global patterns in sandy beach macrofauna species richness abundance biomass and body size abstract global patterns in species richness in sandy beach ecosystems have been poorly understood until comparatively recently because of the difficulty of compiling high resolution databases at continental scales we analyze information from more than 200 sandy beaches around the world which harbor hundreds of macrofauna species and explore latitudinal trends in species richness abundance and biomass species richness increases from temperate to tropical sites abundance follows contrasting trends depending on the slope of the beach in gentle slope beaches it is higher at temperate sites whereas in steep slope beaches it is higher at the tropics biomass follows identical negative trends for both climatic regions at the whole range of beach slopes suggesting decreasing rates in carrying capacity of the environment towards reflective beaches various morphodynamic variables determine global trends in beach macrofauna species richness abundance and biomass are higher at dissipative than at reflective beaches whereas a body size follows the reverse pattern a generalized linear model showed that large tidal range which determines the vertical dimension of the intertidal habitat small size of sand particles and flat beach slope a product of the interaction among wave energy tidal range and grain size are correlated with high species richness suggesting that these parameters represent the most parsimonious variables for modelling patterns in sandy beach macrofauna large scale patterns indicate a scaling of abundance to a body size suggesting that dissipative beaches harbor communities with highest abundance and species with the smallest body sizes additional information for tropical and northern hemisphere sandy beaches underrepresented in our compilation is required to decipher more conclusive trends particularly in abundance biomass and body size further research should integrate meaningful oceanographic variables such as temperature and primary production in deciphering latitudinal trends

some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on venus models of coupled crustal deformation and mantle convection on venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine a previous model which assumed a relatively weak basalt flow law more appropriate for hydrated crust is compared to a similar model that assumes a strong basalt flow law more appropriate for dry crust a principal difference is the timescale for significant crustal thickening of the order of 108 years for the weak basalt model and 109 years for the strong basalt model these results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near surface weakness of the type associated with plate margins on earth the lithospheric overturn time which sets the timescale required for convective thickening of crust scales dominantiy with the rayleigh number defined by the average viscosity of the lithosphere theoretical scalings as well as numerical models suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103 such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present day earth the large values inferred for the thickness of the mechanical lithosphere on venus if correct would thus tend to favor timescales of the order of 109 years if one assumes relatively low convective mantle heat loss over an equivalent time this in turn would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows if instead one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust

how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

viscous forces acting on subducting lithosphere subducting lithosphere is driven by gravitational forces and resisted primarily by viscous forces three independent types of mantle flow combine to create viscous tractions on the surface of a subducting slab which terminates in the upper mantle 1 flow induced by plate convergence and slab subduction 2 flow caused by trench migration and 3 regional upper mantle flow unrelated to convergence or trench migration the equilibrium dip of a slab reflects a balance of viscous tractions with gravitational forces and slab stiffness we present results from a two dimensional finite difference model of upper mantle flow for different slab geometries where mantle viscosity increases exponentially with depth and the slab terminating in the upper mantle is assumed to be infinitely viscous in the overlying mantle wedge the subducting slab creates a strong eddy which is largely unaffected by trench migration or regional flow trench migration and regional mantle flow produce tractions which tend to lift the slab and which are independent of slab dip for a slab terminating in the upper mantle for dips 22 5 the upward torquing force on the slab due to convergence is negligible compared to the pressures created by trench migration and regional mantle flow slabs dipping 87 5 which are in torquing equilibrium require regional flow in the same direction as subduction or trench migration in the opposite direction which simultaneously tends to eliminate most of the horizontal pressure difference across the subduction zone the force calculations based on this model suggest that viscosity at the base of the lithosphere is 2 1020 pa s assuming viscosity increases exponentially with depth to 2 8 1021 pa s just above the lower mantle for force balanced slabs which penetrate halfway into the upper mantle with dip in the range of 30 to 75 slab dip increases linearly with decreasing trench migration rate and decreasing regional mantle flow

crustal anisotropy across northern japan from receiver functions northern japan is a tectonically active area with the presence of several volcanoes and with frequent earthquakes among which the destructive mw 8 9 9 0 tohoku oki occurred on 11 march 2011 tectonic activity leaves an imprint on the crustal structures on both the upper and the lower layers to investigate the crust in northern japan we construct a receiver function data set using teleseismic events recorded at 58 seismic stations belonging to the japanese national hi net network we isolate the signals in the receiver function wavelet that witness the presence of anisotropic structures at depth with the aim of mapping the variation of anisotropy across the northern part of the island this study focuses on the relation among anisotropy detected in the crust stresses induced by plate convergence across the subduction zone and the intrinsic characteristics of the rocks our results show how a simple velocity model with two anisotropic layers reproduces the observed data at the stations we observe a negligible or small amount of signal related to anisotropy in the eastern part of the study area i e the outer arc for both upper and lower crust distinct anisotropic features are observed at the stations on the western part of the study area i e the inner arc for both upper and lower crust the symmetry axes are mostly e w oriented deviation from the e w orientation is observed close to the volcanic areas where the higher geothermal gradient might influence the deformation processes

anomalous low velocity zone and linear alignment of seismicity along it in the subducted pacific slab beneath kanto japan reactivation of subducted fracture zone 1 a detailed investigation of the hypocenter distribution beneath kanto japan reveals a nw se trending linear alignment of seismicity within the subducted pacific slab we estimate the 3d seismic velocity structure in the pacific slab to understand the factors controlling the genesis of such intraslab earthquakes a narrow low velocity zone is imaged within the subducted slab over a length of 150 km which partly penetrates into the mantle portion of the slab the low velocity zone correlates in space with the nw se trending earthquake cluster a reactivation of hydrated fracture zone formed prior to subduction is probably related to the low velocity anomaly dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone accompanied by intraslab earthquakes these observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

geodynamic models of crustal scale episodic tectonic accretion and underplating in subduction zones we investigate the dynamics of crustal scale episodic accretion at convergent margins using a finite element model of subduction the numerical model incorporates effects from flexure subduction loading and large scale variations in material properties the model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction the primary controls investigated are spatial variations in density material strength and thickness of material layers these controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere giving rise to areas of thermally activated ductile creep attempted subduction of small crustal blocks density frictional and or ductile strength variations interaction between well attached basement blocks and overlying detachable sediments and spatial changes in thickness of sediment and crust in the experiments accretion of small continental terranes within a model subduction zone can cause crustal scale fold nappes and shear zones to develop with accompanying tectonic underplating and or frontal accretion the model results may provide insight into real cases of episodic accretion for example the emplacement of the upper penninic nappe stack of the central alps

the depth distribution of mantle anisotropy beneath the tonga subduction zone abstract shear wave splitting recorded by a temporary deployment of broadband seismometers located above the tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the tonga back arc region splitting parameters obtained for local s and teleseismic sks phases were modeled using a ray based method that predicts shear wave splitting on individual phase paths and determines the best fitting values of anisotropic strength orientation and depth extent splitting in teleseismic sks phases is identical to that in s phases from local earthquakes that occur at the base of the transition zone demonstrating that there is no significant splitting due to anisotropy in the lower mantle splitting times for local s phases do not vary significantly with depth and for models in which the strength of anisotropy is laterally uniform they rule out anisotropy in the transition zone finally the observed splitting indicates that anisotropy of more than 0 8 exists in the upper mantle with a fast symmetry axis at 60 w roughly parallel to the absolute plate motion vector of the subducting pacific lithosphere

shear wave polarization anisotropy and subduction induced flow in the mantle wedge of northeastern japan abstract shear wave polarization anisotropy in the central part of northeastern ne japan was investigated we analyzed s phases of 293 intermediate depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters the obtained results show the systematic variation in splitting parameters across the arc the leading shear wave polarization directions fast direction obtained at stations in the western side of the study area are oriented nearly e w which is sub parallel to the direction of relative plate motion between the pacific plate and the north american okhotsk plate in contrast fast directions obtained at stations in the eastern side of the study area are oriented approximately n s stations in the western side have a larger average delay time compared to those in the eastern side these variations suggest that the nature of anisotropy is quite different between the eastern and the western sides both experimental studies on the deformation of olivine and numerical simulations of plate driven flow predict that the lattice preferred orientation of olivine causes the fast direction sub parallel to the flow direction for the conditions in the mantle wedge of ne japan we thus infer that the lattice preferred orientation of olivine generated by flow induced strain is the most likely candidate for anisotropy that produces splitting with e w fast direction three possible causes of anisotropy in the eastern side are considered deformation of water rich olivine in the mantle wedge trench parallel flow in the mantle wedge due to along strike dip variations in the slab and anisotropy in the crust and the slab we could not however determine which causes are dominant further systematic study is required to reveal the cause of anisotropy in the eastern side

fast ductile failure of passive margins from sediment loading sediment loading at continental margins has been interpreted as a very inefficient mechanism for initiating subduction we have re investigated this problem using a time dependent non linear ductile failure analysis in which the effects of power law plastic strain hardening are assessed the results follow classical flexural rigidity solutions until wholesale lithosphere failure occurs sediment loading causes two shear zones to develop from the bottom of the lithosphere upwards for strong strain hardening shear zones nucleate earlier but the plate may never break for small hardening the instability grows much faster and the plate can break under the sediment load with a limiting yield stress lower than 0 4 gpa when the ductile faults growing from the top and the bottom of the plate merge in the center the flexurally supported gravitational energy is released the lithosphere becomes unstable and shears off super exponentially within a few million years

a simple model of plate generation from mantle flow summary a simple model of non newtoniancreeping ow is used to evaluate classes of rheologieswhichallow viscous mantle ow to becomeplate like the model describes shallow layerlithosphericmotion driven by sources and sinks the sources represent spreading ridges while the sinks rep resent subduction zones the sources and sinks thus also prescribe the poloidal component ofthe surface ow eld the toroidal strike slip component of the ow eld is found via thesolution of the stokes equation with non newtonian rheology as a rst basic investigation ofthe model the horizontal divergence from the two dimensional rectangular velocity eld ofolson bercovici 1991 is used for the source sink eld the degree to which the induced uid ow reproducesthe rectangularplate is used to measure the success of differentrheologiesin generating plate like ows results indicate that power law rheologies even in the limit ofvery high power law index can only produce modest plate like ow for example the ratioof toroidal to poloidal kinetic energy for a source sink eld derived from a square plate is atbest 0 65 whereas a perfect square plate has a ratio of 1 0 moreover the power law rheologyappears to reach an asymptotic limit in its ability to produce plate like behavior this impliesthat plate tectonics is unlikely to arise from a power law rheology even in the limit of very high a class of rheologies that yield signi cantly more promisi ng results arise from the carreaupseudo plastic rheology with the power law index taken to be 0 one rheology in thisclass is the continuum model for stick slip earthquake behavior of whitehead gans 1974 which is essentially the carreau equation with 1 this class of rheologies referred to asthe stick slip rheologies induces a toroidal to poloidal kinetic energy ratio for a square plates ssource sink function which can be as high as0 9 the viscosity or strength distribution forthis class of rheologies also appears more plate like showing fairly uniform high viscosity re gions pseudo plates and sharply de ned low viscosity zones pseudo margins in contrast even the most nonlinear power law rheology produces spatially varying high viscosity regionsand relatively smooth low viscosity margins the greater success of the stick slip rheologiesin producing plates is attributed to a self lubricating mechanism in which the transfer of mo mentum from regions of high shear to low shear is inhibited in contrast even in the limit ofin nite power law index a power law rheology can retard but never prohibit momentum trans fer this feature is essential to the sharpening of velocity pro les into plate like pro les whichis illustrated with a simple boundary layertheory key words plate tectonics mantle convection non newtonian ow toroidal poloidal cou pling

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

shear zones as a result of intraplate tectonics in oceanic crust the example of the basal complex of fuerteventura canary islands ductile shear zones affecting igneous rocks pyroxenites syenites gabbros and carbonatites belonging to the basal complex of the island of fuerteventura are described here they develop typical mylonites with their cortege of structures and crystallographic fabrics an evolution from brittle to ductile conditions has been recognized and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks carbonatites syenites along the shear zones local metamorphic changes favour the ductile behaviour via a reaction enhanced softening mechanism the shear zones are arranged in a nearly orthorhombic pattern kinematic criteria suggest an extensional tectonic scenario with a bulk irrotational nonplane deformation the long axis of the finite deformation ellipsoid is e w and horizontal the available data allow us to establish this deformation as being from the late oligocene to the early miocene this is tentatively related to the plate tectonics evolution and mantle anomalies in the northwestern corner of the african plate

low energy sandy beaches in marine and estuarine environments a review this review was undertaken to identify locations where low energy beaches may occur and their diagnostic forms and process controls including waves tides and water levels examples are drawn from the sheltered coastline of western australia near perth and fetch limited estuarine environments on the northeast coast of the united states we suggest that the term low energy be used in locations where 1 non storm significant wave heights are minimal e g 0 25 m 2 significant wave heights during strong onshore winds are low e g 0 50 m 3 beachface widths are narrow e g 20 m in microtidal environments and 4 morphologic features include those inherited from higher energy events micro topographic features can persist in the swash zone of low energy beaches under non storm wave conditions there is little evidence of cyclic cross shore sediment exchange bars excepting transverse forms located seaward of low still water level do not appear to be part of the sediment exchange system with the foreshore developing a better definition of the term low energy requires understanding the occurrence and duration of morphological characteristics and the type magnitude and frequency of hydrodynamic controls that are responsible for these characteristics efforts also should be directed toward 1 discriminating between processes generated within basins in true fetch limited environments and processes generated outside basins that affect sheltered environments 2 identifying the relative contributions of tide and surge related water level fluctuations on low energy beach shape and 3 estimating thresholds for beach change

energetics of a two phase model of lithospheric damage shear localization and plate boundary formation summary the two phase theory for compaction and damage proposed by bercovici et al 2001a j geophys res 106 8887 8906 employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a model for damage void generation and microcracking and a continuum description of weakening failure and shear localization here we examine further variations of the model which consider 1 how interfacial surface energy when averaged over the mixture appears to be partitioned between phases 2 how variability in deformational work partitioning greatly facilitates localization and 3 how damage and localization are manifested in heat output and bulk energy exchange microphysical considerations of molecular bonding and activation energy suggest that the apparent partitioning of surface energy between phases goes as the viscosity of the phases when such partitioning is used in the two phase theory it captures the melt compaction theory of mckenzie 1984 j petrol 25 713 765 exactly as well as the void damage theory proposed in a companion paper ricard bercovici submitted calculations of 1 d shear localization with this variation of the theory still show at least three possible regimes of damage and localization at low stress is weak localization with diffuse slowly evolving shear bands at higher stress strong localization with narrow rapidly growing bands exists and at yet higher shear stress it is possible for the system to undergo broadly distributed damage and no localization however the intensity of localization is strongly controlled by the variability of the deformational work partitioning with dilation rate represented by the parameter f or 1 extreme localization is allowed with sharp profiles in porosity weak zones nearly discontinuous separation velocities and effectively singular dilation rates finally the bulk heat output is examined for the 1 d system to discern how much deformational work is effectively stored as surface energy in the high stress distributed damage cases heat output is reduced as more interfacial surface energy is created yet in either the weak or strong localizing cases the system always releases surface energy regardless of the presence of damage or not and thus slightly more heat is in fact released than energy is input through external work moreover increased levels of damage represented by the maximum work partitioning f make the localizing system release surface energy faster as damage enhances phase separation and focusing of the porosity field thus yielding more rapid loss of net interfacial surface area however when cases with different levels of damage are compared at similar stages of development say the peak porosity of the localization it is apparent that increased damage causes smaller relative heat release and retards loss of net interfacial surface energy the energetics and energy partitioning of this damage and shear localization model are applied to estimating the energy costs of forming plate boundaries and generating plates from mantle convection

shear wave anisotropy of the upper and lower crusts estimated by stripping analysis of ps converted waves abstract the anisotropic structure of the crust is investigated by measuring the shear wave polarization anisotropy of the ps phases converted at the conrad and moho discontinuities the ps converted phases are identified on the p wave receiver functions which are constructed from teleseismic records at the f net and hi net stations in the kinki chugoku shikoku and kyushu districts in southwest japan the ps phase traveling through an anisotropic layer undergoes shear wave splitting which is described by splitting parameters fast polarization direction fpd and split time the upper and lower crustal anisotropies are estimated from the splitting parameters of the conrad and moho ps phases respectively which are determined by a waveform cross correlation method to reliably estimate the lower crustal anisotropy the moho ps phase is corrected for the shear wave splitting effect in the upper crust by a stripping method because the upper crustal anisotropy masks the anisotropic behavior of the ps phase in the lower crust the split times estimated for the upper and lower crusts are less than 0 2 s but their fpd characteristics differ from each other the fpds in the upper crust are predominantly in the e w direction the fpds in the lower crust range from nw se to ne sw in the region with the philippine sea phs slab subduction but from ne sw to se nw in the region without slab subduction thus seismic anisotropy exists in both the lower and upper crusts and slab subduction significantly influences the lower crustal anisotropy the most likely cause for the upper crustal anisotropy is the alignment of vertical cracks induced in the upper crust by the tectonic stress while the lower crustal anisotropy is attributed to the lattice preferred orientation of rock forming minerals along with ductile deformation of the lower crust

dynamic models of subduction geophysical and geological evidence in the tyrrhenian sea summary predictions based on a 2 d finite element model for subduction underneath the calabrian arc in southern italy are compared with a variety of geophysical and geological data such as the present day stress pattern within the slab uplift from the elevation of marine terraces in calabria and subsidence in the tyrrhenian marsili basin from odp leg 107 we model the behaviour of the slab driven by slab pull in agreement with the present tectonic style in this part of the mediterranean as suggested by several investigators the model accounts for the crustal lithospheric and mantle structures in a vertical cross section perpendicular to the calabrian subduction zone the shape of the slab is constrained on the basis of new tomographic images in the southern tyrrhenian sea which were obtained from the regional seismic stations of the istituto nazionale di geofisica while the rheological properties of the mantle are taken from global dynamic models density contrasts between the subducted slab and the surrounding mantle based on petrological models drive the flow in our viscoelastic model stress values displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached our estimates are appropriate for a time window of 100 kyr the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day two families of models with unlocked and locked subduction faults are considered the unlocked models allow for roll back of the trench of about 20 mm yr in agreement with some geological estimates the same family of models predicts uplift of the calabrian arc of about 1 mm yr l and subsidence in the marsili basin of 1 2 mm yr in agreement with geological surveys the deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern tyrrhenian sea with minor concentration within the lithospheric wedge locked models fail to reproduce these geophysical and geological observations predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern tyrrhenian sea deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate dynamic topography and horizontal motions are reduced when compared with the continuous plate with deviatoric stresses concentrated within the relict slab our results indicate that subduction is a major tectonic process in the southern tyrrhenian sea

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

coastal geomorphology and restoration abstract increases in human development of the coastal zone sea level rise and intensity of coastal storms will test the resilience of coastal systems an integrated approach is needed to describe geomorphic biologic dependencies feedbacks between processes and responses and determine how coastal systems can be maintained or restored the 2013 binghamton geomorphology symposium focused on three themes in these research areas 1 the geomorphic response of coastal landforms to changes in sea level and episodic storms 2 the way these stressors alter geomorphic biotic interactions and reduce the resilience of coastal environments and 3 the way natural and human processes are being integrated to restore or maintain dynamic geomorphic and biologic linkages this issue contains eighteen papers presented at the symposium six papers are devoted to each of these themes the papers point to the importance of ecological systems in modulating the responses to natural physical processes and in turn responding to geomorphic change and the role of humans in assisting natural processes

receiver functions of seismic waves in layered anisotropic media application to the estimate of seismic anisotropy abstract we investigate the effect of seismic anisotropy on p wave receiver functions calculating synthetic seismograms for p wave incidence on multilayered anisotropic structure with hexagonal symmetry the main characteristics of the receiver functions affected by the anisotropy are summarized as 1 appearance of seismic energy on radial and transverse receiver functions 2 systematic change of p to s ps converted waveforms on receiver functions as ray back azimuth increases and 3 reversal of the ps phase polarity on the radial receiver function in a range of the back azimuth another important influence is shear wave splitting of the ps converted waves and other later phases reverberated as s wave by numerical experiments using synthetic receiver functions we demonstrate that the waveform cross correlation analysis is applicable to splitting ps phases on receiver functions to estimate the seismic anisotropy of layer structure advantages to utilizing the ps phases are 1 they appear more clearly on receiver functions than on seismograms and 2 they inform us about what place along the seismic ray path is anisotropic real analysis of shear wave splitting is executed to the moho generated ps phases that are identified on receiver functions at six seismic stations in the chugoku district southwest japan the time lags between the two arrivals of the split ps phases are estimated at 0 2 0 7 sec and the polarization directions of the fast arrival components are from north south to northeast southwest this result is consistent with recent results of shear wave splitting measurements and the trend of linear epicenter distributions of crustal earthquakes and active fault strikes in the chugoku district

regional variations of the shear wave polarization anisotropy in the crust and mantle wedge beneath the tohoku district abstract we investigated the regional variations of the shear wave polarization anisotropy in the upper crust lower crust and mantle wedge beneath the tohoku district in the northeast japan arc the shear wave splitting parameters fast polarization direction fpd and split time for the upper and lower crusts are estimated by shear wave splitting analysis of the ps phases which are generated by the conversion of a p wave into an s wave at the conrad and moho discontinuity interfaces the splitting parameters for the mantle wedge are determined by splitting analysis of the slab ps phase converted at the oceanic crust on the subducting pacific slab and the direct s waves from deep focus earthquakes located just below seismic stations the ps phases are identified on the p wave receiver functions constructed from the teleseismic records to accurately determine the splitting parameters for the lower crust from the moho ps phase the ps phase should be corrected for the shear wave splitting effect in the upper crust similarly to estimate the mantle wedge anisotropy the direct s wave and slab ps phase must be corrected for the splitting effect in the crust the anisotropies of the crust and mantle wedge are estimated from the corrected ps phases and direct s waves the anisotropy of the upper crust exhibits a regional variation where fpd of the split shear wave is predominant in the n s direction in the pacific coast area and in the e w direction in the rest of the tohoku district the split time is less than 0 2 s the upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress in the lower crust fpd is predominant in the e w direction with a split time similar to that in the upper crust and the anisotropy of the lower crust is due to the lattice preferred orientation of rock forming minerals in the mantle wedge fpd is predominant in the n s direction trench parallel in the fore arc side of the volcanic front but in the e w direction trench perpendicular in the back arc side the split time in the mantle wedge is similar to that of the upper crust the most likely cause for the trench perpendicular anisotropy in the back arc mantle wedge is the lattice preferred orientation of a type olivine along flows of the secondary mantle convection for trench parallel anisotropy in the fore arc mantle wedge we considered two possible causes the preferred orientation of b type olivine along the secondary mantle convection and that of dry olivine along the trench parallel flow produced by the along strike dip variation of the pacific slab however we could not determine which one is the origin

anomalous deepening of a seismic belt in the upper plane of the double seismic zone in the pacific slab beneath the hokkaido corner possible evidence for thermal shielding caused by subducted forearc crust materials abstract hypocenter relocations of earthquakes in the pacific slab have shown an anomalous deepening of a seismic belt in the upper plane of the double seismic zone at depths of 80 120 km beneath the hokkaido corner while it is located at depths of 70 90 km in the surrounding tohoku and eastern hokkaido areas seismic tomographic inversions performed beneath the hokkaido corner have shown that a low velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the pacific slab and makes direct contact with the upper surface of the pacific slab these observations suggest that 1 the low velocity zone just above the pacific slab is part of the subducted kuril forearc sliver 2 the contact with the subducted and so relatively cold sliver materials prevents the mantle wedge from heating the pacific slab and causes a lower temperature condition in the pacific slab crust beneath the hokkaido corner than in the surrounding areas 3 as a result a delay of eclogite forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

