[S-EM15] Geomagnetism and paleomagnetism

[SEM15-P16] Paleomagnetic study of Aso-2/1 lavas

*Nobutatsu Mochizuki¹, Yuki Onishi², Satoko Ishimaru¹ (1.Department of Earth and Environmental Science, Faculty of Advanced Science and Technology, Kumamoto University, 2.Faculty of Science, Kumamoto University)

Keywords:paleomagnetic direction, paleomagnetic secular variation, Aso-2, Akita lava, Iwato lava, Setaura lava

We conducted a paleomagnetic study on lavas in the Aso caldera wall and out of the Aso caldera, called Aso-2/1 lavas. Stratigraphic levels of these lavas were recognized above Aso-1 pyroclastic flow deposit and below the Aso-2 pyroclastic flow deposit. We collected oriented samples from each of Hokamaki, Matoishi, Togawa, Tamaraigawa, Setaura, and Akita lavas, and the Aso-2 pyroclastic flow deposit. Paleomagnetic directional measurements using alternating field demagnetization were made on the samples. Our paleodirection results combined with the previous data indicate estimates on the temporal gaps between a lava and the Aso-2 pyroclastic flow deposit, and the temporal gaps between lava flows. The paleomagnetic direction of the Akita lava is indistinguishable from that of the Aso-2 pyroclastic flow deposit; this result indicates the Akita lava extruded just before the Aso-2 eruption. In contrast, the paleomagnetic direction of the Setaura lava differs by more than 15 degree from that of the Aso-2 pyroclastic flow deposit; this suggest a temporal gap of 300 year or longer. We will show a modified stratigraphic relationship of the Aso-2/1 lavas based on our paleomagnetic data.