Study on crustal structure of Japanese islands arcs as revealed from magnetic and gravity field analysis.
Average rating
Cast your vote
You can rate an item by clicking the amount of stars they wish to award to this item.
When enough users have cast their vote on this item, the average rating will also be shown.
Star rating
Your vote was cast
Thank you for your feedback
Thank you for your feedback
Author
Ueda, YoshioDate
1994
Metadata
Show full item recordAbstract
This paper consists of seven chapters,① introduction,② description of the method for magnetic and gravity field analysis ,③ crustal structure of Kurile and Honsyu arcs as revealed from magnetic and gravity field analysis,④ crustal structure of the Izu-Ogasawara(Bonin) Arcs as revealed from magnetic and gravity field analysis,⑤ crustal structure of the Ryukyu Arc and its adjacent seas as revealed from magnetic and gravity field analysis,R crustal structure of the Daito ridge as revealed from magnetic and gravity field analysis, and ⑦ discussions and conclusions. In succession of chapter I , the methods used in the present study are explained in chapter II with the results of the numerical examples, i.e. (1) two-dimensional inversion of magnetic anomaly to obtain the magnetic basement relief using the Fourier expansion method, (2) three-dimensional inversion method based on Oldenburg’s two-dimensional formula, (3) definition of the magneto-gravity response function to derive the magnetization direction and ρ/J ratio of the source body, (4) definition of magneto-gravity response filter to extract the magnetic anomalies correlative with gravity anomalies. The method of correlation analysis in the space domain is also described to obtain the magnetization intensity and p/J ratio of the source bodies. The new method described in (3) and (4) will enable us to derive an inclination of magnetization vector without any assumption of the shape and location of the source body. These methods can also discriminate between the magnetic anomaly produced by the upper boundary and that by the lower boundary of the magnetized layer. In chapter III, magnetic basement structures of Kurile and Honsyu arcs consistent with gravity field are presented by selecting typical anomalies such as the Sanriku-Isikari linear anomaly(SILA) in the northeast Honsyu, the Kusiro-Nemuro linear anomaly(KNLA) in the coastal region of the southeastern part of Hokkaido, the southwest Honsyu linear anomaly (SWHLA) in the inner region of southwest Honsyu, and the broad negative anomalies in the outer region of central Honsyu. The model for SILA derived reveals a good agreement between the Conrad surface and the magnetic basement uplift. The model for KNLA also shows the magnetic layer corresponding to the density boundary within the crust.The negative broad magnetic anomaly of the central part of Honsyu seems to be ascribed to the seismic velocity boundary in the crust. A considerably large dimension of the magnetic source is required for SWHLA, but the associated Bouguer gravity anomaly is featureless. The magnetic basement models presented above reveal the magnetic sources in the upper crust with low p/J ratio as well as those in the lower crust with appreciable density contrast as is the case of SILA and KNLA. The former source may be the granitic rocks of magnetite series or serpentinite suites of high magnetic susceptibility. The magnetic structures of northeast Honsyu and Kurile arcs are consistent with lithological model inferred from the magnetization of xenolith samples. In chapter IV, the magnetic basement model covering the Izu-Ogasawara Arc is completed from the geological/ geophysical considerations. Different features of magnetic basement are confirmed between the northern and the southern part of the Izu-Ogasawara Arc with the border at the Sofugan tectonic line (Yuasa, 1992). The magnetic massives of the inner forearc highs of the northern arc show relatively low p/J ratio correlative to acidic to andesitic buried bodies. The magnetic anomalies of the Nisi-Sitito ridge are well explained by the normal magnetization of topographic uplifts. The bulk magnetization per unit km2 along the Quaternary volcanic front of the southern arc becomes 2. 3 times as strong as that of the northern arc. These features may be explained by the stable location of the volcanic front in the southern arc as inferred from old volcanic edifices overlapped by the Quaternary volcanic ones. In chapter V, the magnetic basement model over the Ryukyu Arc and its adjacent seas is completed from the geological/ geophysical considerations. This model reveals the existence of the segmented large magnetic massives having low p/J ratio beneath the Tunghai Shelf break. Besides, the Miocene volcanic basement block located in the trenchward side of the Quaternary volcanic front suggests the southeastward migration of the central Ryukyu. In chapter VI, the magnetization of the Daito ridge is discussed based on the magneto-gravity response analysis. The inclination of the magnetization vector derived shows a good agreement with the present field direction, contrary to the shallow inclination derived from the DSDP sediment core previously. The above features suggest that the magnetization of the Daito ridge is caused by an enhanced magnetic susceptibility or viscous remanent magnetization. Throughout the present study, the characteristic features of the crust are made apparent for the Japanese Island Arcs from the viewpoint of the magnetic, gravity and topographic features. Especially, the distribution of the magnetic massives with low p/J ratio becomes apparent, i.e.in the inner zone of southwest Honsyu Arc, the forearc area of the northern part of the Izu・OgasawaraArc, and the eastern margin of the Tunghai Shelf. The granitic to andesitic rocks rich in magnetite seem to be the most plausible candidate as the source body. Until now, the granitic rock of magnetite series have been thought to be produced in the back-arc side of the arc-trench system. The present study, however, proposes the other case outcropping in the forearc region of the Izu-Ogasawara Arc. Forearc magmatism in the initial stage of subduction and dehydration process of the subducting plate may be related with the origin of these magnetic sources in the forearc region. The magnetic and gravity field analyses on the Izu-Ogasawara and Ryukyu arcs suggest that the undulation of the Moho boundary may be the source of the regional trend of the surface magnetic anomalies. Present study also reveals the significant contribution of magnetic susceptibility to the observed magnetic anomalies. This mechanism may arise from enhanced intial susceptibility at high temperature and/ or the enhanced viscous remanent magnetization of the deep-seated source. The experimental results concerning the enhanced susceptibility may give a significant constrain for constructing the magnetic structure of the crust in the Japanese Islands and the surrounding areas.Journal
Report of Hydrographic and Oceanographic ResearchesIssue/Article Nr
30Page Range
pp.61-174Resource/Dataset Location
https://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/KENKYU/report/rhr30/rhr30-05.pdf