Shimanto Belt Research Articles

Regional Variation of CH4 and N2 Production Processes in the Deep Aquifers of an Accretionary Prism.

Accretionary prisms are mainly composed of ancient marine sediment scraped from the subducting oceanic plate at a convergent plate boundary. Large amounts of anaerobic groundwater and natural gas, mainly methane (CH4) and nitrogen gas (N2), are present in the deep aquifers associated with an accretionary prism; however, the origins of these gases are poorly understood. We herein revealed regional variations in CH4 and N2 production processes in deep aquifers in the accretionary prism in Southwest Japan, known as the Shimanto Belt. Stable carbon isotopic and microbiological analyses suggested that CH4 is produced through the non-biological thermal decomposition of organic matter in the deep aquifers in the coastal area near the convergent plate boundary, whereas a syntrophic consortium of hydrogen (H2)-producing fermentative bacteria and H2-utilizing methanogens contributes to the significant production of CH4 observed in deep aquifers in midland and mountainous areas associated with the accretionary prism. Our results also demonstrated that N2 production through the anaerobic oxidation of organic matter by denitrifying bacteria is particularly prevalent in deep aquifers in mountainous areas in which groundwater is affected by rainfall.

Quantitative estimation of fluid pressure ratio of shear vein in an on-land accretionary complex, the Yokonami mélange, the Cretaceous Shimanto Belt, Kochi, southwest Japan

The fluid pressure ratio and effective friction coefficient of shear veins in an exhumed accretionary complex in the Cretaceous Shimanto Belt, southwest Japan, were constrained by fluid inclusion and stress inversion analyses. The shear veins are considered to be developed along the decollement. Temperature and pressure conditions for shear veins were found to be in the range of 175–225°C and 143–215 MPa, respectively. Fluid pressure ranged between the lines of 70–100% lithostatic pressure, when the geothermal gradient was assumed on the basis of the slab age at the trench. The effective friction coefficient of shear veins was estimated to be 0.1–0.2 by stress inversion analysis, which corresponds to a fluid pressure ratio of 0.67–0.83. Both pressure–temperature and friction coefficient results indicate that the fluid pressure ratio is higher than about 0.7. In this paper, we propose the procedure to constrain paleo-fluid pressure ratio from natural materials. Wedge states were examined using the weak decollement friction coefficient, with shallow and steep taper angles indicating that the wedge state is stable or compressively critical, and is controlled by the fluid pressure ratio within the wedge.

Lithification facilitates frictional instability in argillaceous subduction zone sediments

Previous work suggests that in subduction zones, the onset of large earthquake nucleation at depths>~5–10km is likely driven by a combination of factors associated with the process of lithification. At these depths, lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall-rock. To test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low-grade metamorphism, we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the Shimanto Belt in SW Japan. These meta-sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225°C, which are representative of seismogenic depths along the active Nankai subduction megathrust in the foreland of the Shimanto Belt. We find that intact Shimanto rock samples, which preserve the influence of diagenetic and metamorphic processes, exhibit the potential for unstable slip under in-situ pressure conditions. Powdered versions of the same samples tested under the same conditions exhibit only velocity-strengthening friction, thus demonstrating that destroying the lithification state also removes the potential for unstable slip. Using advanced porosity loss to quantify the lithification process, we demonstrate that increased velocity weakening correlates with increasingly advanced lithification. In combination with documented frictionally stable behavior of subduction zone sediments from shallower depths, our results provide evidence that the sediment lithification hypothesis can explain the depth-dependent onset of large earthquake nucleation along subduction zone megathrusts.

Friction properties of rocks sampled from the Shimanto Belt (Japan) – a fossil accretionary prism

Friction properties of rocks sampled from the Shimanto Belt (Japan) – a fossil accretionary prism

Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan

Accretionary prisms constitute ideal targets to study fluid circulation and fluid–rock interactions at depths beyond the reach of active margin deep drilling. The highest-grade rocks from the Shimanto Belt on Kyushu were buried under 3–5 kbars at ~300°C (Toriumi and Teruya, 1988). They contain abundant quartz veins, formed throughout burial and exhumation and variably affected by brittle and ductile deformation.Cathodoluminescence (CL) reveals the existence of two distinct types of quartz, characterized by a blue and brown color, respectively. CL-blue quartz fills macro-veins (width≥10μm), while CL-brown quartz is present in micro-veins (width ~1−10μm) and ductilely recrystallized domains. On the basis of microstructures, the fluids associated with the CL-blue and CL-brown quartz are interpreted as “external” and “local”, respectively. Quartz growth rims of alternating CL colors as well as mutually cross-cutting veins show that the two fluids cyclically wetted the host rock.From fluid inclusions analysis, the fluid associated with CL-blue quartz has a salinity similar to seawater, while the fluid associated with CL-brown quartz is less saline. In addition, CL-blue quartz is richer in aluminum than the CL-brown one. In contrast to the salinity/aluminum signature, the δ18O isotopic signature of both quartz types is similar and buffered by host rock. The difference between the preservation of the salinity signature of the fluid and the loss of its δ18O signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration.The “local” fluid, associated with CL-brown quartz, reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals. The “external” fluid associated with CL-blue quartz is interpreted as seawater or pore water from shallow (depth <1–2 km below seafloor) sediments. We propose that downward percolation of shallow water to depths ~10 km is a transient process associated with mega-earthquakes.

2014年夏に露出した四万十帯手結メランジュの海食台露頭

2014年夏に露出した四万十帯手結メランジュの海食台露頭

Provenance analysis of clastic sediments of the Chichibu and Shimanto Belts in Okinawa Prefecture using sandstone modal and whole-rock chemical compositions

Provenance analysis of clastic sediments of the Chichibu and Shimanto Belts in Okinawa Prefecture using sandstone modal and whole-rock chemical compositions

Formation process of fracture and fracture filling minerals in Hyuga Group, Shimanto Belt, and its permeability

Formation process of fracture and fracture filling minerals in Hyuga Group, Shimanto Belt, and its permeability

A structural traverse across the Shimanto belt in western Shikoku, Japan

A structural traverse across the Shimanto belt in western Shikoku, Japan

Frictional heating recoded in vitrinite reflectance within coal material concentrated layer:The Cretaceous Shimanto Belt

Frictional heating recoded in vitrinite reflectance within coal material concentrated layer:The Cretaceous Shimanto Belt

Multiple damage zone structure of an exhumed seismogenic megasplay fault in a subduction zone - a study from the Nobeoka Thrust Drilling Project

To investigate the mechanical properties and deformation patterns of megathrusts in subduction zones, we studied damage zone structures of the Nobeoka Thrust, an exhumed megasplay fault in the Kyushu Shimanto Belt, using drill cores and geophysical logging data obtained during the Nobeoka Thrust Drilling Project. The hanging wall, composed of a turbiditic sequence of phyllitic shales and sandstones, and the footwall, consisting of a melange of a shale matrix with sandstone and basaltic blocks, exhibit damage zones that include multiple sets of ‘brecciated zones’ intensively broken in the mudstone-rich intervals, sandwiched by ‘surrounding damage zones’ in the sandstone-rich intervals with cohesive faults and mineral veins. The fracture zones are thinner (2.7 to 5.5 m) in the sandstone-rich intervals and thicker in the shale-dominant intervals (2.3 to 18.6 m), which indicates a preference of coseismic slip and velocity-weakening in the former, and aseismic deformation in the latter. However, the surrounding damage zones observed in the current study are associated with an increase in resistivity, P-wave velocity, and density and a decrease in porosity, inferring densification and strain-hardening in the sandstone-rich intervals and strain-weakening in the mudstone-rich intervals. These observations indicate that the sandstone-rich damage zones may weaken in the short term but may strengthen in the geologically long term, contributing to a later stage of fault activity. In contrast, the mudstone-rich damage zones may strengthen in the short term but develop weak structures through longer time periods. The observed shear zone thickness in the hanging wall is thinner (2.3 to 18.6 m) compared to the footwall damage zones (12 to 39.9 m), possibly because faults in the hanging wall were concentrated and partitioned between the preexisting turbiditic sequence of alternating shale/sandstone-dominant intervals, whereas in the footwall, faults were more sporadically distributed throughout the sandstone block-in-matrix cataclasites. A splay fault may evolve and be characterized by physical property contrasts, the lithology dependence of deformation, and the variability of damage zone thickness due to a heterogeneous lithology distribution in the hanging wall and footwall. The deformation patterns observed in the Nobeoka Thrust provide insights to the strain-hardening/weakening behaviors of sediments along megathrusts over geological timescales.

Elastic wave velocities of sandstones and shales from the Shimanto Belt in the Kanto mountains and greenstones from the Tanzawa group at high pressure

Elastic wave velocities of sandstones and shales from the Shimanto Belt in the Kanto mountains and greenstones from the Tanzawa group at high pressure

Spatial and temporal transition in detrital composition in the Shimanto Belt: Regional comparison between Ryukyu and Mie areas.

Spatial and temporal transition in detrital composition in the Shimanto Belt: Regional comparison between Ryukyu and Mie areas.

ルーフィングとアンルーフィングは砂岩組成にどのように反映されるか？ —紀伊半島四万十層群を例に—

ルーフィングとアンルーフィングは砂岩組成にどのように反映されるか？ —紀伊半島四万十層群を例に—

四国西部四万十帯北帯に産する蛇紋岩中のクロムスピネル

四国西部四万十帯北帯に産する蛇紋岩中のクロムスピネル

Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

Tectonic melange zones within ancient accretionary complexes include various styles of strain accommodation along subduction interfaces from shallow to deep. The ductile-brittle transition at shallower portions of the subduction plate boundary was identified in three tectonic melange zones (Mugi melange, Yokonami melange, and Miyama formation) in the Cretaceous Shimanto Belt, an on-land accretionary complex in southwest Japan. The transition is defined by a change in deformation features from extension veins only in sandstone blocks with ductile matrix deformation (possibly by diffusion-precipitation creep) to shear veins (brittle failure) from shallow to deep. Although melange fabrics represent distributed simple to sub-simple shear deformation, localized shear veins are commonly accompanied by slickenlines and a mirror surface. Pressure-temperature (P-T) conditions for extension veins in sandstone blocks and for shear veins are distinct on the basis of fluid inclusion analysis. For extension veins, P-T conditions are approximately 125 to 220°C and 80 to 210 MPa. For shear veins, P-T conditions are approximately 185 to 270°C and 110 to 300 MPa. The P-T conditions for shear veins are, on average, higher than those for extension veins. The temperature conditions overlap in the range of approximately 175 to 210°C, which suggests that the change from more ductile to brittle processes occurs over a range of depths. The width of the shallow ductile-brittle transition zone can be explained by a heterogeneous lithification state for sandstone and mudstone or high fluid pressure caused by clay dehydration, which is controlled by the temperature conditions.

Changes in paleostress state along a subduction zone preserved in an on-land accretionary complex, the Yokonami mélange in the Cretaceous Shimanto Belt, Kochi, southwest Japan

A change in paleostress along a subduction zone plate interface in the shallow portion of a seismogenic zone was detected in an on-land accretionary complex, the Yokonami melange in southwest Japan, using the microfault inversion method. Microfaults were classified into two groups based on location: those occurring throughout the Yokonami melange and those occurring in the Goshikinohama fault zone, which is considered to be a fossil seismogenic fault and is located at the northern end of the Yokonami melange. Stresses obtained for these classified microfaults indicate that two different stress states exist for each deformation feature: one is subhorizontal σ1 and subvertical σ3 with a smaller stress ratio, and the other is subvertical σ1 and subhorizontal σ3 with a larger stress ratio. The difference between these stress states could be related to the horizontal stress change by stress drop after large earthquakes that were recently observed after the Tohoku-Oki great earthquake.

Long-term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan

The Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic melange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a “simple” giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional.

Hydrocarbon seepage and its sources at mud volcanoes of the Kumano forearc basin, Nankai Trough subduction zone

AbstractTwelve submarine mud volcanoes (MV) in the Kumano forearc basin within the Nankai Trough subduction zone were investigated for hydrocarbon origins and fluid dynamics. Gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from MVs 2, 4, 5, and 10. Molecular ratios (C1/C2 &lt; 250) and stable carbon isotopic compositions (δ13C‐CH4 &gt;−40‰ V‐PDB) indicate that hydrate‐bound hydrocarbons (HCs) at MVs 2, 4, and 10 are derived from thermal cracking of organic matter. Considering thermal gradients at the nearby IODP Sites C0009 and C0002, the likely formation depth of such HCs ranges between 2300 and 4300 m below seafloor (mbsf). With respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of HCs fueling the MVs is derived from sediments of the Cretaceous to Tertiary Shimanto belt below Pliocene/Pleistocene to recent basin sediments. Considering their sizes and appearances hydrates are suggested to be relicts of higher MV activity in the past, although the sporadic presence of vesicomyid clams at MV 2 showed that fluid migration is sufficient to nourish chemosynthesis‐based organisms in places. Distributions of dissolved methane at MVs 3, 4, 5, and 8 pointed at fluid supply through one or few MV conduits and effective methane oxidation in the immediate subsurface. The aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane‐containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth.

Distribution of CO2 fluids in the Shimanto belt on Muroto Peninsula, SW Japan: possible injection of magmatic CO2 into the accretionary prism

Carbon dioxide and methane are major components in geofluids; however, there is little evidence showing how C-H-O fluids evolve in a subduction zone. We investigated fluid inclusions in quartz veins from the Eocene-Oligocene Shimanto belt (Murotohanto subbelt) on Muroto Peninsula, SW Japan using microthermometry and laser Raman spectroscopy. Quartz veins that cut the cleavage of the host rocks in the Murotohanto subbelt contain one-phase carbonic inclusions (CH4) and two-phase aqueous inclusions (CH4 ± CO2 vapor and H2O liquid). The vapor in the two-phase inclusions is essentially CH4 in the northern part of the belt and a CO2-CH4 mixture in the southern part; values of (=CO2 / (CO2 + CH4)) vary from 0 to 0.9. Within a single CO2-bearing vein, values decrease from the vein wall ( = 0.5 to 0.9) to the vein center ( = 0), and the homogenization temperature increases from approximately 180°C to 240°C-250°C, indicating a transition of the carbonic species from CO2-CH4 to CH4 during vein formation. CO2-dominant fluids are rare in most accretionary prisms formed under low-grade metamorphic conditions, and the generation of CO2 cannot be explained by diagenesis of organic matter in sediments under the P-T conditions of formation of the CO2-bearing veins (235°C to 245°C, 165 to 200 MPa). The CO2 fluids are distributed preferentially near an out-of-sequence thrust that brings the Murotohanto subbelt into contact with the late Oligocene-early Miocene Nabae subbelt and its many volcanic and intrusive rocks. We therefore suggest that the CO2 fluids were generated in association with near-trench magmatism during the middle Miocene and that the fluids were injected and mixed with the CH4 pore fluids of the sediments in the accretionary prism.