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To understand the sedimentary development of the Boso Forearc Basin, central Japan, since ~ 3 Ma, we investigated paleothermal structure and consolidation trends in the central and eastern parts of the forearc basin through vitrinite reflectance measurements and consolidation tests. Vitrinite reflectance (Rm) was in the range 0.33 % to 0.61 % for the Miura Group in the central part of the forearc basin and 0.34 % to 0.41 % for the Miura and Kazusa Groups in the eastern. These values suggest a roughly uniform vitrinite reflectance for the Miura Group from the central to eastern parts. No significant vitrinite reflectance difference is observed across the ~ 3 Ma Kurotaki Unconformity in the eastern part of the basin. The consolidation yield stress (pc) was calculated as 27.5 MPa and 32.2 MPa for the Kiyosumi and Amatsu Formations of the Miura Group in the eastern part, respectively. Both the pc values are consistent quantitatively with represent the trend of the maximum overburden pressure estimated from the thickness and density of overlying sediments, and the difference in pc is expected by the maximum burial depths of the strata at the sampling localities. Values of pc in the eastern part of the basin increase with thickness of overlying sediment, showing no break across the Kurotaki Unconformity. Considering the eroded thickness of the Miura Group, the continuous trends in vitrinite reflectance and consolidation between the Miura and Kazusa Groups in the eastern part reflect the greater deposition of the eastern part of the Boso Forearc Basin since ~ 2.3 Ma. [ABSTRACT FROM AUTHOR]

Forearc basin sediments near the ocean-ward margin preserve tectonic information related to plate subduction. The post-middle Miocene Boso forearc basin, central Japan, records major differences in structure, paleo-maximum temperature, and consolidation state between below (Miura Group) and above (Kazusa Group) the Kurotaki Unconformity, which formed at ca. 3 Ma. Many fault systems below the unconformity are characterized by a disaggregation-band-like inner fabric that apparently formed soon after sedimentation, whereas there are few of this type of fault system above the unconformity. Vitrinite reflectance values (R o ) are 0.38%–0.44% and 0.22%–0.25% below and above the unconformity, respectively. The consolidation yield stress ( p c ) in the Miura Group (23.7 MPa in the Anno Formation; 31.0 MPa in the Amatsu Formation) is much greater than that in the Kazusa Group (7.5, 7.6 and 9.6 MPa in the Umegase, Ohtadai and Kiwada formations, respectively). These clear differences in vitrinite reflectance and consolidation characteristics above and below the unconformity are attributed to a forearc basin evolution, which resulted in the Miura Group being geothermally matured, tectonically compacted, uplifted, and eroded (500 m in maximum) before sedimentation of the Kazusa Group. The forearc basin, especially near the trench-slope break, records structural and physical properties reflecting the plate-tectonic environment and the development of the trench-slope. [ABSTRACT FROM AUTHOR]

We have constrained the geothermal structure of the Miura–Boso plate subduction margin, located in central Japan, using maximum paleo-temperature data derived from vitrinite reflectance measurements. We established that higher maximum paleo-temperatures are restricted to the western part of the early Miocene Hota accretionary complex (Hota and Hayama groups), indicating a spatial difference in the amount of slip upon the out-of-sequence thrust potentially associated with the Izu–Bonin Island Arc collision. The weakly deformed sedimentary sequences overlying the highly deformed Hota Group strata have much lower vitrinite reflectance values than the latter. This variation indicates that the sedimentary sequences of the trench slope experienced a markedly lower maximum burial depth than the relatively deep-buried and uplifted Hota accretionary complex. Conversely, maximum paleo-temperatures obtained for tectonic blocks hosted by the neighboring Mineoka ophiolite complex are very high: ca. 140 °C for the large, early Miocene Haccho Formation blocks, and 65–90 °C for the other blocks. This result suggests that the individual tectonic blocks enclosed in the ophiolite complex were subjected exhumation from depths of 3–5 km. [ABSTRACT FROM AUTHOR]

Microorganisms in marine subsurface sediments substantially contribute to global biomass. Sediments warmer than 40°C account for roughly half the marine sediment volume, but the processes mediated by microbial populations in these hard-to-access environments are poorly understood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hot sediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetative cells drop two orders of magnitude and endospores become more than 6000 times more abundant than vegetative cells. Methane is biologically produced and oxidized until sediments reach 80° to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrations demonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zones alternate with zones up to 192 meters thick where microbes were undetectable. [ABSTRACT FROM AUTHOR]

Drilling during International Ocean Discovery Program (IODP) Expedition 370 at Site C0023 encountered instances of hydrothermal mineralization from 775 to 1121 m below seafloor. Fluid inclusion homogenization temperatures measured on barite veins within this interval indicate precipitation from fluids with temperatures up to 220 °C, and salinities ten times higher than interstitial water (16–25 c.f. 2.8–3.6 wt % NaCl). Patches of stratabound mineralization (rhodochrosite, calcite, barite and anhydride) are largely confined to the vicinity of veins and have vertical thicknesses and extents that can be explained by precipitation within the thermal aureoles of veins. Thermal maturities measured on petroleum biomarkers in underthrust sediments record a rise from pre-oil window to early oil window thermal maturities. Basin models show that increases in thermal maturity can be explained by burial metamorphism, and kinetic-based calculations suggest that hot fluids would only have had a minimal effect on hydrocarbon generation. However, the movement of hot fluids still has geochemical implications and creates a complex thermal history where both short-duration, localized heating within fracture zones at hot temperatures (~200 °C for less than 1 yr) and heating during burial over long durations (80–110 °C for 0.5–2 Myrs) need to be considered. • Thermal histories for the underthrust domain of the Nankai Accretionary Complex, IODP 370 "Temperature Limits of the Deep Biosphere". • Hydrothermal and burial metamorphism at IODP Site C0023 with basin models. • Fluid inclusion homogenisation temperatures significantly hotter than present-day formation temperatures. • First direct evidence for anomalously hot fluids in underthrust sediments of Nankai Accretionary Complex. • Biomarkers in underthrust sediments record rise from pre-oil window to early oil window thermal maturities. [ABSTRACT FROM AUTHOR]