HR: 0830h
AN: T31F-0900    [PDF]
TI: Silicification of pelitic schist in the Iwakuni-Yanai area of the Ryoke belt, southwest Japan: Origin of competent layers in the deep crust
AU: * Terabayashi, M
EM: tera@eng.kagawa-u.ac.jp
AF: Kagawa University, Takamatsu, Kagawa, 761-0396 Japan
AU: Yamamoto, H
EM: hyam@sci.kagoshima-u.ac.jp
AF: Kagoshima University, Kagoshima, Kagoshima, 890-0065 Japan
AB: Bright-layer reflections have been imaged at about 15-20 km depth in fore-arc region of southwest Japan, and most of the epicenter distributes above the bright-layer (Ito, 1999). To understand the cause of a seismic bright-layer with the mechanism of large inland earthquake, we studied the low-P/T metamorphic rocks in the Ryoke belt, southwest Japan. The Ryoke belt in the Iwakuni-Yanai area is underlain by Jurassic accretionary complex and their metamorphosed equivalents. They were intruded by Cretaceous granitic rocks. Five metamorphic zones can be difined by mineral assemblages of pelitic rocks as a regional metamorphism before the intrusion; chlorite, chlorite-biotite, biotite, cordierite and sillimanite zones from north to south (Ikeda, 1993). Silicified rocks have been found in pelitic schist of the biotite zone and northernmost part of the cordierite zone. The pelitic schist is dark in color but silicification turned it to be pale gray or milky white. Silicified pelitic schist is mainly composed of fine-grained quartz and minor muscovite and biotite. The silicified pelitic schist forms layers or lenticular bodies several to fifty meters in thickness. The boundary between silicified rock layer and underlying pelitic schist is fairly distinct but that between the overlying pelitic schist is gradual. Quartz veins crossing high angles with schistosity were preferentially developed in the silicified rock layers, while schistosity-parallel quartz veins, which underwent plastic flow, were observed in the pelitic schist. En echelon quartz vein and fishnet-like quartz veins are characteristic of silicified rock layers. This mode of occurrence of quartz veins indicates competence of silicified rock layers relative to pelitic schist. Rock boundary with high competence contrast is probably a good reflector of seismic waves. Bright-layer reflections would arise from silicified rock layers if those are distributed in the deep crust to a considerable extent.
DE: 3660 Metamorphic petrology
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting