• Chapter contents
• Abstract
• Introduction
• Data and methodology
• Results
• Macroscopic observations
• Deformation structures observed in thin sections
• Discussion
• Deformation regime and kinematics
• Brecciation and timing of the veining
• Concluding remarks
• Acknowledgments
• References
• Figures
• F1. Regional location map and schematic cross-section.
• F2. Section 348-C0002P-5R-4.
• F3. Section 5R-4 thin section locations.
• F4. Sandstone and silty claystone.
• F5. Lentil-type clasts.
• F6. TS 78–85.
• F7. Microfaults.
• F8. Drag fold.
• F9. Array of microfaults.
• F10. Vein formed of aggregates of small calcite grains.
• F11. Vein formed of aggregates of small calcite grains.
• F12. Lentil-shaped aggregates.
• F13. Vein formed of swarms of veinlets.
• F14. Veinlets with a rhombic geometry.
• F15. Veinlets sealing cataclastic deformation.
• F16. Microfault cutting a veinlet.
• F17. Elongated calcite grains and wall rock inclusion bands.
• F18. Deformation structures, vein geometry, and apparent orientation in fault zone.
• PDF file

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doi:10.2204/iodp.proc.348.205.2017

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 348, part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) (Stage 3), the D/V Chikyu conducted riser drilling to extend an existing hole at Site C0002 (2005.5 meters below seafloor [mbsf] reached during IODP Expedition 338 [Strasser et al. 2014]) to 3058.5 mbsf (see the “Site C0002” chapter [Tobin et al., 2015]). This site is located 80 km southeast of the Kii Peninsula (Japan) in the Kumano forearc basin, on top of the Nankai accretionary prism (Fig. F1). The prism formed during subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate. The accreted material consists of Shikoku Basin sediments deposited during the late Oligocene to late Miocene rifting and backarc spreading behind the Izu-Bonin arc system (Kobayashi et al., 1995) on the northern margin of the subducting Philippine Sea plate. The Nankai prism is actively accreting in the frontal wedge near the toe of the prism; meanwhile, the subhorizontal sediments of the Kumano forearc basin lie on top of the fold-and-thrust belt of the inner accretionary prism.

Cuttings (875.5–3058.5 mbsf) and cores (2163.0–2217.5 mbsf) were collected in the upper Miocene to Pliocene turbiditic silty claystone with few intercalations of sandstone and sandy siltstone that characterize the accretionary prism lithologic units drilled during Expedition 348 (see the “Site C0002” chapter [Tobin et al., 2015]). A remarkably preserved brittle shear zone, 90 cm long (apparent thickness), was cored around 2205 mbsf (Section 348-C0002P-5R-4, hereafter Section 5R-4). The shear zone is located in lithologic Unit V (Strasser et al., 2014; see the “Site C0002” chapter [Tobin et al., 2015]), formed of silty claystone and sandstone.

This paper describes the macro- and microscopic structures observed both in the core and in the covered petrographic thin sections of this fault zone, which show an anastomosing cataclastic foliation and the occurrence of calcite veins, restricted to the most damaged interval of the cored fault zone. The main objectives of this study were

1. To characterize the brittle deformation during faulting through the type of deformation structures, in particular in terms of kinematics, and
2. To unravel the vein opening mechanism determined from their study, with special emphasis on their geometry, internal texture, and their relationships with the deformation structures of the wall rock.

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