Off Sanriku region is a well known plate subduction zone, which lies on the west side of the Japan Trench. High seismic activities have been observed, but there are also low seismic activity regions. We conducted an OBS-controlled source seismic experiment across the seismic-aseismic boundary in 1996 to examine the relationship between seismic activity variation and crustal structure. A traveltime analysis using refraction and reflection waves was applied to the observed data to determine the 2D crustal structure. An amplitude analysis of reflection waves from the subducting plate boundary revealed a good correlation between acoustic-impedance (seismic reflectivity) and seismic activity; that is, high-impedance region is a low seismic region and vice versa. We propose the hypothesis that high-impedance is caused by fluid or hydrate rocks around the plate boundary, and this hypothesis explains the crustal velocity structure and the variations of observed reflection waves and seismic activities.
To predict earthquakes, huge efforts have been devoted to monitoring earthquakes, crustal deformations and water level during past decades, however, has been found to be extremely difficult. A new approach in marine seismic studies on earthquake prediction proposes focusing on the nature of the subducting plate boundary. Some Ocean Drilling Program (ODP) drillings and seismic reflection studies show fluid flows and the existence of hydrous minerals in the decollement zone. Considering that the subducting plate might contain much water, a number of hydrous minerals might be stable down to 150-300 km, in particular, serpentines and lawsonite. In 1998, the authors carried out a seismic experiment at the Izu-Bonin trench using Ocean Bottom Seismometers (OBSs) and controlled sources. The 130 km long E-W line crosses the Torishima forearc seamount, one of the serpentine diapirs at the eastern terminus. The velocity structure obtained along the line shows a relatively high velocity at the top and alow velocity at the bottom of the serpentine diapir. The velocity of the mantle wedge is slower than that of normal mantle. The dip of the subducting slab is initially quite gentle and abruptly becomes steep around 100 km from the trench axis. Hydration of peridotite in the mantle wedge might occur close to the Izu-Bonin trench axis and serpe ntines seem to be raised upward to the ocean bottom.
Serpentinite diapiric seamounts have been reported exclusively from the forearc region of the Izu-Ogasawara-Mariana (hereafter, IOM) arc-trench system. Petrological characteristics of mantle peridotites constituting these seamounts are summarized in comparison with other trench region peridotites. Mantle peridotites drilled from the Conical seamounts during ODP Leg 125 (Site 779) have distinctive compositions both in bulk rock chemistry and mineral chemistry. Their compositions suggest that they underwent a high degree of partialmelting (more than 30 %), which is thought to be related to island arc volcanism in the mantle wedge. The compositions of mantle peridotites collected by submersibles from other serpentinite seamounts in the IOM forearc are similar to those from the Conical seamount. This indicates that most mantle peridotites from these seamounts are residues derived from similar high degrees of partial melting related to the island arc volcanism. In contrast, mantle peridotites recovered from the southern Mariana exhibit wider range of compositions including more fertile peridotites, suggesting that they are residues of relatively lower degrees of partial melting. It is probable that they are related to volcanism that occurred during the formation of the back arc basin. Furthermore, mantle peridotites found in the Tonga forearc have wider compositional range than peridotites found in the IOM forearc region. These peridotites are considered to be derived from a layered sequence from upper crust through lower crust to the upper mantle rather than serpentinite diapiric seamounts. The wide range of mantle peridotite compositions from the Tonga forearcregion indicate a wider range of partial melting than found in the IOM region. The “remnant mantle diapir (RMD)” hypothesis was previously proposed for the origin of the mantle peridotites constituting serpentinite diapiric seamounts. It is plausible that part of the RMDs constitutes the serpentinite seamounts.
