HR: 0800h
AN: T11B-0374    [Abstracts]
TI: Emplacement of a Layered Mafic Intrusion in the Shimanto Accretionary Complex of Southwest Japan: Evidence From Paleomagnetic and Magnetic Fabric Analysis
AU: * Kodama, K
EM: kdma@cc.kochi-u.ac.jp
AF: Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, 783-8502 Japan
AU: Koyano, T
EM:
AF: Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, 783-8502 Japan
AU: Byrne, T
EM:
AF: Center for Integrative Geosciences, University of Connecticut, 354 Mansfield Rd, Storrs, CT 06269 United States
AU: Lewis, J C
EM:
AF: Department of Geoscience, Indiana University, 113 Walsh Hall, Indiana, PA 15705 United States
AU: Hibbard, J
EM:
AF: Department of Marine, Earth, and Atmospheric Science, North Carolina State University, 1113 Jordan Hall, Raleigh, NC 27695 United States
AB: A middle Miocene, gabbroic intrusive body, which occurs as about 200 m thick and near-vertical sheet in the Muroto area of the Shimanto accretionary complex of southwest Japan, yields anisotropy of magnetic susceptibility (AMS) showing magnetic foliation with minimum (K_min) axis normal to the contact plane. The AMS results suggest that the gabbroic body intruded horizontally into host sedimentary strata prior to tight, intra-prism deformation. A structural correction, assuming the K_min axis as a paleo-vertical axis, leads to the overall mean paleomagnetic direction of D = 152°, I = -56°, α₉₅ = 3°, N = 120, which means that the body underwent after intrusion an about 30° counterclockwise rotation with respect to SW Japan. Based on the paleomagnetic and AMS results, we propose a tectono-magmatic model that the unusual, fore-arc volcanism in the outer zone of SW Japan, including the Muroto area, was a consequence of temporary acceleration of convergence velocity of the Philippine Sea plate with respect to the SW Japan arc, which was caused by rapid CW rotation of SW Japan at 15 Ma. We further suggest that pronounced, cusplike deformation of accreted strata in the Muroto area resulted from subduction of the Shikoku Basin spreading ridge beneath the proto-Nankai trough and thermal softening of prism strata caused by the intrusion of mafic melt produced by the active spreading ridge.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005