three dimensional structure of p wave anisotropy beneath the tohoku district northeast japan 1 we retrieve three dimensional structures of isotropic and anisotropic velocities of p waves of the tohoku district from first p arrival time data assuming azimuthal anisotropy to be caused by hexagonal symmetry axes distributed horizontally in the earth the results show that the high velocity pacific slab is clearly imaged in the isotropic velocity structure even though the azimuthal anisotropy is taken into account in addition small scale low velocity regions and prominent low velocity anomalies are found just below the active volcanoes and in the mantle wedge above the high velocity pacific slab respectively the fast propagation axis of p waves is in mostly e w direction in the upper crust nearly n s and e w directions in the lower crust e w direction in the mantle wedge and n s direction in the descending pacific slab these features of the p wave anisotropy structure are consistent with those of lateral variations of the fast polarization directions measured previously by shear wave splitting observations the plausible factor that causes the crust anisotropy is interpreted as being alignment or preferred orientation of microcracks and crust minerals the mantle wedge anisotropy is attributed to lattice preferred orientation of the mantle minerals arising from present day mantle process such as the mantle wedge convection and the plate motion however the fast propagation axis of p waves in the slab is almost perpendicular to the magnetic lineation of the oceanic plate under the northwest pacific and thus the slab preserves the original anisotropic property that the pacific plate gained when it formed

regional variation in shear wave polarization anisotropy of the crust in southwest japan as estimated by splitting analysis of ps converted waves on receiver functions abstract regional variation in shear wave polarization anisotropy of the crust is investigated in the kyushu chugoku shikoku and kinki districts southwest japan using ps converted phases generated at the moho discontinuity the ps phases on radial and transverse receiver functions are analyzed to determine shear wave splitting parameters of fast polarization direction and time lag by waveform cross correction analysis as a result the fast polarization directions are shown to be obviously different between the chugoku district and the kyushu and shikoku districts which are separated from each other by the seto inland sea in the kyushu shikoku and south kinki districts the ps phases exhibit fast polarization directions ranging from e w to nw se the polarization directions are not only consistent with those of s waves from shallow crustal earthquakes but also parallel to the direction of maximum horizontal compression acting on southwest japan and the strike directions of metamorphic belts accretionary zones and rift zones thus the polarization anisotropy is understood to reflect mainly the upper crustal anisotropy that is attributable to stress induced vertical cracks and geological lineament structure on the other hand in the chugoku and north kinki districts fast polarization directions are in a range from n s to ne sw they are not consistent with the tectonic stress field and the s wave splitting observed for shallow crustal earthquakes the disagreement between the fast polarization directions estimated from the s waves and the ps converted phases is interpreted as arising because the lower crustal anisotropy attributed to mineral preferred orientation has a significant influence on the ps phase splitting the regional variation in ps phase polarization anisotropy of the crust in southwest japan is dependent on which anisotropy of the upper and lower crust is prominently reflected in the moho ps phase splitting

subduction zone seismicity and the thermo mechanical evolution of downgoing lithosphere in this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km with emphasis on the role of temperature and rheology in controlling the deformation of and the seismic energy release in downgoing lithosphere this is done in two steps after a brief review of earlier developments we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth dependent critical valuetcr

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

cambrian explosion triggered by geosphere biosphere feedbacks 1 a new hypothesis for the cause of the cambrian explosion is presented for that the evolution of the planet earth is described by the co evolution of the geospherebiosphere system here we specify our previously published earth system model for the long term carbon cycle by introducing three different types of biosphere procaryotes eucaryotes and complex multicellular life they are characterized by different global temperature tolerance windows the biotic enhancement of silicate weathering by complex multicellular life adds an additional feedback to the system and triggers the cambrian explosion the cambrian explosion is characterized by a sudden increase of biomass and a rapid cooling which amplified the spread of complex multicellular life cooling events in the neoproterozoic however could force a premature appearance of complex multicellular life index terms 0330 atmospheric composition and structure geochemical cycles 8125 tectonophysics evolution of the earth 9699 information related to geologic time general or miscellaneous 3220 mathematical geophysics nonlinear dynamics 0315 atmospheric composition and structure biosphere atmosphere interactions citation von bloh w c bounama and s franck cambrian explosion triggered by geosphere biosphere feedbacks geophys res lett 30 18 1963 doi 10 1029 2003gl017928 2003

seismic anisotropy beneath the shumagin islands segment of the aleutian alaska subduction zone lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction in this study we inferred possible strain geometries in the mantle beneath the eastern aleutians by mapping seismic anisotropy with shear wave splitting we analyzed s phases of local events recorded by the shumagin seismic network and obtained splitting parameters fast direction and splitting time t and their 95 confidence regions to avoid complexity from converted phases data were restricted to paths at incidence angles less than 32 and to events deeper than 50 km the s phases sample the forearc and back arc of the shumagin segment from 159 7 w to 163 8 w at depths up to 216 km in our data set of 84 local events 44 of 127 s phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc roughly 60 the remaining 83 s phases produced null measurements and the fast directions inferred from these phase polarizations agree with the fast axis alignment in the constrained measurements splitting times vary from 0 1 to 0 35 s and show an overall increase with hypocentral depth observed splitting parameters are well matched by predicted values for olivine rich mantle wedge models with 1 sv anisotropy where the olivine b axis is arc orthogonal and the a axis is vertical no arc parallel strain or arcparallel no vertical strain these results are consistent with mantle wedge strain models in which arc normal compression is accompanied by arc parallel or vertical shearing or extension

shear wave anisotropy in the crust mantle wedge and subducting pacific slab under northeast japan to study the anisotropic structure beneath northeast ne japan we made 4366 shear wave splitting measurements using high quality seismograms of many earthquakes occurring in the crust and the subducting pacific slab our results provide important new information on the s wave anisotropy in the upper crust lower crust mantle wedge and subducting pacific slab in the upper crust the anisotropy is mainly caused by the stress aligned fluid saturated microcracks the measured delay times dts increase to 0 10 s at 10 11 km depth the fast velocity directions fvds are parallel to either the tectonic stress or the strike of active faults the maximum dts for the low frequency earthquakes near the moho are 0 15 0 17 s suggesting strong anisotropy at the base of the crust or in the uppermost mantle the measurements for the intermediate depth earthquakes in the pacific slab show dominant e w trench normal fvds in the back arc area and n s trench parallel fvds in the fore arc area the trench normal fvds in the back arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the pacific plate the maximum dts for the slab earthquakes reach 0 30 0 32 s at 100 km depth but only half of the total dts are produced in the mantle wedge the small dts in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge in the fore arc the dominant trench parallel fvds for the slab earthquakes are consistent with those for the upper crust earthquakes and 80 of the total dts can be accounted for by the anisotropy in the crust in the subducting pacific slab the trench parallel fvds may reflect either the original fossil anisotropy in the pacific plate when the plate was produced in the mid ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

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

the concept of stored plastic work or frozen elastic energy in soil mechanics this paper is part of a series that aims to explore systematically the applications of the modern theory of thermomechanics to the constitutive modelling of soils and other geomaterials although the division of the applied work into recoverable elastic energy and irrecoverable plastic energy is straightforward at the level of single grains this is not true at the continuum level because of the highly heterogeneous nature of the stress and deformation fields at the micro level some of the micro level elastic energy is frozen and consequently some of the applied plastic work is stored and is recoverable the concept is explained at the micro level at the meso level of the force chains and at the continuum level it is also explained in terms of a schematic spring slider model these ideas have very important ramifications for constitutive modelling at the continuum level in particular it is shown that there are two causes for dilation in a soil one due to reynolds dilatancy the other due to the

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

shear wave anisotropy in the mariana subduction zone to determine the location strength and orientation of seismic anisotropy in the mariana subduction zone beneath guam we evaluated shear wave splitting in local s regional s and scs and teleseismic core phases such as sks recorded at station gumo fast directions from the local s phases have an average azimuth of 45 and splitting times range from 0 1 s to 0 4 s for local s phases from events within the southeastern half of the subducting slab splitting parameters manifest minimal frequency dependence in both fast direction and splitting time however for the remaining local s phases in the data set fast directions vary with frequency content no well constrained splitting parameters were obtained for the regional and teleseismic phases but the particle motions of these unsplit phases are consistent with an average anisotropic fast direction of 45 anisotropy due solely to olivine oriented by slab entrained flow in the mantle wedge would produce local s fast directions at 66 and anisotropy due solely to fossil seafloor spreading in the subducting slab would yield fast directions at 20 to 30 neither of these predictions is consistent with the observed fast directions however the observed splitting including the frequency dependent fast directions can be explained by models containing anisotropy in both the slab and wedge and possibly although not necessarily anisotropy due to recent extension in the overriding philippine sea plate

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

water and geodynamics hydrogen is the most abundant element fig 1 in the galaxy and our solar system lodders 2003 therefore it is not astonishing that hydrogen is a key player in the geodynamic evolution of planets its fate in the early earth after condensation of the solar nebula the accretion of our planet and hydrogen reprocessing through early asteroidal and cometary bombardment dauphas et al 2000 and segregation of a proto earth into iron core and silicate mantle is described elsewhere in this volume marty and yokochi 2006 figure 1 abundance of elements in the solar system in numbers of atoms per 106 atoms of silicon lodders 2003 this chapter concerns itself with the geodynamics of the modern earth where nowadays hydrogen occupies only a very small mass fraction less than 0 1 wt although its abundance is thus drastically diminished it is still believed to have a profound influence on the geodynamics of the present planet the reason why hydrogen can have a profound influence on geodynamics even when it is diluted to ppm levels relies on its fundamental influence on both rock mechanics kohlstedt 2006 and the melting relationships kohn 2006 however both the micromechanical mechanism and the large scale implications are not yet fully understood in this review geodynamic processes which may be triggered by a rock mechanical hydrogen threshold value are reviewed in addition to resulting styles of planetary convection controlled by melt and rock mechanics this review will conclude with a more speculative discussion of the possible implications for planetary evolution from an early earth to the present day and highlight areas of future work following current practice in the literature the term water will be used interchangeably for hydrogen recent breakthroughs in computational geodynamics are throwing a new light on the possible evolution of planetary tectonics simulations

on the detection and identification of converted and reflected phases from receiver functions summary we develop a simple methodology to extract information related to crustal structure such as the mean vp vs ratio the magnitude of lateral variations and the presence of discontinuities from a selection of receiver functions we show that crustal phases can be detected and exploited on individual receiver functions two filtering techniques based on singular value decomposition and spectral matrix filtering are proposed to enhance the coherent phases in receiver functions additionally the relative traveltimes and slownesses between the different phases are utilized for a more reliable phase identification we apply these methods to data of station pru czech republic and compare the results with a classical time domain waveform inversion we find a good qualitative agreement between the simple crustal models inferred from the two approaches waveform modelling suggests that the identification of converted phases can still be ambiguous and other geophysical or geological constraints are needed

the role of pre existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere an example from the oman ophiolite structural and fabric analysis of the well exposed hilti mantle section oman ophiolite suggests that shear zone development which may have resulted from oceanic plate fragmentation was influenced by pre existing mantle fabric present at the paleoridge detailed structural mapping in the mantle section revealed a gently undulating structure with an east west flow direction a nw se strike slip shear zone cuts across this horizontal structure the crystal preferred orientation cpo of olivine within the foliation is dominated by 010 axial patterns rather than more commonly observed 010 100 patterns suggesting that the horizontal flow close to the moho involved non coaxial flow olivine cpo within the shear zone formed at low temperature is characterized by 001 100 patterns and a sinistral sense of shear the olivine cpo becomes weaker with progressive mylonitization and accompanying grain size reduction and ultimately develops into an ultra mylonite with a random cpo pattern the olivine 010 axis is consistently sub vertical even where the horizontal foliation has been rotated to a sub vertical orientation within the shear zone these observations suggest that the primary mechanical anisotropy mantle fabric has been readily transformed into a secondary structure shear zone with minimum modification this occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling we propose that primary olivine cpo fabrics may play a significant role in the subsequent structural development of the mantle thus the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center

self consistent generation of tectonic plates in three dimensional mantle convection abstract despite the fundamental importance of plates in the earth s mantle convection plates have not generally been included in numerical convection models or analog laboratory experiments mainly because the physical properties which lead to plate tectonic behavior are not well understood strongly temperature dependent viscosity results in an immobile rigid lid so that plates where included at all in 3 d models have always been imposed by hand an important challenge is thus to develop a physically reasonable material description which allows plates to develop self consistently this paper focuses on the role of ductile shear localization in two dimensional geometry it is well established that strain rate softening non newtonian rheologies e g power law visco plastic cause weak zones and strain rate localization above up and down wellings resulting in a rudimentary approximation of plates three dimensional geometry however is fundamentally different due to the presence of transform plate boundaries with associated toroidal motion since power law and visco plastic rheologies do not have the property of producing shear localization it is not surprising that they do not produce good plate like behavior in three dimensional calculations here it is argued that a strain rate weakening rheology previously shown to produce plate like behavior in a two dimensional sheet representing the lithosphere is a reasonable generic description of various weakening processes observed in nature one and two dimensional models are used to show how this leads to shear localization and the formation of faults this rheology is then applied to the high viscosity lithosphere of 3 d mantle convection calculations and the velocity pressure viscosity solution for the entire 3 d domain lid and underlying mantle is solved self consistently it is found that the lithosphere divides into a number of very high viscosity plates separated by narrow sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors broad weak zones with dominant convergent divergent motion above up and down wellings are interconnected by a network of narrow weak zones with dominant strike slip motion passive spreading centers are formed in internally heated cases while the resulting plates are not fully realistic these results show that self consistent plate generation is a realizable goal in three dimensional mantle convection and provide a promising avenue for future research

azimuthal shear wave velocity anisotropy in the basin and range province using moho ps converted phases an eight station large aperture seismometer array was deployed in the basin and range province of west central nevada for the 1988 1989 program for array seismic studies of the continental lithosphere passcal passive source seismic experiment during the 10 months of data collection a total of 100 teleseismic events were recorded 30 source equalized p wave receiver functions show waveform variations in timing amplitude and polarity in the moho ps phase that is diagnostic of shear wave velocity anisotropy information regarding the crustal anisotropic component can be recovered through the ps phase since it is generated at the moho and is sensitive to shear wave splitting in the crust only we utilize single event stacked and array beam formed receiver functions to minimize the influence of structural variations and scattering on the observed converted phases analysis of shear wave splitting of the ps conversion from the crust mantle boundary indicates a fast azimuth of anisotropy oriented approximately nw se with observed time delays of 0 20 s this strongly suggests coherent crustal anisotropy with a fast direction perpendicular to the strike of fault block mountain ranges this fabric is most likely due to the preferred crystallographic orientation of seismically anisotropic minerals in the middle to lower crust rather than the distribution of basin and range upper crustal fracture systems

morphodynamic characteristics and classification of beaches in england and wales abstract the use of beach classification models has become widespread in literature in recent years beach classification models that recognise distinct modal beach states in response to environmental conditions are most widely used these models were developed largely in high energy micro to meso tidal sandy environments and subsequent attention has focussed on extending their use into other beach environments here the applicability of these traditional beach classification models to the highly diverse coastline of england and wales was assessed through collection of detailed morphodynamic characteristics of 92 beaches yielding a comprehensive multi variate data set containing morphological sedimentological and hydrodynamic information the complex and diverse study region incorporates beach morphology covering the full spectrum from reflective to dissipative and non barred to multi barred cluster analysis supplemented by mds ordination resulted in the identification of 9 distinct beach types traditional morphodynamic indices and rtr were found to be effective in discriminating between beach groups providing some support for the beach state models derived using information from australian beaches it was found that absolute wave energy wave power is important as well in controlling beach type for intermediate beaches a wave energy flux p h 2 t value of 3 kw m 1 was found to differentiate between beaches with p 3 kw m 1 and without p 1 three dimensional bar rip morphology a key component controlling recreational beach hazard levels observations presented here inform a beach classification model to be used as the basis for a national beach risk assessment programme beach classification models based on environmental parameters are by necessity simplifications and should be used as tools for understanding morphodynamic systems rather than beach type prediction

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

a tensile flexural model for the initiation of subduction we argue that subduction may be initiated at passive continental margins without shortening the lithosphere overcoming the lithosphere s high compressive strength requires special circumstances and these make it difficult to explain the nearly complete recycling of old sea floor instead we present a model that predicts tensile decoupling of the continental and oceanic lithosphere passive rifting and foundering of the seafloor beneath material welling up in the rift this occurs because the lithosphere in a new ocean basin establishes mechanical continuity with the continent at a depth comparable to mid ocean ridges later subsidence at the margin is therefore inhibited by flexure which implies shear stresses that promote fault slip and tensile stresses necessary to balance the component of the plate s weight directed down the margin slope we show that this tension can more than offset ridge push in our model an important additional tension arises from basal shear tractions resisting the plate s motion away from the mid ocean ridge although these tractions cannot be evaluated with confidence slip on a high angle fault decouples the oceanic and continental lithosphere when shear stresses arising from flexure and the applied tension exceed the lithosphere s shear strength under these loads a passive rift then forms allowing a mantle column to rise to the height of mid ocean ridges over 3 km above the old seafloor and flow onto the surface in a gravity current this load flexes the plate downwards which enhances the flow and lets the old oceanic lithosphere founder this model is consistent with the presence of oceanic material in continental forearcs and the youth of ophiolites when they are obducted as both might be explained by rifting a margin and underthrusting the juvenile crust formed there boninites in ophiolite complexes and tectonites at their bases show depleted and hydrated geochemistries consistent with the melting of rifted mantle lithosphere that receives volatiles from foundering oceanic crust

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

on the trade off between seismic anisotropy and heterogeneity numerical simulations and application to northeast japan p wave tomography has been recently used to study 3 d azimuthal and radial anisotropy of subduction zones and continental regions however the fundamental issue about the trade off between the isotropic and anisotropic structures is still unclear in this study we investigate this issue systematically with comprehensive synthetic tests our results indicate that good ray coverage in the azimuth for azimuthal anisotropy and incidence for radial anisotropy is required for determining reliable anisotropic models the isotropic and anisotropic structures are strongly coupled and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence we therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis i e the ratio of the short axis and long axis lengths for estimating the ray coverage quantitatively applying our novel approach to a large number of high quality arrival time data of local shallow and intermediate depth earthquakes we obtained new tomographic images of 3 d p wave azimuthal and radial anisotropy in northeast japan both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab our results show dominant trench normal and vertical fast anisotropy in the mantle wedge while trench parallel and horizontal fast anisotropy in the subducting slab which indicates different dynamics in different domains of the subduction zone

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

characterising sandy beaches into major types and states implications for ecologists and managers abstract sandy beaches are dynamic systems that host a diverse range of fauna whose habitat is controlled by the interactions among sand waves and tides in recent decades much of the study of sand beach ecology has focused on community type and structure across the range of beach types as well as regional variations in order to compare different studies accurate information is required on the nature of the environment studied sand particle size tide range and wave conditions unfortunately this has not always been provided by beach ecologists our understanding of sandy beach processes has advanced to the stage where ecologists can no longer omit assessing the role of wave climate even if this cannot be done precisely as can be done for sand and tides in this paper we provide a method to quantify wave conditions and in combination with sand particle size and tide range determine the beach type and state for waves we first highlight the concepts sea and swell as distinct from the terms sheltered and exposed we then provide a method to estimate breaker height we then develop a model with two steps first breaker height is combined with tide range to identify whether waves or tides are the primary driver of beach morphology and thereby the define the beach type wave dominated tide modified or tide dominated then using breaker height period and sand particle size the beach state is determined seven main beach morphodynamic types states are defined at the narrow and steep extreme are wave dominated reflective beaches and at the wide and low gradient extreme are tide dominated flats the physical and ecological features typical of the seven types states are presented as well as some management implications

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

lithosphere response to externally and internally derived stresses a viscoelastic stress guide with amplification summary lithosphere subjected to an externally derived horizontal stress undergoes creep in the lower lithosphere resulting in the decay of lower lithosphere stress and the associated amplification of stress within the upper lithosphere this stress response of lithosphere has been investigated for a lithosphere model with power law stress and temperature dependent viscoelastic properties the rate and extent of stress decay and associated stress amplification is greatly dependent on the lithosphere geotherm oceanic lithosphere subject to an applied stress of o 1 kb undergoes upper lithosphere stress amplification of 1 5 1 8 and 2 0 at 104 106 and 108 yr respectively at 106 yr the effective lithosphere thickness is reduced to approximately 40 km the stress decay and amplification proceeds more rapidly for an applied stress of 1 0 kb and in the case of a tensile applied stress results in some upper lithosphere fracture for continental and basin and range type lithosphere the comparable stress amplification at 106 yr for a stress of 0 1 kb is 2 0 and 6 5 with effective lithosphere thicknesses of 60 and 20 km respectively the large values of stress amplification for the basin and range lithosphere result in complete upper lithosphere fracture which gives rise to a cyclic process of upper lithosphere faulting and lower lithosphere creep in which extensive lithosphere deformation can occur the stress amplification process also occurs for stresses generated by lateral density contrasts for an isostatically compensated plateau uplift structure deviatoric stresses of the order of 1 kb can be generated in the upper lithosphere by this process and are sufficient to cause tensile fracture of the upper lithosphere the response of viscoelastic lithosphere to constant geometry bending stresses has also been examined and results in substantial but not complete reduction of the bending stresses

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

multiple layers of seismic anisotropy and a low velocity region in the mantle wedge beneath japan evidence from teleseismic receiver functions 1 a complete characterization of seismic anisotropy can yield powerful constraints on mantle flow and deformation this is particularly important for the mantle wedge above subducting slabs where the geometry of mantle flow remains poorly understood we seek to better characterize the geometry and strength of anisotropy in the mantle wedge beneath northeast honshu and hokkaido both of which overlie the subducting pacific plate previous studies indicate that upper mantle anisotropy in the japan subduction zone is highly complex and exhibits dramatic spatial variations to provide complementary constraints on the along strike variations in anisotropy we analyze teleseismic receiver functions from stations of the broadband f net array using the multitaper correlation receiver function estimator backazimuthal variations in p to sh converted energy provide clear evidence for complicated anisotropic structure in the mantle wedge beneath northeast honshu and hokkaido in northeast honshu forward modeling of receiver functions using synthetic seismograms suggests the presence of an anisotropic layer in the forearc mantle wedge above the subducting slab and a second anisotropic layer beneath the crust of the overriding plate we also see evidence for a region of low isotropic velocity in the central part of the wedge beneath ne honshu comparisons between transverse component receiver functions at stations located in ne honshu and hokkaido reveal striking differences providing further evidence for along strike variation in anisotropic structure in the mantle wedge beneath japan

split s waveforms observed in northern taiwan implications for crustal anisotropy short period stations ent and nsk in northern taiwan have frequently recorded split s waveforms from earthquakes in the underlying subduction zone to determine splitting parameters together with objective straightforward error estimations we employ a waveform cross correlation method the error for each measurement is estimated by translating the lower limit of the 95 confidence interval of the cross correlation coefficient into azimuth and time only measurements with 95 confidence regions excluding zero delay time are accepted as splitting data we also use an energy minimization method to constrain the solutions with minor splitting we interpret the splitting as caused by stress aligned cracks in the upper crust the average delay times are 0 03 0 04 s for ent and 0 07 s for nsk the fast directions at the two stations are predominantly n s which roughly match the stress trajectories predicted by a model simulating the arc continent collision in the taiwan area however at ent there is a subset of data with fast directions oriented in roughly e w the nearly 90 rotation of the inferred crack orientation occurs in a horizontal distance of 10 15 km in the upper crust both the predicted heterogeneous stress regime and the local structural setting in northeastern taiwan favor a lateral variation with a similar length scale

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

shear zones in the upper mantle evidence from alpine and ophiolite type peridotite massifs there is abundant field and microstructural evidence for localization of deformation in alpineand ophiolite type mantle massifs on the basis of field relationships and microstructures we recognize two types of tectonite shear zones mediumto coarseand fine grained as well as two types of mylonitic shear zones anhydrous and hydrous peridotite mylonites in tectonite shear zones softening processes responsible for localization are probably melt related weakening in the medium to coarse tectonites and a change in limiting slip system in the fine grained tectonites in peridotite mylonites the most likely cause for softening and localization is a change in dominant deformation mechanism from dislocation to grain size sensitive creep microstructural and petrological study of mylonite rocks reveals that reactions either continuous net transfer reactions anhydrous and hydrous or melt rock reactions play a key role in the formation of fine grained material that promotes grain size sensitive creep these reactions occur over a broad range of pressure temperature conditions encompassing a large part of the lithospheric upper mantle we conclude that mantle shear zones are widespread and that they reduce the bulk strength of the lithosphere significantly attempts to understand the dynamics of tectonic processes depend critically on quantitative knowledge concerning the mechanical properties of the lithosphere and the relative strength of the crust and upper mantle e g brun 2002 for many years it has been accepted that the upper part of the mantle lithosphere is strong compared to the crust e g kirby 1983 kuznir park 1986 the notion of a strong uppermost mantle is based on extrapolation of laboratory flow laws kohlstedt et al 1995 and the depth distribution of seismicity in the lithosphere chen rc molnar 1983 the inferred high strength of the shallow mantle implicitly refers to homogeneous flow however at large strains deformation of the mantle lithosphere may become localized within weak shear zones fig 1 so the application of low strain flow laws may seriously overestimate the strength of the mantle rutter brodie 1988 handy 1989 vissers et 01 1995 in a recent study on the seismicity and elastic thickness of continental lithosphere magi et al 2000 have suggested that the crust is in fact the strongest layer thus it is emerging that the strength of lithospheric upper mantle may vary considerably over the last decade we have studied a number of peridotite massifs both of orogenic alpine type and ophiolitic origin in order to establish under which conditions and to what extent deformation in mantle rocks is localized in this contribution we present an overview of shear localization features in these peridotite massifs from our data as well as published data from other workers it emerges that under lithospheric conditions localization of deformation in mantle rocks is a common phenomenon fig 1 table i in addition shear localization seems to occur over a wide range of pressure temperature conditions producing a number of distinct shear zone types the range of shear zone types is comparable to types of deformation structures found in crustal rocks e g carreras et al 1980 microstructural work has highlighted the importance of mineral reactions as a softening mechanism in the development of mantle mylonite shear zones handy 1989 drury et al from alsop g i holdsworth r e mccaffrey k j w hand m eds 2004 flow processes in faults and shear zones geological society london special publications 224 11 24 0305 8719 15 00 o the geological society of london 2004 12 a h dijkstra eta l

theoretical analysis of shear localization in the lithosphere deformation in the earth is rarely homogeneous and often occurs in narrow regions of concentrated strain referred to as zones of shear localization mylonites found in the continental crust are the manifestation of shear localization white et al 1980 and at the largest scale shear localization is proposed to be crucial for the generation of tectonic plates from a convecting mantle bercovici 1993 1995b bercovici a 1996 1998 bercovici et al 2000 trompert and hansen 1998 tackley 1998 2000a tackley b c d shear localization is apparent in many field of physics including for example metallurgy lemonds and needleman 1986 rock mechanics poirier 1980 jin et al 1998 granular dynamics scott 1996 geminard et al 1999 e g and glaciology yuen and schubert 1979 however basic solid state rheologies such as elasticity visco elasticity viscous flow and even steady state non newtonian viscous flow are insufficient by themselves to generate shear localization this is because in all such rheologies an increase in deformation or rate of deformation results in greater resistance i e stress instead of self weakening and loss of strength shear localization tends to require dynamic feedback mechanisms wherein self weakening is controlled by a macroscopic variable such as temperature or microscopic structure grain size or microcrack density whose evolution and concentration are themselves determined by deformation in this way deformation can induce weakening which subsequently causes deformation to concentrate on the weak zone being most easily deformed causing further weakening and thus more focusing of deformation and so on one of the most fundamental manifestations of such feedback mechanisms arises from the coupling of viscous heating and temperature dependent viscosity wherein the zone of dissipative heating weakens and thus focuses deformation leading to further heating and weakening this mechanism is thought applicable to problems

modeling shear zones in geological and planetary sciences solid and fluid thermal mechanical approaches abstract shear zones are the most ubiquitous features observed in planetary surfaces they appear as a jagged network of faults at the observable brittle surface of planets and in geological exposures of deeper rocks they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of mylonitized reduced grain sizes behind the overall size of the entire shear network rarely exceeds kilometer scale at depth although mylonitic shear zones are only visible to the observer when uplifted and exposed at the surface they govern the mechanical behavior of the strongest part of the lithosphere below 10 15 km depth mylonitic shear zones dissect plates thus allowing plate tectonics to develop on the earth we review the basic multiscale physics underlying mylonitic ductile shear zone nucleation growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization we also discuss a framework for analytic and numerical modeling including the effects of thermal mechanical couplings thermal elasticity the influence of water and void volatile feedback the physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium positive feedback can amplify strength heterogeneities by cascading through different scales we define basic intrinsic length scales of strength heterogeneity such as those associated with plasticity grain size fluid inclusion and thermal diffusion length scale for an understanding ductile shear zones we need to consider the energetics of deformation shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates for the long time scale deformational energy stored inside the shear zone through plastic dilation or crystallographic and shape preferred orientation consumes only a maximum of 10 of energy dissipated in the shear zones but creates structural anisotropy shear zones become long living features with a long term memory a special role is attributed to the presence of water in nominally anhydrous minerals we show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale we have shown that fully coupled finite element calculations using mechanical data from the laboratory can reproduce the basic mode of deformation of an entire mylonitic shear zone the next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure without losing the large scale constraint numerical methods capable of fulfilling the goal are emerging these are adaptive wavelet techniques and hybrid particle finite element codes which can be run over a computational grid across the net

towards a self consistent plate mantle model that includes elasticity simple benchmarks and application to basic modes of convection one of the difficulties with self consistent plate mantle models capturing multiple physical features such as elasticity non newtonian flow properties and temperature dependence is that the individual behaviours cannot be considered in isolation for instance if a viscous mantle convection model is generalized naively to include hypo elasticity then problems based on earth like rayleigh numbers exhibit almost insurmountable numerical stability issues due to spurious softening associated with the co rotational stress terms if a stress limiter is introduced in the form of a power law rheology or yield criterion these difficulties can be avoided in this paper a novel eulerian finite element formulation for visco elastic convection is presented and the implementation of the co rotational stress terms is addressed the salient dimensionless numbers of visco elastic plastic flows such as weissenberg deborah and bingham numbers are discussed in a separate section in the context of geodynamics we present an eulerian formulation for slow temperature dependent visco elastic plastic flows a consistent tangent incremental formulation of the governing equations is derived numerical and analytical solutions demonstrating the effect of visco elasticity co rotational terms are first discussed for simplified benchmark problems for flow around cylinders we identify parameter ranges of predominantly viscous and visco plastic and transient behavior the influence of locally high strain rates on the importance of elasticity and non newtonian effects is also discussed in this context for the case of simple shear we investigate in detail the effect of different co rotational stress rates and the effect of power law creep the results show that the effect of the co rotational terms is insignificant if realistic stress levels are considered e g deviatoric invariant smaller than 1 10 of the shear modulus say we also consider the basic convection modes of stagnant lid episodic resurfacing and mobile lid convection as applicable to a cooling planet the simulations show that elasticity does not have a significant effect on global parameters such as the nusselt number and the qualitative nature of the basic convection pattern our simple benchmarks show however also that elasticity plays a significant role for instabilities on the local scale of an individual subduction zone

p wave anisotropic tomography beneath northeast japan we determined a 3 d p wave anisotropic tomography beneath northeast japan by using first p arrival times the travel time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming p wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally the geometries of the conrad and moho discontinuities and the upper boundary of the subducting pacific plate are taken into account in the model our extensive synthetic tests show that the anisotropic velocity inversion method is reliable our results show that low velocity zones exist below active volcanoes in the crust in the fore arc mantle wedge and in the central portion of the mantle wedge above the high velocity pacific slab which are similar to the previous tomographic images anisotropic velocity variations are revealed in the crust mantle wedge and the subducting pacific slab the anisotropic patterns are complex and different in the upper and lower crust in the mantle wedge the fast direction of p velocity is generally trench parallel in the fore arc area while it becomes trench normal in the back arc area which agree well with the shear wave splitting results the trench parallel fast direction is revealed beneath the volcanic front which may suggest the existence of complex 3 d mantle flows in the mantle wedge the fast velocity direction in the subducting pacific slab is mostly n s which is likely due to the original fossil anisotropy of the pacific plate formed at the mid ocean ridge our result also suggests that the anisotropy is weak in the upper crust and the average amplitude of the velocity anisotropy in the mantle is no larger than 4

subduction initiation and continental crust recycling the roles of rheology and eclogitization abstract a mechanical model with viscous ductile and plastic brittle rheologies is used to investigate the effect of eclogitization on the dynamics of convergence density increases by 300 600 km m3 during eclogitization of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle we explore cases of intracontinental deformation subduction and continental collision we consider a wide range of parameters for friction activation energy and initial thermal state and cases with or without eclogitization the style of deformation appears to be primarily controlled by the presence or absence of weak zones simulations are run with a constant convergence velocity 1 5 or 4 cm year and the evolution of the compressive force through time is thus a critical test of the model viability for intracontinental deformation when the brittle crust is decoupled from the mantle the mantle deforms by symmetrical bending or subduction and a variable amount of lowermost crust is entrained with the sinking mantle in these cases the compressive force remains in the 1012 3 1013 n m acceptable range oceanic subduction only occurs if a low friction shear zone is specified in the brittle realm during the transition from oceanic subduction to collision the oceanic crust in this shear zone remains trapped between colliding crusts eclogitization has no influence on the initial mode of deformation thus it is probably not an important factor in the initiation of subduction in all simulations the influence of eclogitization increases progressively with time in proportion of the amount of lower continental crust and oceanic crust entrained into the mantle the structural evolution of the orogen model depends on eclogitization if initial decoupling occurs at mid crustal level the simulations thus indicate that eclogitization promotes convergence and also in some cases enables the recycling of the whole continental lower crust into the mantle we also suggest that eclogitization could regulate the long term convergence rate in orogens

a unified description of localization for application to large scale tectonics 1 localized regions of deformation such as faults and shear zones are ubiquitous in the earth s lithosphere however we lack a simple unified framework of localization that is independent of the mechanism or scale of localization we address this issue by introducing the effective stress exponent ne a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization the value of ne is based on micromechanics a localizing regime has a negative ne indicating a weakening behavior and localization is stronger for more negative 1 ne we present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale brittle failure with loss of cohesion elastoplasticity rate and state dependent friction shear heating and grain size feedback in ductile rocks in most cases localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables brittle mechanisms failure and friction are generally described by ne of the order of 100 shear heating requires an already localized forcing which could be provided by a brittle fault at shallower levels of the lithosphere grain size reduction combined with a transition from dislocation to diffusion creep leads to localization only if the grain size departs significantly from its equilibrium value because either large scale flow moves rocks through different thermodynamic environments or new grains are nucleated when shear heating or grain size feedback produce localization 1 ne can be extremely negative and can control lithospheric scale localization

seismic anisotropy from local earthquakes in the transition region from a subduction to a strike slip plate boundary new zealand shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone lower half of the north island of new zealand and its transition to oblique transform faulting marlborough area northern south island in marlborough delay times show almost no increase with depth and it is most likely that the higher frequency phases used in this study respond mainly to lithospheric anisotropy in the central marlborough fault system mfs fast polarizations are subparallel to the faults anisotropy is attributed to the presence of metamorphosed schist eclogite of 30 10 km thickness and located 50 80 km beneath the mfs on the edges of the mfs fast polarizations are parallel to the maximum compressive stress direction consistent with crack induced anisotropy in the crust the shear zone which is as wide as the island in the mantle as inferred from sks phases seems to occur in a narrower zone in the crust in the lower half of the north island fast polarizations from events at all depths are oriented parallel to the strike of the hikurangi subduction zone as well as to the faults polarisations are similar to those of sks phases which mainly sample the mantle this suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike slip shear we calculate 1 2 0 3 velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth in order to match the sks delay times this result requires the presence of anisotropic material down to 580 100 km depth or a change in anisotropy with depth or frequency dependent splitting

initiation of subduction by small scale convection 1 the possibility of initiation of subduction by sublithospheric small scale convection is investigated with the help of fowler s theory numerical constraints on the lithospheric stresses generated by convection and direct numerical simulations of subduction the derived scaling laws show that the critical yield stress for initiation of subduction by small scale convection is of the order of 30 mpa this value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone this implies that small scale convection can play an important role in the initiation of subduction on the present day earth this also implies that small scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early earth and other terrestrial planets the critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the earth since the critical yield stress depends only weakly on planetary parameters water seems to be the key condition required for plate tectonics to occur on a terrestrial planet

the tectonics of venus solid venus has several differences from solid earth a mild variation in topography with marked departures of some kilometres confined to less than 10 of the surface no interconnected system of ridges such as would be associated with a spreading lithospheric boundary layer a high correlation of gravity with topography and a ratio of gravity to topography implying compensation depths in excess of 100 km for major features this high admittance ratio implies a stiff upper mantle as also indicated by the high depth diameter ratio of craters the details of magellan radar imagery enable some inference of event sequences the morphology generally indicates a more regional scale of change than on earth the principal chronometer is impact crater distribution the number of impact craters identified is 914 which is equivalent to about 500 ma s infall of bodies big enough to penetrate the atmosphere but the number of craters per unit area has but slightly more variations from the mean than random and only a mild negative correlation with topographic elevation only one third of the craters evidence tectonic or volcanic modification hence the rate of resurfacing over the last few 100 ma must be quite slight compared to earth s the main debate is whether venerean tectonic activity is in monotonic decline or whether it is episodic on a 500 ma time scale evidencing some current tectonic and volcanic activity are a few limited regions of marked topographic and geoidal highs exceeding 8 km and 80 m respectively with steep slopes clearly plate tectonics does not exist on venus the underlying cause of this different evolution appears to be the lack of water this dryness makes the upper mantle stiff enough to regionalize the tectonics and inhibit recycling of crust the lack of water also prevents erosion and thus the recycling of secondary differentiates these deficiencies in recycling imply a strong net upward differentiation of heat sources and a thick crust now allowed by new results on dry diabase rheology

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

global tectonics and space geodesy much of the success of plate tectonics can be attributed to the near rigidity of tectonic plates and the availability of data that describe the rates and directions of motion across narrow plate boundaries m 1 to 60 kilometers wide nonetheless many plate boundaries in both continental and oceanic lithosphere are not narrow but are hundreds to thousands of kilometers wide wide plate boundary zones cover m 15 percent of earth s surface area space geodesy which includes very long baseline radio interferometry satellite laser ranging and the global positioning system is providing the accurate long distance measurements needed to estimate the present motion across and within wide plate boundary zones space geodetic data show that plate velocities averaged over years are remarkably similar to velocities averaged over millions of years