[S-EM15] 地磁気・古地磁気・岩石磁気

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

劉 浩田¹、*望月 伸竜²、加藤 千恵³、Kidane Tesfaye⁴、Muluneh Ameha⁵、藤井 昌和⁶、吉村 令慧⁷、加々島 慎一⁸、乙藤 洋一郎⁹、石川 尚人¹⁰ (1.熊本大学大学院自然科学教育部、2.熊本大学大学院先端科学研究部、3.九州大学、4.University of KwaZulu-Natal、5.Addis Ababa University、6.国立極地研究所、7.京都大学防災研究所、8.山形大学、9.地球年代学ネットワーク、10.富山大学)

キーワード：古地磁気方位、古地磁気強度、拡大軸、噴出岩、溶岩連続層、磁化構造

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.