s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

finite amplitude convection through a phase boundary summary a finite amplitude numerical model of convective motions through a phase boundary is studied the phase change is assumed to occur over a finite vertical transition zone in the fluid and thus a continuously stratified model is considered the motions are driven by horizontal temperature gradients on the boundaries or super critical vertical stratification in the case of rayleigh instability the numerical results reproduce the neutral curve previously obtained in the analytic solution of the linearized problem when the parameters of the problem are chosen to simulate the olivine spinel transition in the upper mantle the following results are obtained 1 the vertical scale of motion is the entire depth of the fluid 2 the horizontal scale is not significantly changed from the case of a single phase fluid 3 the amplitude of the motion is not significantly changed due to buoyancy changes at the phase boundary being offset by corresponding sources and sinks of latent heat and 4 the phase boundary varies in depth by as much as 30 km when the vertical velocities are of the order of 10 1 cm yr 1

induced compositional layering in a convecting fluid layer by an endothermic phase transition abstract an endothermic phase change in the presence of chemical heterogeneities can act as an efficient filter such that heavy material is sequestered below the phase change and light material is trapped above the phase change due to the fact that the endothermic phase change damps thermal convection it can act as a trap door for chemical heterogeneities ascending flows which are chemically dense will need more thermal buoyancy to rise through the transition than ascending flows whose composition is closer to the background composition likewise descending flows which are chemically light need more thermal buoyancy to sink through the transition the results of a suite of numerical calculations of two dimensional double diffusive convection in a fluid layer with an endothermic phase change suggests that if the 670 km seismic discontinuity has a steep enough clapeyron slope compositional layering may be induced and dynamically maintained by the transition furthermore these calculations also show that the phase change and compositional boundaries will approximately coincide

late quaternary cryptotephra detection and correlation in loess in northeastern japan using cummingtonite geochemistry abstract we detected late pleistocene cummingtonite bearing cryptotephras in loess deposits in ne japan and correlated them with known tephras elsewhere by using major element compositions of the cummingtonite this is the first time cryptotephras have been identified by analysis of a crystal phase rather than glass shards in central ne japan four cummingtonite bearing tephras the ichihasama pumice the dokusawa tephra the naruko nisaka tephra and the adachi medeshima tephra are present in late pleistocene loess deposits because the cummingtonite chemistry of each tephra is different and characteristic it is potentially a powerful tool for detecting and identifying cryptotephras an unidentified cummingtonite bearing cryptotephra previously reported to be present in the late pleistocene loess deposits at kesennuma pacific coast did not correlate with any of the known cummingtonite bearing tephras in central ne japan but instead with the numazawa kanayama tephra erupted from the numazawa caldera southern ne japan although kesennuma is well beyond the previously reported area of the distribution of the numazawa kanayama tephra three new cummingtonite bearing cryptotephras in the mid and late pleistocene loess deposits estimated to be less than 82 ka 100 200 ka and ca 250 ka on the isawa upland were also detected

a marker tephra bed close to the lower middle pleistocene boundary distribution of the ontake byakubi tephra bed in central japan abstract tephrochronology is an exceptionally important tool in the precise regional correlation of early and middle pleistocene sedimentary strata in japan the present study reveals that the yukawa tephra 5 yut5 derived from the older ontake volcano the nezumigawa nzg and mitamitajima mtj tephras of the ina bazin and the byakubi e tephra byk e of the boso peninsula are the same tephra on the basis of their lithofacies bulk grain composition mafic mineral composition major element composition of hornblende and stratigraphic relationships with the dated tephras we propose to call the series of tephras correlated with byk e the ontake byakubi tephra bed on byk tephra following the naming convention in which the tephra name consists of the names of the source volcano and the type location the matuyama brunhes chronozone boundary occurs just above byk e in the type section of the kokumoto formation in the kazusa group which is a candidate global boundary stratotype section and point gssp for the lower boundary of the middle pleistocene subseries therefore on byk tephra becomes a critically important marker tephra bed for the early middle pleistocene boundary in central japan the present study indicates that the major element composition of hornblende can be a useful tool for identification and correlation of strongly weathered tephra layers such as nzg and mtj in which all the volcanic glass shards have been altered

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

analysis of titanomagnetite within weathered middle pleistocene kmt tephra and its application for fluvial terrace chronology kanto plain central japan abstract the widespread kaisho kamitakara tephra kmt derived from the source vent located in the hida mountains is a favorable marker tephra for the chronological study of the middle pleistocene in the kanto plain central japan as the kmt in aeolian sediments is strongly weathered the chemical composition of titanomagnetite resistant to weathering was determined for its identification the age of the kmt was estimated at 0 62 ma mis 16 0 by nakazato 2006 horizon and age of kaisho kamitakara tephra from the inubo group in the northeastern part of chiba prefecture central japan programme and abstracts japan association for quaternary research 36 106 107 in japanese revising a previous estimate of 0 58 0 69 ma by suzuki 2000 kaisho kamitakara tephra erupted from the hida mountains in early half of middle pleistocene and its significance for geomorphic chronology of central japan geographical review of japan 73 1 25 in japanese the stratigraphic position of the kmt in the azuyama and sayama terrace surfaces in southwest kanto and the kitsuregawa upper terrace surface in north kanto indicates that these terraces are correlative it is assumed that the dissected fluvial terrace surfaces were formed a few tens of thousands of years prior to the deposition of the kmt alluvial fan deposits constituting these terrace surfaces were formed during the period from mis 17 3 to 16 2 during a transition from interglacial to glacial geomorphic conditions during this period of the middle pleistocene were stable allowing the formation of broad fans

a tephrochronologic method based on apatite trace element chemistry geochemical correlation of ash fall beds with conventional tephrochronologic methods is not feasible when original glass composition is altered thus alternative correlation methods may be required initial studies of heavily altered paleozoic tephra k bentonites have suggested the potential for employing trace element concentrations in apatite as ash fall bed discriminators to further test the utility of apatite trace element tephrochronology we analyzed apatite phenocrysts from unaltered volcanic rocks with an electron microprobe nine samples from rocks erupted during the quaternary and one sample from a rock erupted during the paleogene the resulting apatite trace element data provide unique bed discriminators despite within crystal variability each of the volcanic rocks studied possesses unique trends in mg cl mn fe ce and y concentrations in apatite the results from this study establish an important tephrochronologic method that can be applied to nearly all portions of the phanerozoic stratigraphic record and greatly assist development of an advanced timescale in addition to establishing a fingerprint for a particular eruption apatite chemistry provides useful information about the source magma

spatiotemporal patterns of clustered and non clustered seismic activity in the shin etsu and tokai regions central japan we investigated spatiotemporal patterns of seismicity in the shin etsu region and tokai region central japan by separating the seismicity into clustered and non clustered activity we found that clustered activity occurred at various sites during short periods in each investigated region and that an alternation of active and quiet periods was clear for the clustered activity in contrast non clustered earthquakes occur at an almost constant rate and active or quiet periods can not be distinguished in the non clustered activity some evidence suggests that an increase of clustered activity indicates a buildup of the regional stress field

関東平野西縁 上鹿山面 狭山面の古地磁気による編年 58 69 kmt kmt brunhes chron

dynamical influences from thermal chemical instabilities at the core mantle boundary two dimensional time dependent calculations show that the thermal and flow structures can be influenced by the lower mantle thermal chemical instabilities the style of the core mantle boundary cmb deformation in thermal chemical convection is different from that obtained in thermal convection the amplitude of cmb hills is reduced greatly from the depth dependence of thermal expansivity found in recent high pressure experiments

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

fluvial archives a valuable record of vertical crustal deformation abstract the study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms but also for identifying the nature of crustal deformation and its history in recent decades geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens here we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions the effects of horizontal deformation are not addressed channel long profiles have received considerable attention in the literature and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady state characteristics then we review the literature dealing with the transient response of rivers to tectonic perturbation through the production of knickpoints propagating through the drainage network inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories finally we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes as well as the erosional imprint of tectonic forcing sedimentary deposits such as fluvial terrace staircases are also considered as a classical component of tectonic geomorphology we show that these studies have recently benefited from rapid advances in dating techniques allowing more reliable reconstruction of incision histories and estimation of incision rates the combination of progress in the understanding of transient river profiles and larger more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation i e extrapolation of local data to entire profiles finally planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the dis equilibrium state of drainage systems across regional drainage divides

a simple model of the 1982 83 californian el nino steric height steric height anomaly sea level and sea level anomaly show that the 1940 41 and the 1982 83 californian el nino events were resonant responses of the california current to the expansion and intensification of the aleutian low and decrease in strength of the north pacific high in both cases the subsurface anomalies were dynamically produced by onshore transport of subarctic water and a downward tilt of the inshore thermocline sea surface temperature anomalies however resulted from thermodynamic as well as dynamic processes most previous californian el nino or anti el nino events probably were resonant or anti resonant responses of the seasonal cycle of the california current to anomalous large scale atmospheric forcing

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

effects of phase transions and possible compositional changes on the seismological structure near 650 km depth if the lower mantle is enriched in fe si relative to the upper mantle the compositional boundary is expected to be an interface with a density jump of 5 2 and with depth varying laterally by 100 300 km but with changes in seismic velocities that are nearly unobservable 0 to 1 0 2 5 discontinuities in compressional vp and shear vs wave velocities in contrast the polymorphic transformation of transition zone minerals to the perovskite bearing assemblage of the lower mantle is estimated to produce a jump of 5 10 percent in density vp and vs near 650 km depth and the depth of the seismological discontinuity due to the phase transition is relatively independent 50 150 km variation of temperature or horizontal position if present a compositional boundary is not in general expected to occur at the identical depth 650 km as the phase transition to the perovskitebearing assemblage although the depths of the two may be linked

use of amphibole chemistry for detecting tephras in deep sea sequences chikyu c9001c cores and developing a middle pleistocene tephrochronology for ne japan abstract using amphibole crystals we detected tephras in deep sea sequences chikyu c9001c cores and correlated marine and terrestrial tephras in ne japan then we reconsidered some previously reported tephra ages by using the age model of the cores we identified 50 spikes in amphibole grains numbered a1 a50 from top to bottom among these 12 spikes a38 49 corresponded to a thick non tephric sand layer we selected 20 spikes containing amphibole grains with glass coatings as likely primary tephras and analyzed their chemistry for tephra characterization on the basis of its amphibole chemistry we correlated spike a34 with the tn cii ciii layers of the tanabu c tephra tn c which erupted from osore volcano on shimokita peninsula we then showed that spike a34 represents primary tephra deposition not secondary deposition through bioturbation or bottom current reworking because amphibole grains with homogeneous chemistry were accumulated only in sediment near the spike within 10 cm we also showed that the chemical variation within each amphibole grain was not as great as the variation between grains which further supports the correlation between spike a34 and tn cii ciii these findings show that amphibole grains which are resistant to weathering can be used for characterization and correlation of weathered tephras from which glass shards have been removed by dissolution the glass shard chemistry of the 16h6a 60 80 tephra at 4 58 m below spike a34 which does not correspond to an amphibole spike suggests that this tephra likely correlates with the amphibole free tn ci layer in the terrestrial sequences non tephric sediments marine sand and lahar deposits occur between tn ci and tn cii ciii suggesting that some time elapsed after the deposition of tn ci before tn cii ciii were deposited by using the stratigraphic information of the 16h6a 60 80 tephra and spike a34 together with the 18o stratigraphy of the deep sea sequence we re assigned the eruptive age of tn c from the previously reported zircon fission track age of 180 40 ka to mis 8 257 263 ka

characterization and correlation of cryptotephras using major element analyses of melt inclusions preserved in quartz in last interglacial marine sediments southeastern shikoku japan although sediments underlying marine terraces in se shikoku southwest japan contain no visible tephra layers late pleistocene cryptotephras were detected in a paleosol within the sediments the cryptotephras contained little or no glass but they were characterized through major element analyses of glass preserved as melt inclusions in quartz grains extracted from the paleosol the analyses enabled correlation of the cryptotephras with known eruptives from volcanoes on kyushu namely the kikai tozurahara tephra mis 5 2 5 3 the aira tn tephra 28 6 cal ka and the kikai akahoya tephra 7 5 cal ka these correlations enabled dating of the marine terrace surface known as the murotomisaki 1 surface in se shikoku as being older than mis 5 2 5 3 probably mis 5 5 in age in the absence of glass or mafic minerals because of weathering the analysis of melt inclusions in resistant quartz grains demonstrably provides a powerful method for characterizing cryptotephras to facilitate their correlation and thus their application as a linking and dating tool

detection and correlation of widespread cryptotephras in middle pleistocene loess in ne japan using cummingtonite geochemistry we used cummingtonite chemistry to detect strongly weathered widespread cryptotephras within middle pleistocene loess and fluvial terrace deposits and to correlate them with previously reported tephras in central and ne japan we correlated a cummingtonite bearing cryptotephra widely distributed in ne japan below the ichihasama pumice 108 146 ka and above the iwadeyama pumice iwp 160 240 ka with the iizuna kamitaru c tephra iz ktc which erupted from iizuna volcano central japan iz ktc is thus a widespread middle pleistocene tephra covering both central and ne japan moreover we detected the adachi medeshima tephra ac md which had been previously inferred to be a late pleistocene tephra 80 100 ka below the iz ktc and iwp tephras 125 150 ka and above the hagimori white ash tephra 210 250 ka therefore ac md must be a middle pleistocene tephra with an eruptive age older than 125 150 ka and younger than 210 250 ka we also detected a cummingtonite bearing tephra older than 210 250 ka and younger than 250 300 ka in the isawa area of ne japan and correlated it with a previously reported tephra in central japan thus like iz ktc this tephra is a widespread middle pleistocene tephra

composition of the upper mantle geophysical tests of two petrological models the elastic properties of candidate mantle phases are used to test the viability of olivine rich pyrolitic and cao al 2o 3 rich eclogitic assemblages for the mantle high temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties both pyrolitic and eclogitic assemblages satisfy the seismic properties between 200 and 400 km between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients the 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite piclogite or phase transformations in the olivine orthopyroxene components of a piclogitic assemblage containing about 16 olivine high velocity gradients in the transition zone may be explained by the transformation of ca rich cpx to majorite garnet seismic properties at the top of the lower mantle are consistent with pyrolite piclogite or perovskite implying that the 670 km discontinuity may be a chemical boundary

constraints on lower mantle composition and temperature from density and bulk sound velocity profiles recent studies comparing density from the seismological model prem to that predicted for various perovskite and magnesiowustite phase assemblages under lower mantle conditions suggest that the lower mantle is a few percent denser than reasonable candidates for upper mantle composition such as pyrolite this has been interpreted as evidence for an iron enriched lower mantle in addition to density bulk sound velocity provides an important constraint upon proposed lower mantle compositions being independent of the shear modulus has several desirable characteristics experimentally it can be determined in the laboratory by density measurements under static compression seismologically it is not affected by attenuative dispersion and is expected to be much less laterally heterogeneous than p or s wave velocities we have calculated and along lower mantle adiabats as functions of iron magnesium content xmg and silica content xpv and compared them to values obtained from prem for a reasonable temperature estimate at the top of the lower mantle tlm of 2000 k we find that the data are most compatible with a lower mantle enriched in fe in agreement with previous studies in addition the data require that silica enrichment accompany this fe enrichment furthermore increasing tlm increases both the required fe and si content with a high assumed tlm 2700 k the data can be satisfied by low xmg values but even a pure perovskite lower mantle is too slow compared to thus assuming no free sio2 in the lower mantle this provides an upper bound on tlm of approximately 2700 k finally conclusions regarding the composition and temperature of the lower mantle are strongly dependent upon uncertainties in estimated thermoelastic parameters and upon choice of a seismological model most importantly neither fe nor si enrichment is required if the zero pressure volume coefficient of thermal expansion of perovskite is about 2 5 10 5k 1 rather than 4 0 10 5k 1 at high temperatures alternatively but less probably fe and si enrichment are not required if the average of the lower mantle is 1 0 less than given by prem the si enrichment is not required if the average of the lower mantle is 0 5 slower than pkem closer to the value given by the body wave model derived from the jeffreys bullen tables or if the lower mantle is anomalously cold with tlm 1700 k

models of mantle convection one or several layers numerical model calculations are used to determine if convection in the earth s mantle could be organized in two or more layers with only limited mass exchange in between the seismic discontinuity at 670 km depth and the top of the d layer at the bottom of the mantle are considered as candidates for internal boundaries if the 670 km discontinuity is caused by an isochemical phase transition it has to have a clapeyron slope of dp dt 6 mpa k 1 to prevent convection currents from crossing this value is improbably low if the discontinuity represents a chemical boundary the intrinsic density difference has to exceed 3 to prevent subducted lithospheric slabs from penetrating deeply into the lower mantle also the condition is possibly hard to meet the least improbable mechanism for a mid mantle barrier for convection currents would be a combination of endothermic phase transition and chemical change the boundary between upper and lower mantle would show considerable topography and a limited material exchange is to be expected at any rate the possibility of a downward segregation of former oceanic crust transformed to dense eclogite is studied in a further model series it requires a region of low viscosity as the delayer probably is and is faciliated by the decrease of the thermal expansion coefficient with pressure about 20 of subducted oceanic crust could accumulate at the core mantle boundary the dense material would concentrate underneath rising therm al plumes and some of it is entrained into the plumes possibly affecting their geochemical signature

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

北関東 東北地方南部のテフロクロノロジー 現状と展望

correlation of fluvial terraces within the hikurangi margin new zealand implications for climate and baselevel controls abstract a correlation of fluvial terraces is presented for eight non glacial catchments of the eastern north island new zealand within the actively uplifting hikurangi margin using a combination of loess and tephra coverbed stratigraphy and radiocarbon and osl dating of fluvial deposits and loess coverbeds we demonstrate correlation of four fill terraces t1 t4 the available age constraints suggest t1 15 30 ka t2 31 50 ka t3 50 70 ka and t4 115 ka but the association and temporal link with loess deposits suggest correlation with cold periods and thus the refinement of t2 late mis 3 31 40 ka t3 mis 4 55 70 ka t4 mi substage 5b 90 ka 5d 110 ka or mis 6 140 160 ka the ability to correlate terraces between catchments plus the lack of independent evidence for tectonic triggering events indicates terraces have probably formed in response to either climate terrestrial or baselevel sea level control climate control is indicated by the temporal link of post glacial incision with re establishment of forest cover and of lgm aggradation with limited grass and shrub cover and periglacial processes aggradation due to increased sediment supply under reduced vegetation is dramatically demonstrated by formation of the taupo pumice alluvium terrace in response to inundation by volcanic deposits unwelded ignimbrite following the 1 8 ka taupo eruption and the response to post settlement

correlation of middle pleistocene crystal rich tephra layers from daisen volcano southwest japan based on the chemical composition and refractive index of mafic minerals abstract analyses of chemical compositions and refractive indices of mafic minerals have been effective correlation tools especially for crystal rich tephra layers containing isolated volcanic glass shards the analyses are applied to the correlation of the middle pleistocene tephra layers in the kinki and chubu districts central japan with those erupted from daisen volcano in the chugoku mountains west of the districts on the basis of chemical compositions of hornblende phenocrysts and refractive indices of hornblende orthopyroxene and cummingtonite phenocrysts four groups of tephra layers in the kinki and chubu districts are successfully correlated to bvs2 cpm gpm and hpm1 or dop tephra layers of the lowermost member of the daisen tephra formation the correlation reveals that two tephra layers bvs2 and cpm with estimated eruption ages of 410 ka and 370 400 ka respectively are present in the kinki district the hpm1 tephra about 210 ka is also widespread and recognized at the eastern foot of yatsugatake volcanoes about 400 km east of the source these tephra layers can contribute to establishing the regional correlation and chronology of the middle pleistocene sequences in central japan especially for the time from mis 7 to mis 11

interaction of plumes with a compositional boundary at 670 km the seismic discontinuity observed at a depth of 670 km may result from a phase change a compositional boundary or both likewise it may mark a change in density viscosity or both between the upper and lower mantle this paper addresses whether upwelling plumes in the lower mantle penetrate into the upper mantle and what effect lower mantle plumes have on convection in the upper mantle if there is a density jump at 670 km due to a change in composition with depth a finite element model of double diffusive convection is used to model flow in a spherical axisymmetric shell heated from below with a rayleigh number of 106 the ability of plumes to rise into the upper mantle is governed by the buoyancy number b c t which is the ratio of the compositional to thermal buoyancy when b 1 flow in the upper layer is shear coupled to flow in the lower layer that is a plume impinging on the compositional boundary will induce a down welling in the overlying layer when b 1 the coupling is more complex strong plumes in the lower layer are shear coupled to flow in the overlying layer but as plumes evolve and weaken the flow in the upper layer switches to thermal coupling with entrainment of lower mantle material

modeling of residual spheres for subduction zone earthquakes 1 apparent slab penetration signatures in the nw pacific caused by deep diffuse mantle anomalies we have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the pacific using regional and global mantle velocity models from tomographic inversions the mantle models explain much of the observed residual sphere data and to a certain extent suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns for most deep events considered the fast slablike residual sphere anomalies are caused by diffuse heterogeneities mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab the region immediately below the deepest earthquakes depths of 650 1500 km has an effect usually smaller than or comparable to the effect of other regions of the mantle without a proper account of the teleseismic effect attributing the long wavelength anomalies of the residual sphere to near source slab effects alone or even primarily is not valid the fast bands in many observed residual spheres agree with seismicity trends once the deep mantle and receiver mantle effects are removed these may give the approximate orientation but not the depth extent of near source fast velocities for most deep earthquakes under japan the predominant fast band is subhorizontal rather than near vertical this type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

development of diapiric structures in the upper mantle due to phase transitions solid state phase transitions in time dependent mantle convection can induce diapiric flows in the upper mantle when a deep mantle plume rises toward phase boundaries in the upper mantle the changes in the local thermal buoyancy local heat capacity and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir this mechanism may explain episodic hot spot volcanism the nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes allowing hot plumes to go through and retarding the tepid ones

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal bearing sediments offshore shimokita peninsula japan iodp site c0020 sediments recovered at integrated ocean drilling program iodp site c0020 in a forearc basin offshore shimokita peninsula japan include numerous coal beds 0 3 7 m thick that are associated with a transition from a terrestrial to marine depositional environment within the primary coal bearing unit 2 km depth below seafloor there are sharp increases in magnetic susceptibility in close proximity to the coal beds superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence we investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record using isothermal remanent magnetization irm thermal demagnetization anhysteretic remanent magnetization arm iron speciation and iron isotopes magnetic mineral assemblages in all samples are dominated by very low coercivity minerals with unblocking temperatures between 350 580 c that are interpreted to be magnetite samples with lower unblocking temperatures 300 400 c higher arm higher frequency dependence and isotopically heavy 56fe across a range of lithologies in the coal bearing unit between 1925 1995 mbsf indicate the presence of fine grained authigenic magnetite we suggest that iron reducing bacteria facilitated the production of fine grained magnetite within the coal bearing unit during burial and interaction with pore waters the coal peat acted as a source of electron donors during burial mediated by humic acids to supply iron reducing bacteria in the surrounding siliciclastic sediments these results indicate that coal bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time within the subsequent deep biosphere