no flat wadati benioff zone in the central and southern central andes abstract the wadati benioff zone wbz is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30 in some areas of the andes where there are gaps in volcanic activity and where heat flow is abnormally low this plane in most studies has nearly horizontal dip at a depth of about 75 100 km and it has been associated to flat subduction of the oceanic lithosphere this situation has been taken as the present day analogue of the laramide orogeny of western north america for which a flat slab episode has been proposed in the past years in this work the observed low heat flow in areas of the andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western south america on the basis of geotherms obtained for areas of ecuador peru chile and argentina and of rheological results describing the partition between brittle and ductile regimes the seismic activity observed both in the lower crust and at depths of about 75 100 km is thoroughly explained at these depths earthquakes occur within the subcontinental upper mantle and then there is no flat wbz associated to subduction of the oceanic lithosphere there is evidence from recent seismological observations that the real wbz lies not horizontally and deeper in the tectonosphere

thermal structure beneath tohoku northeast japan combined with geophysical observations and petrological considerations we have studied the thermal models of the tohoku northeast japan subduction zone the temperature of the deeper part of the mantle wedge is constrained by the petrological requirements that demand a high temperature region 1400 c beneath the volcanic zone to satisfy this condition namely to explain the arc volcanism in the tohoku subduction zone a high temperature upper mantle beneath the japan sea possibly 1400 c at the depth shallower than 100 km is required heat flow distribution which constrains the temperature structure at shallower depth shows that low heat flow values ranging from 20 to 50 mw m2 are distributed between the trench and aseismic front and consistently high heat flow values 80 120 mw m2 prevail in the area landward of the volcanic front including the japan sea recent heat flow measurements indicate that the transition from low to high heat flow occurs between the aseismic front and volcanic front this heat flow distribution restricts the form of the flow in the mantle wedge and shear stress acting at the plate boundary a rigid non moving triangular region bounded by the subducting slab and the vertical plane beneath the aseismic front is necessary to explain the low heat flow zone between the trench and aseismic front shear stress of about 500 to 1000 bar is supposed to act at the interface between the subducting slab and rigid triangular mantle we also suggest that to explain the peak of heat flow near the volcanic front 120 mw m2 magma migration rate of about 0 05 cm3 cm 2 year 1 is necessary a comparison of the thermal structure of the tohoku subduction zone with that of the central south america which was preliminary calculated by the similar scheme shows that the temperature in the mantle wedge of the central south america is probably lower than that of the tohoku subduction zone this difference may result in that of the nature of volcanic rocks

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

cooling following large volcanic eruptions corrected for the effect of diffuse radiation on tree rings 1 the lack of a larger cooling in proxy records of climate change following large volcanic eruptions such as those of tambora in 1815 and krakatau in 1883 has long been a puzzle for climatologists these records however may have been biased by enhanced tree growth for several years following each eruption induced by additional diffuse radiation caused by the stratospheric volcanic aerosol clouds from the eruptions by comparing proxy reconstructions of climate with and without tree ring data this effect is demonstrated for the five largest eruptions for the period 1750 1980 when proxy records of northern hemisphere climate change are corrected for this proposed diffuse effect there is no impact on climate change for time scales longer than 20 years however it now appears that there was a hemispheric cooling of about 0 6 c for a decade following the unknown volcanic eruption of 1809 and tambora in 1815 and a cooling of 0 3 c for several years following the krakatau eruption of 1883

blueschist facies metamorphism shear heating and p t t paths in subduction shear zones the low temperatures recorded by blueschist facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15 50 km in this paper steady state and transient pressure temperature time p t t paths followed by subducted material are calculated using analytical expressions presented by molnar and england 1990 supplemented by two dimensional numerical calculations in the absence of shear heating calculated steady state p t t paths are very cold intersecting 1 gpa 10 kbar at t 200 c for a wide range of subduction parameters for the case of constant shear stress in the subduction shear zone and typical thermal parameters steady state p t t paths intersect the blueschist facies for 10 60 mpa and 0 100 mpa for convergence rates of 10 and 3 cm yr 1 respectively blueschist facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 mpa for the case of shear stress increasing linearly with depth steady state p t t paths intersect the blueschist facies when 1 9 p and 0 14 p for convergence rates of 10 and 3 cm yr 1 respectively subduction zone shear stresses may be considerably lower and possibly zero if blueschist terrains represent material metamorphosed during the early stages of subduction first 5 10 m y while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation subduction zone shear stresses of several tens of mpa at 35 km depth or a few percent of lithostatic pressure are consistent with a steady state model of franciscan metamorphism a progressive underplating model for the formation of the pelona schist inverted metamorphic gradient and surface heat flow measurements in the ne japan subduction zone

numerical modeling of thermal structure circulation of h2o and magmatism metamorphism in subduction zones implications for evolution of arcs abstract numerical models on thermal structure convective flow of solid generation and transportation of h2o rich fluid in subduction zones are consolidated to have a comprehensive view of the subduction zone processes heat balance circulation of h2o magmatism metamorphism growth of arcs and continental margins a large scale convection model with steady subduction of a cold old slab 130 myr old predicts rapid 100 myr cooling of subduction zones resulting in cessation of magmatism the model also predicts that the mantle temperature beneath arcs and continental margins is greatly affected by the effective temperature of the subducting slab i e the age of the subducting slab if subduction of a young hot slab including ridge subduction occurs every 60 to 120 myr as is suggested for eastern asia the average temperature beneath arcs is increased by about 300 c which may explain the long lasting magmatism in eastern asia associated with subduction of young slabs and ridges thermal structure and circulation of h2o are greatly modified to cause a transition from 1 normal arc magmatism 2 forearc mantle melting to 3 slab melting to produce a significant amount 100 km3 of granitic melts associated with both high p t and low p t type metamorphism the last stage of 3 can result in formation of a granitic batholith belt and a paired metamorphic belts synthesis of the numerical models and observations suggest that episodic subduction of young slabs and ridges can explain heat source for generating a large amount of granitic magmas of batholiths synchronous formation of batholith and regional metamorphic belts and p t conditions of the paired metamorphism even the high p t metamorphism requires an elevated geothermal structure in the forearc region associated with ridge subduction although the emplacement of the batholiths and the regional metamorphic belts and the mass balance in subduction zones are not well constrained at present the episodic event associated with ridge subduction is thought to be essential for net growth of arcs and continental margins as well as for the long term heat balance in subduction zones

large scale deformation associated with ridge subduction summary continuum models are used to investigate the large scale deformation associated with the subduction of aseismic ridges formulated in the horizontal plane using thin viscous sheet theory these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction modelling was used to compare the tonga arc and louisville ridge collision with the new hebrides arc and d entrecasteaux ridge collision which have disparate arc ridge intersection speeds but otherwise similar characteristics models of both systems indicate that diffuse deformation low values of the effective stress strain exponent n are required to explain the observed deformation deformation is somewhat insensitive to the vertically integrated strength of the arc inversely proportional to the argand number ar but indicates that the arc lithosphere is not extremely weak ar 100 low values of both ar and n suggest that the thermal structure is typical of cold or normal arcs and that deformation is dominated by flow in the lower crust and mantle in addition low values of n approaching newtonian flow may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere possible mechanisms include low stress grain size dependent creep pyroxenite controlled rheology and mechanisms associated with water weakening changes in the boundary conditions greatly affect deformation within island arcs high rates of arc ridge intersection speed tonga louisville system yield arc parallel tension and crustal thickening in the wake of ridge subduction in contrast low rates of arc ridge intersection speed new hebrides d entrecasteaux system yield compressional deformation directly arcward of the collision zone and transverse strike slip faulting adjacent to the region of compressional deformation localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re entrants

ambient seismic noise tomography in west central and southern brazil characterizing the crustal structure of the chaco parana pantanal and parana basins

a model of post seismic recovery induced by a deep focus earthquake abstract post seismic behavior induced by a deep focus earthquake in subduction zone is closely investigated numerically on realistic upper mantle models taking into account the pressure and temperature conditions for this purpose the post seismic stress changes expected from the deep event are calculated by a two dimensional finite element method 2d fem assuming newtonian and power law creep behaviors as justified by high p t experiments several cases are investigated in detail for various fault depths and thermal structures two cases are considered for the location of the fault one when an earthquake occurs within oceanic crust overlying a subducting plate and the other when an earthquake is located within the plate we also examine the case when the temperature distribution within the plate is lower by about 200 c on average than that in the upper mantle at the same depth in addition the post seismic behavior due to a deep focus event in a petrological thermal model of a mantle wedge is investigated the results show that the post seismic behavior associated with the deep event occurring within the oceanic crust indicates a pattern of relaxation of the coseismic stress changes lower temperatures inside the plate would suppress the post seismic displacements and stress changes effectively the petrological model also has some influence on the post seismic flow pattern in the mantle wedge we also study a possible correlation between deep focus earthquakes and a shallow large event which has been detected in the northwestern pacific region however effective mechanisms to explain the phenomenon could not be identified from the viewpoint of post seismic behavior caused by power law creep

effects of oblique subduction on the 3 d pattern of small scale convection within the mantle wedge 1 we have developed a 3 d numerical model of obliquely subducting slab and mantle flow above using this model we explore the effects of oblique subduction on the small scale convection within the mantle wedge we find that the spatial pattern and time dependent characteristics of oblique subduction are similar to those of non oblique cases whose subduction speed is the same as the subduction velocity component normal to the strike of subducting slab this also implies that the axes of rolls of small scale convection are nearly normal to the plate boundary assuming that the plate boundary represents the strike of subducting plate only when the back arc lithosphere overlying the small scale convection moves along the strike of subduction a significant change of the orientation of roll axes is observed

numerical simulations of mantle flow around slab edges abstract a simple kinematic dynamic model of mantle flow around the slab edge is constructed in order to understand the flow complexity there the flow velocity on the top and the small boundary region around the shallow plate boundary is kinematically imposed in order to achieve a subduction like feature and the flow in other part is dynamically calculated the geometry of the plate mimics the region around the junction of aleutian islands and kamchatka that are examples of the convergent transform fault plate boundaries in a simple model in which the overlying plate is almost stationary the lateral flow from the mantle under the subducting slab to the mantle under the neighboring plate is of minor importance once the slab penetrates into the high viscosity layer where the downward flow encounters the resistance similar situation was found when the trench is advancing that is the trench moves toward the overlying plate for the case with retreating trench that is the trench moves toward the subducting plate a lateral flow exists even after the slab penetrates into the high viscosity layer although its magnitude is significantly smaller than that of the plate velocity the presence of a low viscosity layer just beneath the subducting plate may promote the emergence of lateral flow a significant lateral flow is observed when the high temperature anomaly that is buoyant and low viscosity block carried by the movement of subducting plate approaches the slab these results may have important implications for the possible existence of trench parallel flow in the sub slab mantle

a simple semi dynamic model of the subduction zone effects of a moving plate boundary on the small scale convection under the island arc summary a simple semi dynamic numerical model of the subduction zone which incorporates the plate velocity and a moving plate boundary as a priori constraints has been developed this model may be useful for studying specific subduction zones where a history of kinematic movements of plates is well constrained as an application of this model the effects of a moving plate boundary on the nature of the small scale convection in the mantle wedge were studied under the restriction of given relative velocity between two converging plates the absolute velocity of each plate was changed and i found that the characteristics of small scale convection are not significantly affected by the movement of the plate boundary at least within the parameter range used in this study i also found a small systematic change which may be related to the difference of the heat supply to the mantle wedge caused by the moving plate boundary thus the model with a fixed continental plate and moving oceanic plate with a given relative velocity may be sufficient for studying the present subject this model can be extended into a simple 3d subduction zone model

a hypothesis for the seismogenesis of a double seismic zone summary the seismogenesis of a double seismic zone in particular the lower layer of a double seismic zone has not been adequately explained in the literature on the basis of seismic data and geothermal structures along three well studied cross sections in the kuril kamchatka and japan subduction zones we investigate the temperature pressure conditions associated with seismogenic structures of the double seismic zones the corresponding tip loci seem to suggest that earthquakes observed in the lower layer and in the lower part below approximately 130 20 km of the top layer of a double seismic zone were caused by metastable phase transition a mechanism similar to that responsible for deep focus earthquakes only at lower temperature pressure conditions under this hypothesis the wedge shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition according to theoretical experimental studies and the constraints imposed by our observations a likely candidate for such a phase transition is the metastable al rich enstatite decomposing into the assemblage of al poor enstatite plus garnet earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and or facies change from basalt to eclogite that the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition it is inferred that single seismic zones are also caused by the same mechanisms except that the implicit layer of a supposed double seismic zone is missing due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

introduction to the special section northeast japan a case history of subduction subduction related tectonic events such as strike slip faulting uplift of high pressure metamorphic rocks in the forearc back arc spreading arc magmatism and continental collisions have shaped northeast japan making it a case history for subduction much of the record of these events is preserved and can be used to reconstruct the tectonic history of the region although many geological geophysical and geochemical data have been collected in japan for the last 30 years interpretation of these data into a plate tectonic context did not really begin until about 10 years ago much of the data and interpretation on northeast japan have been published in japanese or as individual local studies the primary goal of this special section is to present new investigations covering a wide range of topics and scales and place them in a tectonic framework the papers in this special section focus on three main subjects mesozoic aspects of northeast japan the relation of japan sea opening with the tectonics and geochemistry of northeast japan in the tertiary and the geophysical setting the purpose of this overview is to link these subjects and provide a tectonic framework for north east japan and eastern asia since the mesozoic

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

heat sources and melting in subduction zones in order to understand the mechanism of melt generation and the origin of high heat flow in subduction zones a series of numerical models of the thermal and flow structures in the mantle have been tested in two dimensional boxes with an inclined subducting slab of constant velocity in contrast to previous models 1 large convecting cells are used without imposing a high temperature profile on the backarc boundary which enables us to discuss the global heat balance and to seek for the heat sources responsible for the melting and high heat flow observed in subduction zones and 2 various mechanical conditions e g coupling between the slab and overlying mantle wedge buoyancy associated with melting and heat sources e g heat flux from below internal heat generation viscous heating are tested in varying proportions in all the calculations steady state or near steady state with a small instability periodically occurring at the upper thermal boundary layer is achieved under these conditions the global heat balance can be described by a simple boundary layer argument the results show that in order to attain a high enough temperature for melting and a high heat flux a large amount of internal heating i e more than 2 5 10 7 w m3 is required if the convection is limited within the upper mantle the high internal heating required may be explained if the radioactive nuclides in fluids expelled from the subducting slab are added to the wedge and circulate in the convection cell for a sufficient time another possible explanation for melting and the high heat flux is that the hot material is supplied from the lower mantle based on the thermal and flow structures obtained melting regimes in subduction zones are discussed in which the following key processes take place 1 melting associated with pressure change of a rock packet with its movement especially compression melting in the downward flow of the mantle wedge along the slab and 2 melting due to compositional change of the rock packet associated with migration of h2o and melt to solve these problems in detail further studies on distribution and migration of the fluids will be required

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

a new regime of slab mantle coupling at the plate interface and its possible implications for the distribution of volcanoes abstract we investigate the effects of a thin low viscosity layer just above the subducting slab on 3d thermal and flow structure in the mantle wedge by taking northeast japan as an example the low viscosity layer assumed here is needed to explain the observed low surface heat flow and low seismic attenuation in the forearc by decoupling the mantle from the subducting slab we find that when the viscosity in the low viscosity layer is sufficiently low along arc component of the flow arises inside the layer and produces along arc temperature variation it can also be considered as the along arc changes in the degree of slab mantle coupling at the plate interface the onset time and the characteristic wavelength of the 3d flow depend on the viscosity and the extent of the low viscosity layer in order to explain the observed spatial and temporal changes in the distribution of quaternary volcanoes in northeast japan the viscosity and the thickness of the low viscosity layer need to be 5 10 18 pa s and 6 km respectively the model proposed here is based on an assumption which is simpler and better constrained by observations compared to previous models therefore it could be an alternative explanation of the distribution of volcanoes in northeast japan

implications of measured thermal gradients for water movement through the northeast japan accretionary prism thermal gradients of 24 36 c km have been found in the slope cover sediments of the accretionary prism off northeast japan deep sea drilling project dsdp sites 438a 439 and 440 these gradients lead to the prediction of temperatures in excess of 400 c at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity such temperatures conflict with the predictions of temperatures of 150 c at even greater depths from conductive thermal models and petrologic studies of low temperature blueschists at similar ancient margins one dimensional steady state conduction and advection via darcian water flow of heat up through the prism was simulated numerically the measured thermal gradients can be reconciled with temperatures less than 125 c at the prism base by water flowing upward at rates of the order of 10 10 m s such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast japan prism would flow up through a 10 km width of the prism perpendicular to the trench and would require the dsdp sites to have been located fortuitously over the localized zone s of water flow smaller velocities will reproduce the measured gradients and thus greater widths through which water flow occurs are predicted if the temperature at the prism base is higher and or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate where water flow is significant as likely wherever abundant water rich sediments are subducted thermal gradients below a few kilometers in accretionary prisms may be much lower than near surface values this has profound implications for the thermal structure of the prism off northeast japan and elsewhere

three dimensional mapping of magma source and transport regions from seismic data the mantle wedge beneath northeastern japan we investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern japan the comparison of the seismic tomography with laboratory velocity data on a partially molten mantle rock yields estimates of melting zones in three dimensions we employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature melting and magma rich zones derived from the velocity structure coincide with observed low q zones the results of the three dimensional mapping indicate that the source of magma in island arc is diapir like melting patches localized within the low velocity zones of the mantle wedge extensive volcanic activity along the volcanic front is due to the presence of vast magma rich zones just beneath the moho those melting zones in the uppermost mantle may in turn cause melting of lower crustal materials and produce felsic magma melt appears to stay at and beneath the moho where crystallization fractionation may proceed melt exists at greater depths in the back arc region which may correlate with across arc variations of chemical compositions of the volcanic rocks observed in northeastern japan we suggest that magma migration in the ductile lower crust may cause low frequency microearthquakes and magma penetration into the brittle upper crust may produce mid crustal s wave reflectors

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

seismic structure and seismogenesis off sanriku region northeastern japan summary in 1997 a seismic experiment using an airgun array and ocean bottom seismographs obss was performed in the forearc region of the northeastern japan nej arc the objectives of this experiment were to clarify seismic structures of the forearc region off sanriku japan using airgun obs data and to understand the nature of the japan trench seismogenic zone fundamental features of the structure are as follows i the subduction angle becomes steeper from east to west from 3 to 8 to 11 with the top of the plate located at depths of 11 km 40 km west from the trench 21 km 120 km west from the trench and 28 km 140 km west from the trench respectively ii the nej moho beneath the forearc is defined at a depth of approximately 20 km iii p wave velocity of the island arc upper mantle is approximately 8 km s 1 iv the p wave velocity of the cretaceous sedimentary layer exhibits significant lateral variability 4 0 5 5 km s 1 i nthe e w direction v the upper crust of nej forearc also has the significant lateral heterogeneity 5 0 6 3 km s 1 i nthe e w direction but not in the n s direction vi the middle crust defined on land extends in some locations 40 km east of the coastline vii lateral variability in the p wave velocity of the lower crust 6 7 7 0 km s 1 i ssmall compared with that of the upper crust according to these results it is suggested that the rupture areas of large earthquakes m7 in this region are limited within the contact zone between the nej arc crust and the subducting oceanic crust although a thin layer with low velocity is probably present within this contact zone the serpentinization of uppermost mantle wedge of the nej arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited this might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km

buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones the thickness of earth s mechanical lithosphere is poorly defined to investigate whether rheology controls the thickness of the overriding plate s mechanical lithosphere in subduction zones the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity it was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge corner leading to unrealistically high heat flow since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere we suggest that it is instead controlled by buoyancy the source of buoyancy we assume is compositional e g buoyant crust two end member models with crustal thickness of 10 and 70 km respectively were then undertaken these had lower heat flow this work supports the assumption of some earlier workers e g plank and langmuir 1988 who equated the mechanical lithosphere with the crust of the overriding plate

modelling the interplate domain in thermo mechanical simulations of subduction critical effects of resolution and rheology and consequences on wet mantle melting abstract the present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate a 2d thermo mechanical model is used to simulate a subduction channel made of oceanic crust free to evolve convergence at constant rate is imposed under a 100 km thick upper plate pseudo brittle and non newtonian behaviours are modelled the influence of the subduction channel strength parameterized by the difference in activation energy between crust and mantle e a is investigated to examine in detail the variations in depth of the subduction plane down dip extent z coup first simulations show that numerical resolution may be responsible for an artificial and significant shallowing of z coup if the weak crustal layer is not correctly resolved second if the age of the subducting plate is 100 myr subduction occurs for any e a the stiffer the crust is that is the lower e a is the shallower z coup is 60 km depth if e a 20 kj mol and the hotter the fore arc base is conversely imposing a very weak subduction channel e a 135 j mol leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions partial kinematic coupling at the fore arc base occurs if e a 145 kj mol if the incoming plate is 20 myr old subduction can occur under the conditions that the crust is either stiff and denser than the mantle or weak and buoyant in the latter condition cold crust plumes rise from the subduction channel and ascend through the upper lithosphere triggering 1 partial kinematic coupling under the fore arc 2 fore arc lithosphere cooling and 3 partial or complete hindrance of wet mantle melting z coup then ranges from 50 to more than 250 km depth and is time dependent if crust plumes form finally subduction plane dynamics is intimately linked to the regime of subduction induced corner flow two different intervals of e a are underlined 80 120 kj mol to reproduce the range of slab surface temperature inferred from geothermometry and 10 40 kj mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near primary arc magmas and seismic tomographies therefore an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations a mantle viscosity reduction by a factor 4 20 caused by metasomatism in the mantle wedge is proposed from these results i conclude that the subduction channel down dip extent z coup should depend on the subduction setting to be consistent with the observed variability of sub arc depths of the subducting plate surface

the thermal effects of pervasive and channelized fluid flow in the deep crust heat advection by upward migrating fluids in one possible mechanism for elevating temperatures within the earth s crust models are presented that compare the thermal impact of a pervasive upward flux of fluids to that of the same flux focused into vertical parallel planar channels in channelized flow the average temperature across channels is lower than that of pervasive flow at comparable times in the systems evolutions as the channel spacing decreases the evolution through time and steady state approach that of pervasive flow with pervasive flow representing the limiting case of zero spacing when channel spacings are small relative to the length of the vertical flow path lateral temperature gradients around channels will be very shallow for any geologically reasonable flux when small fluxes such as may be generated from pelites undergoing progressive metamorphism are focused into very widely spaced channels the resultant temperature increase around channels may be discernable with geothermo

correlation between surface heat flow and elevation and its geophysical implication surface heat flow gives direct information about geothermal gradient but it is significantly affected by erosion and sedimentation the rate of erosion sedimentation is generally in proportion to elevation following such an idea first we examined the relation between observed surface heat flow and elevation along each of three lines crossing the northernmost northeastern and southwestern parts of japan and found a clear positive correlation between them the good correlation indicates essential importance of the effects of erosion and sedimentation on surface heat flow next assuming the linear dependence of erosion rates on elevation we calculated the profiles of surface heat flow expected from the surface topography with a 2 d thermal conduction model the results of calculation well explain gross features of the observed heat flow profiles but some systematic differences still remain from these systematic differences we can extract unbiased information about thermal state at depth

roles of mantle diapir and ductile lower crust on island arc tectonics abstract in order to examine the relationships between surface geological phenomena in island arcs and lower crustal and upper mantle dynamics crustal and upper mantle responses induced by upper mantle density heterogeneities with magnitudes expected from the seismic tomography beneath the northeastern japan arc have been evaluated according to the rheological model with a low viscosity lower crust and low viscosity uppermost mantle low density lower crustal material above mantle diapiric upwelling is mechanically dragged along by asthenospheric shear stresses and then lower crustal material flows from the back arc corner to the fore arc corner the surface subsidence for this process is about 300 m on a geological time scale of 5 50 my and the formation of a sedimentary basin with about the same lateral scale as the mantle diapir can be expected in the area above the mantle diapir stage s the subducted slab in island arc will operate as a barrier to the lateral movement of lower crustal material accumulation of low density lower crustal material may occur at the edge of the overriding plate during this process at a critical stage of the pressure in the mantle wedge of fore arc region high temperature lower crustal material may then be exposed along the tectonic line stage e as the result of discharging lower crustal material along the tectonic line the pressure in the lower crust will be decreased and accelerated subsidence accompanied by volcanism is expected in the sedimentary basin cretaceous geological phenomena in southwest japan may be explained by this physical mechanism its linked surface crustal movement and the flow of lower crustal material a relatively slow deposition in the period of about 140 120 ma observed in the kanmon sedimentary basin may be interpreted as the geological representation of stage s from about 110 ma the kanmon sedimentary basin was supplied with a large quantity of volcanic sediments and simultaneously expanded and suddenly subsided at about the same time low pressure high temperature metamorphism and granitic activity ryoke belt occurred along the areas to the south of the kanmon group it may be possible to interpret these geological phenomena along the median tectonic line mtl as the surface geological events corresponding to stage e it is however stressed that the time and spatial scale for this coupling is determined by the rheological structure of the lower crust and upper mantle and by the scale of mantle diapir

thermal model for approach of a spreading ridge to subduction zones and its implications for high p high t metamorphism importance of subduction versus ridge approach ratio 1 the approach of oceanic ridges to subduction zones finally resulting in ridge subduction is observed on the present earth s surface the thermal effects of the ridge approach event are studied using a two dimensional numerical model around subduction zones with particular emphasis on the implications for the associated high p high t metamorphism the ridge approach occurs when the subduction rate v is larger than the half spreading rate at the ridge the difference between the two rates represents the ridge approach rate to the trench u which is newly incorporated in the present thermal model the calculated thermal structure along the slab surface when a ridge reaches the trench generally depends only on u v when shear heating or mantle flow is not taken into account if u v is small or the ridge approach is slow compared to the subduction rate the thermal effect of the ridge approach is large enough to explain the hot general p t conditions of high p high t metamorphism relative to the steady state conditions predicted by numerical simulations in addition when the ridge is close to the trench the subduction stage p t paths experienced by high p high t metamorphic rocks show a strongly curved trajectory with the dp dt progressively increasing with pressure this suggests that the slope of petrologically determined subduction p t paths may indicate if ridge approach contributed to the formation of the metamorphic rocks

grain size distribution in the mantle wedge of subduction zones 1 mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones including mantle flow and fluid migration to investigate the grain size distribution in the mantle wedge we coupled a two dimensional 2 d steady state finite element thermal and mantle flow model with a laboratory derived grain size evolution model in our coupled model the mantle wedge has a composite olivine rheology that incorporates grain size dependent diffusion creep and grain size independent dislocation creep our results show that all subduction settings lead to a characteristic grain size distribution in which grain size increases from 10 to 100 m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle despite the large variation in grain size its effect on the mantle rheology and flow is very small as 90 of the deformation in the flowing part of the creeping region is accommodated by grain size independent dislocation creep the predicted grain size distribution leads to a downdip increase in permeability by 5 orders of magnitude this increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches 100 km depth compared with shallower depths potentially providing an explanation for the relatively uniform subarc slab depth seismic attenuation derived from the predicted grain size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones without requiring a significant contribution by the presence of melt

interplate coupling in northeast japan deduced from inversion analysis of gps data abstract the spatial distribution of the strength of interplate coupling between the subducting pacific plate and the overlying continental plate in northeast japan was investigated through an inversion analysis of gps data employing akaike s bayesian information criterion the gps data used for the inversion analysis are rates of baseline length changes and vertical displacement rates of crustal movements during the period from april 6 1996 to march 20 1998 which were obtained at 161 continuous gps observation stations operated by the geographical survey institute of japan the result shows the average drag rates of the overriding plate on the model source regions off the tohoku and hokkaido districts are 6 7 cm yr and 6 8 cm yr respectively which are 0 85 and 0 87 as interplate coupling ratio these values are much higher than those in previous studies the average directions of drag rates are oriented n73 w 15 and n78 w 13 on the model source regions off the tohoku and hokkaido districts respectively these directions agree well with the direction of coseismic slip of the foot wall side n75 w of the 1978 miyagiken oki earthquake m 7 4 but differ slightly from the direction estimated from the plate motion model n65 w seno et al j geophys res 101 1996 11305 11315 we also found afterslip of 5 5 cm yr associated with the 1994 sanriku haruka oki earthquake m 7 5 off aomori prefecture interestingly enough large afterslip occurred on the plate boundary even when more than a year had passed after the sanriku haruka oki earthquake the amount of drag rate becomes dramatically small at depths greater than about 50 km on the model source regions indicating weak interplate coupling there

small scale convection under the back arc occurring in the low viscosity wedge abstract water released from subducting slabs through a dehydration reaction may lower the viscosity of the mantle significantly thus we may expect a low viscosity wedge lvw above the subducting slabs the lvw coupled with a large scale flow induced by the subducting slabs may allow the existence of roll like small scale convection whose axis is normal to the strike of the plate boundary such a roll structure may explain the origin of along arc variations of mantle temperature proposed recently in northeast japan we study this possibility using both 2d and 3d models with without pressure and temperature dependent viscosity 2d models without pressure and temperature dependence of viscosity show that with a reasonable geometry of the lvw and subduction speed small scale convection is likely to occur when the viscosity of the lvw is less than 10 19 pa s corresponding 3d model studies reveal that the wavelength of rolls depends on the depth of the lvw the inclusion of temperature dependent viscosity requires the existence of further low viscosity in the lvw since temperature dependence suppresses the instability of the cold thermal boundary layer pressure i e depth dependence coupled with temperature dependence of the viscosity promotes short wavelength instabilities the model which shows a relatively moderate viscosity decrease in the lvw most of the lvw viscosity is 10 18 10 19 pa s and a wavelength of roll 80 km has a rather small activation energy and volume 130 kj mol and 4 cm 3 mol of the viscosity this small activation energy and volume may be possible if we regard them as an effective viscosity of non linear rheology

a consistent model for fluid distribution viscosity distribution and flow thermal structure in subduction zone water plays crucial roles in the subduction zone dynamics affecting the thermal flow structure through the fluid processes we aim to understand what controls the dynamics and construct a model to solve consistently fluid generation fluid transport its reaction with the solid and resultant viscosity and thermal flow structure we highlight the effect of mechanical weakening of rocks associated with hydration the viscosity of serpentinite serp in subduction zones critically controls the flow thermal structure via extent of mechanical coupling between the subducting slab and overlying mantle wedge when serp is greater than 1021 pa s the thermal flow structure reaches a steady state beneath the volcanic zone and the melting region expands until cin initial water content in the subducting oceanic crust reaches 3 wt and it does not expand from 3 wt on the other hand when serp is less than 1019 pa s the greater water dependence of viscosity expressed by a larger fv confines a hot material to a narrower channel intruding into the wedge corner from a deeper part of the back arc region consequently the overall heat flux becomes less for a larger fv when ageba age of back arc basin as a rifted lithosphere 7 5 ma the increase in fv weakens but shifts the melting region toward the trench side because of the narrow channel flow intruding into the wedge corner where as it shuts down melting when ageba 20 ma several model cases particularly those with serp 1020 to 1021 pa s and a relatively large fv for cin 2 to 3 wt broadly account for the observations in the northeast japan arc i e location and width of volcanic chain extent of serpentinite surface heat flow and seismic tomography although the large variability of surface heat flow and seismic tomographic images does not allow us to constrain the parameter range tightly

simple analytic model for subduction zone thermal structure a new analytic model is presented for the thermal structure of subduction zones it applies to the deeper regions of a subduction zone where the overriding mantle is no longer rigid but flows parallel to the slab surface the model captures the development of one thermal boundary layer out into the mantle wedge and another into the subducting slab by combining this model with the analytic model of royden 1993a b which applies to regions in which the overriding plate is rigid a nearly complete analytic model for the thermal structure of a steady state subduction zone can be achieved a good agreement is demonstrated between the output of the combined analytic model and a numerical finite element calculation the advantages of this analytic approach include 1 efficiency only limited computing resources are needed 2 flexibility non linear slab shape and processes such as erosion and shear heating are easily incorporated and 3 transparency the effect of changes in input variables can be seen directly