A three-dimensional seismic survey was carried out at the western Nankai Trough accretionary wedge from June 18 to August 18, 1999. This experiment was Japan-U.S. collaborative investigation on seismogenic zones. The cruise imaged an 8 X 80 km area with 81, 80km-long, high quality, seismic reflection lines, all of which have nearly continuous coverage. The main objective of our experiment was to image the plate boundary fault at which major earthquakes and tsunamis are generated. Our primary goal is to image the thrust and identify the boundary between aseismic and seismic zones for a large inter-plate earthquake. On-board two-dimensional data processing has been carried out. This gives us a significantly clear image of the inner structure of the accretionary wedge at our survey area. One impressive image of the profile shows the large thrust slice zone that is classified by our interpretation. A number of out-of-sequence thrusts have developed and are concentrated in the area where the water depth is approximately 4, 000 to 3, 000m. A decollement plane touches the oceanic plate (layer II) down at there in first. We identified that the boundary between the stable sliding zone (ocean-ward) and the unstable stick slip zone (landward) is located there. We propose that the boundary is the up-dip limit of the seismogenic zone. Further data processing is ongoing. A true three-dimensional structure will reveal much more details and a clear image of seismogenic zone at the Nankai subduction margin.
The thermal structure of the Nankai subduction zone has been investigated through conventional surface heat flow measurements and estimations of heat flow from the depths of gas hydrate BSRs (bottom simulating reflectors). In the middle and western parts of the Nankai Trough, heat flow on the trough floor is generally higher than that expected from the age of the subducting Shikoku Basin lithosphere, while it is consistent with the age in the eastern part. This high heat flow anomaly might be attributed to advective heat transfer by pore fluid flows from deeper parts of the accretionary prism along the decollement zone, but the amount of pore fluid expelled from the accreted sediments may not be enough to produce the observed anomaly. The deficit in the amount of pore fluid compared to the thermal anomalies is a problem common to other accretionary prisms. Local heat flow anomalies around the deformation front off Shikoku and around thrust faults off the Tokai district indicate the existence of channelized fluid flows along fault zones. A detailed heat flow survey was conducted on the Nankai prism off Shikoku in 1999. Over 60 new heat flow values were obtained along a 80km long line perpendicular to the trough axis, which greatly improved the knowledge of the surface heat flow distribution on this accetionary prism. The heat flow profile landward of the deformation front is generally smooth and seems to correlate well with the thickness of the prism. Local high heat flow anomalies were observed in the vicinity of the deformation front and may correspond to thrust faults. This excellent set of data, combined with BSR heat flow data, should be able to give better constraints for the thermal structure of the accretionary prism and the seismogenic zone. More direct evidence of heat transfer by fluid flows has been obtained in the Barbados and Cascadia accretionary prisms. In ODP drill holes within 15km of the deformation front of the Barbados prism, anomalies in temperature and pore water chemistry were detected around the décollement and thrust faults, indicating localized fluid flows. Some of the anomalies must have been produced by episodic flows. The heat flow around the deformation front in the Barbados prism considerably varies along the arc, which is similar to the observation in the Nankai Trough. A marked increase in temperature in the vicinity of a thrust fault was detected during a long-term monitoring experiment in a drill hole in the Cascadi accretionary prism, and is believed to represent an effect of an episodic flow event.
In order to clarify the effect of water on stability or instability of the shear fracture process of rock, it is critical to study the influence of water on the constitutive properties of the shear fracture process. For this purpose, experiments on fracture of intact granite rock samples have been performed under dry and wet conditions at the following two tempera-tures: 270°C and 450°C. The experimental results indicate that the shear fracture stability or instability is not affected by the magnitude of pore water pressure, but it is strongly affected by temperature ; the shear fracture process is stabilized at temperatures higher than 300°C. The peak shear strength increases linearly with the effective normal stress at temperatures below 300°C. At temperatures above 300°C, however, thepeak strength is lower than that expected from the effect of the effective normal stress. It is discussed how water released by a decomposition reaction of hydrous minerals contained in rock affects stability or instability of the shear fracture.
The presence of water can dramatically influence the chemical and physical properties of minerals that comprise the Earth's mantle. In order to understand these effects itis important to identify the phases in which water may exist under mantle conditions. High-pressure experiments on the stability of hydrous minerals likely to be present in the Earth's mantle provide constraints on the distribution of water in the mantle. In this article, the significance of hydrated subducting slab is evaluated as a water carrier, transporting water from the earth's surface into the subduction zone, where it isreleased to the overlying mantle wedge through the breakdown of hydrous minerals. The capacityt store and transport water in the hydrated part of the subducted slab is controlledby the pressure-temperature stability of hydrous phase assemblages in bulk compositions of peridotite, sediment, basalt, and harzburgite.
A review of the pervasive fluid-migration mechanism is presented with newexperimental results in a quartzite-water system. Microgeometry and connectivity of geological fluids in rocks are governed by the balance of interfacial energy between fluid-crystal and crystalcrystal interfaces; i. e., dihedral angle. Fluid with dihedral angle <-60° can migrate through grain edges as pervasive flow. In rocks having a fluid network, the fluid flow velocity under a fluid pressure gradient is determined by Darcy's law. The minimization of the total interfacial energy drives fluid infiltration and segregation (expulsion). Several experimental and theoretical studies on the interfacial energy-driven fluid migration have been reported, where in the systems considered have been extended from a simple triple junction to polycrystalline rocks with or without two lithologies, bimodal grain size and grain growth. The concepts of MEMF (minimum energy melt fraction), lithological partitioning, grain size effect on fluid fraction, and fluid localization in coarsening rocks, all of whichare required to minimize the total interfacial energy of the system, have been developed. The kinetics of fluid infiltration, segregation and grain growth is derived from the difference in pressure over the interfaces with different curvatures that results in different mineral solubility into the fluids. The interfacial tension, which may be easily overwhelmed with fluid pressure gradient as a direct driving force of Darcy flow, significantly affects Darcy flow by controlling porosity and permeability of the rock. Because the rate of aqueous fluid infiltration into quartzite obtained by the infiltration experiments is much higher than that of Darcy flow velocity under geologically plausible permeability and pressure gradient during metamorphism, interfacial energy-driven fluid infiltration might play a major role in some large-scale fluid/rock systems, although long-term assessment of infiltration rate in natural rocks involves large uncertainty arising from phenomena such as grain faceting, choking of the pore network by accessory minerals and precipitated crystals. More experimental studies to clarify the fluid behavior in geologically realistic systems, as well as direct observation of natural rocks, are needed for better understanding of fluid migration in the Earth's interior.
The H2O released from the descending oceanic lithosphere is thought to play an important role in subduction zone magmatism as this H2O might trigger partial melting of the mantle wedge. Previous models for the formation of subduction zone magmas have assumed that H2O flows easily due to percolation in the whole mantle wedge. However, if there is incomplete wetting of mantle mineral grain boundaries (that is, the dihedral angle at triple junctions between grains is more than 60°), fluid does not form an interconnected network. In such a case, H2O might be trapped as an interstitial fluid in the peridotite and transported to deeper parts of the mantle wedge rather than triggering partial melting. In the present study, the geometrical distribution of aqueous fluid in textural equilibrium with polycrystalline olivine is investigated by measuring the dihedral angle at pressures of 3 and 5 GPa and at temperatures of 800 and 1000°C t for the purpose of clarifying the mobility of aqueous fluid in the upper mantle. By combining these data with previously published results, the extent of wetting within the mantle wedge as a function of pressure and temperature is obtained. It is suggested that the connectivity of aqueous fluid in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in subduction zones.
Many sealed cracks in the subduction related metamorphic belt are found in metamorphic zones. They are filled with quartz, albite, chlorite, muscovite, and actinolite, and rarely with omphacite and hornblende, suggesting that they have been mainly formed during the exhumation stage of the metamorphic belt. The sealed cracks occur as a nearly vertical composite crack against the main shear plane defined by schistosity, and there is some shear displacement. The width and the spacing of sealed cracks show distribution patterns of lognormal and power law types. The spatial distribution of sealed cracks appears to be highly clustered in various space scales. The sealed crack density is high in low grade rocks, but is low in high grade rocks, suggesting that sealed crack density is controlled by the fracture or yield strength of rocks depending strongly on temperature. The evolution of the sealed crack-fluid flow system in an accretionary complex between the subducting slab and wedge mantle can be modeled from fluid occupying and flowing cracks with various densities. The behavior of the model system is characterized by a strong non-linear stochastic differential equation having scale- and time-invariant solutions. The index of the scale and time exponents is just two-thirds, which is comparable to the power exponents in the natural sealed crack clustered distribution patterns. These results suggest that the fluid flow in the accretionary complex is spatio-temporary intermittent and clustered. Further, the high crack density in the lower temperature zone near the wedge mantle induces convective fluid circulation reaching the sea floor and a low density near the slab with a unidirectional fluid flow due to the high temperature.
Material recycling in subduction zones, including generation and migration of aqueousfluids and melts, is key to understanding the origin of volcanism in subduction zones. It isalso important for understanding the global mass budget, such as formation and growth ofthe continents. Recent knowledge concerning the phase relationships of hydrous peridotiticand basaltic systems allows us to model fluid generation and migration in subduction zones.Here we present (1) numerical models for the transportation of H2O and melting beneath theJapanese islands, in which generation and migration of aqueous fluid, its interaction with theconvecting solid, and melting are considered, based on the phase relationships, (2) predictionsof the corresponding seismic structures based on the calculated distribution of the fluids (aqueous fluids and melts), (3) 3-D seismic tomographic images beneath the islands, (4) analyses on distribution of the volcanoes and the volcanic chains, and (5) comparisonsbetween the model predictions and the observations. The model calculation suggests that in northeast Japan, nearly all the H2O expelled fromthe subducted Pacific plate is hosted by serpentine and chlorite just above the plate, and isbrought down to 150-200km. Breakdown of serpentine and chlorite at these depths results inthe formation of a fluid column through which H2O is transported upwards, and results inthe initiation of melting in the mantle wedge beneath the backarc. The seismic tomographicstudies suggest the existence of such a melting region beneath the backarc. In central Japan, the subducted Philippine Sea plate overlaps the subducted Pacific plate. This geometry causesslow thermal recovery of the subducted Pacific plate, resulting in dehydration reactions atlevels (200-300km) deeper than in northeast Japan, and bending of the volcanic chaintowards the backarc side. In contrast, in southwest Japan, where a relatively hot part of thePhilippine Sea plate (Shikoku Basin) subducts, the dehydration reactions are predicted tooccur at relatively shallow levels (100km depth). The seismic tomographic image supportswell the predicted distribution of the fluids beneath the volcanic front to the forearc region.These comparisons between the model predictions and the observations suggest that th ethermal structure (or age) of the subducting plate strongly controls the distribution of the aqueous fluids and melts in subduction zones through the position of the dehydration reactions.
Cosmogenic 10Be (t1/2=1.5×106 years) has been revealed to be a quite useful tracer forsediment recycling at convergent margins. This study focuses on Be isotopic ratios of lavasfrom the North-East Japan including the Hokkaido area, which have been regarded as typicalsubducted areas. The results indicate the incorporation of oceanic sediments into the arcmagma. Regional Be isotopic variations in time and space have also been observed, suggestingthe possible occurrence of a more complicated process for Be isotopic ratios than previously considered.
Based on δ 13C values and CO2/ 3He ratios of North Fiji Back-Arc Basin basalt glasses, wediscuss the carbon geochemical cycle in the subduction zone. Among the North Fiji Back-ArcBasin basalt glasses, there is a close correlation among CO2/ 3He ratios, δ 13C value, and143Nd/ 144Nd ratios. The CO2/ 3He ratios and the δ 13C values of North Fiji Basin basalt maybe attributed to binary mixing between the mantle component (low-CO2/ 3He, high-δ 13C, andhigh-143Nd/ 144Nd) and the subducted (recycled) component (high-δO2/ 3He, low-δ13C, and low-143Nd/ 144Nd). From a simple mass balance calculation, it is derived that the subductedend-member (recycled carbon) has 70% carbonate and 30% organic matter in origin.Assuming that complete decomposition of the subducted organic matters has occurred, most (about 90%) carbonates are not decomposed, because the amounts of subducting carbonatesand organic matters throughout the North Fiji subduction zone are estimated in a ratio of20: 1. This suggests that carbonate can be transported into the mantle through the subduction zones.