heat flow along the nantroseize transect results from iodp expeditions 315 and 316 offshore the kii peninsula japan mal conductivity varies between about 1 0 w m 1 k 1 near the seafloor to 1 6 w m 1 k 1 at a depth of 1 km thermal conductivity generally increases with depth and correlates with variations in porosity and lithology thermal gradients along the transect are characterized by 48 sediment temperature measurements from 6 sites thermal corrections for the effects of bathymetric relief and sedimentation improve the confidence with which the heat flow values can be interpreted heat flow generally decreases with landward distance from the deformation front and varies from 70 mw m 2 just landward of the deformation front to 54 mw m 2 at sites characterizing the outer fore arc high and to 57 mw m 2 at the kumano basin site iodp heat flow measurements are significantly lower than nearby seafloor heat flow measurements this difference is most likely due to variations in bottom water temperature that have a large effect on values of seafloor heat flow thus the heat flow of the nankai accretionary prism is lower than previously thought we present thermal models of subduction along this transect and explore the impact of the initial geotherm conductive plate cooling based on the age of subducting seafloor 20 myr under predicts the observed heat flow we find a good fit to the data using a geotherm appropriate for 10 myr seafloor the extra heat is interpreted in terms of back arc thermal environments

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

a new hypothesis on parameters controlling the formation and size of porphyry copper deposits implications on thermal gradient of subducted oceanic slab depth of dehydration and partial melting along the kerman copper belt in iran abstract porphyry copper deposits in the tethyan metallogenic belt broadly occur in early mesozoic late mesozoic paleogene and neogene within a wide range of tectonic setting these deposits in iran often occur within the cenozoic urumieh dokhtar magmatic arc udma mostly in the kerman porphyry copper belt kpcb located in the southeast of this arc these deposits along the udma are mainly associated with the granitoids of miocene age 18 82 9 20 ma in this research geochemical characteristics pb and sr isotopes tectonic environment oxidizing conditions and source of magmas for the most important porphyry copper deposits of the udma have been compared and discussed on the basis of trace element discrimination diagrams the kerman belt granitoids formed in a volcanic arc type setting whereas the jebal barez granitoids seem to have formed in a different tectonic setting the eu eu of granitoids from the kerman belt are greater than 1 2 representing the oxidizing conditions for the crystallization of the granitoid magma whereas jebal barez granitoids were formed under reducing conditions the sr y ratio for kerman granitoids is greater than 60 indicating adakitic affinities and la yb n for these granitoid rocks is between 17 and 35 suggesting that their parental magma was generated from a deep garnet bearing source based on 87sr 86sr i values for kerman belt granitoids of 0 704 0 705 it is suggested that parental magmas originated from a subducted oceanic slab in the sarcheshmeh porphyry copper deposit the thermal gradient of the oceanic slab at depth was less than 9 c km warm cold slab the oceanic slab was highly hydrated and more than 80 of the water within the oceanic slab was released in the deep zone of the sub arc region at a depth of 110 130 km under saturated conditions the oceanic slab partially melted 20 25 a large volume of magma and magmatic water formed sarcheshmeh as a giant porphyry copper deposit in the iju deposit a small porphyry copper deposit the oceanic slab was slightly hydrated and minor water was released at the depth of 110 130 km a small volume of magma was formed due to water undersaturated melting conditions finally a new hypothesis is introduced regarding the roles of down dip thermal gradient of the oceanic slab water content depth of dehydration and degree of partial melting of oceanic slab which are important controls on the formation and size of the porphyry copper deposits within continental volcanic arcs

seismic structure of the northeastern japan convergent margin a synthesis many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern japan arc and its relationship to seismic and volcanic activity spatial distributions of the depths to the conrad and the moho discontinuities estimated from shallow earthquake data and seismic explosion data show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the japan sea and the pacific ocean the pn velocity has a lateral variation it is as low as 7 5 km s beneath the land area while that beneath the japan sea and the pacific ocean is 8 0 8 2 km s it changes abruptly at the transition zones which are located along the coastlines precise structure and location of the subducted pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate the pacific plate is composed of a thin 5 km low velocity upper layer and a thick high velocity lower layer its total thickness being 80 90 km the upper plane seismicity of the double seismic zone is confined to the thin low velocity upper layer which probably corresponds to the subducted former oceanic crust the lower plane seismicity lies at the middle of the high velocity lower layer and the lower half of the plate below it is incapable of generating earthquakes the shallower portion of the upper surface of the plate beneath the pacific ocean along which major seismicity with low angle thrust faultings is actually occurring is also located by seismic observations on land and in the sea the pacific plate subducts at an extremely low angle of 5 for the first 25 km depth and then the dip steepens rather abruptly to 30 normal fault type events at the top of the plate have not been detected in the portion where the downward bending is the largest but have been ditected near the trench axis where it is rather small tomographic inversions for seismic velocity structure clearly delineate the inclined high velocity pacific plate with a thickness of 80 90 km and low velocity zones in the crust and the mantle wedge beneath active volcanoes seismic attenuation tomography also shows similar zones of low q value beneath active volcanoes although its spatial resolution is much lower the low velocity zones with 2 6 velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100 150 km in the mantle wedge and are approximately parallel to the dip of the underlying pacific plate these low velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the earth s surface corresponding to a portion of the subduction induced secondary mantle wedge flow

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

crustal structure and seismicity beneath the forearc off northeastern japan marine seismological and other data in the japan trench area north of 38 5 n were used to infer the relation of subducting plate structure seismicity and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates previous crustal models from ocean bottom seismographic refraction obs surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data a wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting pacific plate crust its dip angle increases to about 7 110 km landward of the trench axis a large increase beneath the deep sea terrace is required to reach the well defined angle of 25 beneath the tohoku east coast a large wave speed gradient within layer 2 commonly observed under normal oceans seems to vanish beneath the inner trench slope at about 10 km depth within the overriding plate apparently brittle material with a p wave speed of 6 km s can be found as near as 35 km from the trench axis the upper limit of the seismogenic zone of interplate low angle thrust events is about 15 km depth from obs seismicity and large event analyses both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates

contrasting seismic reflectivity of the lower crust and uppermost mantle between ne japan and sw japan as illustrated by petrophysical analyses of mafic and ultramafic xenoliths 1 petrophysical analyses of mafic and ultramafic xenoliths reveal large contrasts in acoustic impedance between the lower crustal rocks and uppermost mantle rocks both beneath ichinomegata crater in ne japan and beneath oki dogo island in sw japan in contrast to minor contrasts in acoustic impedance both among the lower crustal rocks and among the uppermost mantle rocks beneath ichinomegata crater relatively large contrasts in acoustic impedance are present beneath oki dogo island between granulite and olivine gabbro in the lower crustal rocks as well as between plagioclase bearing wehrlite and other cumulus peridotites in the uppermost mantle rocks based on the existing petrologic models of the lower crust and uppermost mantle beneath ichinomegata crater and oki dogo island we then constructed model structures of the lower crust and uppermost mantle beneath these two areas composed of units with randomly arranged 100 m thick foliation parallel horizontal layers of xenolith samples and used these model structures to examine the seismic reflectivity beneath these two areas the results suggest the presence of strong seismic reflectors between the lower crust and the uppermost mantle in both areas although no other recognizable seismic reflector is expected beneath ichinomegata crater many seismic reflectors due to the above contrasts in acoustic impedance are expected both in the lower crust and in the uppermost mantle beneath oki dogo island thus a remarkable contrast in seismic reflectivity in the lower crust and uppermost mantle is likely present between these two areas as well as between ne japan and sw japan

runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope the tsunamigenesis of the 2011 tohoku earthquake off the east coast of northern japan abstract the gigantic 2011 march 11 mw 9 tohoku earthquake is examined from the viewpoint of the pre seismic forearc structure the seismic reflection properties of a megathrust around the usual up dip limit of the seismogenic zone the thermal state of a shallow subduction zone and the dehydration of underthrust sediments at the japan trench the pacific plate is subducting westward beneath northeast japan at a dip angle of 4 6 the middle and lower slopes of the landward side dip eastward at angles of 2 5 and 8 0 respectively the forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states respectively based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes the rapid uplift of the forearc that caused the 2011 tohoku tsunami may have been associated with this internal deformation of the prism the critical state of the prism indicates that the effective basal friction b of the plate boundary megathrust is 0 08 for the lower prism the megathrust especially under the middle slope is characterized by a prominent reflector with negative polarity one of multiple possible explanations is that the megathrust hosts highly pressurized fluids underthrust sediments in this part of the japan trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay the dehydration kinetics of opal a to quartz the clay transformation of smectite illite and the thermal structure of the japan trench suggest that maximum dehydration of the sediments would take place at 50 60 km horizontally from the deformation front where the temperature along the megathrust is 100 120 c the zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity we hypothesize a possible free slip along this portion of the megathrust during the 2011 tohoku earthquake caused by anomalously high fluid pressure resulting from fluid accumulation over centuries

resolving depth dependent subduction zone viscosity and afterslip from postseismic displacements following the 2011 tohoku oki japan earthquake abstract we developed a 3 d viscoelastic finite element model of the m9 2011 tohoku oki japan earthquake capable of predicting postseismic displacements due to viscoelastic relaxation and afterslip we consider seismically inferred slab geometries associated with the pacific and philippine sea plate and a wide range of candidate viscoelastic rheologies for each case we invert for afterslip based on residual surface displacements observed gps minus that predicted due to viscoelastic relaxation to develop combined viscoelastic relaxation and afterslip models we are able to find a mechanical model that fully explains all observed geodetic on land and seafloor horizontal and vertical postseismic displacements we find that postseismic displacements are in about equal parts due to viscoelastic relaxation and afterslip but their patterns are spatially distinct accurately predicting both horizontal and vertical on land postseismic displacements requires a mantle wedge viscosity structure that is depth dependent reflecting the manner in which temperature pressure and water content influence viscosity no lateral heterogeneities within the mantle wedge viscosity structure beneath northern honshu are required westward directed postseismic seafloor displacements may be due flow via low temperature plastic creep within the lower half of a pacific lithosphere weakened by plate bending the distribution of afterslip is controlled by the location of coseismic slip from the tohoku oki and other regional historic earthquakes the paradigm by which afterslip is thought of as the dominant postseismic mechanism immediately following earthquakes with viscoelastic relaxation to follow in later years is shown to no longer be valid

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

seismic wave attenuation determined from tectonic tremor in multiple subduction zones abstract tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground motion prediction and hazard mapping traditionally recorded earthquakes of magnitude 3 8 are used to develop ground motion prediction equations however typical earthquake records may be sparse in areas of high hazard in this study we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor which is plentiful in some regions tectonic tremor occurs in the frequency band of interest for ground motion prediction i e 2 8 hz and is located on the subducting plate interface at the lower boundary of where future large earthquakes are expected we empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones nankai japan cascadia united states canada jalisco mexico and southern chile with the large amount of data available from tremor we show that in the upper plate the lower crust is less attenuating than the upper crust we apply the same analysis to intraslab events in nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic crust this suggests that high pore fluid pressure is present in the tremor source region these differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface

on the importance of offshore data for magnetotelluric studies of ocean continent subduction systems 1 the presence of water in the mantle wedge overlying a subduction zone is expected to significantly enhance electrical conductivity raising it above that of the cold subducting slab or surrounding regions of dehydrated mantle this suggests that magnetotelluric mt transects across subduction systems measuring regional electrical conductivity structure might be able to indirectly trace the pathways of water migration into the mantle for ocean continent subduction it is logistically simpler to collect mt transects on the continental side of the system however we show that such land data are relatively insensitive to details of the electrical connections between the ocean and mantle in contrast seafloor measurements on the landward and seaward side of the trench are very sensitive to these electrical connections and are essential to understanding the electrical structure of the entire subduction system in particular the conductivity structure of the hydrated mantle wedge overlying the slab can only be studied using offshore mt data we demonstrate this result using a model of an ocean continent system although our results can be generalised to other subduction geometries

adakite differentiation and emplacement in a subduction channel the late paleocene sabzevar magmatism ne iran this study describes the structural setting petrogenesis and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the tertiary ophiolitic suture zone of the sabzevar range ne central iran these intrusive bodies show tabular geometries with solid state fabrics documenting syntectonic emplacement during crustal shortening in the total alkalis silica tas diagram their compositions define a medium k calc alkaline suite spanning from basaltic andesite to the dacite and rhyolite fields they show characteristic low mgo 0 15 0 60 wt and ni 40 ar 39 ar white mica and amphibole geochronology constrains the sabzevar magmatism to the late paleocene at ca 58 ma genesis of the sabzevar magmatic suite is interpreted in terms of prograde wet amphibolite melting during oceanic subduction within a pressure temperature range between a plagioclase out and a hornblende out boundary magma differentiation and high pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the sabzevar structural zone implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the neotethyan subduction during the mesozoic tertiary time span

the importance of blueschist eclogite dehydration reactions in subducting oceanic crust the metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure temperature p t paths with a model of metabasalt phase equilibria in steady state subduction zones with high rates of shear heating the upper parts of the subducting oceanic crust progress through the greenschist amphibolite granulite eclogite facies whereas lower parts of the subducting oceanic crust progress through the blueschist eclogite facies in steady state subduction zones with moderate rates of shear heating most of the subducting oceanic crust passes through the blueschist eclogite transition in steady state subduction zones with low rates of shear heating the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km for oceanic crust containing 1 2 wt h 2 o dehydration will not begin until the onset of eclogite or amphibolite facies metamorphism depending on the p t path for many subduction zones the most important dehydration reactions in the subducting oceanic crust occur at the blueschist eclogite facies transition associated with the breakdown of lawsonite or clinozoisite glaucophane and chlorite large amounts of h 2 o released by blueschist eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas

anomalous deepening of a seismic belt in the upper plane of the double seismic zone in the pacific slab beneath the hokkaido corner possible evidence for thermal shielding caused by subducted forearc crust materials abstract hypocenter relocations of earthquakes in the pacific slab have shown an anomalous deepening of a seismic belt in the upper plane of the double seismic zone at depths of 80 120 km beneath the hokkaido corner while it is located at depths of 70 90 km in the surrounding tohoku and eastern hokkaido areas seismic tomographic inversions performed beneath the hokkaido corner have shown that a low velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the pacific slab and makes direct contact with the upper surface of the pacific slab these observations suggest that 1 the low velocity zone just above the pacific slab is part of the subducted kuril forearc sliver 2 the contact with the subducted and so relatively cold sliver materials prevents the mantle wedge from heating the pacific slab and causes a lower temperature condition in the pacific slab crust beneath the hokkaido corner than in the surrounding areas 3 as a result a delay of eclogite forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there

insights into the causes of arc rifting from 2 d dynamic models of subduction author s billen mi abstract 2017 american geophysical union all rights reserved back arc spreading centers initiate as fore arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting two models have been proposed for triggering fore arc arc rifting rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone this paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high viscosity boundary which separates the weak fore arc from the hot buoyant mantle wedge is removed buoyant upwelling mantle causes arc rifting drives the overriding plate away from the subducting plate and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism for spontaneous fore arc arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces

thermal regime of the southwest japan subduction zone effects of age history of the subducting plate abstract during a mid miocene at about 15 ma tectonic reorganization an ocean spreading ridge perpendicular to the strike of the nankai trough subduction zone stopped spreading since that time there has been subduction of the cooling fossil spreading ridge in the present study we have developed a time dependent thermal subduction model for this region using the finite element method there are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 ma boundary conditions are specified so that the oceanic plate subducting at the nankai trough becomes cooler as it gets older the model results agree with the present heat flow trend that decreases landward and explain the paleothermal regime of high mid miocene temperatures inferred from land geological studies the tectonics of the region prior to 15 ma has some uncertainties assuming subduction of an active spreading ridge or an 80 ma old lithosphere prior to 15 ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region 250 km the thermal history of the forearc since 15 ma can be summarized as a rapid warming period as a consequence of ridge subduction followed by a cooling trend to the present as a result of the aging of the subducting plate the results illustrate the thermal consequences of one type of ridge subduction they also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere especially if it is young as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust this dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

subduction initiation by thermal chemical plumes numerical studies abstract prior suggestions for the initiation of subduction have included sediment loading compression and plate reconfiguration as potential triggers here we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume numerical testing of this hypothesis has been conducted in 2d with the i2vis thermo mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal chemical or purely thermal plume we demonstrate that a mantle plume can break the lithosphere and initiate self sustaining subduction provided the plume causes a critical local weakening of the lithospheric material above it the intensity of the required weakening depends on the plume volume plume buoyancy and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes another necessary condition is the presence of high pressure fluids at the slab upper interface reducing the effective friction coefficient there to very low values based on our results we suggest that sheet like instabilities of the archean mantle convection could have initiated subduction on earth where ocean was already present in less stable tectonic settings provided that mantle plumes sheets at that time were rich in water and melt which could drastically reduce the effective friction coefficient in the lithosphere above the plume our numerical models are also in good agreement with suggested concepts for corona formation on venus

nature of the low velocity zone in cascadia from receiver function waveform inversion abstract a dipping low velocity zone lvz is a ubiquitous structural element of subduction zones worldwide in this study we map seismic attributes characterizing the lvz beneath the cascadia subduction zone from northern vancouver island to northern california using receiver function waveform inversion throughout this region the lvz is characterized by high v p v s ratios mean 2 77 strong s velocity contrasts 50 and thicknesses averaging 3 37 km the lvz is immediately underlain by a second weaker layer exhibiting moderate v p v s ratios mean 1 85 with mean thickness of 4 62 km we interpret the combined structure in terms of subducting oceanic crust based on classical structural petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores the lvz is identified with pervasively hydrated high porosity pillow basalts and sheeted dikes of layer 2 with possible contributions from sediments layer 1 fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above and a low porosity gabbroic mafic cumulate dominated layer 3 below

geochronological geochemical and sr nd hf isotopic constraints on the petrogenesis of late cretaceous a type granites from the sibumasu block southern myanmar se asia abstract the late cretaceous to paleogene granitoids occur widespread in the sibumasu block within myanmar se asia which show a close association with tin tungsten mineralization however the precise timing petrogenesis and tectonic significance of these granitoids are poorly constrained so far in this study we present a detailed study on geochronology elemental and sr nd hf isotopic geochemistry for the hermyingyi and taungphila granites in southern myanmar with the aim of determining their petrogenesis and tectonic implications la icp ms u pb dating of zircon grains from the two granites yield ages of 69 70 ma indicating a late cretaceous magmatic event these granitic rocks are weakly peraluminous and belong to the high k calc alkaline series they are both characterized by high sio2 k2o na2o feot feot mgo and ga al ratios and low al2o3 cao mgo p2o5 and tio2 contents enriched in rb th u and y but depleted in ba sr p and eu suggesting an a type granite affinity moreover they display prominent tetrad ree patterns and non charac trace element behavior which are common in late magmatic differentiates with strong hydrothermal interaction or deuteric alteration the granites belong to a2 type and probably formed at a high temperature and anhydrous condition they have zircon ehf t values from 12 4 to 10 0 and whole rock end t values from 11 3 to 10 6 with paleoproterozoic tdm2 ages 1741 1922 ma for both hf and nd isotopes geochemical and isotopic data suggest that these a type granites were derived from partial melting of the paleoproterozoic continental crust dominated by metaigneous rocks with tonalitic to granodioritic compositions without significant input of mantle derived magma and followed by subsequent fractional crystallization by integrating all available data for the regional tectonic evolution in se asia and adjacent regions we attribute the formation of the late cretaceous a type granites to a back arc extension in the hinterland behind the subduction zone which is induced by the rollback of the flat neo tethyan subducting slab around ca 70 ma

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

three dimensional seismic structure beneath southwest japan the subducting philippine sea plate abstract a three dimensional seismic structure beneath southwest japan has been investigated in detail by applying an inversion method this analysis clearly reveals the present and possibly the past subduction of the philippine sea plate beneath southwest japan beneath the izu peninsula the philippine sea plate does not appear to subduct but collides against the continental asian plate here and its thickness appears thinnest at most about 30 km from the tokai to the shikoku regions the high velocity zone corresponds well to the zone of subcrustal earthquakes down to a depth of about 60 70 km indicating that the subduction of the present philippine sea plate does not reach below this depth here however a small high velocity portion appears at depths between 100 km and 150 km beneath a part of the kinki region and probably extends to the chugoku region where no corresponding seismic activities exist these high velocities might be an indication of the remnants of the past subduction of the philippine sea plate in the kyushu region the high velocity zone continues down to a depth of about 150 km or even to 200 km in the southern part corresponding to a steeply inclined seismic zone the philippine sea plate in this region appears to be thicker than in regions east of the kyushu palau ridge it was also found that there exist narrow low velocity zones intervening within the philippine sea plate where two aseismic ridges the kyushu palau and shichito iwojima ridges extend northwards and the nankai trough shows warping these low velocity zones give support to the buoyant subduction hypothesis it also appears that lateral heterogeneities existing within and around the subducting plate are closely related to the mode of rupture of large earthquakes occurring around there

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

seismic waves guided by untransformed oceanic crust subducting into the mantle the case of the kanto district central japan a model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the kanto district central japan a distinct pair of later p and s phases xp and xs commonly observed at specific stations for these events in a depth range of 40 60 km and is considered to take place in association with the subducting motion of the philippine sea plate the xp phase is found from its particle motions to be a longitudinal seismic wave a principal component analysis of wavelets including the initial p and the xp phases shows that the latter arrives at the earth s surface with a slightly shallower angle relative to the former close agreement between the relative arrival time ratio of xs s xp p and the vp vs ratio of crustal or mantle rocks suggests that both xp and xs travel along the same path as longitudinal and tangential waves respectively one can clearly observe these later phases at stations located where the upper surface of the philippine sea plate is apparently in direct contact with the bottom of the crust of the honshu island apparent velocities of the xp and the xs phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous travel time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region the hypocentral distribution of the events related to the subducting motion of the philippine sea plate does not form such a thin seismic zone as observed in southwest japan but extends over the entire thickness of the philippine sea plate the earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone the deeper portion does not generate such events these characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the philippine sea plate taking the result of our earlier work into account we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the philippine sea plate

cascadia subducting plate fluids channelled to fore arc mantle corner ets and silica deposition in this study we first summarize the constraints that on the cascadia subduction thrust there is a 70 km gap downdip between the megathrust seismogenic zone and the episodic tremor and slip ets that lies further landward there is not a continuous transition from unstable to conditionally stable sliding seismic rupture occurs mainly offshore for this hot subduction zone ets lies onshore we then suggest what does control the downdip position of ets we conclude that fluids from dehydration of the downgoing plate focused to rise above the fore arc mantle corner are responsible for ets there is a remarkable correspondence between the position of ets and this corner along the whole margin hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore arc mantle laboratory data show that such fore arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone at the fore arc mantle corner these fluids are released upward into the more permeable overlying fore arc crust an indication of this fluid flux comes from low poisson s ratios and vp vs found above the corner that may be explained by a concentration of quartz which has exceptionally low poisson s ratio the rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

mantle wedge structure in the miyagi prefecture forearc region central northeastern japan arc and its relation to corner flow pattern and interplate coupling 1 we applied three dimensional 3 d seismic tomography to both active source data and local earthquake data recorded at onshore and offshore seismic stations in the miyagi prefecture forearc region of central northeastern japan to obtain a detailed seismic velocity model of the mantle wedge the velocity model showed a clear boundary that divides the mantle wedge into an oceanward high velocity toe and a landward low velocity region that extends trenchward from beneath currently active arc volcanoes the top of the seaward dipping boundary is about 20 30 km landward from the coastline and its bottom is approximately beneath the coastline we consider that this velocity boundary represents the trenchward limit of corner flow and the high velocity toe represents a stagnant mantle wedge the high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences we also identified a swarm of active microseismicity in the mantle wedge well above the seismogenic plate boundary this activity may be related to dehydration of a detached seamount

reduction in bet surface area of nojima fault gouge with seismic slip and its implication for the fracture energy of earthquakes abstract a common view concerning the energetics of seismogenic fault motion is that at least part of the fracture energy is consumed in grain crushing in the fault zone and that this part may be estimated by grain size analysis of fault rocks we address this problem by conducting room dry friction experiments on nojima fault gouge at subseismic to seismic slip rates 0 009 1 31 m s and at normal stresses up to 3 64 mpa and by measuring the bet surface area of the gouge before and after the experiments where it cuts granite the nojima fault zone has bet surface area of about 65 10 6 m 2 per unit fault area 1 m 2 clayey and granular fault gouges composed mainly of quartz plagioclase kaolinite and smectite were collected from a granitic fault zone at a new outcrop in funaki awaji island southwest japan both clayey and granular gouges exhibit dramatic weakening at high slip rates specific bet surface areas of clayey and granular gouges decreased with increasing slip rate from 46 0 and 15 4 m 2 g before deformation to about 20 and 5 m 2 g after deformation 55 70 reduction respectively microstructural observations revealed that grain welding within the slip zones at high slip rates reduced grain surface area the energetics of seismic fault motion should be examined with broader views taking into account grain crushing grain welding decomposition and frictional melting

microseismicity at the seaward updip limit of the western nankai trough seismogenic zone 1 the seismogenic zone at subduction zones does not normally extend to the trench axis the shallowest part of the plate interface is considered to be seismically decoupled the location of the seaward updip limit of the seismogenic zone and its relation to the crustal structure are important for understanding the transition process from aseismic slip to the seismic rupture at the plate interface the nankai trough seismogenic zone southwestern japan is one of the most well studied subduction seismogenic zones in the world however the offshore seismicity around the nankai trough is very low and hypocenters are not determined accurately by the on land seismic network we performed microseismicity observations using ocean bottom seismographs off cape muroto hypocenters were determined by using a three dimensional vp and vs structure based on seismic survey results the observed microseismicity seems to be classifiable into two types the first is earthquakes that occur around the plate interface the second is a group of earthquakes that occur in the uppermost mantle of the subducting oceanic plate the seismicity around the plate interface forms several clusters the seaward limit of the clusters is characterized by earthquakes with a similar waveform these similar waveform earthquakes are considered to occur at small asperities in the aseismic seismogenic transition zone of the plate interface the seismicity in the uppermost mantle may be related to dehydration embrittlement of subducting serpentinized mantle

consequences of progressive eclogitization on crustal exhumation a mechanical study summary the very low water content of the granulitic unit of holsnoy in the bergen arcs norway caused its partial metastable preservation throughout caledonian burial end exhumation leading to the observed mixture of completely eclogitized and uneclogitized rocks the eclogitization of the granulite facies protolith led to a density increase 10 but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction we address the consequences on subduction dynamics of the concomitant evolution of large scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel flow model large scale buoyancy and density are integrated in a single adimensional parameter the exhumation number which describes the capacity of the crust to exhume for a crust whose eclogitization causes a very large viscosity decrease burial resistant end member goes through a maximum in the partially eclogitized zone that is there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle for this partially eclogitized crust with max the maximum possible downward flow is very low this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming this return flow zone is bounded downwards in the partially eclogitized zone by max while its upper limit propagates upwards at a constant rate the curve eclogite fraction controls the maximum proportion of eclogite of the crust that can be exhumed we also demonstrate that large exhumation rates reached by certain uhp units are incompatible with any stationary flow regime whatever the nature of subducted crust the additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones but does not qualitatively change the dynamics of the subduction channel this study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large scale crustal properties with progressive eclogitization

imaging subducted slab structure beneath the sea of okhotsk with teleseismic waveforms the structure of subducted slabs is not well imaged in most global tomographic models with anomalies typically less than 1 synthetic waveforms for such models are not noticeably different from their 1d reference models in contrast we find that observed teleseismic waveforms sampling slabs in the down dip direction display multipathing features indicative of substantial structure such waveform complexity patterns appear distinctly different for various source locations with outer rise events generally displaying relatively simple waveforms here we model these waveform patterns for a portion of the kuril subduction zone the best fitting slab models have relatively fast cores 5 with smooth edges compatible with thermal modeling in shape complete seismograms for these 2d models indicate that multibounce p and s waves are slab sensitive and can be used in refining models

structural and magmatic responses to spreading ridge subduction an example from southwest japan subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries the late oligocene early miocene rocks of the shimanto accretionary prism at cape muroto shikoku island record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms these accreted strata have been imprinted with an unusual late stage event that involved 1 regional cusp like flexing of structural trends in the prism 2 near trench magmatism and 3 pervasive faulting this event overlaps in time with the opening of the shikoku back arc basin circa 26 14 ma the shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of cape muroto the regional flexing of structural trends in the prism affects rocks as old as cretaceous although the flexure is most tightly developed in the late oligocene early miocene portion of the prism where it is steeply plunging kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature at cape muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid ocean ridge basalts morb from the shikoku back arc basin preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development these data combined with plate reconstructions for the area at 15 ma indicate that indentation of the prism most likely resulted from early miocene orthogonal subduction of the topographically elevated shikoku basin spreading ridge in the muroto area morb magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction

slab melting as an explanation of quaternary volcanism and aseismicity in southwest japan two volcanoes on the quaternary volcanic front in southwest japan have erupted medium k andesite and dacite which are chemically similar to adakites i e high al 2 o 3 sr y low y and steep rare earth element ree patterns with no negative eu anomalies unlike other subduction related volcanoes in japan earthquake data and the extrapolated position of the philippine sea plate from its near trench position indicate that the leading edge of the plate is 75 100 km deep beneath these volcanoes melting of the plate at this depth would leave a garnet eclogite residual accounting for the y and heavy ree depletion the lack of seismic data and active volcanism in southwest japan can be attributed to the attenuated seismic response of the plate due to melting and to the atypical nature of magma generation

