convective interactions between oceanic lithosphere and asthenosphere influence of a transform fault abstract current estimates of the asthenospheric viscosity favor the occurrence of small scale convection ssc beneath oceanic lithosphere of sufficient age since transform faults tfs juxtapose lithospheres of distinct ages and thicknesses they are likely to influence their dynamics in this paper we describe 3d numerical models of oceanic spreading for two ridge segments offset by a tf in domains that are sufficiently large for ssc to fully develop a framework based on thermal convection with temperature dependent viscosity and both internal and basal heating is adopted and parameters such as surface velocity age offset at the tf activation energy mantle viscosity and internal heating rate are explored in our numerical experiments the onset time for ssc is identical on each side of the fracture zone fz it is not modified by the presence of a tf moreover scaling relationships for the onset time obtained in 2d experiments remain valid earlier studies focused on the influence of a tf based on 2d experiments corresponding to slices parallel to the ridge axis they noted the rapid development of a sinking thermal instability located at the step in the lithospheric relief our results indicate that the wake of the tf is characterized by the development of a lateral edge driven buoyant flow from the younger thinner side to the older thicker side of the fz however due to the global shearing context this instability does not sink before the onset of ssc characterized by small scale instabilities on each side of the fz as observed in earlier studies the essentially 3d velocity field combining this lateral flow and the prescribed surface spreading velocity results in an angle formed by the tf trail with the spreading direction we describe the parameters controlling this angle in our experiments and propose a scaling relationship in oceanic settings the specific geometry of the ridge axis segment length orientation of the tfs may affect this angle in addition a large scale mantle flow of other geodynamical origin could interact with the spreading flow and also modify the tf trail angle this departure at depth of the boundary between the thick and thin lithospheres from the surface fz may explain the presence of oceanic volcanoes on the older side of the fz usually considered as thicker

protoanthophyllite from three metamorphosed serpentinites this is the first report of a natural mg rich protoanthophyllite it is common in metamorphosed serpentinites from three japanese ultramafic complexes and some crystals contain anthophyllite pnma lamellae the mg mg fe ratios of the hayachine tari misaka and takase protoanthophyllites are 0 90 0 92 and 0 91 respectively the samples have identical optical properties x a y b z c and 2 v x 64 5 their space group is pnmn or pn 2 n as revealed by systematic extinctions in selected area electron diffraction patterns the protoanthophyllite and anthophyllite have similar compositions and orthorhombic symmetry they are difficult to distinguish using optical microanalytical and powder x ray diffraction measurements this problem raises the possibility that some of the published data on geological and synthetic anthophyllite samples may be of misidentified materials potentially leading to errors in the published stability relations of anthophyllite we provide a method to identify protoanthophyllite and differentiate it from its polymorphs using selected area electron diffraction and high resolution transmission electron microscopy methods

strength of fracture zones from their bathymetric and gravitational evolution fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip this is an improvement over past formulations where bathymetric offsets were imposed kinematically the models use a viscoelastoplastic rheology that incorporates the influence of fault friction on fracture zone slip history using viscoelastic plates we assess the role of small scale convection on removal of the lowermost thermal lithosphere beneath fracture zones through a comparison of synthetic gravity to free air gravity across fracture zones we find that the amplitude of the gravity jump across fracture zones is best fit by models with weak faults that have depth averaged yield strengths 10 mpa fracture zones with such low strengths can convert to subduction zones with 100 km of convergence many fracture zones do not fit plate subsidence models with locked or slipping faults but are better fit by systems that are tectonically deformed by modest amounts of extension

the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

crystal structure of protoanthophyllite a new mineral from the takase ultramafic complex japan protoanthophyllite mg fe 7 si 8 o 2 2 oh 2 is a newly discovered amphibole species from the takase ultramafic complex in japan it occurs as prismatic crystals up to 5 mm in length in a thermally altered serpentinitethat experienced contact metamorphism the protoanthophyllite is associated with forsterite talc serpentine minerals chlorite chromian spinel magnetite pentlandite and calcite some protoanthophyllite crystals contain minute lamellae of anthophyllite other pyriboles or both protoanthophyllite is biaxial negative with refractive indices n 1 593 2 n calc 1 609 n 1 615 2 and 2vx 64 5 electron microprobe analyses give an empirical formula of mg 6 3 1 fe 0 6 1 na 0 0 6 mn 0 0 1 ni 0 0 1 7 0 0 si 7 9 0 al 0 1 4 8 0 4 o 2 2 oh 2 it is orthorhombic with space group pnmn the unit cell dimensions are a 9 3553 8 b 17 9308 15 and c 5 3117 4 a with v 891 0 3 a 3 and z 2 a single crystal x ray structure determination shows that following the convention of thompson 1981 protoanthophyllite has an x configuration the topology of the silicate tetrahedral chains is similar to that of the anthophyllite a chains silicate tetrahedral chains are o rotated in protoanthophyllite whereas those in protoferro anthophyllite are s rotated iron atoms are concentrated in the 4 coordinated m4 sites the unit cell volume is 1 5 larger than the equivalent volume of anthophyllite with mg mg fe 0 885 suggesting a high temperature or low pressure stability relative to anthophyllite assuming that protoanthophyllite is not metastable

small scale instabilities below the cooling oceanic lithosphere summary interpretation of satellite altimetry data as well as ship bathymetry data revealed strongly elongated anomalies roughly perpendicular to the mid ocean ridges in the indian and east pacific oceans a spectral analysis of gravity altimetry data along profiles parallel to the east pacific rise indicated wavelengths of about 150 180 km close to the ridge and about 250 km further away a simple model of rayleigh taylor instabilities developing at the base of the cooling lithosphere is discussed and applied to the data by considering thermal divusion and comparing rayleigh taylor growth rates to the velocity of the thermal front in the cooling lithosphere we are able to explain the observed anomalies by instabilities developing below the lithosphere in a layer with a viscosity of about 1019 pa s above an asthenospheric layer with a viscosity reduction of 2 3 orders of magnitude

viscosity and thickness of the asthenosphere detected from the fennoscandian uplift the earth s response to the deglaciation in fennoscandia is modelled using a layered viscous model with an elastic lithosphere the modelled tilting of palaeoshorelines at particular locations peripheral to the former ice load and the pattern of the present rate of uplift consistent with the observations strongly suggest a low viscosity asthenosphere earth models without a low viscosity asthenosphere give significant mismatch to the present rate of uplift and are unable to mimic the observed tilting of palaeoshorelines models with a viscosity of the lower mantle of 1 0 2 0 1021 pa s and a viscosity of the upper mantle of 0 7 1 0 1021 pa s overlain by low viscosity asthenosphere with a thickness ranging from 25 to 100 km fit equally well with the observations it is suggested strongly that the asthenosphere has a thickness of less than 150 km and a viscosity of less than 7 0 1019 pa s

mantle upwelling and melting beneath slow spreading centers effects of variable rheology and melt productivity abstract we examine the effects of non uniform rheology and melting models on the 3d structure of mantle upwelling beneath spreading centers our numerical models identify a range of conditions for which 3d upwelling can be considered as a possible mechanism for magmatic segmentation of slow spreading ridges in a highly viscous shallow region due to dehydration of the solid by melt extraction flow results essentially from the plate spreading this contrasts with a deeper buoyant low viscosity region where melt is present but upwelling solid is not yet dehydrated and where both solid and melt flows acquire their 3d nature the thickness of this buoyant region depending mainly on the temperature interval between the wet and dry solidi results in a specific characteristic length scale for the upwelling smaller segmentation wavelengths occur for smaller values of this thickness in contrast to isoviscous models where minimum segmentation wavelengths were large 150 km minimum wavelengths obtained in models with non uniform rheology and melt production 40 70 km are comparable to smallest observed segment lengths a single wavelength may be preferred for a given set of parameters but such a preferred wavelength is difficult to achieve because a range of initially prescribed wavelengths can persist for long times the persistence of 3d solutions for tens of millions of years of model time indicate that segment lengths observed at slow mid ocean ridges may be inherited from the initial stages of spreading

buoyant decompression melting a possible mechanism for intraplate volcanism 1 volcanic activity away from plate boundaries occurs in a variety of settings linear age progressive volcanic chains have been explained as the manifestation of fixed hot spots possibly generated by plumes originating deep in the mantle however important examples of oceanic intraplate volcanism cannot be explained in this way including short lived chains and chains violating the predicted age distance behavior furthermore a significant fraction of volcanism does not occur in linear chains and observations suggest control by lithosphere structure in many cases in this study melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism numerical models of this buoyant decompression melting in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small random perturbation for a range of model parameters a predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process buoyant melting could occur in a number of geologic settings since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled spontaneous buoyant decompression melting may be possible only in regions where a large scale mantle upwelling can counteract conductive cooling keeping the mantle at its solidus temperature over some depth range where the mantle is at or sufficiently near its solidus buoyant decompression may spontaneously generate convective storms of melting this may explain the abundance of volcanism associated with the south pacific superswell where mantle is slightly cooler than its melting temperature buoyant melting may not occur spontaneously but may be triggered by some initial upwelling this may provide a physical explanation for volcanism that is controlled by lithosphere structure

retrograde hydration sequence in disordered mg amphiboles a tem investigation acicular mg amphibole in thermal metamorphic serpentinites from elba island italy is affected by polytypic disorder with the possible coexistence of pnmn pnma and c 2 m structures chain width defects and retrograde hydration reactions based on hr tem nanostructures we documented three main hydration stages 1 formation of chain multiplicity faults cmfs 2 formation of fibrous topotactic talc elongated parallel to 001 tlc and resulting from progressive polymerization of cmfs and 3 pervasive serpentinization incipient serpentinization produces isolated inclusions within mg amphibole rhombic in shape and parallel to 210 mg am or elongated parallel to 100 mg am the inclusions are filled by not topotactic curved 001 srp layers or chrysotile partial fibers as the mg amphibole to serpentine replacement proceeds the isolated serpentine inclusions merge together producing a sponge like texture with interconnected fluid paths from which serpentine can rapidly grow at the expense of enclosing amphibole we suggest that whereas the first hydration reactions to cmfs and fibrous talc take place at relatively high t and low fluid rock ratio fluid circulation confined to microfractures cleavage planes and subgrain boundaries subsequent serpentinization requires significant changes in physical conditions with definitely lower t and pervasive fluid circulation

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

sublithospheric small scale convection and its implications for the residual topography at old ocean basins and the plate model 1 the seafloor topography and heat flux differ significantly from the predictions of the half space cooling hsc model at old ocean basins understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large scale mantle dynamics a widely used model that explains significant fraction of the deviations is the plate model but the dynamical feasibility of the plate model has never been demonstrated in this study we investigated the effects of sublithospheric small scale convection ssc and of internal heating on seafloor heat flux and topography and mantle thermal structure and we examined the dynamic feasibility of the plate model by formulating high resolution two dimensional numerical models of mantle convection with strongly temperature and depth dependent rheology we found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere especially when significant internal heating is present this trapped heat may exist in the middle mantle when the ssc is absent and it may also be redistributed by the ssc to shallow depths to reheat the lithosphere and to homogenize mantle temperature when internal heating accounts for 60 of the total heat output the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via ssc while maintaining uniform mantle temperature we suggest that the trapped heat and the ssc are responsible for the residual heat flux and topography at old ocean basins relative to the hsc model predictions our models also show that for the plate model to be dynamically viable both the ssc and significant internal heating 60 are necessary this is because only the ssc in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere which is the basic assumption of the plate model with the viscosity structure and internal heating rate for the present day mantle we think that the plate model is dynamically viable

influence of rheology on realignment of mantle convective structure with plate motion after a plate reorganization 1 small scale convection ssc rolls below the oceanic lithosphere have the tendency to align with the large scale shearing direction and thus with the plate motion direction relative to the deep mantle understanding the timescales of and processes responsible for realignment would contribute significantly to our understanding of the unresolved phenomena in the pacific such as gravity lineations small scale seismic velocity variations and intraplate volcanism that cannot be explained by hot spots in this study we examine the evolution of those convection rolls when this relative plate motion direction is suddenly changed as suggested by the kink in the hawaii emperor seamount chain using three dimensional numerical flow models we investigate the realignment of ssc rolls after a change in plate motion direction from the nature of the ssc it is expected that rheological parameters dominate the characteristics of this realignment our results show that this is indeed the case we find that 1 using constraints from onset timing of ssc realignment of rolls can occur as fast as within 20 ma but might also take much longer dependent on the rheology 2 the realignment period is strongly correlated to the sum of large scale shear stress induced by plate motion and small scale shear stress from the ssc itself 3 in a mantle deforming by dislocation creep realignment occurs faster than by diffusion creep because dislocation creep ssc is more vigorous and 4 activation energy has little influence on the realignment time possible evidence for the realignment period might come from precise age determination of intraplate volcanism or azimuthal seismic anisotropy

flattening of the oceanic topography and geoid thermal versus dynamic origin summary the origin of the flattening of the seafloor bathymetry and geoid at old ages is still a matter of debate is this departure from the square root of age law induced by dynamic uplift of the lithosphere linked to the flow in the mantle or by cooling of the lithosphere different from that predicted for an infinite half space the amplitude of the dynamic uplift should strongly decrease at short wavelengths and should not be correlated with the seafloor age on a local scale therefore in order to eliminate the dynamic signal the cooling of the lithosphere is studied here by means of depth age and geoid age relationships computed from high pass filtered bathymetric geoid and age fields or from local dtopography d and dgeoid dage slopes the geoid age and depth age relationships obtained here are in good agreement with those previously deduced from global non filtered data at all observation scales a significant departure from the half space solution is found however the data are not sufficiently accurate to discriminate between the plate and chablis constant heat flow at the base of the lithospheric isotherm models our results support the existence of small scale convection supplying heat at the base of the old oceanic lithosphere

thermal evolution of the oceanic lithosphere an alternative view abstract the most common model used for representing the evolution with age of the oceanic lithosphere is the plate model where the temperature is set at a fixed depth called the base of the plate this base of the plate has no physical meaning but this model provides a mathematical substitute for a system where small scale convection occurs through instabilities growing at the base of the cooling lithosphere and becomes effective only below old ocean another possible view is that convection provides heat at the base of the lithosphere whatever the age of the overlying plate this last process can be modeled by a constant heat flow applied on the bottom lithospheric isotherm chablis model a good fit to the observables bathymetry and geoid as function of age and old age heat flow can be obtained both for plate and chablis models in spite of an experimentally determined thermal expansion coefficient much larger than assumed in previous plate models these models have important consequences for several geodynamic processes the plate at an age of 100 ma is only 80 km thick for both models melting above a hot spot can then occur in the garnet spinel transition field without much plate thinning in the plate model the subsidence is stopped at an age of about 80 ma while according to the chablis model several hundred meters of subsidence are expected after 100 ma thus the two models predict quite a different long term pattern of subsidence in the sedimentary basins finally in the chablis model the global cooling of the mantle coming from cold material eroded by secondary convection at the base of the plates is considerably larger than in plate models it amounts to 40 the remaining 60 being due to the subduction process

convection beneath young oceanic lithosphere implications for thermal structure and gravity small scale convection under the oceanic lithosphere which begins in the first 5 m y of cooling can produce a gravity signal with the amplitude and wavelength observed for large areas of the central pacific and southern indian oceans using seasat altimeter data the trend of the observed anomalies is parallel to the direction of plate motion as might be expected if they were produced by small scale convection models predict that the wavelengths of gravity anomalies increase more rapidly with age than is observed the persistence of short relatively uniform wavelength anomalies 200 km to crustal ages of 50 ma may indicate that they were produced when the lithosphere was very young and thin and were frozen in as cooling thickened the elastic lithosphere small scale convection which begins under very young lithosphere does not violate other geophysical data such as the rate of seafloor subsidence and variations of geoid height with age after convection has begun the subsidence due to thermal contraction within the lithosphere varies linearly with age in the absence of mantle heat sources although the rate of change of these quantities is affected by convection much of the variation of the geoid height across fracture zones can be fit by a model which includes small scale convection

deep seated rockslide avalanches preceded by mass rock creep of sedimentary rocks in the akaishi mountains central japan abstract seven large rockslide avalanches of sedimentary rocks are known in japan almost all of these major landslides were preceded by gravitational deformation of rocks mass rock creep mrc as inferred from geologic and geomorphic investigations of five rockslide avalanches in the akaishi mountains the mrc that preceded the rockslide avalanches in the akaishi mountains was characterized by a slow but steady downslope bowing of steeply dipping foliations which accompanied intensive deformation and fragmentation of the rock mass by shearing along foliations this deformation and fragmentation of the rock mass provides a basic cause for the occurrence of rockslide avalanches which occur when a creeping rock mass has lost its support from the lower part of a slope by stream erosion this type of mrc continues for a long time after a part of the creeping area has slid and results in longterm generation of debris by the landslide sear of the rockslide avalanche

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

numerical simulations of the cooling of an oceanic lithosphere above a convective mantle abstract numerical simulations of two dimensional rayleigh benard convection are designed to study lithospheric cooling above a convective mantle a strongly temperature and pressure dependent viscosity fluid is heated from below or from within an imposed velocity at the surface of the box mimicks the plate motion between the ridge on one side and the subduction zone on the other side as the lithosphere cools its upper part remains rigid and therefore conductive while its bottom part is convectively unstable dripping instabilities are not observed close to the ridge nevertheless the material flows along the slope defined by the lower part of the lithosphere and feeds the first descending drip afterwards cold downgoing instabilities develop continuously and randomly at the base of the lithosphere and are replaced by hot material from the convecting core of the box the lithosphere continues to thicken even after the onset of the first instability surface heat flow subsidence and lithospheric temperature structure obtained by the convective simulations are compared to the predictions of three conductive models the plate chablis and modified chablis models these models differ by their applied bottom boundary condition which represents the lithosphere asthenosphere convective coupling i e by the presence or absence of instabilities developing at the base of the lithosphere the conductive model which best explains the lithospheric cooling obtained by convective simulations is the modified chablis model in this model a variable heat flow depending upon the viscosity at the base of the lithosphere is applied along an isotherm located in the lower unstable part of the lithosphere

upper mantle low anisotropy channels below the pacific plate a new 3d anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals new insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure the model is best resolved in the pacific plate the largest and the most active tectonic plate superimposed on the large scale radial parameter and azimuthal anisotropy of vsv velocity within and below the lithosphere correlated with present or past pacific plate motions are smaller scale 1000 km lateral variations of anisotropy not predicted by plate tectonics channels of low anisotropy down to a depth of 200 km hereafter referred to as lac are observed and are the best resolved anomalies one east west channel between easter island and the tonga kermadec subduction zones observed on both radial and azimuthal anisotropies and a second one only observed on azimuthal anisotropy extending from the south west pacific up to south east hawaii and passing through the polynesia hotspot group for plate older than about 40 ma these features provide strong constraints on the decoupling between the plate and asthenosphere they are presumably related to cracking within the pacific plate and or to secondary convection below the rigid lithosphere predicted by numerical and analog experiments the existence and location of these lacs might be related to the current active volcanoes and hotspots possibly plumes in the central pacific lacs which are dividing the pacific plate into smaller units might indicate a future reorganization of plates with ridge migrations in the pacific ocean

three dimensional numerical simulations of mantle flow beneath mid ocean ridges we perform three dimensional 3 d numerical simulations of an oceanic lithosphere cooling above a convective mantle to investigate mantle flow geometry and its associated lithospheric features a constant horizontal velocity half spreading velocity of 2 or 4 cm yr is applied at the box surface to mimic plate motion in all cases because of the temperature dependent viscosity a rigid conductive lithosphere cools progressively beneath the two plates separated by the ridge after 16 40 ma cold downgoing instabilities develop at the base of the lithosphere small scale convection is then superimposed on the large scale circulation it produces and then interacts with a slowly evolving short wavelength isotherms topography at the base of the lithosphere the influence of the imposed ridge geometry on the mantle flow appears by comparison of simulations with case a a linear ridge perpendicular to the plate motion case b a linear ridge strongly oblique to the plate motion and case c a ridge with transform faults along the spreading center with a mean orientation strongly oblique to the plate motion all simulations reveal complex interaction between the isotherm topography within the base of the lithosphere the small scale flow generated at or just below the base of the lithosphere and the large scale flow dominating in the mantle below the large scale flow appears always controlled by the mean ridge axis direction and thus may be oblique to the imposed plate motion direction it is enhanced by the instabilities development we argue that the large scale flow orientation results from the interaction with the small scale velocity field which flows down the isotherm topography at the base of the lithosphere the latter is either inherited lithospheric cooling transform faults or develops as boundary layer instabilities

azimuthal anisotropy of the pacific region azimuthal anisotropy is the dependence of local seismic properties on the azimuth of propagation we present the azimuthally anisotropic component of a 3d sv velocity model for the pacific ocean derived from the waveform modeling of over 56 000 multi mode rayleigh waves followed by a simultaneous inversion for isotropic and azimuthally anisotropic vsv structure the isotropic vsv model is discussed in a previous paper a maggi e debayle k priestley g barruol multi mode surface waveform tomography of the pacific ocean a close look at the lithospheric cooling signature geophys j int 166 3 2006 doi 10 1111 j 1365 246x 2006 03037 x the azimuthal anisotropy we find is consistent with the lattice preferred orientation model lpo the hypothesis of anisotropy generation in the earth s mantle by preferential alignment of anisotropic crystals in response to the shear strains induced by mantle flow at lithospheric depths we find good agreement between fast azimuthal anisotropy orientations and ridge spreading directions recorded by sea floor magnetic anomalies at asthenospheric depths we find a strong correlation between fast azimuthal anisotropy orientations and the directions of current plate motions we observe perturbations in the pattern of seismic anisotropy close to pacific hot spots that are consistent with the predictions of numerical models of lpo generation in plume disturbed plate motion driven mantle flow these observations suggest that perturbations in the patterns of azimuthal anisotropy may provide indirect evidence for plume like upwelling in the mantle

post glacial rebound and asthenosphere viscosity in iceland during the weichselian glaciation iceland was covered with an ice cap which caused downward flexure of the earth s surface the post glacial rebound in iceland was very rapid being completed in about 1 000 years the length of this time interval constrains the maximum value of asthenosphere viscosity in iceland to be 1 times 10 sup 19 pa s or less further clarification of the ice retreat and uplift history may reveal lower viscosity current changes in the mass balance of icelandic glaciers must lead to measurable elevation changes considering this low viscosity expected current elevation changes around the vatnajoekull ice cap are of the order of 1 cm per year due to mass balance change in this century

onset of small scale instabilities at the base of the lithosphere scaling laws and role of pre existing lithospheric structures summary numerical simulations of 2 d rayleigh benard convection are designed to study the devel opment of convection at the base of the cooling lithosphere a zero temperature is suddenly imposed at the mantle surface which has initially a homogeneous temperature the strongly temperature dependent viscosity fluid is heated from within in order to balance the internal temperature drift resulting from global fluid cooling for a while the lithosphere cools approx imatively as a conductive half space and the lithospheric isotherms depth grows as the square root of age as instabilities progressively develop at the base of the lithosphere lithospheric cooling departs from the half space model we propose two different parametrizations of the age of the first dripping instability using boundary layer marginal stability or quantifying the characteristic timescale of the instability exponential growth as a function of the rayleigh number and of the viscous temperature scale both parametrizations account very well for our numerical estimates of onset times but with a slightly better adjustment of the viscous tem perature scale dependence in the second case the absolute value of the onset time depends on the amplitude and location of initial temperature perturbations within the box and on the initial temperature structure of the thermal boundary layer tbl furthermore thermal perturba tions of finite amplitude located within the lithosphere such as the ones induced by transform faults for example strongly reduce the age of the first dripping instability however the onset time parametrization derived from transient cooling experiments well adjusts the lithospheric age of the first drip instability below a lithosphere cooling perpendicularly to the ridge axis this study emphasizes the role of the initial topography of the lithospheric isotherms on the development of instabilities

local lithospheric relief associated with fracture zones and ponded plume material 1 buoyant plume material ponds at the base of oceanic lithosphere it is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside down drainage pattern geomorphic and geochemical studies of near ridge and on ridge hot spots indicate as expected that the oceanic lithosphere thickens as it ages corridors of thin lithosphere channel plume material along the axis while the thick lithosphere at the end of ridge segments dams this along axis flow channel hot spots extend from plumes toward ridge axes however a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins such a correlation would be expected if the lithosphere cooled only by conduction then pressure release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere this concentration of volcanic edifices does not occur this lack of correlation between lithosphere thickness and plate age is an expected consequence of small scale convection at the base of the lithosphere scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age typically the step in lithospheric thickness moves toward the old side of the fracture zone

non hotspot volcano chains originating from small scale sublithospheric convection 1 some oceanic volcano chains violate the predictions of the hotspot hypothesis for geographic age progressions one mechanism invoked to explain these observations is small scale sublithospheric convection ssc in this study we explore this concept in thermo chemical 3d numerical models melting due to ssc is shown to emerge in elongated features 750 km parallel to plate motion and not just at a fixed spot therefore volcanism occurs in chains but not with hotspot like linear age progressions the seafloor age at which volcanism first occurs is sensitive to mantle temperature as higher temperatures increase the onset age of ssc because of the stabilizing influence of thicker residue from previous mid ocean ridge melting mantle viscosity controls the rate of melt production with decreasing viscosities leading to more vigorous convection and volcanism calculations predict many of the key observations of the pukapuka ridges and the volcano groups associated with the line cook austral and marshall islands