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ASAHIKO TAIRA SANEATSU SAITO KAN AOIKE SUMITO MORITA HIDEKAZU TOKUYAMA KIYOSHI SUYEHIRO NARUMI TAKAHASHI MASANAO SHINOHARA SCHOICHI KIYOKAWA JIRO NAKA & ADAM KLAUS 《Island Arc》1998,7(3):395-407
The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by 72 ) that the continental crust was created by arc magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of arc crust addition obtained by 50 ) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic ( 4 ; 49 ). New data from the Northern Izu–Bonin arc are presented here which support the 72 ) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P wave crustal structure obtained from the Northern Izu–Bonin arc by 66 ) indicates that the arc crust has four distinctive lithologic layers: from top to bottom: (i) a 0.5–2-km-thick layer of basic to intermediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2–5-km-thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2–7-km-thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4–7-km-thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu–Bonin arc is estimated to be similar to the average continental crust by 73 ). The rate of igneous addition of the Northern Izu–Bonin arc since its initial 45-Ma magmatism was calculated as 80 km3/km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km3/year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km3/year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km3/year, 24 ), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by arc magmatism. This conclusion indicates that arc magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level. 相似文献
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Building spatial models within GIS through Geo-Algebra 总被引：3，自引：0，他引：3
MASANAO TAKEYAMA 《Transactions in GIS》1997,2(3):245-256
This paper describes Geo-Algebra, a mathematical framework for supporting geo-computational modelling in conjunction with GIS-based spacial data manipulation capabilities. Geo-Algebra overcomes the discrepancy between spacial modelling and GIS in the modes of representation as well as in their underlying concepts of space by using a common representational framework for (1) mathematical models expressing spatial relationships and (2) data models of georeferenced information. Geographic models of spatial structure and static as well as dynamic spatial interaction are formulated consistently within Gco-Algebra through a limited set of generic operations on map layers which are used simultaneously for GIS dam manipulation and analysis. A significant advantage of Geo-Algebra over other approaches to integration, such as high-level computational languages, is the development of theoretical concepts of the most general kind which allows the derivation of general properties of these in a deductive manner. In particular, Geo-Algebra formalizes and extends the mathematical notion of Map into the novel concepts of relational and metarelational Maps. These extensions lead to the novel concepts of space bridging the absolute and relative, as well as static and dynamic views of space. Such theoretical concepts are also implemented in a dynamic simulation tool called Geocellular . 相似文献
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The results of a controlled source seismic reflection–refraction experiment carried out in 1992 reveal the following characteristics of the northern Izu–Bonin (Ogasawara) oceanic island arc–trench system. (1) The crust rapidly thickens from the Shikoku back-arc basin to the arc, is thickest beneath the active rifts, and then gradually thins to the forearc. The thickness of the crust beneath the arc rift zone and the back-arc basin are ∼ 20 km and 8 km, respectively. (2) The Moho vanishes beneath the forearc. Velocities rapidly decrease eastwards beneath the inner trench wall. (3) The velocity of the lower crust of the arc and the back-arc basin is 7.1–7.3 km/s. This velocity is higher than the typical oceanic lower crust whose velocity is ∼ 6.7 km/s. (4) The velocity of the middle crust of the arc is ∼ 6 km/s. This layer does not exist beneath the back-arc basin. (5) A slight difference in the velocity gradient of the middle crust exists between the arc rift zone and the forearc. Based on these findings and previous studies, it is inferred that: (i) the middle crust is probably granitic rock and formed in more than two episodes; (ii) the lower crust formed by igneous underplating which may also have affected part of the back-arc basin; and (iii) the root of the serpentinite diapir on the inner trench wall is a low-velocity mantle wedge that was probably caused by large amounts of water released from the subducting Pacific plate at depths shallower than 30 km. 相似文献
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