strength and water weakening of mantle minerals olivine wadsleyite and ringwoodite rheological properties of olivine and its high pressure polymorphs both dry and hydrous have been studied by monitoring x ray diffraction peak broadening as a function of pressure temperature and time the measurements were carried out up to a pressure of 10 gpa and a temperature of 600 c for olivine and wadsleyite and a pressure of 20 gpa and a temperature of 1000 c for ringwoodite the observations indicate that olivine is much weaker than the other two minerals for this range of conditions and furthermore water weakens olivine dramatically but only slightly weakens wadsleyite and ringwoodite when the temperature increases from 25 c to 400 c the yield strength in the olivine drops by 39 without water and 62 with water however there is hardly any change in strength for wet or dry wadsleyite and ringwoodite

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

global mantle heterogeneity and its influence on teleseismic regional tomography abstract a new global p wave tomography model is determined using a flexible grid parameterization this new model better reveals the mantle structure under the polar regions than the previous tomographic models the subducting slabs are generally imaged clearly as high velocity high v zones the young slabs are still subducting in the upper mantle and the mantle transition zone mtz whereas the old and ancient slabs are either stagnant in the mtz or have subducted down to the lower mantle even reaching the core mantle boundary low velocity low v anomalies are generally revealed in the mantle under the hotspot regions it seems that a variety of mantle upwelling plumes exist some strong plumes are visible in the whole mantle under the long living hotspots such as those in south central pacific africa hawaii and iceland whereas weak plumes are visible in only some depth range under the minor hotspots under the intraplate volcanoes in east eurasia bering sea and west alaska significant low v anomalies are revealed in the upper mantle which may reflect hot and wet upwelling associated with corner flows in the big mantle wedge bmw above the stagnant pacific slab in the mtz and perhaps deep slab dehydration as well the subduction triggered magmatism in the bmw may be a new class of mantle plumes we also used the new global model to investigate the influence of whole mantle heterogeneity on the determination of upper mantle tomography under japan with a teleseismic inversion method the results show that the mantle heterogeneities outside the target volume of regional tomography can cause significant changes 0 2 0 4 s to the observed relative travel time residuals of a distant earthquake the pattern of regional tomography remains the same even after correcting for the whole mantle heterogeneity but there are some changes in the amplitude of velocity anomalies in the regional tomography hence it is necessary to correct for the mantle heterogeneities outside the target volume so as to obtain a better regional tomography

deep sea fluid and sediment dynamics influence of hill to seamount scale seafloor topography abstract deep sea sediments play a central role in a wide range of subject areas a number of important controls on the formation of sedimentary deposits have been studied however to date the impact of submarine landscape geometry as a possible control has received comparatively little attention this seems to be particularly true for intermediate scale topographic features such as abyssal hills knolls and seamounts that can be found in many regions of the global seafloor recent estimates suggest that in the deep open oceans away from continental margins there might be as many as 25 106 abyssal hills knolls and seamounts despite this large number very little is known about how they influence environmental complexity and patchiness biogeochemical fluxes and the formation of sedimentary records this paper reviews the currently known types of fluid flow interactions with abyssal hills knolls and seamounts that could potentially influence the way sediments are formed the main types of relevant flow components are quasi steady to eddying background flow internal lee and near inertial waves barotropic and baroclinic tides and seamount trapped waves previous studies looking into systematic links between fluid dynamics and sediments at hills knolls and seamounts are reviewed finally a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid flow components leaves a detectable imprint in the recent sediments on and around a short seamount the main conclusions and implications are as follows 1 topographically generated flow field geometries that are composed of a number of different prevailing fluid flow components can be reflected and detected in properties of the underlying sediments 2 tidal and other higher frequency lee wave near inertial components of deep ocean currents can be essential for locally driving total current velocities across threshold values for non deposition erosion resuspension of freshly deposited deep sea sediments moreover there is evidence suggesting that not only maximum current speeds but also intensities of higher frequency tidal and or near inertial current direction variability might control sediment dynamics and sediment formation this relativises the view that current speed is the main or even only controlling factor for sediment dynamics and sediment formation 3 when it comes to the reconstruction of paleo flows these findings imply that certain sedimentary records may well reveal more about variability in the higher frequency flow components than about variability in the basin scale net flow component that often is the focus of paleoceanographic studies 4 single core paleo records from hill seamount or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features to arrive at unbiased paleo records for non fluid dynamic parameters the influence of the flow field geometry would have to be removed from the record first 5 it seems the mechanistic understanding of hill and seamount related flow topography interactions and their links to sediment dynamics is approaching a level that may a facilitate improved interpretation of topographically controlled sedimentary paleo records b help fill in the knowledge gap that exists for functional deep sea biodiversity at intermediate space scales and c improve predictive capabilities for exploration of economically relevant iron manganese fe mn crusts on seamounts

a complex 660 km discontinuity beneath northeast china abstract we present a detailed seismic study of the 660 km discontinuity beneath northeast china using the receiver function technique we use seismic data collected from 24 broadband stations in northeast china analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude 128 0 e to 130 5 e and latitude 40 0 n to 44 0 n and then it splits into multiple discontinuities in the surrounding regions the complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab we speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area

pn wave geometrical spreading and attenuation in northeast china and the korean peninsula constrained by observations from north korean nuclear explosions we investigate the geometric spreading and attenuation of seismic pn waves in northeast china and the korean peninsula a high quality broadband pn wave data set generated by north korean nuclear tests is used to constrain the parameters of a frequency dependent log quadratic geometric spreading function and a power law pn q model the geometric spreading function and apparent pn wave q are obtained for northeast china and the korean peninsula between 2 0 and 10 0 hz using the two station amplitude ratios of the pn spectra and correcting them with the known spreading function we remove the contributions of the source and crust from the apparent pn q and retrieve the p wave attenuation information along the pure upper mantle path we then use both pn amplitudes and amplitude ratios in a tomographic approach to obtain the upper mantle p wave attenuation in the studied area the pn wave spectra observed in china are compared with those recorded in japan and the result reveals that the high frequency pn signal across the oceanic path attenuated faster compared with those through the continental path

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

a deep sea benthic community exposed to strong near bottom currents on the scotian rise western atlantic abstract this paper gives an overview of the structure of a benthic community at 4626 m depth on the nova scotian continental rise here abundances of polychaetes bivalves isopods and tanaids are conspicuously high compared to those reported from comparable depths bacterial numbers and atp concentrations are also high we suspect that these anomalous abundances result from enhanced food availability caused by the strong near bottom currents that flow through the area the polychaete and bivalve faunas have few adults and high species dominance suggesting that currents create large scale sediment transporting disturbances the composition of the crustacean fauna supports this inference in that it is dominated by forms that can enter the seabed and thereby escape adverse conditions on the sediment surface

sedimentation under deep sea storms fast currents high concentrations of suspended sediment and grooved mud beds are associated with erosion in frequent abyssal storms where a fast deep mean flow is augmented or reversed by intense intermittent currents this occurs about 5 km below the gulf stream or its rings the waning phase of a storm results in development of bedforms and rapid deposition of a mud blanket several other regions of the world ocean display evidence of abyssal storm activity

detailed moho variations under northeast china inferred from receiver function analyses and their tectonic implications abstract in this study we investigate detailed moho variations beneath northeast china by applying the arithmetic mean back projected and fresnel zone migration imaging methods to a total of 169 602 high quality p wave receiver functions from seismograms of 2903 teleseismic events recorded at 127 necessarray stations and 321 china earthquake administration stations our results show that the moho depth variations are correlated with the surface geology in the study region the moho is deeper 34 0 42 0 km under the great xing an range the lesser xing an range the zhangguangcai range and the changbaishan mountain whereas it is shallower 26 0 32 0 km under the songliao basin our results also reveal obvious moho variations across the north south gravity lineament the moho offsets up to 5 0 km are clearly observed under the nenjiang balihan yilan yitong and dunhua mishan faults indicating that they are lithospheric scale faults a deeper moho is revealed under the volcanoes such as the jingpohu wudalianchi and changbaishan volcanoes and abaga and halaha volcanic groups in particular the moho under the changbaishan volcano reaches 40 0 km depth and the observation varies with the teleseismic back azimuths suggesting a complicated magma system in the crust in addition the moho under the songliao basin varies significantly from 26 0 km depth in the east to 32 0 km depth in the west which could be related to the lithosphere extension and thinning all these results suggest that there exists a hot and wet mantle upwelling in the big mantle wedge formed by the deep dehydration of the long stagnant pacific slab in the mantle transition zone under northeast china

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

pan african post collisional ferro potassic granite and quartz monzonite plutons of eastern nigeria abstract three pan african hypersthene bearing monzogranitic and quartz monzonitic plutons from the eastern terrane of nigeria have been investigated in detail new major trace and ree data used to constrain their origin and nature indicate that they display chemical features of ferro potassic trans alkaline affinity further trace element discrimination suggests i production of calc alkaline medium k diorite magmas by partial melting of fluid metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust ii simultaneously production of the granite quartz monzonite ferro potassic magmas from partial melting of hornblende bearing granodioritic crustal sources iii mixing of the two magmas sr initial ratios of 0 707 to 0 711 witness that the source of the granite magmas is the lower crust ages of the lower crustal granulitic protoliths is bracketed by nd model ages between 1 9 and 2 2 ga pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 ma 40 ar 39 ar ages of amphiboles at about 560 ma suggest cooling rates around 15 c ma extensive field work has established that pluton emplacement occurred during a regional north south dextral strike slip tectonics following the 630 610 ma stage of oblique continent continent collision in this part of west africa

turbulent suspension of sediments in the deep sea the high energy benthic boundary layer experiment demonstrated the existence of high energy events capable of suspending large amounts of sediment at the base of the nova scotian rise the currents that cause these storms are episodic pulses of 25 35 cm s 1 flows lasting four to seven days the build up and decay of the currents is too rapid for local equilibrium of the suspended sediment distribution to be achieved therefore a fully time dependent model of the turbulent boundary layer and the suspended sediments was developed to describe the events in detail the period of high flow is erosive for only a few hours the surface erodible bed sediments are quickly removed the dominant processes resulting in the development of the suspended sediment profile are then restricted to turbulent diffusion and entrainment the depth of penetration of the suspended sediments into the water column is limited by stratification induced by suspended sediments after the shear generated turbulence collapses most of the eroded sediment remained in suspension far above the expected equilibrium height for a non storm turbulent boundary layer scaling arguments and the model show that fine clay particles kept in suspension by turbulent diffusion dominate settling during the low level turbulence present during non storm conditions level 2 and 2 textstyle frac 1 2 energy closure models with stratification predict quite different structures of the nepheloid layer

mantle transition zone structure beneath the changbai volcano insight into deep slab dehydration and hot upwelling near the 410 km discontinuity we study the detailed mantle transition zone structure beneath the active changbai intraplate volcano in northeast china by using a receiver function method a total of 3005 teleseismic receiver functions recorded by 70 broadband stations are obtained by using a common conversion point stacking method for conducting the time to depth conversion we use a three dimensional velocity model of the study region so as to take into account the influence of structural heterogeneities our results reveal significant depth variations of the 410 520 and 660 km discontinuities a broad depression of the 410 km discontinuity and a low velocity anomaly are revealed beneath the changbai volcano which may reflect a large scale hot mantle upwelling around the 410 km discontinuity with a positive clapeyron slope the 520 km discontinuity is identified clearly and its uplift occurs above the stagnant pacific slab we also find a prominent depression of the 660 km discontinuity which is elongated along the trend of deep earthquake clusters in a range of 39 n 44 n latitude and the depression area has a lateral extent of about 400 km because the 520 and 660 km discontinuities correspond to positive and negative clapeyron slopes respectively we think that the 520 uplift and the 660 depression are caused by the cold subducting pacific slab a part of the pacific slab may have penetrated into the lower mantle and so caused the large scale 660 depression in front of the deep earthquake clusters our results also reveal a part of the upper boundary of the subducting pacific slab in the mantle transition zone

diffusion transport model for pelagic sediments on the mid atlantic ridge the diffusion model is potentially useful for quantifying the effect of downslope gravity transport on sedimentation rate variations which are commonly found between deep sea drilling project and ocean drilling program sites if appropriate the model and stratigraphy could be used for example to constrain the amount and timing of fault block rotation deep tow profiler records from the french american mid ocean undersea study area of the mid atlantic ridge are examined to determine whether they are consistent with a diffusion model sedimentary contacts with basement show that a variety of processes may be operating some of which are inconsistent with the diffusion model for example there are moats around some contacts which are typical of scouring or nondeposition due to high current velocities more than half of the contacts however show onlapping which is qualitatively consistent with the diffusion model sediments generally fill low areas of the topography and have smooth surfaces as expected from a diffusion model where the fluxes of sediment entering a basin are constant the diffusion model predicts that the sediment surface should evolve to a parabola the steady state solution some 20 curved surfaces in the profiler data were digitized and least squares parabolas fitted to them with rms errors of less than 1 m the slopes of the model parabolas provide values for the ratio of sediment flux to diffusivity at the edges of the basins steep surfaces are produced by low diffusivity or high fluxes this ratio is combined with estimates of the fluxes to determine the apparent sediment diffusivity kapp for eacri basin flux is estimated by assuming the abyssal hill topography acts as a simple sediment trap so that the lateral flux equals the width of a basin s pelagic catchment area times the area s mean sedimentation rate s using this method median kapp is 0 04 0 11 m2 yr 1 assuming s 10 30 m m y 1 variations in kapp and the assumptions behind the diffusion model are discussed and alternative causes of sediment surface tilts are considered in particular differential compaction which is modelled using a simple porosity depth function

the transition from magmatic to high temperature solid state deformation implications from the mount stuart batholith washington criteria for syntectonic emplacement of plutons are commonly ambiguous the strongest single criterion is probably the preservation of a continuous transition from submagmatic to high temperature solidstate deformation this transition is not commonly documented an exception is the mount stuart batholith which displays a variety of syntectonic structures within its sheared northeastern margin including the following 1 s c fabrics that range from submagmatic to high temperature solid state 2 solid state foliation and lineation defined by amphibolite facies assemblages that overprint and are parallel to magmatic foliation and lineation 3 folds of magmatic and solid state foliation that have hinge lines parallel to magmatic and solid state lineation and to equivalent structures in wall rocks 4 dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite 5 pegmatite dikes and mineralized joints that are sub perpendicular to magmatic and solid state lineation and 6 submagmatic and high temperature solidstate ductile shear zones we suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation syntectonic features are most likely preserved in plutons cooled at slow to moderate rates or in plutons deformed at high strain rates during emplacement the optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid crust such as the mount stuart batholith and in intrusions at deeper levels that were rapidly exhumed and or intruded during the waning stages of regional deformation

u pb dating of precambrian rocks from northern cameroon orogenic evolution and chronology of the pan african belt of central africa abstract northern cameroon has been classically divided into an old basement complex a middle proterozoic cover series intermediaires and granites emplaced during the pan african orogeny new u pb geochronological data on different rock units from this region are presented a rhyolite from the poli schists series intermediaires gives an age close to 800 ma migmatitic gneisses of the basement complex seem to derive from different detrital or magmatic sources the metadiorites and foliated granites of the basement complex give ages around 630 ma the 800 ma age is interpreted as dating volcanic activity probably related to the initial stage of the pan african evolution ages around 630 ma date a syn tectonic plutonism dominated by a regional scale basic to intermediate plutonic suite bip zircons of the bip suite are almost concordant and show no evidence of inheritance the suite shows calc alkaline affinity and relatively low 87 sr 86 sr initial ratios thus the bip suite represents material newly added to the crust during the panafrican orogeny these data are at variance with previous interpretations of this part of the mobile belt as old precambrian crust reactivated during the pan african orogeny the previous distinction between the series intermediaires and a regional basement complex must be reappraised similar ages being found in both units a chronological succession of the pan african evolution is established and tentatively compared with that of the trans saharan belt in nigeria and hoggar

a study of the deep electrical structure of the northern segment of the tan lu fault zone ne china abstract we studied a magnetotelluric transect from tongliao inner mongolia to baishan jilin that crosses the northern segment of the tan lu fault zone based on the profiling data we calculated and analyzed the two dimensional 2d deviation degree and the tectonic strike inverted the data using the 2d nonlinear conjugate gradient inversion method and obtained an electrical structure model of the crust and upper mantle along the transect our analysis reveals that the western portion of the transect is within and at the margin of a typical sedimentary basin and a high conductivity layer is present in the lower crust in comparison the eastern portion of the transect is in the jihei fold zone and has a relatively stable electrical structure but its resistivity is one to two orders of magnitude higher than that of the western portion a distinct electrical anomaly exists between the western and eastern sides the yilan yitong fault of the northern segment of the tan lu fault zone dips to the northwest has strike slip characteristics extends to a depth of 40 50 km and induces upwelling of upper mantle materials the dunhua mishan fault of the northern segment of the tan lu fault zone has a clear electrical gradient and similar to the yilan yitong fault it also dips to the northwest however the dunhua mishan fault is mostly fractured extends to a depth of approximately 20 km and is slightly smaller than the yilan yitong fault the inferred distribution of deep resistivity values indicates that the lithosphere is generally thinner at the center of the transect and thicker at the edges the lithosphere is approximately 150 km thick on the western end thins eastward to approximately 60 km thick near the tan lu fault zone and then gradually thickens eastward to 100 km thick

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

characterization of deep sea storms abstract to describe and quantify the physical forcing of sediment transport events at the hebble site velocity stress and sediment concentration data were obtained during a year long deployment through the year a variety of intense sediment transporting events were recorded careful analysis of these events reveals that only a small proportion of the peaks in suspended sediment concentration correspond to local erosion most sediment events were clouds of fine suspended material advected into the region analysis of progressive vector diagrams reveal that these advected clouds were bounded by regions with concentration gradients as high as 200 g 1 1 km 1 the high gradients suggest streams of sediment bearing water with sharp fronts of sediment concentration along their sidewalls were veering across the region the largest concentrations of suspended sediment were observed during local suspension events the growth and decay of the sediment clouds during the local events did not exactly track the boundary shear stress rapid erosion occurred as the stress exceeded a critical shear stress maximum suspended concentrations occurred shortly after erosion began indicating a source limited erosion process the depositional period lasted for several weeks whereas the erosion occurred in only a few days photographs of the bottom reveal depositional features crag and tail dunes which are aligned with the post storm flow not the erosional flow direction

thermal expansion of wadsleyite ringwoodite hydrous wadsleyite and hydrous ringwoodite abstract the thermal expansions of wadsleyite ringwoodite hydrous wadsleyite and hydrous ringwoodite have been determined by high temperature powder x ray diffraction at ambient pressure wadsleyite and ringwoodite persist up to 700 c and their thermal expansion coefficients are measured as 34 0 5 10 6 and 30 7 6 10 6 k 1 respectively values that are slightly larger than those measured by j phys earth 27 1979 53 j phys earth 28 1980 273 hydrous wadsleyite and hydrous ringwoodite were dehydrated at 450 and 400 c respectively below their dehydration temperatures their thermal expansion coefficients are 30 1 14 10 6 and 27 3 9 10 6 k 1 respectively which are slightly lower than those of their anhydrous forms using the present results combined with the previous thermoelastic data the seismic velocity contrast at the 410 km discontinuity has been calculated for both dry and wet mantle if a pyrolite composition is assumed a hydrous mantle transition zone better explains seismic constraints

suspended sediment storm modeling abstract a variety of sediment transporting storms was observed in the hebble long deployment indicating several distinct forcing mechanisms in particular some storms appear to originate locally gross and williams this issue in this paper a simple model of a locally forced storm is presented to elucidate the time dependent dynamics of erosion and deposition events in the deep sea based on these modeling efforts we make the following inferences storms exhibit peak sediment concentration before peak velocity this behavior is due to a limited amount of erodible bed sediment after near instantaneous erosion of the bed is complete fine sediment concentration in the water column drops due to dilution throughout a thickening turbulent mixed layer in the accelerating flow during and immediately after a storm event turbulence intensity above the 40 m thick momentum mixed layer can be sufficient to suspend fine particles in low concentration to elevations of 100 m above bottom or more following the storm the rate of sediment clearing depends on gravitational settling and the potential importance of additional turbulence driven deposition and aggregation mechanisms syn and post storm overthickening of the suspended sediment mixed layer relative to the ekman momentum mixed layer thickness as well as sensitivity of the post storm concentration profile to turbulence driven deposition can explain some complex features of near bed nepheloid layers in the hebble region observed by other workers validation of suspended sediment transport models is based on field observations obtained with light transmissometers transmissometer measurements of suspended sediment concentration depend in turn on calibration of instrument response to both mass concentration and particle size distribution by analyzing the temporal and spatial changes in mass concentrations with the model presented herein we calculate an operational calibration function during hebble storms this calibration function does not change by more than 30 due to particle sorting by turbulent suspension

seismic evidence of on going sublithosphere upper mantle convection for intra plate volcanism in northeast china abstract a 3 d crustal and upper mantle s wave velocity model of ne china is constructed by inversion of phase velocity dispersion curves at 6 140 s periods from ambient noise tomography and two plane surface wave tomography the seismic data used in this study are collected from 120 china earthquake administration cea permanent stations and 127 portable stations of necessarray we observe strong low s wave velocity beneath the changbaishan volcano in the upper mantle to at least 200 km depth which is interpreted as a mantle upwelling beneath the changbaishan volcano that is consistent with the body wave tomographic image the songliao basin is dominated by a high velocity extending to at least 200 km depth built upon the observed velocity anomalies we propose a sub lithosphere mantle convection model for ne china in which the upwelling of upper mantle materials from the mantle transition zone to the changbaishan volcano could induce a local sub lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the songliao basin corresponds to the downwelling limb of this convection cell the downwelling beneath the songliao basin could also induce secondary local convection in the asthenosphere to the west leading to local asthenospheric upwelling beneath the abaga and halaha volcanoes in the xing an mongolia orogenic belt

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

u pb zircon and microfabric data of meta granitoids of western cameroon constraints on the timing of pluton emplacement and deformation in the pan african belt of central africa abstract striking characteristics of the western neoproterozoic belt of cameroon nfbc are the large volume of granitoids and crustal scale shear zones new structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the atlantic ocean three different complexes have been identified in the study area the migmatitic complex of foumbot mcf the metagranitoid complex of bangwa bc and the batie pluton bp the mcf was intruded by the bc while the bp cuts through the bc u pb zircon dating of metaleucogranite and metagranodiorite of the bc yielded concordant to subconcordant ages of 638 2 ma and 637 5 ma respectively a concordant u pb zircon age of 602 1 4 ma has been obtained from porphyrogranite of the bp these ages are interpreted as emplacement ages continuous deformation from magmatic to solid state flow along the bp margins and the sub parallelism of the steep solid state foliation in the bp margins with the foliation in the surrounding bc and mfc suggest synkinematic emplacement of the bp along crustal scale nne to ene trending strike slip shear zones subhorizontal foliations in migmatitic gneiss xenoliths found in the bc suggest that the major transcurrent motion was preceded by thrusting the new data confirm previous assumptions that the western nfbc is equivalent to parts of the borborema province of brazil there are geochronological correlations between the studied meta granitoids and brasiliano pre to syn transcurrent granitoids of the borborema province

the 3 4 3 5 ga sao jose do campestre massif ne brazil remnants of the oldest crust in south america abstract the oldest fragment of continental crust recognized in south america occurs as an isolated archean enclave in northeastern brazil s borborema province ca 600 ma brasiliano pan african orogenic belt this archean fragment the sao jose do campestre massif is surrounded by large tracts of 2 2 2 0 ga paleoproterozoic gneisses and is located more than 600 1500 km from the much larger assemblages of archean rocks found in the sao fransciso and amazonian cratons located to the south and west respectively geochronological studies of the sao jose do campestre massif show that its oldest rocks contain zircons with u pb ages up to 3 5 ga and sm nd tdm model ages of more than 3 7 ga indicating that they represent reworked crust this older nucleus is flanked by both reworked and juvenile 3 25 and 3 18 ga rocks which are intruded by both 3 00 and 2 69 ga plutonic bodies the protracted evolution the sao jose do campestre massif is consistent with that of a larger continental mass as opposed to a small crustal fragment that grew in isolation as such the sao jose do campestre massif is interpreted as representing a detached piece of an evolved craton that became entrained with younger rocks during a subsequent paleoproterozoic accretionary orogenic event this hypothesis is bolstered by the presence of paleoproterozoic gneisses that envelop the sao jose do campestre massif as well as the existence of ca 2 0 ga metamorphic zircon and monazite within its rocks the occurrence of several different archean cratonic basement inliers within the greater paleoproterozoic crustal framework of the borborema province suggests that cratonic slices spalled off one or more larger archean masses prior to the ca 2 2 2 0 ga paleoproterozoic orogenic collage a important challenge is to link these older fragments to their parent cratons although results are not unique the pattern of ages and isotopic signatures observed in the sao jose do campestre massif is similar to that seen in parts of the sao francisco craton and it is possible that the sao jose do campestre massif is a fragment of an archean continental fragment formed during an episode of continental breakup prior to 2200 ma

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

water transport into the deep mantle and formation of a hydrous transition zone abstract the phase relations in the peridotite water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone results of preliminary experiments to clarify the effect of water on kinetics of the olivine wadsleyite transformation are given and show that water enhances the transformation kinetics significantly there are potential dehydration sites in the deep mantle four sites along the descending slabs depending on their temperature profiles i e the mantle wedge of the subduction zone mantle transition zone top of the lower mantle where hydrous ringwoodite and superhydrous phase b decompose and deep in the lower mantle around 1200 1500 km depth where decomposition of phase d g occurs we also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated the transition zone can serve as a water absorber for the mantle scale water circulation system some of the recent seismic works including the p wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone elevation of the 410 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions

seismic structure and origin of active intraplate volcanoes in northeast asia abstract three dimensional p wave velocity structure beneath the changbai and other intraplate volcanic areas in northeast asia is determined by inverting 1378 high quality p wave arrival times from 186 teleseismic events recorded by 61 broadband seismic stations low velocity low v anomalies are revealed beneath the changbai longgan xianjindao volcanoes high velocity high v anomalies are found in the mantle transition zone where deep focus earthquakes under hunchun occur at depths of 500 600 km the high v anomaly reflects the deep subduction of the pacific slab under ne asia which may have contributed to the formation of the changbai longgang xianjindao and jingpohu intraplate volcanoes a low v anomaly is also revealed in the mantle transition zone which may have a close relationship with the occurrence of deep earthquakes under the hunchun area our results support the big mantle wedge bmw model by zhao et al zhao d lei j tang y 2004 origin of the changbai volcano in northeast china evidence from seismic tomography chin sci bull 49 1401 1408 zhao d maruyama s omori s 2007 mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics gondwana res 11 120 131 who proposed that the intraplate volcanoes in ne asia are caused by the back arc magmatism associated with the deep dehydration process of the subducting slab and convective circulation process in the bmw above the stagnant pacific slab

east asia seismotectonics magmatism and mantle dynamics abstract in this article we review the significant recent results of geophysical studies and discuss their implications on seismotectonics magmatism and mantle dynamics in east asia high resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes which may reflect magma and fluids that affected the rupture nucleation of large earthquakes in subduction zone regions the crustal fluids originate from the dehydration of the subducting slab magmatism in arc and back arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab the intraplate magmatism has different origins the continental volcanoes in northeast asia such as changbai and wudalianchi seem to be caused by the corner flow in the big mantle wedge bmw above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well the tengchong volcano in southwest china is possibly caused by a similar process in bmw above the subducting burma microplate or indian plate the hainan volcano in southernmost china seems to be a hotspot fed by a lower mantle plume associated with the pacific and philippine sea slabs deep subduction in the east and the indian slab s deep subduction in the west down to the lower mantle the occurrence of deep earthquakes under the japan sea and the east asia margin may be related to a metastable olivine wedge in the subducting pacific slab the stagnant slab finally collapses down to the bottom of the mantle which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab plume interactions some of these issues such as the origin of intraplate magmatism are still controversial and so further detailed studies are needed from now

seismic imaging of the amur okhotsk plate boundary zone in the japan sea abstract we determined high resolution images of p and s wave velocity and poisson s ratio under the japan sea off honshu island using a large number of arrival time data from local earthquakes to understand the interaction of the amur and okhotsk plates our new data collected from the crustal earthquakes under the japan sea relocated precisely with sp depth phase are crucial to make this work possible our results show that strong lateral heterogeneities exist in the crust and upper mantle under the amur okhotsk plate boundary zone which may have affected the seismotectonics in the region the crustal velocity variations under the japan sea may reflect the complicated geologic structures that were produced during the opening of the japan sea and the present compressional stage of the honshu arc associated with the collision of the amur plate with the okhotsk plate low velocity zones in the mantle wedge are found to extend westward under the japan sea rather than just confined under honshu island suggesting that the back arc magmatism and tectonics are part of the complex geodynamic system in east asia

crustal structure in the changbaishan volcanic area china determined by modeling receiver functions abstract during 1998 1999 we installed a temporary broadband seismic network in the changbaishan volcanic region ne china we estimated crustal structure using teleseismic seismograms collected at the network we detected a near surface region of strong anisotropy directly under the main volcanic edifice of the volcanic area we modeled 109 receiver functions from 19 broadband stations using three techniques first we used a slant stacking method to model the principal crustal p reverberation phases to estimate crustal thickness and the average crustal p to s speed ratio vp vs assuming an average p wave velocity in the crust we then estimated crustal s wave velocity vs and vp vs profiles by modeling stacked receiver functions using a direct search finally we inverted several receiver functions recorded at stations closest to the main volcanic edifice using least squares to estimate vs velocity profiles assuming a vp vs value the results from the three estimation techniques were consistent and generally we found that the receiver functions constrained estimates of changes in wave speeds better than absolute values we resolved that the crust is 30 39 km thick under the volcanic region and 28 32 km thick away from the volcanic region with a midcrust velocity transition at about 10 15 km depth we estimated that the average crust p wave velocity is about 6 0 6 2 km s surrounding the main volcanic region while it is slightly lower in the vicinity of the main volcanic edifice the estimates of vp vs were more ambiguous but we inferred that the bulk crustal poisson s ratio which is related to vp vs ranges between 0 20 and 0 30 with a suggestion that the poisson s ratio is lower under the central volcanic region compared to the surrounding areas we resolved low s wave velocities down to about 3 km s in the middle crust in the region of the main volcanic edifice the low velocity anomaly extends from about 5 10 to 15 25 km below the surface probably indicating a region of elevated temperatures we were unable to determine if partial melt is present with the data we considered in this paper

characteristics structural styles and tectonic implications of mesozoic cenozoic faults in the eastern heilongjiang basins ne china abstract the eastern heilongjiang basins ehbs are the assemblage of a series of meso cenozoic residual basins located in the northeastern corner of china the deformation pattern of the ehbs has significant implications for the history of the pacific plate subduction beneath the eurasia since the late mesozoic in this paper research on the characteristics and structural styles of the meso cenzoic faults in the ehbs has been conducted on the basis of a comprehensive analysis of field geology drilling data and seismic reflection profiles as a result five different stages of the meso cenozoic faults in the ehbs have been recognized these are in accordance with the time and relevant characteristics of fault movements i e the early stage of the early cretaceous normal fault the early stage of the late cretaceous thrust fault the late stage of the late cretaceous thrust fault the cenozoic synsedimentary normal fault and the late stage cenozoic shear fault a regional geological section has been generated across the ehbs by linking four local seismic profiles together a step by step reconstruction has been made to help better understand the mesozoic cenozoic tectonic evolution of the ehbs two phases of extension rifting in the early cretaceous period and the paleogene respectively are demonstrated to be interfered with two phases of regional uplift compression and erosion in the late cretaceous period the complicated development of multiple fault systems within the ehbs has reflected the evolution of a complex tectonic subduction of the pacific plate beneath the eurasia since the cretaceous period

charnockitic and monzonitic pan african series from north central nigeria trace element and nd sr pb isotope constraints on their petrogenesis abstract major and trace element studies indicate that the evolution of the calc alkaline charnockitic and sub alkaline monzonitic pan african rocks in north central nigeria cannot be explained in terms of a unique or classical process of fractional crystallisation at toro the crystallisation history of the more differentiated members appears more complex with the participation of accessory phases the quartz fayalite monzonites show evidence of a cumulative phase during their differentiation the fayalite bearing quartzmonzonites are not common rocks and their occurrence in the nigerian basement remains the only reported exposure in the pan african belt of west africa like the charnockitic suite at toro they were emplaced during the pan african and at deep structural levels unlike the adjacent high level jurassic peralkaline granites of jos plateau the application of multiple radiogenic tracers sr nd and pb indicates the involvement of pre existing crust in the formation of these rocks sr i 30 to 86 nd i 15 5 to 5 5 206 pb 204 pb i 16 34 17 71 and 207 pb 204 pb i 15 48 15 58 for toro charnockitic complex and sr i 19 to 51 and nd i 8 0 to 4 0 for the quartz fayalite monzonites of bauchi long crustal residence ages tnd of between 2 0 and 1 5 ga preclude their complete derivation from the mantle the bulk of evidence is consistent with the partial melting of a u poor source typical of the lower crust during the pan african orogeny and in particular the involvement of older precambrian crust

quartz fabric transition in a cooling syntectonic granite hermitage massif france abstract the hermitage granite situated in the northwest massif central france is a syntectonic hercynian leucogranite emplaced along an active transcurrent shear zone during emplacement and cooling the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands c s mylonites increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline a slip and rotation recrystallization migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction we discuss relevant deformation mechanisms and rheological implications for syntectonic plutons in particular we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid state flow

seismic anisotropy and mantle dynamics beneath china abstract we analyzed the shear wave splitting at 138 permanent seismograph stations to study seismic anisotropy and mantle dynamics under mainland china to obtain reliable results we used three different methods to measure the shear wave splitting parameters using core phases sks skks skiks and pks as well as the direct s waves from regional and distant earthquakes our results show that the fast orientations of the anisotropy wnw ese in eastern china are generally consistent with the absolute plate motion apm direction of the eurasian plate suggesting that the anisotropy is mainly located in the asthenosphere resulting from the lattice preferred orientation of olivine due to the shear deformation there the fast axes in western china generally agree with the strikes of the orogens and active faults while they are perpendicular to the direction of the maximum horizontal stress suggesting that the anisotropy in the lithosphere contributes significantly to the observed shear wave splitting the fast axes in western china are also consistent with the apm direction suggesting that the apm driven anisotropy in the asthenosphere is another source of the shear wave splitting there these results suggest that apm driven anisotropy commonly exists under continents similar to that under oceanic regions even though the continental lithosphere has suffered extensive deformation

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

grain size reduction and foliation development in a deformed granitoid batholith abstract the predominantly peraluminous wologorong batholith in eastern new south wales australia has undergone heterogeneous deformation it always has at least one gneissic foliation but in strongly deformed zones it has two foliations one of these is a gneissic foliation s 1 consisting of relatively discontinuous fine grained micaceous and feldspathic folia anastomosing between feldspar relics and elongate aggregates of recrystallized quartz the other foliation s 2 is more continuous than s 1 and the folia are fine grained and commonly laminated as in mylonites the two foliations could have formed siimultaneously or sequentially during a single deformation s 2 formed oblique up to 30 to s 1 in order to accommodate an imposed shear component of the strain in strongly deformed parts of the batholith s 2 folia appear to have initiated on suitably oriented parts of anastomosing s 1 folia continued deformation locally reduced the angle between s 1 and s 2 and resulted in the development of mylonite zones grain size reduction by recrystallization and or neocrystallization occurred in all minerals during the deformation but quartz appears to have attained a steady state grain size of around 0 15 mm which was not reduced during more intense deformation deformation mechanisms are difficult to determine but microfracturing associated with offsetting and microboudinage intragranular slip and solution transfer of material may have operated in the earlier stages of deformation and grain boundary sliding and or solution transfer of material may have dominated the later stages the material transfer may have been associated with chemical and mineralogical changes that accompanied neocrystallization

is there a big mantle wedge under eastern tibet abstract the big mantle wedge bmw model was originally proposed to explain the origin of the intraplate changbaishan volcano in ne china where the subducting pacific slab becomes stagnant in the lower part of the mantle transition zone mtz the bmw is defined as a broad region in the upper mantle and upper part of the mtz overlying the long stagnant pacific slab under eastern tibet prominent low velocity low v anomalies are revealed in the upper mantle whereas an obvious and broad high velocity high v anomaly is imaged in the mtz from the burma arc northward to the kunlunshan fault zone and eastward to the xiaojiang fault zone in addition receiver function analyses clearly illustrate a thickened mtz in a similar area hence we conclude that there is a bmw structure under eastern tibet which can explain the deep origin of the tengchong volcano and generation of large crustal earthquakes in the region the tengchong volcano is caused by hot and wet mantle upwelling in the bmw and fluids from dehydration reactions of the stagnant indian slab in the mtz the 2008 wenchuan earthquake ms 8 0 took place in a transitional belt from the low v songpan ganzi block to the high v sichuan basin suggesting that the occurrence of the wenchuan earthquake was affected by the bmw structure these results shed new light on the mantle structure and dynamics under eastern tibet

mantle transition zone stagnant slab and intraplate volcanism in northeast asia 3 d p and s wave velocity structures of the mantle down to a depth of 800 km beneath ne asia are investigated using similar to 981 000 high quality arrival time data of local earthquakes and teleseismic events recorded at 2388 stations of permanent and portable seismic networks deployed in ne china japan and south korea our results do not support the existence of a gap or a hole in the stagnant slab under the changbai volcano which was proposed by a previous study of teleseismic tomography in this work we conducted joint inversions of both local earthquake arrival times and teleseismic relative traveltime residuals leading to a robust tomography of the upper mantle and the mantle transition zone mtz beneath ne asia our joint inversion results reveal clearly the subducting pacific slab beneath the japan islands and the japan sea as well as the stagnant slab in the mtz beneath the korean peninsula and ne china a big mantle wedge bmw has formed in the upper mantle and the upper part of the mtz above the stagnant slab localized low velocity anomalies are revealed clearly in the crust and the bmw directly beneath the active changbai and ulleung volcanoes indicating that the intraplate volcanism is caused by hot and wet upwelling in the bmw associated with corner flows in the bmw and deep slab dehydration as well

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

shape and microstructure of microgranitoid enclaves indicators of magma mingling and flow abstract at tarandore point 328 km south of sydney new south wales australia two tonalite phases are in contact with a gabbroic diorite all belonging to the moruya batholith lobate to crenulate contacts pillowing of the more mafic phase in the more felsic phase net veining of the more mafic phase by the more felsic local chilling of the more mafic phase against the more felsic and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement therefore all phases were emplaced before the others solidified and an order of intrusion cannot be determined the variety of microgranitoid enclaves suggests that repeated magma mingling has occurred linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the moruya suite of granitoids direct evidence of mixing is not present but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible restite unmixing is unlikely to be the prime process because near minimum melts which would occur in all granitoid phases according to that model should mix easily rather than mingling in fact mingling has occurred between tonalite phases of very similar composition which would not be expected if the liquid parts of enclaves were minimum melts available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at tarandore point owing to inadequately constrained parameters but mixing generally should be favoured by higher temperatures which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves in which the distortion has been accomplished by magmatic flow without solid state deformation as evidenced by strong dimensional preferred orientations coupled with a lack of microstructural evidence of intragranular plastic strain this suggests that elongate enclaves should not be used as indicators of solid state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place

obliquely convergent tectonics and granite emplacement in the trans saharan belt of eastern nigeria a synthesis abstract the eastern nigeria terrane belongs to the 3000 km long trans saharan belt which was formed in the neoproterozoic between 750 and 500 ma by continental collision between the converging west african craton congo craton and east saharan block the study area consists mostly of gneisses and migmatites that underwent granulite facies metamorphism 800 c 800 mpa in contrast with western nigeria and cameroon no basement cover relationship has been identified which is in agreement with a monocyclic metamorphic history around 640 ma the area underwent a nappe tectonics with eastward displacement emplacement of bt ms granites and granulite facies peak later on around 615 ma nappes were towards the north numerous hbl bt granite plutons were emplaced around 585 ma during north south strike slip regional deformation the anisotropy of magnetic susceptibility fabric helps in defining strain localization along narrow north south to nne ssw vertical shear zones during the exhumation history exhumation of the terrane assessed using u pb ar ar and rb sr methods was presumably slow with uplift rates around 0 2 0 5 mm year preliminary estimates of contemporaneous horizontal movements suggest that they were one order of magnitude larger

brasiliano pan african granitic magmatism in the pajeu paraiba belt northeast brazil an isotopic and geochronological approach the neoproterozoic brasiliano pan african orogeny in the borborema province bp ne brazil is characterized by intense granitic magmatism spatially associated with continental scale shear zones and metamorphism under high temperature conditions u pb zircon and rb sr whole rock data for 14 granite intrusions from the paje u parabelt have ages that suggest more than 100 ma of intrusive magmatism which can be divided in four events the oldest granites between 620 and 600 ma are medium to slightly high k calc alkaline i type granitoids intruded into metagreywackes and gneiss granites of 0 9 1 0 ga they are related to the peak of metamorphism and to the development of a flat lying foliation the youngest intrusions 540 520 ma have geochemical signature of a type post orogenic extension related granites and are associated with sub volcanic bimodal magmatism probably contemporaneous with deposition of small sedimentary basins in the north iara jaibaras graben and sa ori and central fatima bet ania and carnatectonic domains of the borborema province they reflect post tectonic relaxation of the brasiliano orogeny between these episodes two other intrusive events were identified 1 high k calc alkaline granitoids and shoshonitic granitoids associated with mafic syenites meladiorites and hornblende biotite diorites intruded between 590 and 581 ma associated with a transcurrent deformation event and 2 alkaline post collisional granitoids having u pb zircon ages of ca 570 ma marking the final stage of the brasiliano orogeny in the south domain of the borborema province migmatization took place between 610 and 600 ma although similar ages were not found in the granitoids of the central tectonic domain field evidence suggest that migmatization followed the intrusions of the oldest 620 600 ma granitoids 2004 elsevier b v all rights reserved

the benthic boundary layer and sediment transport the major problems facing scientists studying marine boundary layer fluid mechanics in 1979 concerned the specific mechanismic operating in the regions of the top and the bottom of the boundary layer at the top the mechanisms of fluid escapement from the boundary layer and mixing into the ocean interior could only be postulated approximations of diffusivity across the top of the layer were being debated the values of which were more articles of faith than physically validated estimates cf armi 1979 garrett 1979 at the bottom the intermittent generation of stress cf gordon 1975 heathershaw and simpson 1979 was held responsible for the wide variability observed in the drag coefficient and the thickness of the viscous sub layer was claimed to be greater than that traditionally found from laboratory studies caldwell and chriss 1979

in situ measurements of particle settling velocity in the deep sea two instruments the remote optical settling tube and the benthic autonomous settling tube have been deployed on the nova scotian continental rise at a depth of 4820 m the few results obtained suggest that optical and gravimetric methods give comparable results assumptions about particle density little affect the settling velocity distribution but do scale the total magnitude by using a distribution of declining density with increasing size indicating increasing water content the total concentration inferred from the light attenuation by using mie theory is quite close to the total found from gravimetric data the coarser end of the size distribution changes more with overall change in concentration than the fine end the settling velocity distribution shows the presence of several modes the two principal ones can be matched with modal sizes of 3 3 m and 60 m in coulter counter data allowing inference of stokes density contrasts of 1 5 and 0 061 g cm 3 respectively at concentrations of about 200 mg m 3 more than 50 of the material has a settling velocity less than 2 10 4 cm s 1 probably 3 m in size but this percentage appears to decrease with an increase in total concentration

modeling the benthic boundary layer abstract a second order turbulence closure model is used to study the development of the benthic boundary layer results are presented on the effects of a time dependent oscillatory forcing flow and an initially stably stratified density gradient using typical values for the deep ocean the model suggests a development time for the layer of 10 days the results of the model show that for a neutrally stratified layer although the flow is oscillating the turbulence is essentially in local equilibrium and that an eddy viscosity approach is appropriate to determine the equilibrium boundary layer height the time development of the two models was however different for an initially stratified case although local shear production of turbulence is suppressed nest the top of the layer diffusive effects enable the boundary layer to continue growing past a height set by a critical value of the richardson number based on shear flow stability arguments attempts to relate the growth rate of the boundary layer t

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

metamorphic evolution of the high himalayan crystallines in se zanskar india the high himalayan crystallines hhc of se zanskar consist of biotite paragneisses of orthogneisses that derive from early palaeozoic granitoids of minor metabasics and of post metamorphic leucogranites of miocene age two main metamorphic events have been documented in the hhc the first event occurred at p 12 0 0 5 kbar and t 750 50 c in rare metabasics intruded by early palaeozoic granitoids in the biotite paragneisses thermobarometric estimates of the first event point to comparable t at p 4 5 kbar lower the first event is followed by a pervasive syn tectonic crystallization characterized by lower p and t on the basis of the cooling ages of the metamorphic minerals and on the geological evidence the second event is referred to the tertiary himalayan crystallization further petrological and geochronological studies are necessary to prove whether a few mineral relics ascribed to the first event define a polyphase himalayan evolution or if they record the incomplete obliteration of an older history during the himalayan event the hhc of se zanskar show a decrease in metamorphic grade from the middle structural levels upward close to the kade unit and downward close to the lesser himalaya from sillimanite k feldspar biotite bearing assemblages to kyanite staurolite muscovite bearing assemblages this metamorphic zonation is probably a consequence of the polyphase history of intracontinental thrusts and of the tectonic emplacement of hot crustal slabs within shallower and colder thrust sheets at relatively late stages of the continental collision between india and eurasia

receiver functions in northeast china implications for slab penetration into the lower mantle in northwest pacific subduction zone abstract seismic studies of the subducting lithosphere and the upper mantle discontinuities in the northwest pacific subduction zone beneath japan and northeast china have suggested contrary subduction scenarios there was little consensus on the issue whether the subducting slab penetrates the upper mantle discontinuities into the lower mantle or it is deflected atop of the 660 km discontinuity over several hundred kilometers we calculate receiver functions from a recent seismic broadband station network located in northeast china and find topographic variations of the upper mantle discontinuities a deeper than normal 660 km discontinuity is observed over an area of 400 km and it coincides with the stagnant slab imaged by seismic tomography the 660 km discontinuity is locally depressed by more than 35 km and the transition zone is thickened by more than 20 km in the east of the region where it encounters the slab these observations provide evidence of the slab accumulating in the mantle transition zone and locally penetrating into the lower mantle

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics abstract recent results of high resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of western pacific and east asia the most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation corner flow processes in the mantle wedge the combination of the two processes may have caused the back arc spreading in the lau basin affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan and contributed to the formation of the continental rift system and intraplate volcanism in northeast asia which are clearly visible in our tomographic images slow anomalies are also found in the mantle under the subducting pacific slab which may represent a small mantle plumes b upwellings associated with the slab collapsing down to the lower mantle or c sub slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite water we proposed a petrologic model for arc volcanism arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge a large amount of fluids can be released from hydrated morb at depths shallower than 55 km which move upwards to hydrate the wedge corner under the fore arc and never drag down to the deeper mantle along the slab surface slab dehydration reactions at 120 km depth are the antigorite related 5 reactions which supply water rich fluids for forming the volcanic front phase a and mg surssasite breakdown reactions at 200 and 300 km depths below 700 c cause the second and third arcs respectively moreover the dehydration reactions of super hydrous phase b phases d and e at 500 660 km depths cause the fluid transportation to the mantle boundary layer mbl 410 660 km depth the stagnant slabs extend from japan to beijing china for over 1000 km long indicating that the arc trench system covers the entire region from the japan trench to east asia we propose a big mantle wedge bmw model herein where hydrous plumes originating from 410 km depth cause a series of intra continental hot regions fluids derived from mbl accumulated by the double sided subduction zones rather than the india asia collision and the subsequent indentation into asia are the major cause for the active tectonics and mantle dynamics in this broad region

p wave tomography and origin of the changbai intraplate volcano in northeast asia abstract we present the first seismic image of the upper mantle beneath the active intraplate changbai volcano in northeast asia determined by teleseismic travel time tomography the data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations our results show a columnar low velocity anomaly extending to 400 km depth with a p wave velocity reduction of up to 3 high velocity anomalies are visible in the mantle transition zone and deep focus earthquakes occur at depths of 500 600 km under the region suggesting that the subducting pacific slab is stagnant in the transition zone as imaged clearly by global tomography these results suggest that the intraplate changbai volcano is not a hotspot like hawaii but a kind of back arc volcano related to the deep subduction and stagnancy of the pacific slab under northeast asia

theoretical modelling of deep ocean sediment transport abstract predicting the fate of suspended material requires knowledge of both sediment concentration and velocity fields these are governed by the advection and diffusion of mass and momentum respectively and are coupled by the turbulent diffusion process crucial to both the link between the two is the boundary shear stress which determines force exerted on the bottom by the flow and vice versa the response of sediment to fluid forcing is complicated by its cohesiveness both from physical and biological processes this is important not only in the bed but also in suspension where flocculation can occur during eroding events the friction velocity is much greater than the mean settling velocity so the sediment essentially behaves as a passive contaminant however in the low velocity periods the suspension is a collection of different settling velocity classes to be treated separately but coupled by the aggregation process the deposition process can be complex as is evidenced by the formation of longitudinal ripples these features may be formed due to helical circulation arising from non uniform turbulence field the cause of the variation in turbulence is postulated to be inhomogeneities in the concentration field that damps the turbulence through the effect of stratification

a new tectonic model for the cameroon line central africa abstract the cameroon line a major geological feature in central africa has been considered successively as a series of horsts and grabens a continental rift and a mega shear zone it is marked out by about 60 anorogenic complexes and a dozen volcanic centres all of which have alkaline affinity remote sensing allows us recognition of the main lineament trends n70 n s n135 and e w while autocorrelation analysis reveals a major fault zone striking n30 in western cameroon and n15 in the northern region a mega left lateral shear zone is the model that best accounts for the fracture pattern and associated features such as linear and circular structures alignment of subvolcanic complexes syntectonic leucogranites marking out older shear zones and vein dykes the n70 adamawa fault zone a pan african fracture reworked during albian aptian times is the only shear zone of continental scale that could have initiated en echelon mega tension gashes within the cameroon line during a cainozoic left lateral transcurrent movement

deep structure and origin of the baikal rift zone p wave velocity images are determined under the baikal rift zone in siberia by using teleseismic tomography our results show prominent low velocity anomalies in the upper mantle under the baikal rift zone and high velocity anomalies in the lithosphere under the siberian craton the low velocity anomalies are interpreted as a mantle upwelling plume which has played an important role in the initiation and evolution of the baikal rift zone the rift formation may be also controlled by other factors such as older prerift linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far field forces caused by the india asia collision