[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-P06] Paleomagnetic study on the transitional geomagnetic field variations recorded in lava sequence in the Afar Depression‚ Ethiopia (a preliminary report)

*Ryuichiro Hidaka¹, Nobutatsu Mochizuki², Chie Kato³, Tesfaye Kidane⁴, Ameha A. Muluneh^5,6, Naoto Ishikawa⁷ (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.Faculty of Social and Cultural Studies, Kyushu University, 4.Wayne State University, 5.GFZ German Research Centre for Geosciences, 6.Addis Ababa University, 7.Faculty of Sustainable Design, Toyama University)

Keywords:paleomagnetic intensity, paleomagnetic direction, Tsunakawa-Shaw method, Afar Depression, Tendaho Graben, Matuyama-Brunhes polarity reversal

Reconstructing temporal changes in paleomagnetic intensity as well as paleomagnetic direction are important to better understand the variation of the Earth's magnetic field during geomagnetic reversals and geomagnetic excursions. Lava sequences, which provide a continuous record of the earth's magnetic field, are useful in reconstructing temporal changes in absolute paleomagnetic intensity. The Afar depression in Ethiopia is located at the triple junction of three expanding plates and is an area where aspects of ocean floor expansion can be seen above ground. Tendaho Graben is a 50 km wide rift formed by the extension of the Red Sea Rift propagator to the center of the Afar depression, and is dominated by normal faults with northwest-southeast strike. Lava sequences are exposed on those normal fault planes. Previous studies have reported the ages and paleomagnetic directions from the lavas in the Tendaho Graben. Our research has also clarified that the normal polarity zone was observed at the lavas of the central area and the reverse polarity zone was recorded in the lavas at both relatively narrow outsides of the normal zone in the Tendaho Graben. Based on the above data, we now can search for the lava sequence that potentially records the Matuyama-Brunhes polarity reversal of c. 770 ka. We collected samples from two sections of lava sequences (Section 1: 18 lavas; Section 2: 10 lavas) that may have recorded the Matuyama-Brunhes reversal. In addition, one section (Section 3: 5 lavas) was sampled for the c. 600 ka excursion reported in Kidane et al. (2003). In this study, we started to measure absolute paleomagnetic intensity using the Tsunakawa-Shaw method for three sections of the Tendaho Graben lava sequence. For the 18 lavas of Section 1, about one sample of each lava was measured; the results show paleomagnetic polarity changed from reverse to normal between Units 5 and 6 and it also changed normal to reverse between Units 10 and 11. Absolute paleomagnetic intensity showed a large increase between Units 6 and 7 and a decrease between Units 8 and 9. This presentation will provide a preliminary report on the paleointensity results of the samples from the three sections.