DELGADO Hugo Instituto de Geofisica, Universidad Nacional Autonoma de Mexico Professor, 地球物理学研究所, 助手
URRUTIAーFUCU メキシコ大学, 地球物理学研究所, 教授
伴 雅雄 山形大学, 理学部, 助手 (50208724)
長谷中 利昭 東北大学, 理学部, 助手 (50202429)
大槻 憲四郎 東北大学, 理学部, 助教授 (70004497)
吉田 武義 東北大学, 教養部, 助教授 (80004505)
藤巻 宏和 東北大学, 理学部, 助教授 (90133933)
URRUTIA-FUCUGAUCHI J. Instituto de Geofisica, Universidad Nacional Autonoma de Mexico Professor
BAN Masao Faculty of Science, Yamagata University Assistant Professor
OTSUKI Kenshiro Faculty of Science, Tohoku University Associate Professor
HASENAKA Toshiaki Faculty of Science, Tohoku University Assistant Professor
FUJIMAKI Hirokazu Faculty of Science, Tohoku University Associate Professor
YOSHIDA Takeyoshi College of General Education, Tohoku University Associate Professor
|Budget Amount *help
¥4,000,000 (Direct Cost : ¥4,000,000)
Fiscal Year 1991 : ¥4,000,000 (Direct Cost : ¥4,000,000)
The conditions of magma generation and the processes of magma transportation through crust to the surface are quite variable, and make the diversity in the modes of volcanic eruptions and chemistry of magma in the world. After studying volcanoes in northeast Japan, we felt the need of comparative subduction magmntism study, because the diversity of magma reflects the difference in conditions of their formation and transportations. We chose western Mexico as the study field of comparative volcanism, because of the quite different mode of the volcanism and of some similarity of tectonic settings. Proposed rift-rift-rift triple junction characterizes the tectonics of the western Mexico. Some tectonic features may be analogous to that of Japan of at time of Japan Sea opening. Our study area, Michoacan, Guanajuato, and Chapala, contains over 1, 000 small volcanoes and over 300 middle-sized volcanoes in an area of 40, 000 km^2, somewhat equivalent to the size of Shikoku. The E-W graben struc
tures are prominent across the volcanic field.
Our project includes structural geology, petrology, geochemistry, K-Ar age determination, paleomagnetism, gravity and geomagnetism measurements. Information on age is indispensable for our purpose of Comparative subduction volcanism, because we need to find the transition of magma chemistry and eruption mode under different tectonic settings at different times. Thus we concentrated on the K-Ar dating, paleomagnetism measurement, as well as estimation of volcano ages from geomorphology.
Structural analyses of faults and topography did not reveal any supportive evidences of recent rift movement in Chapala and Guanajuato regions. Rapid subsidence as observed in northeast Japan after the Japan Sea opening did not occur in the study area. Geochemistry of volcanic rocks did not show any characteristics related with rifting, either. Most show characteristics typical of subduction zones. Graben structures there probably represents failed rift, and the rest of two rifts have been active until now.
Age estimation by K-Ar datings and paleomagnetism showed interesting transition of volcanism. In Michoacan and Guanajuato, the volcanoes older than I Ma are all found in the northern part, whereas those younger than I Ma are all found in the southern part. Volcanism seems to migrated toward south as a response to the change in tectonics. It will be our next project to find out the cause of migration.
Despite the occurrence of both small cinder cones and middle-sized shield volcanoes in the study field, their distribution, hence the distribution of their conduits are shown to be fractal. The magma transportation in the study field is analogous to the fluid transport in the porous media.
We are finding more primitive or less fractionated magma in western Mexico. Across-arc variations of magma are different between western Mexico and northeast Japan. We believe to find more interesting contrast of geochemistry of volcanic rocks and their relations with tectonics as we obtain more analyses. Gravity data compiled by the Mexican member showed a rather thick lower crust of metamorphic nature in the western part of Mexico. This is one of the few estimates of crustal structures in Mexico, which is vital to making magma genesis models of Mexican volcanoes.
Besides scientific research, it was also significant to show first-time Japanese participants how many volcanoes are distributed in a limited area, a volcanic field completely different from Japan. Our project also initiated fruitful cooperation and exchange of ideas between Japanese and Mexican scientists.