著者

竹下 徹 奥平 敬元

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.47, no.4, pp.453-467, 1995-01-24 (Released:2010-03-11)

参考文献数

79

被引用文献数

1

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Dynamics and thermal modeling in low-pressure/high-temperature metamorphic belts (LPMs) are reviewed. LPM found in the world is formed under such P-T conditions that the pressure is lower than that of the aluminosilicate (Al2SiO5) triple point (about 400MPa), and the peak temperature ranges between 500 and 700°C. Such anomalously high temperatures at relatively shallow depth indicate that the geothermal gradient in the upper crust exceeded 50°C km-1 at the time of LPM formation. Although the steady state geothermal gradients in the upper crust necessary for the formation of LPM could be reached by continuously supplying a huge amount of magma in the upper crust [OXBURGH and TURCOTTE (1971)], such a steady state model is unlikely because of no observation of entire melting of the lower crust which is the outcome of the steady state model. Rather, transient heat sources such as the intrusion of magma, circulation of hot fluid, subduction of young oceanic plate or spreading ridge and convective thinning of the mantle lithosphere have been proposed to be the possible heat sources for LPM. Based on the heat transfer calculations, the duration of high-temperature condition by the intrusion of magma which is several kilometer wide, is less than 1Ma, and that by the circulation of hot fluid is poorly constrained. Even in the thermal model where the highest-temperatures in the country rocks attained by the intrusion of magma are assumed to be recorded as the metamorphic temperatures, the volume of magma necessary for the formation of LPM is more than 50% of the total volume of the crust [HANSON and BARTON (1989)]. Among these thermal models of LPM, subduction of young oceanic plate or spreading ridge is a likely model of LPM, not only because there are many occurrences of LPM evolved from accretionary sedimentary piles at trench, but because the ridge subduction can cause volcanism in the forearc [e. g., DELONG et al. (1979)] in addition to the anomalous heat supply to sediments from the asthenosphere. In order to further constrain the dynamics and thermal model of LPM in future, the P-T path and the duration of low-pressure type metamorphism must be accurately estimated petrologically, and the relative timing between metamorphism and deformation must be thoroughly investigated.