Faults configuration around the eastern Nankai trough deduced by multichannel seismic profiling

Abstract

The Japan Marine Science and Technology Center (JAMSTEC), the Ocean Research Institute, University of Tokyo and the Laboratoire de Géologie, Ecole Normale Supérieure at Paris carried out in collaboration a 120-channel reflection seismic survey using an airgun array with total 4200 cubic inch capacity around the eastern Nankai trough in 1996. Data obtained by R/V Hakuho, University of Tokyo were primarily processed by JAMSTEC and were shared by the above three main bodies of this program to complete geoscientific interpretation. The fault configuration around the eastern Nankai trough is very complicated due to the collision of the Izu Peninsula against central Japan. The major remarkable faults around the eastern Nankai area are the Zenisu, the Kodaiba and the Tokai thrusts. The main objective of this paper is to clarify these fault configurations. The Zenisu thrust dips gently northward from the southern slope of the Zenisu ridge, and reaches the oceanic Moho. The location of the Tokai thrust may jump at the Tenryu canyon, and appears to reach the top of the subducting oceanic plate. The Kodaiba fault penetrates a 5.5–5.6-km/s layer, which is the crust of the central Japan forearc. A large fault in the trough axis cuts the entire oceanic crust and has a flower structure, which indicates that it has strike-slip component. Its strike is NW–SE, parallel to the plate motion and perpendicular to the strike of the Zenisu ridge. The top of the subducting oceanic crust of the Philippine Sea plate was defined beneath the continental slope of the central Japan forearc. The angle of subduction becomes abruptly steeper 50 km north of the deformation front. We also observed at the southern roof of the Zenis thrust a drape fold structure which is proof of subduction. We suggest a subduction on the southern side of the Zenisu ridge, strongly deformed oceanic crust at the trough axis and some major faults in the forearc region reaching subducting oceanic crust.