HR: 14:10h
AN: GP12A-03    [PDF]
TI: Magnetic Fabric, Paleomagnetism and Rock Magnetism of Mafic Intrusive Rocks in the Shimanto Accretionary Complex in Southwest Japan
AU: * Kodama, K
EM: kdma@cc.kochi-u.ac.jp
AF: Center for Advanced Marine Core Researc, Kochi Univ, Monobe B-200, Nankoku, 783-8502 Japan
AB: The Shimanto Belt in Southwest Japan is unique in that a part of this accretionary complex was characterized by anomalous, post-accretion, near-trench igneous activity associated with regional folding and faulting. Our study focuses on a large intrusive body (c. 14 Ma) and nearby mafic rocks as well as surrounding sedimentary rocks near Cape Muroto of Shikoku Island. We measured remanent magnetizations, anisotropy of magnetic susceptibilities (AMS) and rock magnetic properties in order to clarify how the mafic body was intruded and settled in the present attitude, and to assess the thermal effect of volcanic intrusion on the accretionary prism. The intrusive body was sampled several meters interval from the chilled margin over about 120 m thick in total, and showed stable remanent magnetizations with moderate magnetic coercivity and unblocking temperature spectrum indicating fine-grained magnetite in the groundmass is the major magnetic carrier. The in-situ directions point moderately upward in the northwest quadrant, showing remarkable contrast from the representative direction of contemporaneous rocks in Southwest Japan. There is a systematic variation in inclinations with distance from the margin: the in-situ inclinations of 30-50 degrees near the margin appears to steepen gradually up to 70-80 degrees toward the core of the body. AMS study recognized two zones with relatively well-grouped, minimum susceptibility axes; the one zone, which has the most tightly clustered minimum axes, is located about 20-30 m away from the chilled margin, and the other one is in the central portion of the body with minimum axes much more scattered. The oblate magnetic fabric is consistent with the bedding attitude of the surrounding sedimentary strata, indicating that the body intruded nearly horizontally into the sediment. Magnetizations of the sedimentary rocks nearby show similar directions of reversed polarity as the volcanic intrusion, suggesting the sediments are likely to have been remagnetized on intrusion of the mafic rocks. These AMS and paleomagnetic results suggest that the intrusive body experienced considerable deformation as well as the host accretionary prism.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting