long base line measurements of short period mesospheric gravity waves during the seek campaign simultaneous observations of short period 1 hour mesospheric gravity waves were made over an extended base line 660 km from two sites in japan at shigaraki and yamagawa during the seek sporadic e experiment over kyushu campaign 9 23 august 1996 coincident image data obtained on four nights provided a rare opportunity to investigate the horizontal spatial extent and propagation characteristics of the waves in detail surprisingly out of a total of 12 events observed from shigaraki and 10 from yamagawa at most only three exhibited similar propagation characteristics at each site the majority of the wave displays 70 75 had quite dissimilar characteristics suggesting a preponderance for localized wave motions of limited lifetimes and geographical extent a marked preference for wave progression towards the n ne indicates that the gravity waves imaged in the near infrared oh and oi 557 7 nm nightglow emissions were probably not directly associated with the observed e region irregularities

metastable olivine wedge and deep dry cold slab beneath southwest japan abstract oceanic plates subducted at trenches penetrate into the deep mantle and encounter a structural boundary at a depth of 410 km where olivine the dominant element of mantle rocks transforms into a higher density form wadsleyite this transformation may be delayed within the coldest core of subducting plates slabs due to kinetic effects and it has been suggested that metastable olivine may persist deeper than 410 km using high density seismic array data in japan we show the direct image of the structure corresponding to this metastable olivine wedge mow beneath southwest japan numerical simulation of a subducting slab including the kinetic effect of water h 2 o on the olivine wadsleyite transformation indicates that the presence of the imaged mow requires an insignificant amount of water less than 100 wt ppm be present in the slab mantle thus a deep dry cold slab we infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab but no significant amount within the slab itself we also demonstrate that a numerical simulation including the kinetics of 660 km phase transformation can reconcile the observed deep depression of the 660 km discontinuity with a gentle clapeyron slope

a 2500 year record of natural and anthropogenic soil erosion in south greenland the environmental impact of the norse landnam in greenland has been studied extensively but to date no study has quantified the soil erosion that norse agricultural practices are believed to have caused to resolve this problem a high resolution sedimentary record from lake igaliku in south greenland is used to quantitatively reconstruct 2500 years of soil erosion driven by climate and historical land use an accurate chronology allows for the estimation of detritic fluxes and their uncertainties land clearance and the introduction of grazing livestock by the norse around 1010 ad caused an acceleration of soil erosion up to 8 mm century in 1180 ad which is two fold higher than the natural pre landnam background from 1335 ad to the end of the norse eastern settlement in the mid fifteenth century the vegetation began to recover from initial disturbance and soil erosion decreased after an initial phase of modern sheep breeding similar to the medieval one the mechanization of agriculture in the 1980s caused an unprecedented soil erosion rate of up to 21 mm century five times the pre anthropogenic levels independently a suite of biological and geochemical proxies including ti and diatom concentrations c n ratio 13c and 15n of organic matter confirm that the medieval and modern anthropogenic erosion far exceeds any natural erosion over the last 2500 years our findings question the veracity of the catastrophic scenario of overgrazing and land degradation considered to have been the major factor responsible for norse settlement demise they also shed light on the sustainability of modern practices and their consequences for the future of agriculture in greenland

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

a subsolidus olivine water solubility equation for the earth s upper mantle the pressure and temperature sensitivity of the two most important point hydrous defects in mantle olivine involving si vacancies associated to trace amounts of titanium tichu pd or exclusively to si vacancies si was investigated at subsolidus conditions in a fluid saturated natural peridotite from 0 5 to 6 gpa approximately 20 200 km depth at 750 to 1050 c water contents in olivine were monitored in sandwich experiments with a fertile serpentine layer in the middle and olivine and pyroxene sensor layers at the border textures and mineral compositions provide evidence that olivine completely recrystallized during the weeklong experiments whereas pyroxenes displayed only partial equilibration a site specific water solubility law for olivine has been formulated based on the experiments reconciling previous contradictory results from low and high pressure experiments the site specific solubility laws permit to constrain water incorporation into olivine in the subducting slab and the mantle wedge as these are rare locations on earth where fluid present conditions exist chlorite dehydration in the hydrated slab is roughly parallel to the isopleth of 50 20 ppm wt h2o in olivine a value which is independent of the pressure and temperature trajectory followed by the slab hydrous defects are dominated by si under the relevant conditions for the mantle wedge affected by fluids coming from the slab dehydration slab adjacent low viscosity seismic low velocity channel p 3 gpa in cold subduction zones at 5 5 km from the slab surface the storage capacity of the mantle wedge at depths of 100 250 km ranges from 400 to 2 000 ppm wt h2o

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

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

on the dynamics of a hydrous melt layer above the transition zone 1 the transition zone water filter bercovici and karato 2003 model relies on the presence of a dense hydrous melt above the 410 km discontinuity that is formed by dehydration melting as wet wadsleyite undergoes a phase change to low water solubility olivine recent studies suggest that particularly in the pacific there is sufficient water in the transition zone for dehydration melting to occur here we construct a model for melt spreading and entrainment along the 410 km discontinuity in particular we propose a mechanism for melt entrainment in downwelling regimes at ambient mantle temperatures we find 1 that this entrainment mechanism is efficient in that it does not require melt to reach subducting slabs in order to recycle most of the upwelling water 2 that in steady state the efficiency of entrainment does not necessitate the existence of a thick melt layer 3 that the entrainment of hydrous buoyant material into the transition zone significantly affects the bulk mantle flow field thereby diminishing entrainment efficiency and allowing the melt to spread further 4 that these interactions help to recirculate water from the entrained regions through the transition zone and 5 that the adjustments in bulk mantle velocity due to water entrainment are characterized by a broadening of upward flow with a corresponding decrease in amplitude our model predicts regions of high water content in the transition zone at ambient mantle temperatures which may contribute to seismic observations of high attenuation near slabs

mechanism of backarc opening in the japan sea role of asthenospheric injection abstract the process of backarc extension in the japan sea includes two modes of spreading the parallel and rotational or fan shape opening data from paleomagnetism for rocks on the northeast japan arc radiometric age determination for dredged volcanic rocks and heat flow in the basin have suggested that the parallel opening of the japan sea took place earlier than the rotational opening at 21 14 ma the mechanism of opening of the japan sea has been examined based on the secular change in the geometry of the subducted slab and in the volcanic activity on the northeast japan arc mechanisms of backarc rifting including mantle diapirism mantle flow induced by the slab subduction or interaction of the over and the underriding plates cannot reasonably explain the process of opening of the japan sea the most plausible mechanism operating in the spreading process of the japan sea is injection of the asthenosphere into the mantle wedge that forcefully pushed the subducted slab to cause both steepening of the slab and the backarc extension the asthenospheric current towards the northeast japan arc trench system may have been associated with the large scale upwelling of the asthenosphere beneath the northeastern china region that caused intra plate volcanism and produced continental rift systems in the region the present mechanism can also explain the active backarc rifting in the okinawa trough in the western pacific and the taupo depression in north island new zealand

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

a charcoal rich horizon at o69 greenland evidence for vegetation burning during the norse landnam it is often assumed that the colonisation of greenland by norse settlers in c a d 985 had a sudden and dramatic effect on the environment involving substantial vegetation clearance and environmental degradation consequently it has been argued that charcoal rich horizons visible in many sections in greenland represent the initial burning of the vegetation by norse farmers to create land suitable for agriculture in this study a charcoal rich layer visible in a modern drainage ditch beside the norse farm of o69 was analysed using archaeobotany sedimentary analysis and radiocarbon dating to test the date and formation processes of the horizon it is demonstrated that the charcoal rich layer at o69 was not derived from in situ vegetation burning in the 10th century and concluded that the layer was probably formed by the addition of midden material to the infields around o69 in the 13th and 14th centuries cal ad perhaps as part of a soil amendment strategy it is argued that caution must be exercised when interpreting charcoal rich horizons as time specific chronological markers in palaeoenvironmental sequences in greenland

statistical study of short period gravity waves in oh and oi nightglow images at two separated sites 1 using airglow images of the near infrared oh band 720 910 nm and oi 557 7 nm line we investigated seasonal latitudinal and local time variations of short period gravity waves the images were obtained at two locations in japan that are 1200 km apart rikubetsu 43 5 n 143 8 e and shigaraki 34 9 n 136 1 e between october 1998 and october 1999 our analysis has focused on small scale gravity waves with wavelengths less than 40 km and dominant phase speeds of 20 50 m s wave occurrences for both oh and oi at rikubetsu and shigaraki are significantly higher than 60 with a slightly larger value in summer the occurrences increase from evening to midnight there are no obvious local time dependencies in horizontal wavelength propagation direction and phase speed the propagation directions in summer are either northward or northeastward at both locations however in winter the propagation directions at rikubetsu are generally westward nw w and sw whereas those at shigaraki are only southwestward from simultaneous wind observation by the mf radars at wakkanai 45 4 n 141 7 e and yamagawa 31 2 n 130 6 e we discuss possible influences of doppler and thermal ducting wind filtering and source distribution of gravity waves propagating from the lower atmosphere to the airglow heights in the mesopause region

big mantle wedge anisotropy slabs and earthquakes beneath the japan sea abstract the japan sea is a part of the western pacific trench arc backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the japan sea which was determined using p and s wave arrival times of suboceanic earthquakes relocated precisely with sp depth phases ambient noise tomography revealed a thin crust and a thin lithosphere beneath the japan sea and significant low velocity low v anomalies in the shallow mantle beneath the western and eastern margins of the japan sea observations with ocean bottom seismometers and electromagnetometers revealed low v and high conductivity anomalies at depths of 200 300 km in the big mantle wedge bmw above the subducting pacific slab and the anomalies are connected with the low v zone in the normal mantle wedge beneath ne japan suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back arc magmatism the pacific slab has a simple geometry beneath the japan sea and earthquakes occur actively in the slab down to a depth of 600 km beneath the ne asian margin teleseismic p and s wave tomography has revealed that the philippine sea plate has subducted aseismically down to the mantle transition zone mtz 410 660 km depths beneath the southern japan sea and the tsushima strait and a slab window is revealed within the aseismic philippine sea slab seismic anisotropy tomography revealed a nw se fast velocity direction in the bmw which reflects corner flows induced by the fast deep subduction of the pacific slab large deep earthquakes m 7 0 depth 500 km occur frequently beneath the japan sea western margin which may be related to the formation of the changbai and ulleung intraplate volcanoes a metastable olivine wedge is revealed within the cold core of the pacific slab at the mtz depth which may be related to the deep seismicity however many of these results are still preliminary due to the lack of seismic stations in the japan sea the key to resolving these critical geoscientific issues is seismic instrumentation in the japan sea for which international cooperation of geoscience communities in the east asian countries is necessary

phase relations of peridotites under h2o saturated conditions and ability of subducting plates for transportation of h2o phase relations of peridotites under h2o saturated conditions up to 28 gpa and 2600 c have been clarified based on the high pressure experimental results in the mgo feo sio2 h2o mgo sio2 al2o3 h2o cao mgo al2o3 sio2 h2o and natural systems based on the phase assemblages deduced and the chemical compositions of the phases the maximum h2o contents for two bulk rock compositions lherzolite and harzburgite have been calculated using mass balance then the potential ability of subducting plates for transportation of h2o is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a 15 130 ma the subduction velocity u0 2 25 18 cm year and angle 30 and 60 and the mantle potential temperature tp 1300 and 1350 c the results confirm the importance of choke point a cusp around 6 2 gpa and 550 c at which the stability field of major hydrous phases is minimized in terms of temperature although the choke point is exceeded by the mg sursassite bearing assemblage that has the maximum h2o content of 0 4 0 7 wt around 5 7 gpa and 550 700 c if the geotherm of the coldest part across the subducting plate passes below the choke point maximum 4 6 wt h2o can subduct whereas h2o less than 0 4 0 7 wt can subduct above the choke point the critical conditions are clarified as follows if 30 and tp 1300 c a 15 ma with u0 9 cm year or a 30 ma with u0 2 25 cm year is the critical condition if 60 and tp 1300 c or 30 and tp 1350 c the critical conditions slightly shift to an older age or a greater velocity e g a 30 ma with u0 3 5 cm year the slab thermal parameter au0sin is useful to predict roughly the critical condition however within the younger plate the spatial extent of a cold region with major hydrous phases is thinner hence less h2o is subducted even with the same slab thermal parameter this indicates that the age of the plate and the subduction velocity act differently on the h2o transportation ability of subducting plates

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

seismic evidence for deep water transportation in the mantle we report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern japan our data indicate that water is released from the hydrated oceanic crust at shallow depths approximately 100 kilometers and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment

effect of solid flow above a subducting slab on water distribution and melting at convergent plate boundaries 1 hydrous fluids derived by dehydration of the downgoing slab at convergent plate boundaries are thought to provoke wet melting in the wedge above the downgoing plate we have investigated the distribution of hydrous fluid and subsequent melt in the wedge using two dimensional models that include solid mantle flow and associated temperature distributions along with buoyant fluid migration and melting solid mantle flow deflects hydrous fluid from their buoyant vertical migration through the wedge melting therefore does not occur directly above the region where hydrous fluids are released from the slab a melting front develops where hydrous fluids first encounter mantle material hot enough to melt wet melting is influenced by solid flow through the advection of fertile mantle material into the wet melting region and the removal of depleted material the region of maximum melting occurs where the maximum flux of water from slab mineral dehydration reaches the wet melting region the extent of melting f and melt production rates increase with increasing convergence rate and grain size due to increased temperatures along the melting front and to increased fractions of water reaching the melting front respectively the position of isotherms above the wet solidus varies with increasing slab dip and thereby also influences f and melt production rates applying the understanding of wet melting from this study to geochemical studies of the aleutians may help elucidate the processes influencing fluid migration and melt production in that region estimates of the timescale of fluid migration seismic velocity variation and attenuation are also investigated

melting and seismic structure beneath the northeast japan arc transportation of h2o associated with subduction of the pacific plate beneath the northeast japan arc is modeled to predict distribution of aqueous solution and melt and the consequent p wave velocity structure the observed velocity structure coincides well with the model for equilibrium transport of h2o most of h2o subducted is brought down to 150 200 km depth along the slab then the aqueous fluid generated ascends to initiate melting which explains the low velocity regions 6 reduction observed beneath the backarc rather than beneath the volcanic front if equilibrium transport of h2o occurs initiation of melting beneath the backarc with deep subduction of h2o is likely to be the case also for other subduction zones with slabs older than several tens of m y cold enough to stabilize serpentine to a great depth

propagation directions of gravity wave patterns observed in oh ccd images during the seek campaign oh airglow about 87 km altitude was observed with a ccd imager at shigaraki 34 9 n 136 1 e in august 1996 during the seek campaign period from nine night observations the characteristics of the gravity wave modulation of the oh airglow layer were extracted and analyzed the distributions of the horizontal wavelength observed wave period and horizontal phase speed were 10 30 km 5 30 min and 20 60 m s respectively the horizontal propagation direction was mainly north eastward this preferential direction is similar to previous mu radar observations of mesospheric gravity waves 68 80km in summer months but different from the propagation direction of gravity waves expected from fai theories and observations in the e region

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

depth of the decoupling plate interface and thermal structure under arcs subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate at depth below this mechanically decoupling plate interface the subducting plate couples with the overlying mantle and flow is induced in the mantle wedge this induced flow is the main factor that controls thermal structure under arcs depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern japan arc using a two dimensional numerical simulation of thermal structure the estimated depth is about 70 km the termination depth of thrust earthquakes agrees well with the depth of the coupling transition which indicates that these earthquakes occur at the decoupling plate interface the mechanism of this decoupling plate interface is discussed in this paper it is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2 0 gpa and have little temperature dependence termination depths of thrust earthquakes estimated from seismic studies are nearly the same 60 80 km for various arcs this indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust across arc heat flow profiles observed in various subduction zones show similar features heat flow is very low in the forearc region and increases steeply at the volcanic front considering that the depth of the slab is about 100 km at the volcanic front for various arcs this heat flow feature is closely related to the depth of the slab thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust

imaging coherent scatter radar incoherent scatter radar and optical observations of quasiperiodic structures associated with sporadic e layers 1 during june and july 2002 a 30 mhz imaging coherent scatter radar was installed and operated on the island of st croix to view the e region ionosphere above arecibo puerto rico during the observing period 10 events with discernible quasiperiodic echo structure were observed with the coherent scatter radar in six of those events simultaneous measurements were made with the arecibo 430 mhz incoherent scatter radar the imaging coherent scatter radar allows us to locate and track the echo structures within the volume illuminated by the transmitter which shows structures that are generally aligned along wavefronts a slight preference for motion of the structures toward the southwest is evident throughout the period but the propagation directions and speeds vary greatly the incoherent scatter radar measurements show a close correspondence between the occurrence of the coherent echoes and the location of the enhanced electron density structures in particular the coherent echoes occur when the electron density layers show uplifts

palaeoecological and historical evidence for manuring and irrigation at garðar igaliku norse eastern settlement greenland palaeoenvironmental data are presented from the site of gar ar modern igaliku the location of the cathedral and the bishop s farm in the norse eastern settlement of greenland the latter was founded from c ad 985 and abandoned some time during the fifteenth century inspection of drainage ditches located in close proximity to the settlement ruins revealed inter alia an organic rich unit containing cultural debris worked wood animal bone stone and charcoal dated by ams radiocarbon dates on seeds to the period c ad 1110 1370 fossil insect and pollen assemblages contained within the deposit appear representative of natural environments primarily wet eutrophic meadows but are mixed with high frequencies of a range of synanthropic insects including human and animal ectoparasites that could only derive from indoor habitats this is strongly indicative of the manuring of fields with waste from houses and byres in order to increase yields of hay large amounts of hay would have been necessary to prov

stagnant slab wet plumes and cenozoic volcanism in east asia abstract data from seismology and mineral physics indicate the existence of low velocity zones in the deep part of the upper mantle wedge and the mantle transition zone under east asia zhao and ohtani 2009 zhao et al 2009 it suggests a link between the deep subduction and stagnation and deep dehydration of the pacific slab below this area and the cenozoic volcanism observed at the surface here we present a numerical model describing the dynamics of a hydrated layer covering a slab that is stagnating in the earth tm s mantle transition zone specifically our model includes the effects of water on density and viscosity of the mantle minerals we conclude that the presence of water is likely to induce rayleigh taylor type instabilities wet plumes that are able to transport water up to the base of the surficial lithosphere in a relatively short time scale a few million years under initial conditions representative of the subducting pacific slab under east asia melting is induced where wet plumes reach the base of the lithosphere according to our results and analysis of the known composition of cenozoic basalts from southwest japan iwamori 1991 iwamori 1992 wet plumes and their melting induced by water related to the stagnant slab provides an appealing hypothesis to explain cenozoic volcanism in east asia

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

environmental responses of past and recent agropastoral activities on south greenlandic ecosystems through molecular biomarkers paleoenvironmental studies previously performed on lake igaliku revealed two agropastoral phases in south greenland the norse settlement from ad 986 to ca ad 1450 and the recent installation of sheep farmers since the 1920s to improve the knowledge of the timing and magnitude of the greenlandic agropastoral activities a lipid inventory was realized and compared with biological and geochemical data during the 12th century a major increase in deoxycholic acid doc and coprophilous fungal spores revealed a maximum of herbivores synchronously a minimum of the n c29 n c31 alkane ratio and tree and shrub pollen and a maximum of triterpenyl acetates showed a reduction in the tree and shrub cover because of grazing activities lupanone produced by angiosperms appeared simultaneously in the molecular content probably revealing an introduction of plant species by the norse as it has been the case for rumex spp no major erosion was recorded by trimethyl tetrahydrocrysenes tthcs and titanium ti fl

water flow to the mantle transition zone inferred from a receiver function image of the pacific slab variations in seismic velocity near the subducting slab provide constraints on the thermal structure and the distribution of hydrous minerals and water in a subduction zone we investigated the seismological structure in the upper mantle beneath japan using both radial and transverse receiver functions rfs radial rf can image horizontal layers of velocity contrast while transverse rf can image dipping layers the combination allows us to simultaneously image both horizontal and dipping velocity discontinuities we investigated the records of 45 teleseismic events observed with tiltmeters at 678 hi net stations the frequency band we used is from 0 02 to 0 16 hz the resultant rf image shows an elevated 410 km discontinuity and more importantly the top surface of the pacific slab down to below the 410 km discontinuity with forward modeling we determined that the mantle wedge is about 8 slower in shear wave speed than the subducting slab at depths deeper than 200 km the seismic velocity contrast is presumably caused by a sequence of hydrous minerals at the base of the mantle wedge which receives water released by dehydration reactions in the oceanic crust we employed numerical simulations to determine the distribution of water in and around the subducting slab the result suggests that hydrous minerals are continuously stable above the subducting slab in relatively cool conditions and carry water to the mantle transition zone

problematic but promising ponds palaeoenvironmental evidence from the norse eastern settlement of greenland pollen and related proxy based studies of human environment interactions during the norse and later periods within greenland have primarily involved the investigation of peat lake and soil deposits all of which have taphonomic and sampling problems many small ponds exist but they seem to have been relatively ignored in investigations of palaeoecologically based environmental history to evaluate their usefulness for studies of ecologically related cultural history especially where sites are in intimate association with the archaeology a pond in the norse eastern settlement has been examined to investigate three principal questions i does such a site contain a useful record of environmental history ii does it offer a record of sufficient resolution and sensitivity for the study of anthropogenic landscape impacts iii are there any apparent drawbacks to these sequences using data obtained from palynological diatom sedimentological and radiocarbon analyses it is concluded that environmental data for each proxy aside from 14c dating are clearly capable of being explained in a reasonably straightforward and compatible manner and fulfil the hopes for the sampling medium copyright 2011 john wiley sons ltd

experimental sytectonic dehydration of serpentinite under conditions of controlled pore water pressure we have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine talc water at temperatures ranging between 300 and 600 and at total pressures from 100 to 300 mpa constant stress stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction at 500 and 600 c however the strength was linearly dependent on the inverse of the deformation rate and independent of effective confining pressure over the range investigated microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine grained 0 1 m olivine produced in the breakdown reaction the enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion accommodated grain boundary sliding these experimental results suggest that metamorphic transformations affect the deformability of rocks and demonstrate enhancement of deformability through the transient existence of fine grained reaction products the results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain size reduction by dynamic recrystallization in plastically deforming rocks

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

airglow imaging of gravity waves 2 critical layer filtering images of the variations of oh intensities have been recorded by a three station network of scanning radiometers in southwestern ontario during the period from may until september 1998 the ensemble of gravity waves seen in all three instruments is analyzed and observed to have a strong dependence in their propagation direction with the majority of waves seen at each of the three sites having a horizontal wave vector in the northeast direction in order to understand these results a model of the transmission of gravity waves through the atmosphere was employed the gravity waves observed by the oh scanning radiometers ohscrs are assumed to originate in the troposphere with an isotropic spectrum meaning that the average wave amplitudes are equal in all directions the gravity waves propagate upward where they will undergo a selection process owing to the effect of the background winds using the horizontal winds from the horizontal wind model hwm 93 the transmission of gravity waves from a tropospheric source is modeled and shown to be consistent with the observations the strong zonal winds account for the summer eastward and winter westward propagation but it is the tidal components of the meridional wind that account for the northward tendency of wave propagation and the corresponding relative absence of southward propagating waves in order to further validate the transmission model winter measurements from the ohscr data set are examined and shown to be consistent

a systematic 2 d investigation into the mantle wedge s transient flow regime and thermal structure complexities arising from a hydrated rheology and thermal buoyancy 1 arc volcanism at subduction zones is likely regulated by the mantle wedge s flow regime and thermal structure and hence numerous studies have attempted to quantify the principal controls on mantle wedge conditions in this paper we build on these previous studies by undertaking a systematic 2 d numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge s flow regime and associated thermal structure we quantify the role of a range of plausible i water contents 0 5000 h 106si ii subduction velocities 2 10 cm yr and iii upper plate ages 50 120 myr finding that small scale convection ssc resulting from rayleigh taylor instabilities or drips off the base of the overriding lithosphere is a typical occurrence the morphology of ssc varies with viscosity and subduction parameters with drips at their most prominent when subduction velocities and wedge viscosities are low our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner flow regime and strong upper plate erosion below the arc region by contrast we find that back arc upper plate erosion by ssc is largely controlled by wedge viscosity occurring when i viscosities are 5 1018 pa s and ii the length of the upper plate available for destabilization exceeds the characteristic wavelength of instabilities thus if hydrous weakening of wedge rheology extends at least 100 150 km from the trench our 2 d models predict an unstable flow regime resulting in temperature fluctuations of 50 100 k which are sufficient to influence melting and the stability of hydrous minerals

quantitative analysis of the inclined low velocity zone in the mantle wedge of northeastern japan a systematic change of melt filled pore shapes with depth and its implications for melt migration abstract travel time tomography beneath the northeastern japan arc reveals that an inclined low velocity zone exists in the mantle wedge sub parallel to the down dip direction of the slab this zone distributed continuously along the arc as a single inclined sheet has been considered as the main source of arc magma a quantitative interpretation of both p and s wave velocity structures precisely determined from the recent travel time tomography is done in terms of thermal heterogeneity and fluid content the combined analysis of both p and s wave velocity structures makes it possible to independently derive the information on the shape and volume fraction of the fluid filled pores the reference p and s wave velocities representing the host rock velocities at reference temperature t 0 are estimated from the tomographic data while an alternative method based on high temperature and high pressure elasticity of upper mantle minerals is used to confirm the validity of the present method the calculated velocity anomalies are corrected for the thermal effect using the three dimensional thermal structure estimated from the p wave attenuation data showing that the observed low velocity anomalies cannot be explained by the thermal effect alone the remaining velocity anomalies are explained by the existence of melt filled pores and effective aspect ratio and volume fraction of the pores are determined the results show a systematic change in melt filled pore shapes with depth suggesting the existence of 3 6 vol melts as grain boundary tubules at a depth of 90 km 0 04 0 05 vol melts as thin cracks or dikes with aspect ratio of 0 001 at a depth of 65 km and 1 2 vol melts as cracks or dikes with aspect ratio of 0 02 0 04 at a depth of 40 km

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

the seek sporadic e experiment over kyushu campaign the seek sporadic e experiment over kyushu campaign was conducted in late august 1996 from the southern region of kyushu japan to investigate the mechanism for the generation of quasi periodic qp radar backscatter from field aligned irregularities imbedded in nighttime sporadic e es layers seek was designed to determine in situ small scale electrodynamical properties using two sounding rockets and large scale dynamics and electrodynamics using ground based sensors which included a transportable radar and other radio and optical instruments deployed in the vicinity of the rocket range it was observed by this campaign that es layers existed in a convergent wind shear region where large electric fields were induced and when active atmospheric gravity waves existed in the mesosphere however there was little evidence which positively supported the hypothesis that es layers were deeply modulated in altitude

grain size variations in the earth s mantle and the evolution of primordial chemical heterogeneities 1 chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection one of the factors which can cause viscosity variations is the grain size analysis of various physical processes in the earth s mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs the largest grains are formed in the early molten earth where ostwald ripening is controlled by liquid state diffusion the smallest grains are formed due to recrystallization by the spinel perovskite phase transformation in downwelling flow and subsequent ostwald ripening the viscosity contrast between primordial large grain size material and recrystallized material is likely to be many orders of magnitude preliminary numerical simulations show that strongly time dependent mantle convection can process primordial large grain size heterogeneities through the spinel perovskite phase boundary only after a few overturns this reduces the viscosity contrasts and leads to rapid mixing by mantle convection a more steady flow in the lower mantle may lead to the formation of stagnant poorly mixed regions which may never have had a chance to cross the 660 km discontinuity and be recrystallized by the spinel perovskite phase transformation heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle in this case they accumulate at the core mantle boundary

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

observational evidence of wave ducting and evanescence in the mesosphere a collaborative radar and imaging study of gravity waves over the hawaiian islands was performed during october 1993 as part of the airborne lidar and observations of hawaiian airglow 1993 coupling and dynamics of regions equatorial aloha 93 cadre campaign to investigate the propagation characteristics of short period 1 hour waves at nightglow altitudes the horizontal wavelengths and apparent phase speeds of quasi monochromatic wave events were measured in four separate nightglow emissions using data obtained by a high resolution ccd imager this information was correlated with simultaneous mf radar wind measurements over the same height interval 80 100 km to infer intrinsic wave parameters in each case correlating the two data sets allowed the determination of the local vertical wavenumber for each event in particular whether it be real indicative of freely propagating waves or imaginary indicative of ducted or evanescent waves the results of this study indicate a preponderance of ducted or evanescent waves at 80 100 km during the time of the observations with up to 75 of the events recorded exhibiting ducted or evanescent behavior also noted was a tendency for ducted behavior to be more prevalent among waves with shorter horizontal wavelengths in agreement with doppler ducting theory these results suggest that ducted waves are relatively common in the upper mesosphere and lower thermosphere region at least over the mid pacific ocean as small scale waves which are ducted have the potential to travel much longer horizontal distances than freely propagating waves the frequency of their occurrence should be taken into account in efforts to quantify gravity wave effects at these altitudes

application of the model of small scale convection under the island arc to the ne honshu subduction zone 1 we compare the results of 2 d 3 d time dependent models of small scale convection under the island arc with the geophysical and geologic data observed in the ne honshu subduction zone northeast japan assuming that the temperature anomalies have a close connection with the seismic anomalies we may constrain the geometry of the low viscosity wedge lvw overlying the slab which may be produced by the water dehydrated from the subducting slab our preferred model predicts step like low velocity anomalies above the subducting slab rather than smooth anomalies subparallel to the subducting slab since the shallow nature of the lvw is required for the small scale convection to occur at the back arc end of the lvw a movement of cold plumes generated at the back arc end of the lvw may be related to a possible migration of volcanism from back arc to volcanic front side to be consistent with the observation which may suggest the migration rate of 2 cm yr models require weak viscous couplings between the mantle wedge and the underlying subducting slab whose speed of subduction is 10 cm yr three dimensional models showing 3 d temperature variation consist of fairly continuous and strong temperature anomalies several hundreds of degrees under the volcanic front and weak finger like temperature anomalies several tens of degrees behind them which are similar to the pattern of the seismic tomography the time dependent behavior of the temperature field shows that the pattern of fingers flip flops with a timescale equal to the ratio of the horizontal extent of the lvw to the migration speed of cold plumes this change of pattern of fingers may have an important implication for understanding the past distribution of volcanism further studies related to 1 the detailed morphology of the low velocity zone above the subducting slab 2 the spatial and temporal evolution of volcanism and 3 the dynamic 3 d model studies which self consistently determine the distribution of water volatiles and the resultant weak rheology are important for checking the validity of our model or developing alternative models

stabilities and equations of state of dense hydrous magnesium silicates abstract the thermodynamic stability of high pressure phases in the ternary system mgo sio2 h2o is reviewed with special emphasis on hydrated phases including the alphabet phases on the basis of recent experimental data a stability diagram for these phases along a geotherm appropriate for a subducting slab is constructed this suggests that at realistic water contents the breakdown of antigorite will produce olivine plus phase a in the upper mantle but that the ability of wadsleyite to accommodate wt levels of water means that phases e and d are unlikely to occur in the upper portion of the transition zone for the lower part of the transition zone in which ringwoodite becomes stable the coexisting hydrous phase would be superhydrous phase b provided ringwoodite is not able to accommodate significant amounts of water at the top of the lower mantle phase d is stable but only at temperatures below 1300 c as the maximum solubility of h2o in magnesium silicate perovskite is also quite low slabs may dehydrate somewhat as they enter the lower mantle the available thermodynamic data for these phases is also reviewed from the most recent measurements of equations of state it is concluded that the elasticity of the phases in the mgo sio2 h2o ternary system is primarily dependent upon the density and not upon the water content it is therefore concluded that the presence of even significant amounts of hydrated phases in subducting slabs could not be unequivocally identified from seismological observations

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

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

effect of pore fluid pressure on the frictional strength of antigorite serpentinite abstract triaxial deformation experiments on antigorite serpentinite show a systematic correlation between shear stress and effective normal stress over a wide range of pore pressures 50

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone