Department of Environmental Sciences, Faculty of Education and Human Sciences, Yokohama National University
Department of Environmental Sciences, Faculty of Education and Human Sciences, Yokohama National University
抄録(英)
The Tanzawa plutonic complex, which is composed mainly of tonalite with minor quartz gabbro and gabbro, is exposed at the northern end of the Izu-Bonin island-arc. It is interpreted as exposed middle crust of the Izu-Bonin arc which was uplifted during the collision of Izu-Bonin arc with the Honshu arc in Japan. Determination of elastic wave velocity in these rocks is crucial to interpret the seismic velocity structure of the Izu-Bonin arc. Compressional-wave (Vp) and shear-wave (Vs) velocities from Tanzawa plutonic rocks were measured at 1 atm and room temperature. The P-wave and S-wave velocities are strongly dependent upon the lithology : gabbro (Vp=4.89-5.44km/s, Vs=2.49-2.86km/s), quartz gabbro (Vp=4.90-4.97km/s, Vs=2.39-2.50km/s), tonalite (Vp=4.25-4.71km/s, Vs=2.40-2.48km/s), granitic aplite (Vp=3.82km/s, Vs=2.11km/s). The Tanzawa plutonic rocks show a positive correlation between elastic-wave velocity and density, and a rough positive correlation between Vp and Vs. Theoretical Voigt-Reuss-Hill average Vp values were also calculated for tonalites, quartz gabbro and gabbros at 1.0 GPa pressure. Theoretical Vp is about 2km/s faster than measured Vp at atmospheric pressure. The measured Vp of plutonic rocks (4.25-5.44km/s) is faster than the refraction-determined P-wave velocity in 2km-thick uppermost crust (2.5-4.0km/s) of the Izu-Bonin island arc, implying that plutonic rocks are not present at 0-2km depth. Theoretical Vp values of tonalite and quartz gabbro are consistent with the velocity structure of middle crust (6.0-6.7km/s velocity). The theoretical Vp of hornblende gabbro (7.0-7.2km/s) can explain the lower crustal P-wave velocity structure (7.1-7.2km/s). The high P-wave velocity (7.2-7.3km/s) in the lowermost crust implies a high content of orthopyroxene (more than 20%).
雑誌書誌ID
AA00733561
雑誌名
Memoirs of National Institute of Polar Research. Special issue
Rock velocities at atmospheric pressure and room temperature in Tanzawa plutonic rocks from central Japan
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KJ00000012897
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概要