lateral variation of the cutoff depth of shallow earthquakes beneath the japan islands and its implications for seismogenesis abstract we estimated the spatial distribution of the cut off depth of shallow seismicity beneath the japan islands based on data from a recently constructed nationwide dense seismic network in order to avoid the effect of heterogeneous seismic velocity structure we determined the 3d seismic velocity structure for the whole of japan which was then used to relocate the hypocenters the cut off depth was determined by calculating d90 the depth above which 90 of the earthquakes occur a spatial distribution of estimated d90 values shows a considerable lateral variation ranging from about 5 km to about 40 km the d90 s are deep along the pacific coast of eastern japan locally shallow d90 s extend along belt like stretches of volcanic areas heat flow is low along the pacific coast in eastern japan and high in the volcanic areas this indicates that the lateral variation of d90 s is formed mainly by lateral geotherm variation cooling by the subducting old very cold pacific plate may cause the deep d90 s along the pacific coast in eastern japan heating by magma supplied from the mantle wedge below is considered to be the main cause of the shallow d90 s in the volcanic areas magma and or aqueous fluids derived from solidified magma contribute not only to locally shallow d90 s but also to the weakening of ambient rocks in the lower crust resulting in stress concentrations in the upper crust just above them large shallow inland earthquakes tend to occur in such areas suggesting that a model of shallow earthquake generation applied to tohoku can be used in other areas in japan as well

seismic body wave constraint on mechanisms of intermediate depth earthquakes 1 source parameters of six large earthquakes 7 1 mw 7 7 in the depth range between 112 and 277 km from five different subduction zones are derived from teleseismic body waves recorded by global seismic networks except for the 21 october 1995 mexican event which is consistent with the model of simple rupture on a planar surface the investigated earthquakes show complex rupture processes that can be explained by irregularities in the focal region as is the case of shallow events the inferred velocity of the moment release varies between 2 5 and 4 km s corresponding to 40 90 of the shear velocity at the focus four of the six earthquakes show stress drops in the range observed for shallow events 1 10 mpa ruptures extend mainly parallel to the strike of subducting slabs with dimensions reaching up to 50 km the extent of rupture perpendicular to slab dip of generally less than 15 km is comparatively small this suggests that the rupture during large intermediate depth earthquakes propagates predominantly parallel to the strike of the subducting plate and that the seismogenic zone perpendicular to the dip direction is limited to a narrow width the relatively small rupture size perpendicular to slab dip suggests that the rupture process does not extend into regions of significantly different temperatures the spatial extent of the investigated earthquakes is consistent with the orientation of trench parallel faults that are generally observed in near trench outer rise regions suggesting that these events may result from reactivation of such preexisting faults at intermediate depths through dehydration of hydrous minerals concentrated in these zones of weakness depths of most of the earthquakes are consistent with the dehydration embrittlement of serpentine the bolivian earthquake of 23 january 1997 seems to be too deep 277 km for such an explanation

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

nucleation of frictional instability caused by fluid pressurization in subducted blueschist pore pressure is an important factor in controlling the slip instability of faults and thus the generation of earthquakes particularly slow earthquakes are widespread in subduction zones and usually linked to the occurrence of high pore pressure yet the influence of fluid pressure and effective stress on the mechanics of earthquakes is poorly understood therefore we performed shear experiments on blueschist fault rocks which likely exist at depth in cold and old subduction zones to investigate the influence of effective stress on frictional behavior our results show potentially unstable behavior at temperatures characterizing the seismogenic zone as well as a transition from stable to unstable behavior with decreasing effective normal stress which is mechanically equivalent to increasing fluid pressure this transition is a prerequisite for generating slow earthquakes our results imply that high pore pressures are a key factor for nucleating slip leading to both megathrust and slow earthquakes

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

the low velocity layer in subduction zone structure and elasticity of glaucophane at high pressures abstract we investigated the structure equation of state and elasticity of glaucophane na2mg3al2si8o22 oh 2 up to 9 gpa which encompasses its experimentally observed stability field we find that the pressure volume results for glaucophane are well represented by a third order birch murnaghan formulation with k 0 81 gpa k 0 4 5 and v 0 899 4 a3 the full elastic constant tensor reveals significantly larger stiffness along the 1 0 0 plane the 1 0 0 direction is the relatively softer this could be rationalized in terms of the stacking of the stiffer tetrahedral units along 0 1 0 and 0 0 1 directions within the crystal structure glaucophane is a dominant mineral constituent of blueschist facies rock and has significantly lower velocities compared to garnet bearing eclogites in addition glaucophane is anisotropic and could account for the observed low velocity layer in the subducting slabs at depth range within the thermodynamic stability of glaucophane at high pressures beyond stability of glaucophane hydrous phase such as lawsonite could account for the observed low velocity layers in certain subduction zones

cloud top height determination using atsr data abstract a method for computing cloud top heights using multiangle satellite data has been developed by exploiting the dual angle capability of the ers 1 along track scanning radiometer atsr cloud top heights can be determined to a nominal accuracy of 1 km the method uses a spatial correlation function to determine the best fit between the nadir and forward images of any channel the resulting horizontal shifts are then converted to cloud top heights using a simple but accurate approximation to the atsr view geometry the technique is presented and some examples of cloud top height determination are given the possibility of determining cloud top winds using this technique is explored

low frequency tremors intraslab and interplate earthquakes in southwest japan from a viewpoint of slab dehydration 1 low frequency tremors lft recently found in the southwest japan fore arc likely occur due to hydro fracturing from fluids leaving the dehydrating philippine sea slab however there are some places without such lft they are n izu kanto e shikoku and s kyushu these are the places where island arc type crust is subducting we propose that dehydration of the subducted crust does not occur beneath these regions because it is composed mainly of tonalite lacking the quantity of hydrous minerals seen in normal subducting oceanic crust we show that almost no earthquake occurs within the subducted crust in such regions consistent with dehydration embrittlement hypothesis for intraslab seismicity the lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes because pore fluid pressure is more difficult to rise at the thrust zone this might be reflected in the irregular occurrence of great earthquakes in these regions

seismic shear wave reflectivity structure of the mantle beneath northeast china and the northwest pacific 1 using multiple scs reverberations we examine the mantle reflectivity structure beneath northeast china and the northwest pacific we find several upper mantle discontinuities including a melt layer with a mean thickness of 64 km atop the 410 km discontinuity present on both sides of the subducting slab near the nankai trench the transition zone contains a split 520 km discontinuity in several paths and tomographic images show stagnant slabs at this depth we believe that this may be slab related based on experimental work a negative reflector is found in one path beneath the northeast china craton at a depth of 598 km mid mantle reflectors are found in all of our paths and are present throughout a wide depth range 750 1600 km

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

geochemical characteristics of the rio verde complex central hispaniola implications for the paleotectonic reconstruction of the lower cretaceous caribbean island arc abstract new geochronological trace element and sr nd isotope data for metabasalts dolerites and amphibolites from the rio verde complex central hispaniola are integrated with existing geochemical data for mafic volcanic rocks and metamorphic derivatives from the los ranchos amina and maimon formations giving new insights into magma petrogenesis and paleotectonic reconstruction of the lower cretaceous caribbean island arc back arc system u pb and 40ar 39ar age data show that the rio verde complex protoliths were in part coeval with volcanic rocks of the los ranchos formation upper aptian to lower albian the geochemical data establish the existence of gradients in trace element parameters nb yb th yb zr yb zr ba and normalized ti sm y and yb abundances and nd isotope compositions from throughout hispaniola which reflect differences in the degree of mantle wedge depletion and contributions from the subducting slab the rio verde complex mafic rocks and some mafic sills and dykes intruding in the loma caribe peridotite have a transitional iat to n morb geochemistry and a weak subduction related signature and are interpreted to form in a rifted arc or evolving back arc basin setting the los ranchos amina and maimon formations volcanic rocks have arc like characteristics and represent magmatism in the volcanic front trace element and nd isotope modeling reproduce observed data trends from arc to back arc and suggest that the variations in several geochemical parameters observed in a sw direction across the caribbean subduction system can be explained from the progressively lower subduction flux into a progressively less depleted mantle source the low nb contents and high end i values in both arc and back arc mafic rocks imply however the absence of a significant lower cretaceous plume enriched component in order to explain these observations a model of proto caribbean oceanic lithosphere subducting to the sw at least in the 120 110 ma interval is proposed to cause the observed magmatic variations in the lower cretaceous caribbean island arc back arc system in this context arc rifting and initial sea floor spreading to form the rio verde complex protoliths occurred in the back arc setting of this primitive island arc built on the ne edge of the caribbean plate

petrogenesis of a voluminous quaternary adakitic volcano the case of baru volcano the origin of adakite magmas remains controversial because initially the term adakite had petrogenetic significance implying an origin by direct melting of the eclogitized subducting oceanic crust many models have been produced for their origin and until now there has not been a straightforward method to discriminate between these models in a given adakite suite here we use detailed chronological and geochemical studies of selected adakitic edifices that allows for the determination of the magmatic output rate parameter qe which has been correlated with the rates of magma generation deep within subduction zones by providing temporal and eruption rate estimates we provide constraints on the possible petrogenetic processes involved in the generation of adakite like signatures adakite magmas derived from the melting of the subducting slab should be volumetrically insignificant when compared to the adakite like magmas produced by typical arc magma generation processes in this study we use this observation and the extraordinary stratigraphic exposure from miocene to present in an adakitic volcano in panama and to study the temporal and chemical variation in erupted magmas to estimate rates of magma generation detailed chemical and geochronological analyses of baru volcano indicate that the volcanic edifice was constructed in its entirety during the quaternary and magmas display adakite like features such as steep rare earth elements patterns pronounced depletions in the heavy rare earth elements low y high sr and high sr y the magmatic output rates qe that we have calculated show that compared to other typical adakitic volcanoes most of the volcanic edifice of baru volcano was constructed extremely rapidly 213 k a and in time frames that are similar to typical arc volcanoes the observed chemical and mineralogical variation coupled with the high magma production rates indicate that baru volcano is more representative of a typical arc volcano than a small volume melt of the subducting oceanic crust the technique we outline may have broader application in determining the petrogenetic conditions of other adakite suites

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting