When a rigid, elastic oceanic plate subducts from a trench, an outer bulge is formed due to flexure of the elastic plate. In addition, rough seafloor topographic features caused by plate bending-related normal faulting, such as horst and graben structures, are observed in the vicinity of the trench axis.
(Above) Schematic illustration of plate subduction in the north-western Pacific margin.
(Bottoms) Bathymetric maps around the Japan trench at northern latitudes of 37-41 degrees (from off-Miyagi to–off-Aomori). Left bottom, water depth; middle bottom, inclination of slope; right bottom; aspect of slope.
* Bathymetric data are ETOPO1 (above), and a compilation of multi-narrow beam echo sounder data collected by Japan Coast Guard and JAMSTEC (bottoms).
(Illustration & Explanation: Gou FUJIE and Shigeaki ONO)
An important site of hydration and alteration of the incoming oceanic plate prior to subduction, the trench-outer rise region determines the distribution and amount of water transported into the Earth's mantle by subduction of the oceanic plate. Recent seismic surveys conducted in the northwest Pacific Ocean indicate reductions of seismic velocities within the oceanic crust and the uppermost mantle of the incoming Pacific plate toward the trench. These velocity reductions probably reflect hydration and alteration of the Pacific plate prior to subduction, although there are regional variations in the crustal structures of the oceanic lithosphere. Besides structural change, the stress regime within the incoming Pacific plate, which is derived from seismicity observations in the trench-outer rise region of the Japan Trench after the 2011 Tohoku-Oki earthquake, suggests that extensional stress of the incoming Pacific plate extends down to a depth of about 40 km. However, normal-faulting earthquake activity within the incoming Pacific plate was limited at depths shallower than about 20 km before the 2011 Tohoku-Oki earthquake. The change of the depth extent of normal-faulting earthquake activity suggests that the 2011 Tohoku-Oki earthquake changed the stress regime within the incoming Pacific plate. Distribution and focal mechanisms of shallow seismicity within the oceanic crust indicate that seismicity is affected by the pre-existing structure, which probably formed at the mid-ocean ridges. Heterogeneity and pre-existing structure of the incoming oceanic lithosphere should be taken into account when considering hydration and alteration of the oceanic lithosphere subducting into the trench.
A newly compiled bathymetric map including parts of the Kuril, Japan, and northern Izu–Ogasawara trenches in the northwestern Pacific Ocean demonstrates that most bending-related topographic structures are limited to less than 80 km from the trench axis. This observation contrasts with one that bending-related structures of eastern Pacific trenches are limited to less than 50 km from the trench axis. The discrepancy may be due to differences in the ages of subducting oceanic plates. Bending-related topographic structures of the western Kuril and southern Japan trenches are not parallel to the trench axis, but instead are parallel to magnetic anomaly lineations. Those of the northern Izu–Ogasawara Trench are parallel to fracture zones. These observations indicate the rule that the inherited seafloor spreading fabric is reactivated instead of forming new faults when the degree of obliquity between inherited seafloor spreading fabric and trench axis reaches about 30°. This rule is applicable to most trenches around the Pacific Ocean, except for some parts of curved trenches and trenches near seamounts or other volcanic edifices constructed by off-ridge volcanism. Most bending-related topographic structures near off-ridge volcanic edifices are parallel to the trench axis. This observation suggests that inherited seafloor spreading fabric around the volcanic edifies was disrupted by volcanism.
Recent heat flow surveys in the Japan Trench and Nankai Trough areas delineate prominent heat flow anomalies correlated with the structure of the subducting oceanic crust. On the seaward side of the Japan Trench, anomalously high heat flow values are pervasively observed within 150 km of the trench axis. The broad high heat flow zone can be attributed to trenchward thickening of a permeable layer (aquifer) in the oceanic crust associated with fracturing due to plate bending. Fluid circulation in the thickening aquifer efficiently pumps heat up from the underlying unfractured part. Overlapping the broad anomaly, local variations at a scale of a few kilometers are detected, indicating heterogeneous development of fractures. On the floor of the Nankai Trough off the Kii Peninsula, observed heat flow varies from extremely high and variable values on the western side to normal values for the seafloor age on the eastern side. The transition occurs at around 136°E, which may correspond to the structural boundary in the Shikoku Basin between the seafloor formed by spreading in the NE–SW direction and that formed by spreading in the E–W direction. The high heat flow west of the boundary is considered to result from heat transport along the plate interface by fluid circulation in an aquifer in the subducted oceanic crust. The heat flow transition around 136°E may reflect a change in permeability structure across the boundary between the two spreading directions. The two types of hydrothermal heat transport, fluid circulation in the subducted aquifer in the case of the Nankai Trough and circulation in the thickening aquifer in the case of the Japan Trench, have distinctive features. The subducted-aquifer type is effective for heat transport in a young subducting plate with a highly permeable aquifer, while the thickening-aquifer type can transport heat even in an old plate. The subducted-aquifer type has a more significant influence on the temperature structure of the plate boundary. Some heat flow anomalies observed in other trenches may also be caused by either of these types of hydrothermal heat transport.
The source mantle of the ocean crust on the Pacific Plate is examined using Pb–Nd–Hf isotopes and compared to a global isotope database of ocean basalts. The entire eastern half of the Pacific Plate, formed from an isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. Conversely, the western half of the Pacific Plate becomes younger westward and is thought to have formed from the Izanagi–Pacific Ridge (IPR). The ridge subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 70-65 Ma and currently forms the leading edge of the Pacific Plate stagnated in the mantle transition zone beneath China. The subducted IP formed from both Pacific and Indian mantles. Isotopic compositions of the basalts from borehole cores of 165-130 Ma in the western Pacific show that these are of Pacific mantle origin. However, the scraped-off ocean floor basalts (80-70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates: (1) the younger western Pacific Plate of IPR origin formed from the Indian mantle, (2) the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous, and (3) the IPR moved over the boundary. The Indian mantle is thought to have formed from a depleted MORB source mantle (DMM) due to an ancient melt depletion event (2-3 Ga) and subsequent isotopic growth and mixing with a sub-continental lithospheric mantle. In contrast, the Pacific mantle originated from a primitive mantle at 3-1.5 Ga followed by isotopic growth alone. These different formation processes may relate to the formation of the supercontinent and superocean where the Indian mantle was formed in a sub-continental environment whereas the Pacific mantle formed in an oceanic ridge environment.
Anomalous arc volcanoes tend to exist where a linear topographic feature (fracture zone, line of seamounts, or ridge) on the oceanic plate is subducting. “Dots-and-lines tectonics” are proposed for clarifying this tendency and understanding the mechanisms that generate it. State-of-the-art studies on along-arc variations of volcanism around the Pacific ring of fire and possible effects of inhomogeneities on the subducting oceanic plate are reviewed. Mechanisms generating the clustered distribution of active volcanoes in North-eastern Japan and the extraordinary volcanism at Mount Fuji are reconsidered from a “dots-and-lines” tectonics point of view. Although there still remain many unsolved problems, “dots-and-lines” tectonics could offer a unified explanation of along-arc variations in volcanism, combining processes in mid-ocean ridges, hot spots, and subduction zones.
Geoscientists, who previously had limited direct knowledge of the petrological/geochemical mantle below oceanic regions, were largely restricted to areas near mid-ocean ridges, back-arc spreading centers, and hotspots. Petit-spot lavas and xenoliths provide direct information on the asthenosphere and the lithosphere of subducting plates because the magma that erupts from petit-spot volcanoes originates from the asthenosphere and ascends along the concavely flexed zone prior to the outer-rise along the trench. Such volcanoes have been reported at subduction zones worldwide (e.g., the Japan, Chile, Java, and Tonga trenches). The isotopic composition of petit-spot lavas indicates a heterogeneous asthenosphere, and geobarometric analyses of xenoliths show a higher geothermal gradient in the lithosphere than that predicted previously by the GDH1 model, meaning that conventional theory about the subducting lithosphere needs to be revised in the light of recently obtained petit-spot data. Melt fractionation is thought to occur in the middle lithosphere, given that bulk compositions show fractionation trends in the absence of phenocrysts, in spite of raising lherzolitic xenoliths from ∼45 km depth. The most important indicators of petit-spot input to the lithosphere are high levels of carbon dioxide (CO2) in petit-spot magma, which might explain the low seismic velocity and high electrical conductivity of the oceanic asthenosphere just below the subducting oceanic plate. Because carbon-rich melt ascends through the lithosphere to the seafloor as a petit-spot, it is likely to metasomatize the lithosphere just prior to its subduction.
Seismicity accompanying subduction of one plate under the other is active in subduction zones. This seismicity is not uniformly distributed along strike; aseismic regions may be identified in a band of active seismicity. Such an uneven distribution may be attributed to the heterogeneity of frictional properties along the plate interface. Variations in the physical properties of the top-most layer over the subducting plate and rough topography along the plate interface are considered to be factors determining frictional properties. Recent active-source seismic surveys have revealed characteristics of the plate interface in detail and provide an understanding of how regional seismicity and slip distributions of major earthquakes are controlled. Seismicity around the northern limit of the 2011 Tohoku-oki earthquake shows a good correlation with seismic reflectivity along the plate interface, and suggests that the water content and/or the amount of clay minerals in a thin layer over the top of the subducting plate is a major factor. A subducting seamount was identified from a seismic survey around the southern limit of the 2011 Tohoku-oki earthquake off Ibaraki Prefecture. Repeating M 7 earthquakes share the same source at the subduction front of the seamount, while the source area of the largest aftershock of the 2011 Tohoku-oki earthquake lies next to it. The fault slip of these earthquakes, once initiated at the base of the seamount, did not propagate over the seamount. The same relationship between fault slip and subducted seamount is identified for a large slow slip at the Hikurangi margin. Physical or structural properties controlling seismicity or fault slip along the plate interface appear to be characteristics of the subducting plate. The importance of geophysical surveys of the incoming plate can now be further emphasized to provide a better understanding of seismicity along the plate interface.
To better understand seismic fault rupturing mechanisms, recent research results from scientific ocean drilling projects in seismogenic subduction zones such as Japan Trench and Nankai Trough carried out under the International Ocean Discovery Program (IODP) are reviewed and summarized. Stress states in the vicinity of faults and in wall rocks penetrated by drillings are determined or constrained using borehole wall resistivity images and drill core samples. The stress measurement methods are summarized and the stress results are interpreted. Fluid transportation properties and thermal properties within and around the faults are determined from retrieved core samples. The results are both useful and necessary for examining fault rupturing lubrication mechanisms, e.g., coseismic thermal pressurization. Dynamic frictional behavior of fault gouges is elucidated through rotary frictional experiments over a wide range from intermediate to high velocities covering the real coseismic fault slip velocity at around 1 m/s; the results indicate that frictional coefficients of plate boundary fault gouges in subduction zones are very low. In addition, high-temperature history examined with vitrinite reflectance measurements and minor element movements from isotopic analyses using the fault core samples give important evidence of the frictional heat of faults. Such historic high-temperature signals also enable inversion estimations to reveal past fault-rupturing parameters. It is hoped this review paper will be valuable and helpful for planning and implementing future scientific fault drilling programs.
It is generally accepted that hydration due to plate bending-induced normal faults (bend-faults) occurs in the region between a trench and an outer rise. Hydration of the oceanic plate has played a major role in global deep water circulation, as well as co-seismic megathrust slip at subduction zones. It is, however, emphasized that little is known yet about the degree and mode of hydration in the oceanic plate at outer rises. Investigating several subduction zones with various conditions is crucial to expand our knowledge of bend-fault hydration processes. The northwest Pacific (NW Pacific) region is one of the oldest, thus coldest, and most studied oceanic plates. Water circulation and hydration through bend-faults in the NW Pacific region are supported by the results of extensive recent geophysical surveys: (1) best-developed horst and graben structures, (2) high VP/VS ratio beneath the outer rise region, and (3) anomalously high heat flow. It is noted that tensional stress in the incoming plate is assumed to have extended to depths of 40 km during and after the 2011 Tohoku Earthquake, whereas it is shallower than 40 km for previous earthquakes. The projections of epicenters of microearthquakes after the 2011 Tohoku Earthquake are aligned with topographic lineation of horst and graben structures in the outer rise region. These lines of evidence suggest that hydration has progressed extensively to deeper sections of the oceanic plate since the 2011 Tohoku Earthquake. To address (a) hydration processes and their extents along the bend-fault and (b) physical properties of the bend-fault, in-situ physical properties and lithofacies are best obtained by ocean drilling in the NW Pacific region.