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Geological Society, London, Special Publications; 2003; v. 218; p. 299-314;
DOI: 10.1144/GSL.SP.2003.218.01.16
© 2003 Geological Society of London

Magmatic, Metamorphic and Tectonic Processes in Ophiolite Genesis

The nature of faulting and deformation in the Mineoka ophiolite, NW Pacific Rim

Akiko Takahashi1, Yujiro Ogawa2, Yasufumi Ohta3 & Naoto Hirano4,5

1 Doctoral Program in Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8571, Japan
2 Institute of Geoscience, University of Tsukuba, Tsukuba 305-8571, Japan yogawa{at}arsia.geo.tsukuba.ac.jp
3 Doctoral Program in Geoscience, University of Tsukuba, Tsukuba 305-8571, Japan
4 Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan
5 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro, Tokyo 152-8551, Japan

A belt of disrupted ophiolitic rocks occurs on the Boso Peninsula (Japan), currently located north of the oblique subduction boundary between the Philippine Sea and North American Plates, under which the Pacific Plate has been subducting westwards. This ophiolitic belt (Mineoka Belt) is composed of mafic-ultramafic rocks together with Tertiary chert and limestone and island-arc volcaniclastic rocks. Our detailed structural studies in and around the basaltic rock bodies within the ophiolite reveal three phases of deformation. The first phase is further divided into three stages, all related to oblique normal faulting associated with extensional tectonics at or near a spreading axis. Fluid pressures appear to have fluctuated in association with faulting and veining during this phase. The second phase of deformation is characterized by thrust-related shear zones with a significant strike-slip component and is probably related to the final emplacement of the ophiolite by oblique subduction-obduction processes. The third and final phase of deformation affected not only the ophiolite but also later terrigenous and island-arc pyroclastic rocks. This deformation involved large-scale transpressional dextral slip on forearc sliver faults, which are still active today.





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N. Hirano, Y. Ogawa, K. Saito, T. Yoshida, H. Sato, and H. Taniguchi
Multi-stage evolution of the Tertiary Mineoka ophiolite, Japan: new geochemical and age constraints
Geological Society, London, Special Publications, 2003; 218: 279 - 298.
[Abstract] [PDF]