Northern Fossa Magna Research Articles

Crustal architecture and Cenozoic tectonics of the northern Fossa Magna, central Japan

Crustal architecture and Cenozoic tectonics of the northern Fossa Magna, central Japan

Facies model of the prograding shelf edge on the junction of the northeast and southwest Japan arcs: an example of Neogene strata in Northern Fossa Magna Region, Nagano prefecture, Central Japan

Facies model of the prograding shelf edge on the junction of the northeast and southwest Japan arcs: an example of Neogene strata in Northern Fossa Magna Region, Nagano prefecture, Central Japan

訂正：北部フォッサマグナに産する後期新生代火山岩のSr-Nd 同位体組成からみた 2 つのソースマントルの分布

訂正：北部フォッサマグナに産する後期新生代火山岩のSr-Nd 同位体組成からみた 2 つのソースマントルの分布

Prograding clinoform and depositional sequences of Neogene strata in Northern Fossa Magna Region, central Japan

東北日本弧と西南日本弧の会合部に位置する北部フォッサマグナ地域には，日本海形成時に形成された堆積盆地を埋積して，厚い新第三系が堆積している．その内，長野県上水内郡小川村から大町市にかけての高府向斜西翼地域に分布する中新統から鮮新統は，砂岩を主体とする層準と泥岩を主体とする層準が周期的に繰り返している．小川村小川川沿いでは周期的に繰り返す砂岩卓越部と泥岩卓越部が観察できる．泥岩卓越部に挟まれる砂岩卓越部は，a砂岩タング，b砂岩タング，c砂岩タング，d砂岩タングの順に累重する．砂岩タングの堆積相は陸棚斜面環境から陸棚上の外浜環境を示し，次第に浅海化していく様子が観察できる．小川川で観察される沖側の堆積環境に対して，大町市美麻を流れる金熊川では陸側の環境が観察でき，デルタ上の前浜，網状河川や海進時のエスチュアリーで堆積した砂岩が卓越する地層を断続的に観察することができる．今回の巡検では北部フォッサマグナ地域における中新統の斜面堆積物と陸側のデルタ堆積物を海水準変動と関連させたシーケンス層序学的なアプローチをもとに観察する．

Tectonic provinces of the Northern Fossa Magna region depicted by means of the crustal movement and seismic activity

Tectonic provinces of the Northern Fossa Magna region depicted by means of the crustal movement and seismic activity

Crustal structure of northern Fossa Magna and off Hokuriku area: Does the plate boundary between North America and Eurasia plate exist in central Japan?

Crustal structure of northern Fossa Magna and off Hokuriku area: Does the plate boundary between North America and Eurasia plate exist in central Japan?

北部フォッサマグナに産する後期新生代火山岩のSr-Nd 同位体組成からみた 2 つのソースマントルの分布

We analyzed the Sr-Nd isotopic and geochemical compositions of 112 late Cenozoic volcanic rocks from the northern Fossa Magna, central Japan. The volcanic rocks are divided, on the basis of Nd isotopic compositions, into a high-Nd group (HND) and a low-Nd group (LND). The HND has higher Zr/Nb and Hf/Nb ratios than the LND volcanic rocks. The isotopic ratios and high field strength element (HFSE) ratios indicate that the HND volcanic rocks were derived from a relatively depleted mantle compared with the LND rocks. The relationship between the HND and LND rocks, in terms of their isotopic and Zr/Nb, Hf/Nb ratios, is similar to that observed between Quaternary rear-arc and frontal-arc volcanic rocks of the NE Japan arc. The rear-arc rocks from NE Japan have higher Nd isotopic and HFSE ratios than the frontal-arc rocks, indicating the injection of depleted asthenospheric mantle beneath the rear-arc, associated with Miocene opening of the Sea of Japan. The HND rocks occur in the central part of the Fossa Magna region in the form of a NW-trending wedge. This wedge shape of the HND area is consistent with the wedge-shaped rifting that occurred perpendicular to the strike of the arc and that opened the Fossa Magna contemporaneously with the opening of the Sea of Japan. The Sr and Nd isotopic compositions of the HND and LND rocks fall within the mantle array and extend into the high-Sr isotopic field. Some of the LND rocks contain lower-crustal mafic xenoliths with Sr-Nd isotopic compositions that overlap those of the LND rocks. In the Sr-Nd isotope diagram, the trend of the HND rocks can be extrapolated to the most radiogenic Sr isotopic compositions obtained for the xenoliths. This result suggests that the lower-crustal materials, which have the most radiogenic Sr isotopic compositions, contributed to the Sr isotopic enrichment of the LND and HND magmas.

Crustal structure of the Northern Fossa Magna Basin and the 22 November 2014 Nagano-ken-hokubu earthquake of Mw 6.2, central Japan

Crustal structure of the Northern Fossa Magna Basin and the 22 November 2014 Nagano-ken-hokubu earthquake of Mw 6.2, central Japan

Petrochemistry and mineralogy of the Pliocene volcanic rocks, northern Fossa Magna

Petrochemistry and mineralogy of the Pliocene volcanic rocks, northern Fossa Magna

Magma process of the volcanic rocks of Arakurayama formation and isotopic mapping in the northern Fossa Magna

Magma process of the volcanic rocks of Arakurayama formation and isotopic mapping in the northern Fossa Magna

Deep to shallow structure of the northern Fossa Magna, based on multi-scale seismic reflection profiling

Deep to shallow structure of the northern Fossa Magna, based on multi-scale seismic reflection profiling

長野県北部フォッサマグナ地域における原油・天然ガスの地球化学

長野県北部フォッサマグナ地域における原油・天然ガスの地球化学

Investigating the role of the Itoigawa-Shizuoka tectonic line towards the evolution of the Northern Fossa Magna rift basin

Abstract The Itoigawa-Shizuoka tectonic line (ISTL) fault system is considered to have one of the highest probabilities for a major inland earthquake occurrence in the whole of Japan. It is a complex fault system with the dip directions of the local fault segments changing from north to south between an east-dipping low-angle thrust fault, a strike slip fault and a west-dipping thrust fault. The tectonic relations between the different parts of the fault system and the surrounding geological units are yet to be fully explained. This study aims to reveal the juncture of the northern and central parts of the ISTL and investigate its contribution towards the shaping of the Northern Fossa Magna rift basin. We conducted 3 deployments of 1 or 2 linear arrays of seismic stations across the central and northern ISTL regions and observed local micro-earthquakes for a period of 3 years. Each deployment recorded continuous waveform data for approximately 3 months. Using arrival times of 1193 local earthquakes, we jointly determined earthquake locations and a 3D velocity model, applying the tomography method. We were able to image the regional crustal structures from the surface to a depth of 20 km with a spatial resolution of 5 km. Subsequently, we used the obtained 3D velocity model to relocate the background local seismicity from 2003 to 2009. The juncture of the northern and central parts of the ISTL was well constrained by our results. The depth extension of the northern parts of the ISTL fault segments follows the bottom of the Miocene Northern Fossa Magna rift basin (NFM) and forms an east-dipping low-angle fault. In contrast, the central parts of the ISTL fault segments are estimated to lie along the eastern boundary of the Matsumoto basin forming an oblique strike slip fault (Fig. 1).

Upward shallowing sequence and gravely sediments in the Miocene Ogawa Formation, Northern Fossa Magna, central Japan

Upward shallowing sequence and gravely sediments in the Miocene Ogawa Formation, Northern Fossa Magna, central Japan

Slump scars and slump deposits on the fan-delta slope, Late Miocene rocks in Saigawa Anticline area, Northern Fossa Magna Region, Japan

Slump scars and slump deposits on the fan-delta slope, Late Miocene rocks in Saigawa Anticline area, Northern Fossa Magna Region, Japan

Sedimentary facies of Late Miocene to Pliocene fan delta system in Saigawa Anticline area, Northern Fossa Magna Region, central Japan

Sedimentary facies of Late Miocene to Pliocene fan delta system in Saigawa Anticline area, Northern Fossa Magna Region, central Japan

Nearshore depositional process of the Miocene Aoki and Ogawa Formations in the Northern Fossa Magna Region

Nearshore depositional process of the Miocene Aoki and Ogawa Formations in the Northern Fossa Magna Region

Provenance from Chemical Composition of Heavy Mineral Grains of Neogene Sandstone in Otari-Sasagamine area of Northern Fossa Magna

Provenance from Chemical Composition of Heavy Mineral Grains of Neogene Sandstone in Otari-Sasagamine area of Northern Fossa Magna

北部フォッサ・マグナに位置する鉾ヶ岳半深成岩と江星山火山岩類のK-Ar 年代と岩石化学的研究

北部フォッサ・マグナに位置する鉾ヶ岳半深成岩と江星山火山岩類のK-Ar 年代と岩石化学的研究

Interferometric seismic imaging of crustal structure using scattered teleseismic waves

The deployment of a dense seismic array of portable broadband stations has presented a number of imaging opportunities using scattered teleseismic waves. Interferometric seismic imaging (ISI) of surface‐related multiple reflections extracted from cross‐correlations of teleseismic waves among pairs of receiver stations was investigated for possible applications and compared to receiver function(RF) method. Synthetic seismograms simulated by the elastic pseudospectral method for a simple 2‐D crustal model demonstrate the potential imaging capabilities of ISI for the Moho structure with high spatial resolution. ISI has also been applied to teleseismic data from dense‐array experiments conducted at the Northern Fossa Magna Basin in central Japan. The results indicate that the Moho phase, with positive polarity at a depth of 38–40 km in the ISI profile, is consistent with the lower crustal structure estimated by RF imaging.