yoneyama volcanic rocks.

北部フォッサマグナ，新第三系下部～中部中新統の地質年代と古環境

北部フォッサマグナ，新第三系下部～中部中新統の地質年代と古環境

High-K tholeiitic volcanic rocks of Yoneyama Formation from the northern Fossa Magna, central Japan: Derivative from high water content magma

High-K tholeiitic volcanic rocks of Yoneyama Formation from the northern Fossa Magna, central Japan: Derivative from high water content magma

特集「北部フォッサマグナの地殻変動と地質災害およびその火成−造構史的背景」の 発刊にあたって

特集「北部フォッサマグナの地殻変動と地質災害およびその火成−造構史的背景」の 発刊にあたって

米山層火山岩類のSr, Nd, Hf同位体組成

米山層火山岩類のSr, Nd, Hf同位体組成

O-19 北部フォッサマグナ北西縁-西頸城山地から直江津沖海域-の断層関連褶曲とスラストテクトニクス(3. 堆積盆テクトニクス(液晶有),口頭発表,一般講演)

O-19 北部フォッサマグナ北西縁-西頸城山地から直江津沖海域-の断層関連褶曲とスラストテクトニクス(3. 堆積盆テクトニクス(液晶有),口頭発表,一般講演)

北部フォッサ・マグナ地域,別所層からのArgonautinae化石の発見とその地史学的意義

北部フォッサ・マグナ地域,別所層からのArgonautinae化石の発見とその地史学的意義

北部フォッサマグナ,上田市南部の中新統小川層から見出された安山岩溶岩のK-Ar年代

北部フォッサマグナ,上田市南部の中新統小川層から見出された安山岩溶岩のK-Ar年代

北部フォッサマグナ,下〜中部中新統内村層の地質学的研究

北部フォッサマグナ,下〜中部中新統内村層の地質学的研究

The geological age of the middle Miocene formations in the Northern Fossa Magna region, central Japan : The foraminiferal and fission-track ages of the Uchimura and Bessho Formations

The geological age of the middle Miocene formations in the Northern Fossa Magna region, central Japan : The foraminiferal and fission-track ages of the Uchimura and Bessho Formations

北部フォッサマグナ地域,糸魚川-静岡構造線の西側に分布する中新統山本層安山岩のK-Ar年代

北部フォッサマグナ地域,糸魚川-静岡構造線の西側に分布する中新統山本層安山岩のK-Ar年代

北部フォッサマグナ信濃川地震帯周辺の震源分布と構造規制( 島弧の深部構造-地質・地震・地震波トモグラフィによる解析(I))

北部フォッサマグナ信濃川地震帯周辺の震源分布と構造規制( 島弧の深部構造-地質・地震・地震波トモグラフィによる解析(I))

P2-21 北部フォッサマグナ周辺における新生代沈み込み帯マグマ活動(ポスターセッション)

P2-21 北部フォッサマグナ周辺における新生代沈み込み帯マグマ活動(ポスターセッション)

Neogene volcanism associated with back-arc basin tectonics at the northern Fossa Magna, NE Japan

Neogene volcanism associated with back-arc basin tectonics at the northern Fossa Magna, NE Japan

北部フォッサマグナ中央隆起帯における更新世の運動—そのII，内村渓谷の形成

北部フォッサマグナ中央隆起帯における更新世の運動—そのII，内村渓谷の形成

Miocene-Pliocene mantle depletion event in the northern Fossa Magna, western NE Japan

New isotopic and trace element data presented here imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna of NE Japan from the Miocene to the Pliocene. Less radiogenic 176Hf/177Hf and 143Nd/144Nd, high Zr/Hf, and little or no Hf anomaly characterize the Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge consisted of chemically heterogeneous mantle source. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. This caused the isotopic signature of the rocks to change from enriched to depleted. Then, the mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (<∼780°C) to lose zircon as a residual phase. The Late Miocene and Pliocene volcanism at the post stage of the back-arc opening is best explained by a partial melting of subducted metasediment saturated with trace quantities of zircon and rutile.

Seismological evidence of an active footwall shortcut thrust in the Northern Itoigawa–Shizuoka Tectonic Line derived by the aftershock sequence of the 2014 M 6.7 Northern Nagano earthquake

A destructive M 6.7 earthquake struck Northern Nagano prefecture on November 22, 2014. The main shock occurred on the Kamishiro fault segment of the northern Itoigawa–Shizuoka Tectonic Line (ISTL). We used data recorded at 41 stations of the local seismographic network in order to locate 2118 earthquakes that occurred between November 18 and November 30, 2014. To estimate hypocenters, we assigned low Vp models to stations within the Northern Fossa Magna (NFM) basin thus accounting for large lateral crustal heterogeneities across the Kamishiro fault. In order to further improve accuracy, the final hypocenter locations were recalculated inside a 3D velocity model using the double-difference method. We used the aftershock activity distribution and focal mechanism solutions of major events in order to estimate the source fault area of the main shock. Our analysis suggests that the shallow part of the source fault corresponds to the surface trace of the Kamishiro fault and dips 30°–45° SE, while the deeper part of the source fault corresponds to the downdip portion of the Otari–Nakayama fault, a high angle fault dipping 50°–65° SE that formed during the opening of the NFM basin in the Miocene. Along its surface trace the Otari–Nakayama fault has been inactive during the late Quaternary. We verified the validity of our model by calculating surface deformation using a simple homogeneous elastic half-space model and comparing it to observed surface deformation from satellite interferometry, assuming large coseismic slip in the areas of low seismicity and small coseismic slip in the areas of high seismicity. Shallowing of the source fault from 50°–65° to 30°–45° in the upper 4km, in the areas where both surface fault traces are visible, is a result of footwall shortcut thrusting by the Kamishiro fault off the Otari–Nakayama fault.

Temporal and spatial changes of the conglomerates source of the Sarumaru formation, northern Fossa Magna

Temporal and spatial changes of the conglomerates source of the Sarumaru formation, northern Fossa Magna