Rock velocities at atmospheric pressure and room temperature in Tanzawa plutonic rocks from central Japan

• Main Authors: Keigo Kitamura, Masahiro Ishikawa
• Format: Report
• Language: English
• Published: Department of Environmental Sciences, Faculty of Education and Human Sciences, Yokohama National University 1998
• Subjects: compressional-wave; shear-wave; Poisson's ratio; acoustic impedance; island-arc; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=2345; http://id.nii.ac.jp/1291/00002345/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=2345&item_no=1&attribute_id=18&file_no=1

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%).