[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-09] Paleomagnetism of volcanic rocks in the Tendaho Graben, Afar Depression, Ethiopia: Implications for formation of volcanic rocks at the spreading center

Haotian Liu¹, *Nobutatsu Mochizuki², Chie Kato³, Tesfaye Kidane⁴, Ameha Muluneh⁵, Masakazu Fujii⁶, Ryokei Yoshimura⁷, Shin-ichi Kagashima⁸, Yo-ichiro Otofuji⁹, Naoto Ishikawa¹⁰ (1.Department of Earth and Environmental Science, Graduate School of Science and Technology, Kumamoto University, 2.Department of Earth and Environmental Science, Faculty of Advanced Science and Technology, Kumamoto University, 3.Kyushu Univeristy, 4.University of KwaZulu-Natal, 5.Addis Ababa University, 6.NIPR, 7.Kyoto University, 8.Yamagata University, 9.Japan Geochronology Network, 10.The University of Toyama)

Keywords:paleodirection, paleointensity, spreading axis, effusive rocks, lava sequence, magnetic structure

In order to clarify the formation of magnetic structure at a sea-floor spreading center at a divergent plate boundary, we are performing an on-land geoelectromagnetic research project in Afar Depression, Ethiopia, where we can investigate a subaerial sea-floor spreading center. In this study, paleomagnetic measurements were made on the samples from volcanic rocks of 54 sites across the spreading center in the Tendaho Graben in Afar. Alternating field demagnetization and thermal demagnetization were applied to 216 specimens and 31 specimens, respectively. Consistent characteristic remanent magnetizations were obtained for 47 sites out of the studied 54 sites. Combined with the data reported in the previous studies, the paleomagnetic polarities of the lava flows across the Tendaho Graben are recognized: a normal polarity zone is observed at the spreading center with a width of about 40 km and the reverse polarity zones are recognized at both sides of the normal zone. The normal and reverse polarity zones can be correlated to Brunhes and Matuyama chrons, respectively. We also report paleomagnetic data from two lava sequences outcropping at normal faults near the spreading center. The paleointensity and paleodirectional data from the two lava sequences suggest that the duration for formation of a sequence of several lavas is several hundred years. Combining these paleomagnetic data of effusive rocks with the magnetization model of intrusions from magnetic anomaly observation, we discuss the magnetic structure and formation of volcanic rocks at the spreading center in the Tendaho Graben.