Data report: a brittle (normal?) shear zone from Hole C0002P: deformation structures and their relationship with calcite veins (IODP Expedition 348, Nankai Trough accretionary prism)

Abstract

A brittle shear zone was sampled from Hole C0002P in the Nankai accretionary prism during Integrated Ocean Drilling Program Expedition 348. Macro-and microscopic structures observed both in the core and the covered petrographic thin sections of this fault zone permit characterization of the deformation during faulting. The observed kinematic indicators are coherent with an apparent normal fault sense with respect to its present-day position. Calcite veins are restricted to the most damaged interval of the cored fault. Descriptions of the vein geometry and relationship with the cataclastic foliation present in the fault suggest that these are extension veins that opened during a crack-seal. The various generations of veins suggest that a reiterative history of brecciation and veining took place. IntroductionDuring 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-andthrust 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] 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. Data and methodologyMacroscopic observations on cores were made offshore during the expedition....

Linking Faults, Fractures, and Clay Mineral Occurrence With Fluid Transport in the Accretionary Prism of the Nankai Trough, Japan

“…This is most likely the situation in Hole C0002P between 2,162.5-3,058.4 mbsf, because the settings here change from diagenetic into anchizone source conditions (e.g., Hüpers et al, 2019;Saffer et al, 2008;Underwood, 2017aUnderwood, , 2017b. In fact, the minerals in the cuttings investigated in this study, as well as minerals investigated in core samples by others (e.g., Crespo-Blanc et al, 2017) show distinct alteration features due to dissolution-precipitation reactions (Figures 3 and 4), and both quartz and feldspar clasts have highly sutured grain boundaries that are attributed to intense dissolution. Clay formation occurs particularly in the areas of strong fracturing and faulting (2,350 mbsf, 2,600 and 2,800 mbsf in Hole C0002P).…”

“…These observations are confirmed by indurated and, in some specific areas, highly fractured mudstones, siltstones, and sandstones observed in some cuttings and spot‐cores of Hole C0002P at 2,163.0–2,217.5 mbsf (Tobin et al, ). In fact, a single brittle shear zone at around 2,205 mbsf was investigated in detail in the spot‐core by Crespo‐Blanc (). This shear zone is approximately 90 cm long (apparent thickness) and consists of silty claystone and sandstone.…”