Cretaceous Shimanto Belt Research Articles

Temperatures of Vein Formation Associated With Plate Interface Deformation Constrained by Oxygen and Clumped Isotope Thermometry

AbstractTectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions.

Frictional Properties and Healing Behavior of Tectonic Mélanges: Implications for the Evolution of Subduction Fault Zones

AbstractThe frictional velocity dependence and healing behavior of subduction fault zones play key roles in the nucleation of stick‐slip instabilities at convergent margins. Diagenetic to low‐grade metamorphic processes such as pressure solution are proposed to be responsible for the change in frictional properties of fault materials along plate interfaces; pressure solution also likely contributes to the acceleration of healing according to previous studies. Here, we report friction studies for temperatures of 20–100°C and normal stresses from 20 to 125 MPa on samples collected from ancient subduction fault zones, the Lower Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two mélanges correspond to the updip and downdip limits of the seismogenic zone and include deformation features that indicate lower and higher degrees of pressure solution. Our data show that the Lower Mugi mélange exhibits velocity‐weakening to velocity‐neutral frictional behavior under low normal stress and that the Makimine mélange sample shows velocity‐strengthening behavior under high normal stress. We suggest that mineralogical changes due to diagenesis and metamorphism influence fault slip behavior. We measure frictional healing in slide‐hold‐slide experiments for the Lower Mugi mélange sample and document the role of pressure solution in fault healing. Our results show that frictional healing increases at higher temperatures. The microstructures related to pressure solution found in the post‐experimental gouges support the idea that the enhanced healing is related to pressure solution.

Relationship between deformation structure and vein in the accretionary complex; example from Miyama Formation in the Cretaceous Shimanto Belt

Relationship between deformation structure and vein in the accretionary complex; example from Miyama Formation in the Cretaceous Shimanto Belt

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.

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

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.

Accreted Kula plate fragment at 94Ma in the Yokonami-melange, Shimanto-belt, Shikoku, Japan

Abstract The Cretaceous Shimanto belt that is distributed along the Pacific side of southwest Japan contains one of the best records of accretionary processes and history of oceanic plate subduction in the circum-Pacific region. Yokonami melange, Shimanto belt, Shikoku, SW Japan is composed of pillowed basalt that erupted in the equatorial region, about 200 m-thick bedded radiolarian chert sequence that contains radiolarians with age determinations spanning ca. 135 Ma (Upper Valanginian) to 93.9 Ma (Cenomanian), younging southward with repetition, and alternated sandstone and mudstone. Although the chronology of the bedded chert sequence is well studied using the radiolarian, the chronology of the clastic units was not reported sufficiently. We performed a detailed geologic mapping investigation of rock outcrops along the Goshiki-no-Hama Coast, the Yokonami melange. We have determined U–Pb ages of zircon grains extracted from tuffs and sandstones to determine the depositional age of the turbidite and hemipelagic units. The youngest depositional age of these clastic sediments is interpreted to be the arrival time at the trench. We reconstructed the ocean plate stratigraphy with our new chronological data, which includes mid-oceanic basalt, bedded chert, hemipelagic sediments, and trench-turbidite deposits. We recognize that the turbidite sequence has been repeated by layer-parallel thrust in the study area. We have delineated the travel trajectory of the oceanic lithosphere based on (1) the relative plate motion model of the Paleo-Pacific Ocean, (2) the arrival time of the subducted lithosphere at the trench (94.32 Ma) and (3) paleomagnetic measurement of the basalt and turbidite. The trajectory suggests that the MORB in the Goshiki-no-Hama Coast, Yokonami melange, erupted at the Kula–Pacific Ridge in the Paleo-Pacific Ocean, traveled northward, and eventually arrived at the continental margin of East Asia at 94.32 Ma.

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

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

Difference in acoustic properties at seismogenic fault along a subduction interface: Application to estimation of effective pressure and fluid pressure ratio

Fluid pressure along subduction plate boundaries plays a role in seismogenesis and tsunami genesis because it is strongly related to the physical properties of faults. In this study, we conducted P-wave velocity (Vp) and S-wave velocity (Vs) measurements for the hanging wall and footwall of a fossil subduction interface with pseudotachylyte located at the northern edge of the Mugi mélange in the Cretaceous Shimanto Belt, Shikoku, southwest Japan. This area corresponds to the depth of the shallow seismogenic zone in the transition zone between the inner and outer wedges. By combining the acoustic properties with parameters obtained from Amplitude Variation with Offset (AVO) analysis on the Nankai seismic profile, we estimated the fluid pressure at the seismogenic fault. The Mugi mélanges are composed of shale matrices and are juxtaposed with the coherent unit of the Hiwasa formation in the north, which is composed mainly of sandstone. We collected 5 sandstone samples from the hanging wall (Hiwasa formation) and 4 mudstone samples from the footwall (Mugi mélange). We conducted velocity measurements while controlling both the fluid pressure and confining pressure by using two pumps. The effective pressure in each measurement ranged from 5 to 65 MPa with intervals of 5 MPa. The Vp and Vs of sandstone increase exponentially with effective pressure from ~ 4500 to ~ 5000 m/s and ~ 2500 to ~ 3000 m/s, respectively. The Vp and Vs of the mudstone also increased exponentially from ~ 4100 to ~ 4500 m/s and ~ 1900 to ~ 2200 m/s, respectively. We used AVO parameters along the décollement based upon a seismic profile of the Nankai trough, which is off Muroto 40–45 km landward from the trench axis, corresponding to approximately 66 MPa of effective pressure under a hydrostatic condition. By combining the velocities obtained from this study and the AVO parameters derived from the Muroto seismic data, we estimate the mean effective pressures for the hanging wall and the footwall as approximately 10–20 MPa and 8–10 MPa, respectively. The normalized fluid pressure ratios for the hanging wall and the footwall correspond to approximately 0.82–0.91 and 0.91–0.93, respectively. This high fluid pressure indicates a very low effective friction coefficient along the décollement in the transition zone, possibly causing a rupture to propagate to the shallower outer wedge and thus generating a large tsunami. • Vp and Vs were measured from hanging-wall and footwall at a fossil seismogenic décollement. • Very high fluid pressure ratio was estimated using seismic data and obtained properties. • The target depth was at transition zone, deeper than that of previous works.

Shear veins under high pore pressure condition: Yokonami mélange, Cretaceous Shimanto Belt, Shikoku, Southwest Japan

Shear veins under high pore pressure condition: Yokonami mélange, Cretaceous Shimanto Belt, Shikoku, Southwest Japan

Large amount of fluid migration around shallow seismogenic depth preserved in tectonic mélange: Yokonami mélange, the Cretaceous Shimanto Belt, Kochi, Southwest Japan

AbstractStudying subduction zone fluid at shallow seismogenic depths is important to understand the nature of fault rocks at the updip limit of the seismogenic zone because fluid–rock interactions affect heat and mass transfer, and fault strength. In this study, we conducted detailed analyses of distribution of shear veins, and estimation of pressure–temperature conditions for shear vein formation for the Yokonami mélange, Shikoku, Southwest Japan, which is tectonic mélange zone in an on‐land accretionary complex. We found a seismogenic fault at the upper boundary of the Yokonami mélange, indicating that the Yokonami mélange was active at seismogenic depth. The field‐transect distribution of shear veins was examined. The frequency, the total and mean thicknesses of the shear veins were about 3.7 per meter, about 10 mm per meter, and about 3 mm per shear vein, respectively. Quartz within the shear veins shows elongate‐blocky textures, suggesting precipitation from advective flow. The pressure and temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging 175–225°C and 143–215 MPa, respectively. The temperature is almost consistent with the paleotemperature determined from vitrinite reflectance, suggesting that the shear veins were formed at up to the maximum depth. The depth might be consistent with that where the seismogenic fault was formed. On the basis of the pressure and temperature conditions and the distribution of shear veins, we estimated the minimum volumetric ratio of fluid to host rocks, assuming that the shear veins had precipitated from advective flow. The estimated amount of fluid is about 106 m3 per cubic meter of host rocks. The results suggest that a large amount of fluid migrates through mélange zones at shallow seismogenic depths. This fluid possibly originates from the dehydration of clay minerals from underthrusted sediments and an altered subducting slab.

Gap of elastic velocities at geological boundaries and examination of AVO parameters in the Cretaceous Shimanto Belt, Shikoku

Gap of elastic velocities at geological boundaries and examination of AVO parameters in the Cretaceous Shimanto Belt, Shikoku

九州西端部からの四万十高圧変成岩類の発見

206Pb/238U dating of detrital zircons by LA-ICP-MS clarified that the majority of high-P/T Nagasaki metamorphic rocks in western Kyushu contain Late Cretaceous igneous zircon of 89 ± 3 Ma (Turonian-Coniacian) and 86 ± 2 Ma (Coniacian-Santonian). These data indicate that Nagasaki metamorphic rocks are composed mainly of Upper Cretaceous Shimanto metamorphic rocks, i.e. high-P/T metamorphosed accretionary complex of the Cretaceous Shimanto belt. This find strongly supports the subhorizontal geotectonic structure of the Japanese Islands previously inferred from surface geological mapping and deep seismic images.

A geochemical estimation of fluid flux and permeability for a fault zone in Mugi mélange, the Cretaceous Shimanto Belt, SW Japan

The fluid flux and permeability along seismogenic subduction interfaces were estimated roughly based on geological evidences. A decrease in silica from the host rocks to the deformed rocks due to rock–fluid interaction within the fault–fluid system was qualitatively examined. The area under study was the northernmost part of the Mugi mélange in the Cretaceous Shimanto Belt, Shikoku, SW Japan. A fault zone with pseudotachylytes is located at the area as the roof thrust of the Mugi mélange. The pressure–temperature conditions of the fault zone correspond to the depth around the onset of the seismogenic zone along the subduction interface. On the basis of deformation texture, rocks are classified into three types—mélange, fault breccia, and ultracataclasite. The bulk chemical analysis of the abovementioned rocks was conducted by employing the X-ray fluorescence (XRF) analysis method. The average mass loss from the mélange host rocks to the fault rocks was estimated by the isocon method. In particular, a decrease in silica was observed clearly. Based on the decrease in silica and certain other geological constraints such as the pressure–temperature conditions of fluid, length of the fault, and the duration between deposition and exhumation, the amount of fluid reacting with rocks and the fluid flux were examined roughly. The estimated fluid volume was approximately thousand times the rock volume, and the fluid flux was about 10 −8–10 −6 m/s. Based on the fluid flux and some constraints in temperature pressure conditions of fluid and pressure gradient, the permeabilities of fault zone rocks were estimated, which was approximately of the order of about 10 −17–10 −16 m 2 and about 10 −16–10 −14 m 2 in the fault breccia and ultracataclasite, respectively. While the estimated value of the permeability of the fault breccia is similar to that obtained in laboratory experiments, the estimated value of the permeability of the ultracataclasite is considerably larger than that obtained from laboratory experiments. This probably indicates that the permeability of the ultracataclasite is not constant as observed in the laboratory experiments but can evolve during seismic cycle.

Deformation Style of Slickenlines on Mélange Foliations and Change in Deformation Mechanisms Along Subduction Interface: Example from the Cretaceous Shimanto Belt, Shikoku, Japan

Accretionary complexes record the histories of changes in physical properties of sediments from unlithified sediments to lithified rocks through the deformation processes along subduction interface. The trench sediment suffered various deformation of particulate flow, pressure solution deformation and cataclastic faultings from ductile to brittle regime during accretion in subduction zone. Tectonic mélange is a characteristic rock in on-land accretionary complexes. The dominant deformation mechanism of tectonic mélange formation is pressure solution on the basis of microscopic observation. However, brittle slickenlines are also commonly observed on mélange foliations at the outcrop scale. Although the slickenlines as a brittle failure is common on the surface of the pressure solution foliation, the relationship of their kinetic are still uncertain. Detailed observations of slickenlines suggest that they are formed by reactivation of the mélange foliations, which indicates that the slickenlines are developed after formation of block in matrix texture characterized in mélange. In addition, mélange foliations are cut by faults related to underplating of oceanic materials. Therefore, formation of slickenlines occur before underplating in a relatively deep portion along subduction interface. On the basis of P-T conditions reported from other parts of the Cretaceous Shimanto Belt, the mélange formation and underplating is inferred to have occurred around the seismic front or within the seismogenic zone. The change in deformation mechanisms from pressure solution to brittle failure may be the first change in physical properties from plastic to brittle around seismic front.

P-T conditions of cataclastic deformation associated with underplating: An example from the Cretaceous Shimanto complex, Kii Peninsula, SW Japan

P-T conditions of cataclastic deformation associated with underplating in subduction zone are estimated from fluid inclusions within the post-melange veins in the Cretaceous Shimanto Belt, Kii Peninsula, SW Japan. Both cataclasite and post-melange veins cut the melange fabric and are distributed along the thrusts in duplex structure. The homogenization temperatures range from 200 (±10)°C to 270 (±42)°C corresponding to entrapment pressures over a wide range from 145 (±5) MPa to 304 (−20) MPa. Our results have important implications in evaluating the relationship between underplating processes by duplexing and coseismic deformation in accretionary complexes.

四万十帯北帯に分布する現地性玄武岩の希土類元素組成とその起源に関する示唆

四万十帯北帯に分布する現地性玄武岩の希土類元素組成とその起源に関する示唆

Underplating process from melange formation to duplexing: Example from the Cretaceous Shimanto Belt, Kii Peninsula, southwest Japan

Accretionary complexes are considered to form through off scraping and underplating, which results in the lateral and vertical growth of an accretionary prism. Accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process, although the origin of melange is considered controversial. In order to better understand the origin of melange and its role in accretion, in particular during underplating, we have studied the relationship between the formation of melange and duplexing in the Miyama Assemblage. Here the melange is composed dominantly of shale, basalt, chert, and sandstone. This study stresses the following features: (1) the structure of the Miyama Assemblage appears to be a composite suite of landward and seaward dipping duplexes; (2) the melange fabric displays a systematic asymmetry, which may have resulted through shearing along the decollement; and (3) facing of horses within duplexes changes from north to south, coinciding with a change in the sense of shear inferred from the melange fabric, which suggests that it formed prior to duplexing.

Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite bo, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.