[S-SS30_28AM2] New perspective of great earthquakes along subduction zones

[SSS30-08] Paleomagnetic analyses of core samples from the plate-boundary thrust obtained during the IODP JFAST

*Toshiaki MISHIMA¹, Tao YANG², Kohtaro UJIIE³, James KIRKPATRICK⁴, Frederick CHESTER⁵, Casey MOORE⁶, Christie ROWE⁷, Christine REGALLA⁸, Francesca REMITTI⁹, Jun KAMEDA¹⁰, Monica WOLFSON¹¹, Santanu BOSE¹², Tsuyoshi ISHIKAWA¹³, Virginia TOY¹⁴ (1.Osaka City University, 2.China Earthquake Administration, 3.University of Tsukuba, 4.Colorado State University, 5.Texas A&M University, 6.University of California Santa Cruz, 7.McGill University, 8.Pennsylvania State University, 9.Universita di Modena e Reggio Emilia largo, 10.Hokkaido University, 11.University of New Hampshire, 12.University of Calcutta, 13.JAMSTEC, 14.University of Otago)

IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST), drilled through the plate-boundary dècollement of the Japan Trench, where large slip occurred during the 11 March 2011 Tohoku-Oki Earthquake. In order to reconstruct the deformation of the sediments at the cm scale and less, we conducted paleomagnetic measurements of the core sample from the plate-boundary dècollement zone.The plate-boundary core sample has a scaly fabric and is composed of bicolored clay layers with sharp contacts. We prepared slabs for thin sections across the contacts with typical dimensions of 3x3x5 cm³ from the core sample. We measured remanent magnetization of 16 slabs. The slabs were subjected to natural remanent magnetization (NRM) measurements in 0.5-1 cm intervals and progressive alternating field demagnetization (AFD) up to 80 mT with a 2G755 pass-through superconducting rock magnetometer at Kochi University.Typically, two paleomagnetic components were isolated during the AFD of slab samples up to 80 mT. One component (`soft' component) was demagnetized below 20-30 mT, and another component (`hard' component) was not demagnetized even up to 80 mT. For multiple slab samples cut from the same whole-round sample, the hard component generally has a consistent paleomagnetic direction. Contrastingly, the direction of the soft component is not so consistent between adjacent slabs, and even varies within a single slab. The direction variation of the soft component possibly reflects the cm-scale rotation of competent phaccoids during deformation within the slab samples from the plate-boundary thrust zone. The consistency of the hard component directions implies that the hard component was remagnetization during/after the rotation, and was possibly carried by newly-formed magnetic minerals during the deformation.