5:15 PM - 7:15 PM
[SCG55-P15] Distribution and Formation Process of Filled Mineral Belt Developing Beneath the Nankai Trough Seafloor
Keywords:Nankai trough, XCT, filled mineral, barite, rhodochrosite, hydrothermal mineral
Oceanic crust is formed at mid-ocean ridges and undergoes various processes before subducting at plate convergence zones (e.g., Underwood and Pickering, 2018). Among these processes, mineral-filled zones formed by fluid circulation create heterogeneities in strength and stress, influencing the deformation style of the oceanic crust. However, due to the limited material evidence, their spatial distribution across a large area remains unclear.
The Nankai Trough is one of the most extensively studied plate convergence zones, with numerous scientific research expeditions providing a wealth of geological data. Core samples collected from Site 808 during the Ocean Drilling Program (ODP) Leg 131 in 1991 and Site C0023 during the International Ocean Discovery Program (IODP) Expedition 370 in 2016 off Muroto, Kochi Prefecture, have reported mineral fillings within hemipelagic sediments located just below the plate boundary fault (décollement) (Kastner et al., 1993; Tsang et al., 2020).
In this study, we investigated the spatial distribution of subsurface mineral-filled zones in the Nankai Trough and the Shikoku Basin using X-ray computed tomography (XCT) data from core samples obtained through international ocean scientific drilling programs. The analyzed sites include those drilled in the Shikoku Basin (Deep Sea Drilling Program (DSDP) Sites 297, 442, 443, 444), off Shikoku (ODP Sites 808, 1173, 1174, 1177, and IODP Site C0023), and off Kumano (IODP Sites C0011, C0012). Additionally, we identified mineral compositions and examined formation processes using X-ray diffraction analysis (XRD), XCT image analysis, and macroscopic observations.
Core samples from the four drilled sites in the Shikoku Basin exhibited relatively low CT numbers, suggesting a low abundance of high-density mineral fillings. In contrast, five sites drilled off Shikoku showed concentrated high CT numbers, particularly in hemipelagic mud facies, while two sites drilled off Kumano exhibited localized CT number spikes at Site C0012. Further analysis identified these high CT number minerals as rhodochrosite (MnCO3) and barite (BaSO4).
Based on these results, we mapped the distribution of mineral-filled zones beneath the Nankai Trough seafloor. The findings indicate that mineral-filled zones are concentrated in hemipelagic mud facies within the plate convergence zone and that their abundance varies significantly by location. We propose a model in which Mn and Ba were supplied by hydrothermal activity during and after sediment deposition in the Shikoku Basin. As the sediments approached the convergence zone, these elements combined with carbonate and sulfate ions from high-temperature fluids originating from the landward side, leading to mineral precipitation (Figure 1). The crystallization is estimated to have occurred after 3.9–2.0 Ma, when the uppermost hemipelagic mud facies were deposited. However, it remains uncertain whether high-temperature fluids were indeed rich in carbonate and sulfate ions, necessitating further geochemical investigations.
The Nankai Trough is one of the most extensively studied plate convergence zones, with numerous scientific research expeditions providing a wealth of geological data. Core samples collected from Site 808 during the Ocean Drilling Program (ODP) Leg 131 in 1991 and Site C0023 during the International Ocean Discovery Program (IODP) Expedition 370 in 2016 off Muroto, Kochi Prefecture, have reported mineral fillings within hemipelagic sediments located just below the plate boundary fault (décollement) (Kastner et al., 1993; Tsang et al., 2020).
In this study, we investigated the spatial distribution of subsurface mineral-filled zones in the Nankai Trough and the Shikoku Basin using X-ray computed tomography (XCT) data from core samples obtained through international ocean scientific drilling programs. The analyzed sites include those drilled in the Shikoku Basin (Deep Sea Drilling Program (DSDP) Sites 297, 442, 443, 444), off Shikoku (ODP Sites 808, 1173, 1174, 1177, and IODP Site C0023), and off Kumano (IODP Sites C0011, C0012). Additionally, we identified mineral compositions and examined formation processes using X-ray diffraction analysis (XRD), XCT image analysis, and macroscopic observations.
Core samples from the four drilled sites in the Shikoku Basin exhibited relatively low CT numbers, suggesting a low abundance of high-density mineral fillings. In contrast, five sites drilled off Shikoku showed concentrated high CT numbers, particularly in hemipelagic mud facies, while two sites drilled off Kumano exhibited localized CT number spikes at Site C0012. Further analysis identified these high CT number minerals as rhodochrosite (MnCO3) and barite (BaSO4).
Based on these results, we mapped the distribution of mineral-filled zones beneath the Nankai Trough seafloor. The findings indicate that mineral-filled zones are concentrated in hemipelagic mud facies within the plate convergence zone and that their abundance varies significantly by location. We propose a model in which Mn and Ba were supplied by hydrothermal activity during and after sediment deposition in the Shikoku Basin. As the sediments approached the convergence zone, these elements combined with carbonate and sulfate ions from high-temperature fluids originating from the landward side, leading to mineral precipitation (Figure 1). The crystallization is estimated to have occurred after 3.9–2.0 Ma, when the uppermost hemipelagic mud facies were deposited. However, it remains uncertain whether high-temperature fluids were indeed rich in carbonate and sulfate ions, necessitating further geochemical investigations.